Engineering Excellence

Storage tanks

2020-07-29T22:43:00.059+05:30

Introduction

Steel containers are used for storing liquid or gas in a plant. This blog discusses the most common type of storage vessels, plot plan arrangements, safety considerations, spill containment, pumping, and general piping layout, Tank Settlement for Piping Stress Analysis, Tank Bulging effect or bulging effect of tank shells & stress analysis as per API 650 using CARSAR II.

Tanks

A storage take is a container, usually for holding liquids, sometimes for compressed gases (gas tank). Since most liquids can spill, evaporate, or seep thorough even the smallest opening, special consideration must make for their safety and secure handling. This usually involves building a boundary, or containment dike, around the tank, so that any leakage may be safely contained.

Some storage tanks need a floating roof in addition to or in lieu of the fixed roof and structure. This floating roof rises and falls with the liquid level inside the tank, thereby decreasing the vapor space above the liquid level. Floating roofs are considered a safety requirement as well as a pollution prevention measure for many industries including petroleum refining.

Type of Storage Tanks

There are various storage tanks used for storage of raw material and for storage of final products etc. they are mainly fix roof tanks, floating roof tanks, horizontal bullets and sphere and the details are as follows.

A) Fix Roof Tank

Fix roof tanks are used for liquids where tendency of evaporation is very less and very high flash point. (E.g. fuel oil, water, bitumen etc.) The tank design code is APL 650 and main part are bottom plate, shell and top roof which is conical in shape and used steel section for supporting roof called rafters. Nozzles data will be as per process requirement. Normally nozzles are inlet for tank feeding, outlet which normally connected to pump, drain and vent which are required at the time of maintenance, manholes for ease of maintenance, nozzles level indicator and some spare nozzles. Cone roofs, dome roof and umbrella roofs are usual. These are insulated to prevent the clogging of certain materials, wherein the heat is provided by steam coils within the tanks. Cone roof or fix roof tank is atmospheric tank used for very law pressure. Whereas Dome roof tanks are meant for tanks having slightly higher storage pressure than that of atmosphere (E.g. slop oil).

B) Floating Roof Tank

Floating roof tank is used for high volatile fluids where evaporation rate is very high. Roof will float above liquid like ship float above the water and the surface of the roof is touched above the liquid surface which will minimize vaporization of the fluid. There is no any special code for roof design, but shell design is as per API 650

Floating roof tanks are broadly divided into external floating roof tanks (usually called as floating roof tanks: FR tanks) and internal floating roof types (IFR tanks).

IFR tanks: IFR tanks are used for liquids with law flash-points (E.g. ATF, MS. gasoline, ethanol). These tanks are nothing but cone roof tanks with a floating roof inside which travels up and down along with the liquid level. This floating roof traps the vapor from low flash-point fuels. Floating roofs are supported with legs on which they rest.

FR tanks: FR tanks do not have a fixed roof (it is open in the top) and has a floating roof only. Medium flash point liquids such as naphtha, kerosene, diesel, crude oil etc. are stored in these tanks.

C) Dome Roof Tank

Dome roof tank is one type of fix roof tanks instead of conical roof, dome roof (Dish type) roof is used which is stronger that conical roof and hence will hold slightly higher pressure than conical roof tank and used for petroleum industry for storage of very high amount of liquid.

D) Horizontal Tanks /Horizontal Bullet

Horizontal tank design is as per ASME sec Vlll div.I. horizontal tanks installation is very easy and can be installed above ground or if there is space problem then can install underground also. There type of tanks minimizes evaporation loss. Installation of instruments is very easy. Maintenance can be possible by giving platform and ladder or staircase. Mounting of the tank is on saddle.to take care of thermal expansion one saddle should be fix and another should be sliding. foundation height can be decided as per the operating requirement and piping requirement.

E) Spherical Tanks

As flash-points of fuels go very law the tanks are usually spherical (known as spheres), for storage of LPG, hydrogen, hexane, nitrogen, oxygen etc. Spherical tanks are used for holding very high-pressure liquid or gases. Pressure holding capacity is very high because of equal pressure distribution on surface. construction cost of sphere is very high. Also weld points are more in spherical tank which decreases efficiency of the equipment’s. There are some reasons some countries /companies avoid using spherical tanks and horizontal tanks are preferred instead of spherical tanks.

Other classification which can be made for storage tanks are based upon their location in a refinery:

1. COT- crude oil tankages

2. PIT-product and intermediate storage tankages

3. Dispatch- dispatch area tankages

4. Utilities- tanks made in the power plant area, for storage water etc.

5. OSBL tanks- the first 3 types come under out side battery limit tankages

6. ISBL tanks- these are usually mini tanks which are found in the production units of a refinery (as neutralization tanks,) water tanks e

Typical Arrangement of Tank Piping.

1. Determination of piping route

2. Process piping should not cross any other pipes if possible.

3. Formation of vent pockets

4. Piping from and to the tanks should run through the shortest routes.

5. A minimum distance of 500mm shall be secured between the dike and pipe support.

6. Tank block valves shall be provided with platforms which can also be used as a walkway

7. Valves shall be arranged by aligning the flange surfaces

8. Space between two crossing pipes shall be at least 300 mm.

9. Valve accessories should be considered in determining spaces between closely arranged valves

Links For Reference

Plastic Theory of Bending

2020-07-26T22:14:00.013+05:30

Plastic Theory of Bending

It the

Though the actual stress-strain relationship in the plastic range is complex, an idealized elastoplastic material such as mild steel may be considered to have a stress- strain diagram as shown in figer.

Consisting of two straight-line segments. As long as the loading is in elastic range, the value of stress is less than the

Practically, a structure does not fail as soon as the stress at an edge of the weakest cross-section reaches the yield point. The structure continues to sustain the load as long as a

The assumptions made in the plastic theory are as follows:

1. The material has a single yield point and can undergo considerable strain at yield without further increase in stress.

2. The value of yield stress is the same in tension and compression.

3. Transverse sections remain plane throughout the loading so that the strain is proportional to the distance from the neutral axis.

4. The collapse of the whole structure does not take place till the formation of the plastic hinges at all the required points is not complete.

5. As a plastic hinge develops at any cross-section, the moment of resistance remains constant at that point till the collapse of the whole structure takes place.

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Factor of Safety

2020-07-26T21:11:00.008+05:30

Factor of Safety

From the stress-strain curve we note that ultimate stress is the maximum value of stress, the material withstands before failure.

So, should we allow the material to reach its ultimate stress value while in operation?

The answer is No..! as we would like to keep some scope for safety, since the estimated operation loads may be exceeded at time. Also, the ultimate stress value may be different from that of the tested specimen.

Hence, the actual stress is kept much below the ultimate stress limit. In fact, ideally the stresses should be kept within the elastic limit. The upper limit up to which the stresses are permitted is known as the permissible stress or allowable stress or safe stress or working stress.

The ratio of Ultimate stress to safe stress/Working Stress know as Factor of Safety. It works as a protective shield against failure of material under stress.

The factor of safety for a component or structural part has to the be chosen very carefully. If chosen too small, the possibility of failure becomes dangerously large and if chosen very large, the design can become uneconomical. Generally, the choice of factor of safety for a component is influenced by the flowing considerations:

Material Properties:

Material used for a component may not be homogeneous, i.e., it may not have the same properties throughout. The composition, strength and its dimensions may have small variations during its manufacture. The properties are also altered when a material undergoes various heat treatments.

Type of Loading:

A designer may keep in mind the type of loading (static, impact, reversal, etc.) a structure is going to withstand; yet there are possibilities of inaccuracy in his estimates. There can also be alterations of use of a structure with time.

Number of Loading:

The actual number of loading during the life of a component may be different from the expected loading at the time of designing. The ultimate stress usually decreases with increase in the number of loading. This phenomenon is known as fatigue and plays a key role in designing.

Type of Expected Failure:

Sometimes, the collapse of a structure may occur suddenly and there is no prior indication as in case of brittle materials. In such cases where there is possibility of sudden failure, a higher factor of safety has to be used then where there is possibility of pre-warning in the form of yielding of the material.

Method of Analysis:

As all relations and analyzing methods are based on certain assumptions, it is appropriate to include some factor of safety for such type of assumptions and simplifications.

Maintenance:

All mechanical components and structures require good maintenance during their lifetime. However, to consider poor maintenance, rusting, corrosion, decay and other natural uncertainties in future, some margin has to be included.

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Mechanical Properties of Piping Materials

2020-07-26T02:39:00.006+05:30

Introduction

The primary objective of study properties of piping materials is to provide an piping engineer with tools of analyzing and designing piping components. To determine the suitability of a material under the given loading conditions requires the knowledge of various types of stresses incurring on the components as well as the mechanical properties of the materials. No material can possess all the desired properties in one material. Depending upon the requirements, sometimes a compromise may have to be made while choosing a particular material for a part of a machine and Piping. This blog describes some of the properties most commonly to determine these properties.

The main properties of materials are as follows:

Ductility

It is the property of a material by virtue of which it can be drawn into wires under the action of tensile force. A ductile material must have a high degree of plasticity and strength so that large deformations can take place without failure or rupture of the material. In ductile exhibits a certain amount of elasticity along with a high degree of plasticity.

Brittleness

It is opposite to ductility, i.e., when a material. Cannot be drawn out by tension to smaller sections. A brittle fails instantly under the load with out exhibiting any significant deformation. Examples of brittle materials are cast iron, concrete, glass, stone, etc.

Malleability

This property of a material allows it to expand in all directions without rupture. A malleable material has to be highly plastic, though it may not possess high strength. This property is of great use in processes such as forging, hot rolling, etc.

Hardness

The resistance of a material to indentation including scratching or surface abrasion is termed as hardness.

Strength

It may be defined as the capability of a material to withstand load. It is obtained by divided the load by area. The ultimate strength of material is the load. Required to cause fracture divided by the area of the test specimen.

Toughness

It is the capacity of a structure to withstand an impact load, i.e.., the capacity to absorb without fracture. It depends upon the ductility of a material and its ultimate strength. toughness is represented by the area under the stress- strain diagram and is the energy per unit volume required to cause the material to rupture. This property is highly desirable in components subject to shock or cyclic loading. A general test for toughness is the bend test in which the capability of a material is tested for angular bending.

Fatigue

When loading are repeated thousands or millions of times, rupture occurs at a stress much below the static breaking strength. This phenomenon is known as fatigue. Consideration of fatigue is an integral part of design if the structural or machine components are subjected to fluctuating or repeated loads.

Creep

If the stress in a material exceeds the yield point, the strain caused in the material by the application of load does not disappear totally on the removal of load. The plastic deformation caused to the material is known as creep. At high temperatures, the strain due to creep is quite appreciable.

More detail about the creep is already covered in article; Time Dependent Stresses: Creep

Time Dependent Stresses: Creep

2020-07-26T01:48:00.002+05:30

If the stress in a material exceeds the yield point, the strain caused in the material by the application of load does not disappear totally on the removal of the load. The plastic deformation caused to the material is known as Creep. At high temperature, the strain due to creep is quite appreciable.

Creep is the property by which, at elevated temperatures, metals continue to elongate continuously under a constant stress. The stress may be much less than the ultimate tensile stress. Creep is measured by the rate of strain per hour at a certain temperature under a given stress. When a tensile test is carried out in a shorter period of time on a meter specimen at some specific temperature, a definite value of ultimate tensile stress can be obtained. However, under creep a material can also fail even at lower stress values but at higher temperatures if enough time is allowed to pass. It is also observed that there is limiting stress value below which creep does not take place at a certain temperature and the material does not fracture even if the stress is applied for indefinite time. Thus, it becomes necessary that the materials which are exposed to high temperatures for longer periods at certain stress values, the design must be based on limiting creep stress.

To determine the limiting creep stress requires very long periods which may be months or even years for investigations. It is also observed that the rate of creep diminishes with time. Thus, to have a fair value of limiting creep stress at a certain temperature, effort is made to find the stress at which, after a shorter period of time, a definite very small rate of creep takes place. Usually, a typical stress value is obtained which causes a creep of 1 millionth per hour after 40 days.

Some creep resistant alloy steels have been developed by adding small percentages of molybdenum, cobalt, vanadium and tungsten for high-temperature applications such as gas turbines and high-pressure steam fitting.

Classical Creep Curve:

The Classical creep curve is shown in following diagram.

The rate of deformation is called the creep rate. It is the slope of the line in a Creep Strain vs. Time curve.

As per the diagram, Creep has following three stages;

Primary Creep: starts at a rapid rate and slows with time.

Secondary Creep: has a relatively uniform rate.

Tertiary Creep: has an accelerated creep rate and terminates when the material breaks or ruptures. It is associated with both necking and formation of grain boundary voids.

Magnetic Particle Testing -XI: Type of Discontinuities

2020-07-23T21:37:00.017+05:30

Magnetic particle testing is one of the most widely utilized NDT methods since it is fast and relatively easy to apply, and part surface preparation is not as critical as it is for some other methods. The method is used to inspect a variety of product forms including castings, forgings, and weldments. This blog will discuss types of discontinuities that can be evaluated with the magnetic particle method.

Type of Discontinuities

Discussing can be divided into three general categories: Inherent, Processing, and Service.

A. Inherent Discontinuities

Inherent Discontinuities are usually formed when the metal is molten. Inherent wrought discontinuities relate to the melting and solidification of the original ingot before it is formed into slabs blooms, and billets.

Inherent cast discontinuities relate to the melting, casting, and solidification of a cast article. Usually caused by inherent variables such as inadequate feeding, gating, excessive pouring temperature, and entrapped gases.

B. Processing Discontinuities

Processing discontinuities are usually related to the various manufacturing processes such as machining. Forming, extruding, rolling, welding, heat treating, and plating.

C. Service Discontinuities

Service discontinuities are related to the various service conditions such as stress, corrosion, fatigue, and erosion.

During the manufacturing process, many discontinuities that were subsurface (and not detectable by magnetic particle inspection) will be made open to the surface by machining, grinding, etc.

Remember that discontinuities are not necessarily defects.

Any indication that is found by the inspector is called a discontinuity until it can be identified and evaluated as the effect it will have on the service of the part.

The greatest aid in interpretations is a knowledge of what is likely to be present in any given instance. Knowing the history of a part, what it is made of and what processes it has been through all form important considerations.

Type of Indication

Every magnetic particle pattern produced is due to some magnetic disturbance set up in a leakage field the inspector must be able to determine whether there is a serious crack, or some insignificant or unimportant non-relevant indication.

Surface indications produce sharp, distinct, clean-cut, and tightly held indication patterns. Subsurface indications tend to produce indications which are less distinct, forming diffused or fuzzy patterns.

Non-relevant Indications

This is a group of non-relevant magnetic disturbances not due to discontinuities or actual breaks in the metal a common non-relevant indication could be caused by, a constriction in a metal through which lines of force must pass, such as the shaft with a key way shown below.

Non-relevant indications can be caused by the following:

Excessive magnetizing current.
Structural design of the article.
Variances of permeability within the article.

Over-magnetization could cause a non-relevant indication due to the leakage field attracting particles as shown below.

Excessive magnetization can also cause non-relevant indications on a simple square object as shown on the circularly magnetized part below.

Abrupt changes of section thickness of a longitudinally magnetized the part will causes flux leakages where the line of force leave or enter the part, causing non-relevant indications.

Permeability differences in the part can also cause non-relevant indications. Cold working a metal can change the permeability.

Example: Bending and Straightening a nail will cause the metal to become hard at the point of bending. When the nail is magnetized there will be a flux leakage where the permeability is changed. (see below)

A non-relevant indication would also appear across the shank of a cold chisel where the heat -treated portion ends and the soft shank begins. (see below)

Problems identifying non-relevant indications can be reduced if the operator remembers that:

They can be identified by a fuzzy, rather than a sharp, indication.
They are usually associated with some feature of construction like a keyway or sharp corner.
They are usually uniform in direction and size.

False Indication

False indication are caused when particles are accumulated and held mechanically or by gravity. If the part has a rough surface this may cause false indications.

Inherent Discontinuities

Inherent Discontinuities relate to the original melting and solidification of the metal in the ingot or in a casting. Typical discontinuities found in the ingot are inclusions, blowholes, pipe, and segregation.

Nonmetallic inclusions such as slag, oxides, and sulphides are present in the original ingot.
Blowholes (porosity) are formed by gas which is insoluble in the molten metal and is trapped when the metal solidifies.
Pipe is a discontinuity in the center shrinkage during solidification.
Segregationoccur when the distribution of the various elements is not uniform throughout the ingot. This condition is called “banding” and is not usually significant.

When an ingot is further processed into slabs’ blooms, and billets, it is possible for the above discontinuities to change size and shape. The discontinuities after rolling and forming are called lamination, stringers, or seams depending on the type of processing and the original type of discontinuity.

The “Hot Top” is usually cropped off to remove most of the discontinuities, before the ingot is further processed.

Typical Inherent Discontinuities found in castings are Cold shuts, Hot tears, shrinkage cavities, micro shrinkage, blowholes, and porosity.

A Cold Shut is caused when molten metal is poured over solidified metal as shown below:

Hot tears (shrinkage cracks) occur when there is unequal shrinkage between light and heavy sections as shown below:

Shrinkage cavities are usually caused by lack of enough molten metal to fill the space created by shrinkage, similar to pipe in the ingot.

Micro shrinkage usually appears as many small subsurface holes at the gate of the casting. Micro shrinkage can also occur when the molten metal must flow from a thin section into a thicker section of a casting.

Blow holes are small holes at the surface of the casting caused by gas which comes from the mold itself. Many molds are made of sand and when molten metal comes into contact with the mold. The water in the sand is released as steam.

Porosity is caused by entrapped gas. Porosity is usually subsurface but can occur on the surface depending on the design of the mold.

Processing Discontinuities

Processing discontinuities are those found or produced by forming or fabrication operations including rolling, forging, welding, machining grinding, and heat treating.

As a billet is flattened and spread out, nonmetallic inclusions may cause a lamination. Pipe and porosity could also cause lamination in the same manner as shown below

As a billet is rolled into bar stock, nonmetallic inclusions are squeezed out into longer and thinner discontinuities called stringers.

Welding discontinuities

The following are types of processing discontinuities.

Forging Discontinuities

Forging discontinuities occur when metal is hammered or pressed into shape, usually while the metal is very hot. A forged part gains strength due to the grain flow taking the shape of the die. This process is shown below.

A Forging Lap is caused by folding of metal on the surface of the forging, usually when same of the for forging metal is squeezed out between the two dies.

A Forging Burst is a rupture caused by forging at improper temperatures. Bursts may be either internal or open to the surface as shown below.

Grinding cracks are a processing Type discontinuity caused by stresses which are built up from excess heat created between grinding wheel and metal.

Grinding Cracks will usually occur at right angles to the rotation of the grinding Wheel.

Heat Treating Cracks are often caused by the stresses built up during heating and cooling. Unequal cooling between light and heavy sections may cause heat treat cracks. Heat treat cracks have no specific direction and usually start at sharp corners which act as stress concentration points.

Service Discontinuities

Service discontinuities are probably the most important types to considered extremely. Article which may develop defect due to Metal Fatigue are considered Extremely Critical and demand close attention.

Fatigue cracks are service- type discontinuities that are usually open to the surface where they start from concentration points.

Fatigue cracks are possible only after the part is placed into service, but may be the result of porosity, inclusions, or other discontinuities in a highly stressed metal part.

Preservation of Indications

It is often desirable to preserve magnetic particle indications for future reference. There are several methods used to accomplish this purpose.

Lacquer Technique requires the indication to be sprayed with a clear lacquer and later photographed.
Direct Photography is used to record the indication immediately after performing the test.
Magnetic Rubber uses a process where the magnetic particles are in solution with a liquid rubber. The rubber is allowed to harden on the part after the magnetic field has arranged the magnetic particles.
Transparent Tape Transfer is commonly used to lift the indication from the part. The tape can be photographed or placed directly into a permanent record book.

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Magnetic Particle Testing

Magnetic Particle Testing -X: Different Methods of MPT

2020-07-20T21:15:00.011+05:30

Magnetic Particle Applications

Prior to discussing the applications, some previously mentioned principles are recapped below. several different bases are used for classifying magnetizing methods:

Whether or not the magnetizing force is maintained during the application of the medium. This includes the residual and continuous methods.
The character of the field utilized for magnetization which includes the circular and longitudinal methods.
The type of magnetizing current used which may be either AC, DC, or HWDC.

Residual Method

The medium is applied after the specimen has been magnetized and the magnetizing force removed. This method is not used on specimens that have low Retentivity.

Continuous Method-

The magnetizing operation is conducted simultaneously with the application of the Dry Powder or Wet Suspension.

Circular Magnetization Procedures

Where it is necessary to pass a current through the specimen, care must be exercised to prevent arcing or overheating at the contact areas. All contact areas must be clean, and suitable head pressure must be exerted to insure uniform magnetization.

Longitudinal Magnetization Procedures

When a solenoid or coil is used to magnetize the specimen, it should be no larger than necessary to accommodate the specimen.

Direct Current : To obtain indications of discontinuities that may be subsurface, Direct current (DC) or halfwave rectified current (HWDC) should be used.

Alternating Current : AC is used when the discontinuities are suspected to be on the surface of the specimen.

Testing Medium (Powders and Suspensions)

Dry Powder is commonly used for testing weldments where the prod method is employed. The powder is sprinkled on the surface while the magnetizing current is flowing. Liquid medium (Non-Fluorescent) can be used with both wet residual and wet continuous methods.

Liquid Medium(Fluorescent)can also be used with both wet residual and wet continuous methods. The particles are coated with a fluorescent dye which, when inspected under a black light, fluoresce brilliantly.

The Bath Strength can be checked by the settling test described in last blog,Magnetic Particle Testing -X: Magnetic Particle Mediums & Preparation. The frequency of the test is determined by the degree of bath usage with the strength usually dependent upon the manufactures’ specifications.

Surface Preparation

Prior to magnetic particle testing, the specimen should be thoroughly cleaned. Cleaning may involve removal of flake, slag, heavy build-up of paint, test results. The smoother the surface and the more uniform the color, the more favorable are the conditions for formation of a magnetic particle indication.

Location of Discontinuities

Discontinuities can be located either on or directly below the surface of the specimen. Discontinuities located on the surface appear as sharp, distinct lines, whereas, discontinuities located below the surface appear as irregular, rough, hazy indications.

Demagnetization Requirements

If a specimen is to be magnetized in a second direction, I. e. circular magnetization followed by longitudinal magnetization, the last applied field will drive out the residual field from the preceding magnetization. However, this will happen only if the magnetizing force last applied is equal to or higher than the previous residual field.

A Field Indicator is used after performing demagnetization on an article that has been circularly magnetized to Demagnetize if the field strength is reduced to the desired level. Large specimens can be found difficult to demagnetize completely. shifting the specimen to align it in an east-west position from an original north-south position facilitates demagnetization. The reason for this is the influence of the earth’s magnetic field.

Application of MPT Procedures

The magnetic particle technician must have a good understanding of the reasons for applying the different techniques to the same part. It is even more important to be able to select the proper technique and procedure to obtain the best results.

Magnetization of A Solid Cylindrical Specimen

What methods must be used to find all of the discontinuities shown in the bar below?

Head Shot (Circular Magnetization): Inspect for discontinuities showing a longitudinal indication.

Coil Shot (Longitudinal Magnetization): Inspect for transverse indications.

Magnetization of A Large Gear

What method must be used to find all of the discontinuities in the gear below?

Head Shot (Circular Magnetization) with central conductor: Inspect for discontinuities perpendicular to the circular field.

Head Shot (Circular Magnetization) using at least two shots and turning the gear 90 degree.

Magnetization of Short Hollow Cylinders

What methods must be used to find the discontinuities on the inside and outside of this ring?

Head Shot (Circular Magnetization) with central conductor

If necessary, the ring can also be magnetized by two head shots across the diameter or by placing in a coil. This will produce a longitudinal field which will detect discontinuities in the direction shown below.

Magnetization of Hollow Cylindrical Specimens

What method must be used to find the discontinuities on the inside and outside of this cylinder?

Coil Shot (Longitudinal Magnetization): Inspect for discontinuities showing transverse indications on the inside and outside of the cylinder as shown below.

Head Shot (Circular Magnetization) with central conductor: Inspect for longitudinal discontinuities on the inside and outside of the cylinder as shown below.

Magnetization of Irregularly Shaped Specimens

Each specimen must be thoroughly analyzed to determine which methods will be necessary to find discontinuities in all directions. the part shown below requires the use of a central conductor and two additional head shots.

Magnetization of Large Weldments or Castings

Circular magnetization of large specimens is usually accomplished with the use of Prods.Longitudinal magnetization of large specimen is usually done with either a Yoke or by Wrapping the part with the cables.

The Weldment shown below requires a crisscrossed pattern in applying the magnetizing current with prods to insure 100 percent coverage of discontinuities.

The large shaft shown below requires the cables to be wrapped at several different locations to insure 100 percent coverage. However, longitudinal discontinuities will not be detected with this method.

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Magnetic Particle Testing

Magnetic Particle Testing -IX: Magnetic Particle Mediums & Preparation

2020-07-12T23:26:00.010+05:30

When the medium, whether dry or liquid, is applied to the specimen while the magnetizing current is flowing, the procedure is known as The Continuous Method.

If the medium is applied after the magnetizing current is shut off, the procedure is known as The Residual Method.

In the dry method, the powder is applied by sprinkling or dusting the specimen.

dry method

In the wet method, the medium is mixed with a liquid (kerosene or oil) to make a bath, which is then applied to the surface of the specimen.

wet method

Particles of the testing medium must possess two important properties:

1.High Permeability

2.Low Retentivity

Magnetic particles containing these characteristics will give maximum response in a leakage field but will not remain magnetized when the field is removed. The size of the particles used in the dry method varies but they will usually all pass through a 100-mesh screen.

The shape of the magnetic particle should be spherical and must have a high degree of the magnetic mobility and still have substantial attractive power.

Round smooth particles offer good mobility but have low attractive power. Long, slender, jagged particles have excellent attractive power but do not have the mobility to move in leakage field.

In the Wet Method, Magnetic Oxides of iron are used because they are extremely fine and have a lower Permeability than the metallic Dry particle.

The particle size must be small in the wet method to permit the particles to remain in the suspension in the liquid.

Mobility is important because when the particles are brought into a leakage field they must be able to move to form a pattern or indication.

In the dry method, mobility is assisted by dusting or blowing the particles over the surface of the specimen.
Mobility can also be assisted by vibrating the specimen after the particles the have been dusted on the surface.
Alternating current also assists mobility, since the alternating field causes the particles to “dance”.
In the wet method mobility is greatly assisted because the particles are suspended in a liquid bath.

Visibility is important in magnetic particle testing and a good light source is essential. Magnetic particles are usually available in grey, red and black, and the choice of color is determined by the best contrast with the specimen surface.

Fluorescent particles are commonly used in the wet method to aid visibility, but this requires the use of a black light. Light.

The wet suspensions (bath) used in the wet method consist of a liquid in which the particles are suspended.

The particles used are obtainable in a highly concentrated from and may be either fluorescent or non-fluorescent. To achieve the required test sensitivity, the degree of particle concentration in the bath must be correct.

Agitation must be constant while the bath is in used to maintain the particles in suspension. Agitation is usually accomplished by electrically driven pumps. The bath should be checked daily due to evaporation and loss of particles that are removed from the bath by the specimen.

The settling test is essential to check the strength of the bath and is accomplished by gravity settling in a graduated pear-shaped centrifuge tube as shown above.

1. Agitate the suspension thoroughly to assure particle distribution.

2. Fill 100 cc sample from the delivery hose into the centrifuge tube.

3. Demagnetize, if necessary (when clumping occurs in the tube).

4. Allow settling for 30 minutes.

5. Take reading and record log.

6. Adjust bath, either by adding particles or vehicles, if necessary.

The reading should be between 1.5 and 2.cc for a non-fluorescent bath and between 0.2 and cc for a fluorescent bath as shown above. (these data are guidelines and will vary from one specification to another.)

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Magnetic Particle Testing

Magnetic Particle Testing -VIII: Safety Precaution & Limitations

2020-07-12T21:54:00.002+05:30

Magnetic Particle Testing : Safety Precaution & Limitations

Safety Precaution

A. Electrical Arcing is Hazard.

It is usually caused by poor contact or by using excessive current.
It can also be caused by allowing the prods to slip which may result in arcing or damaging the surface of the part.
The arcing causes sparks which may result in a fire.
Electrical Arcing

B. The Dust from the magnetic Particles is nontoxic, but excessive amounts should not be inhaled. Use a dust respirator.

C. Smoking or Open Flame should not be permitted near the wet bath process.

D. The Black Light which is used with fluorescent particles will not damage the skin or eyes if the filters are used on the light.

It is possible to experience a clouding of vision if the black light is directed into the eyes.
This happens because the fluid in the eyes will momentarily fluoresce.

E. Burns can be caused due to the resistance heating in the part or in the prods. Caution should be used in handling the parts or touching the immediately after the magnetic particle test.

Limitations of Magnetic Particle Testing.

Nonmagnetic materials cannot be testing.
Magnetic particle testing will not detect discontinuities that deeper than approximately ¼ inch below the surface. However, the penetration depends on the permeability of material, type of discontinuity, and amount & type of current used.

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Magnetic Particle Testing

Magnetic Particle Testing -VII: Magnetic Particle Testing Equipment.

2020-07-12T12:29:00.007+05:30

Magnetic Particle Testing Equipment.

The following should be considered when selecting Magnetic Particle Testing

Is Equipment for Wet or Dry method.
Magnetization requirement.
Demagnetization- Incorporated or separated unit?
Amperage Required.
Line Voltage Requirements.
Accessories Needed or Required.

Wet Horizontal Equipment

To obtain Circular Magnetization, the specimen is clamped between the Head and Tail Stocks.

For Longitudinal Magnetization, the coil is moved so that the area to be tested is encircled by the coil.

A typical Wet Horizontal Unit usually accommodates both AC and HWDC Magnetization. The Wet continuous field method has three basic steps;

Flow bath through nozzle and over entire surface of part.
Stop bath flow.
Apply current at the instant bath flow is stopped.

Dry continuous-field method

With this method the powder is usually applied from a shaker, bulb, or blower and follows these steps:

Apply magnetizing current
Blow powder particles over magnetized area.
Blow excess powder off part.
Shut off magnetizing current.

The powder should float to the magnetized area so the particles will be attracted to any flux leakages. The dry powder method is typically used with both mobile and portable equipment which will be discussed on the following.

Mobile Equipment

In many cases it is necessary to bring the equipment to the specimen.

Typical Mobile Equipment

Typical Mobile Equipment such as the one above usually operates on 220/440 volts AC and will produce about 4000 amperes. Mobile equipment will usually produce both AC & HWDC magnetizing current.

The Cables used on the above vary from 15 feet to 100 feet. Shorter cable will permit the maximum current output.

Prods and Cables are usually used with the mobile equipment. It often takes two technicians to manipulate the Prods & apply the magnetic powder.

However, Longitudinal Magnetization can be produced by wrapping the cable into a coil. It is also possible to use a central conductor clamped between the two cables to produce Circular Magnetization.

Typical Mobile equipment uses the Dry Powder Method but can be use with Aerosol cans or External Tanks.

Portable Equipment

Portable Equipment is lighter and less expensive than the other types of magnetic particle testing equipment.

Typical Portable Equipment

Typical Portable Equipment as shows above operates on 110/200 volts AC with an output of between 500 & 1200 amperes. These units usually have a choice of either AC or HWDC.

As with mobile equipment, the cables can be used for prods, wrapping into a coil or connecting to a central conductor.

Portable Equipment also used the Dry Powder Method for most application.

Demagnetizing Equipment

The most common type of Demagnetizing Equipment consists of an open coil through which AC is flowing.

A typical unit is shows below and includes a track that will carry the part through the coil. As the part is carried away from the coil the magnetic field is reduced. The continuously AC current in the coil completes the demagnetization.

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Magnetic Particle Testing

Magnetic Particle Testing -VI: Demagnetization

2020-07-11T11:57:00.023+05:30

Demagnetization

Demagnetization requirements

A field Indicator is used after performing demagnetization on an article that has been circularly magnetized to Demagnetize if the field strength is reduced to the desired level.

Large specimens can be found difficult to demagnetize completely. shifting the specimen to align it in an east-west position from an original north-south position facilitates demagnetization. The reason for this is the influence of the earth’s magnetic field.

A residual magnetic field may not be desirable in the part for several reasons:

Residual fields will affect magnetic compasses or create problem with delicate instruments.
Residual fields in rotating pars will attract metal particles. Causing excessive wear or binding.
Parts are also demagnetized so that all magnetic particles can be removed for further processing.
Residual fields can cause” Arc Blow” which deflects the molten metal during DC weldingoperations.

Review of residual magnetism

The residual filed is in the same direction as the magnetic field.
The residual filed is weaker that the magnetizing field.
The original magnetizing force caused the residual filed.
When an article has been magnetized in more than one direction. The second field applied completely overcomes the first field however, this is only true if the second field is stronger than the first.

When the second field applied is not stronger than the first field. Then a combination circular-longitudinal residual field will result.

It is difficult to tell whether a circularly magnetized bar is demagnetized because the flux lines do not normally leave the bar. On the other hand, it is easy to tell if a longitudinally magnetized bar is still magnetized.

Therefore, it is often recommended that a circularly magnetized part be longitudinally magnetized and then go through the demagnetization procedure.

Demagnetizing a part that has been longitudinally magnetized each time the magnetizing field is reduced and reversed, the residual field is reduced.

Reversing the magnetic field

Reversing the part in the magnetic field.
Reversing the current through the coil.
Reversing the coil (turn the coil 180).

Reducing the magnetic field

Reduce the magnetizing current.
Move the part away from the coil.
Move the coil away from the part.

Any method of demagnetizing will combine one of the methods to reduce the magnetizing field with one of the methods to reverse the magnetizing field.

Demagnetization Definition

The removal of residual magnetism by simultaneously or alternately reducing the strength and reversing the direction of a magnetic field.

Demagnetization procedures

A. Alternating current coil method

Alternating current is electrical current flowing through a wire, first in one direction, then in the opposite direction. Each time the current reverses direction, the magnetic field of the coil reverses. (this meets one of the two requirements for demagnetization.)

To complete the demagnetization process, the part is placed in the reversing magnetic field as shown below, and the current is slowly reduced which reduces the strength of the magnetic field.

A Rheostat is often used to reduce the current through the coil.

Reducing the magnetizing field in an AC coil demagnetize is usually done by slowly moving the article away from the coil.

B.Demagnetization using direct current

With DC current is not automatically reversed. It is therefore necessary to have some mechanical means to reverse the current. Because DC is more penetrating than AC, it is used on large parts. The maximum degree of demagnetization can be obtained with DC when the field is reversed at a frequency of one reversal per second.

In DC demagnetization, the magnetizing field should be reduced first, then reversed.

A rule of thumb for demagnetizing soft iron: At least 10 reversals, but not over 30.

When an article is demagnetized, the earth’ s field will leave a smaller amount of residual magnetism in the article if the demagnetizing field is also in a north- south direction.

where complete demagnetization is required, the demagnetization field must be placed in an east -west direction (the coil opening facing east and west).

Leakage field indicators

There is no satisfactory method by which the magnetic field can measured inside the article without destroying it.

There must be leakage field in order to determine whether the article is magnetized.

The field indicator

Compares the strength of the external field of the article with a fixed field inside the indicator.
Is used more to locate flux leakage than to measure field strength.
Is used to show when the part is demagnetized.

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Magnetic Particle Testing

Magnetic Particle Testing -V: Current Requirement For MPT

2020-07-11T07:01:00.009+05:30

Introduction:

In our last article, Magnetic Particle Testing-IV; we have discuses about Current Field Distribution Around the Conductor. In this article we are going to learn about the Current requirement for magnetization for Magnetic Particle Testing.

Current Requirements (Circular Magnetization)

The amount of current will vary with the shape and permeability or the material being tested. At test specimen with a typical indication is a good method to assure that only enough current is used to show the indication.

Too much current will burn the part or may cause heavy accumulations of iron particles.
Too little current may not provide enough flux leakage to attract the iron particles.

The following rule is used to determine the current needed:

800-1000 amperes per inch of article thickness or diameter.to use this rule on article of greater thickness, just multiply the 800 and 1000 by the number of inches of article thickness. What amperage would be used on the following part?

Steel bar 10 X 3 X 2 inches thick

Answer: 1600 to 2000 amps

What amperage range is required to circularly magnetize the bar shown the head shot below?

Answer: 800 to 1000 for first shot

2000 to 2500 for second shot

The rule of using 800-1000 amperes per inch of thickness also applies to circular magnetization with a central conductor. Article thickness is taken from the outside diameter of the article.

what would the ampere ranges be for the three parts shown on the central conductor below?

Answer: 800 to 1000 for 1 inch Nut

1600 to 2000 for 2 inch Ring

2400 to 3000 for 3 inch Spacer

Current requirements (longitudinal magnetization)

When a coil is used to produce longitudinal magnetization, the effective filed it creates is determined by the product of the number of amperes and the number of turns in the coil. for example, a current of 800 amperes through a five-turn coil creates a magnetizing force of 4000 amperes turns.

The amount of current needed for longitudinal magnetization with a coil is controlled by the formula:

Where,

I=Current in amperes

N=Number of turns in coil

L=Length of article

D=Diameter or thickness of article

The figure 45,000 is a constant for all computations. L\D is the length to diameter or thickness, ratio of the pat in is the ampere- turns.

When the number of ampere-turns is found using the above formula. The next step is to divide the Ampere-Turn by the number of turns is the coil. This will determine the magnetizing current needed most coils typically have three to five turns.

What magnetizing current would be need for a part 16 inches long with a diameter of 2 inches, using a five- turn coil?

Answer: 1125 Amperes

Following Assumption are made, while using the formula

An article greater than 18 inches requires more than one coil shot.
The cross section of the article is not greater than one-tenth the area of the coil opening.
The article has an L/D ratio of between 2 and 15.
The article is placed against the inside wall of the coil, and not in the center where the flux density is zero.

Prod magnetization

Prods are current- carrying conductors (usually copper) which are used to magnetize localized areas as shown below.

Caution: the use of prods may be restricted for some applications due to the possibility of burns at the point of contact.

Prod magnetization creates a circular magnetic field in the part. In the sketch above can you use the left- hand rule to determine if the current is flowing from A to B or from B to A?

A guideline for determining how any given testing problem is shown below.

When Half-Wave direct current (HWDC) is used, the field strength per ampere is about the same as with Direct Current (DC) at typical prod spacing (6 to 8 inches).

However, since HWDC consumes less power and produces lower heating effects at the prod contact points, it is often recommended. HWDC also produces better powder mobility than DC.

The prods should be placed on the part so that the resultant circular field is at 90 degrees to the suspected discontinuities.

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Magnetic Particle Testing

Magnetic Particle Testing -IV: Current Field Distribution Around The Conductor

2020-07-10T23:43:00.030+05:30

Introduction

In our last article, Magnetic Particle Testing -III; we have discuses about Hysteresis Loop and it behavioral relation with Alternating Current, Direct Current and Half-Wave Alternating Current. In this article we are going to learn about Magnetic Field Distribution around the testing material. This help to understand the method of magnetization and Demagnetization as per the requirement.

Direct current filed distribution in Solid Conductor

A. Solid nonmagnetic conductor

When direct current is passed directly through a solid nonmagnetic conductor, such as a copper bar, the following can be observed:

The magnetic field strength varies from zero at the center to a maximum at the surface.
The field strength outside the conductor diminishes with the distance from the center conductor ‘s center (e.g. the field at two times the radius from the center is half the field at the surface).

B. Solid magnetic conductor

Distribution of a direct current magnetic field within a magnetic article can be illustrated graphically as shown below.

The flux density increases evenly from zero until it reaches peak strength at the surface.
Because of the permeability of steel, the field strength is greater within a magnetic conductor as compared to a nonmagnetic conductor.
The flux density drops rapidly just outside the surface of the steel bar.
The field strength outside a solid conductor is the same with either a magnetic or nonmagnetic conductor.

Hollow conductor

A. Hollow magnetic conductor

Permeability is again a factor in determining field strength. If the outer diameter and current flow are identical when comparing a solid and hollow conductor, the filed strength will be the same.

B. Hollow nonmagnetic conductor

In both magnetic and nonmagnetic hollow conductors, the filed strength is zero at the inner surface and increases to maximum at the outer surface.

As with both solid and hollow, and both magnetic and nonmagnetic, the field strength outside the conductor diminishes with the distance from the central conductor’s center. However,

The difference between the permeability of magnetic and nonmagnetic materials affects the field strength within the conductor.

Because of the low permeability of a nonmagnetic conductor, the field strength is relatively low. as shown below, direct current flowing through a central conductor will produce a maximum field on the inside surface of the tube being inspected.

Since the magnetizing force is from the field external to the central conductor, it is obvious (by comparing previous sketches) that either a magnetic or nonmagnetic bar could used for a central conductor.

However,material such as copper is often recommended as a central conductor because there is less heat build-up due to better conductivity.

Alternating current filed distribution

Up to this point, all filed distributions have assumed the use of Direct Current (DC)

Alternating Current (AC) tends to flow near the surface of a conductor. This phenomenon is known as “skin effect.” It is shown below that AC provides a concentrated flux density near the surface which provides for good detection of surface discontinuities.

It is also shown above that DC provides the best conditions for locating subsurface discontinuities because of the distribution of the flux density.
The field strength outside the conductor is comparable for both AC and DC.
As with DC both conductivity and permeability affect the field strength and distribution.

Sensitivity of methods

It is well established that AC method is best suited for finding surface defects.

However, the chart below illustrates the ability of various currents using both wet and dry magnetic particles in locating subsurface discontinuities.

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Magnetic Particle Testing

Magnetic Particle Testing -III : Relation Between Current Type & Hysteresis Loop

2020-07-08T21:47:00.008+05:30

Alternating Current (AC) is the most widely use for conducting magnetic particle testing. AC can be readily converted to the low voltages used in magnetic particle inspection using transformers. AC has little penetrating power and provides the best detection of surface discontinuities. It is not effective for subsurface. Since AC is continuously reversing direction. The magnetic field has a tendency to agitate or make the iron particles more mobile.This causes the iron particles to be more responsive to the flux leakage.Current reversal is illustrated below.

Direct current (DC)-single phase ac can be rectified to produce half-wave alternating current (HWAC) commonly called half-wave direct current (HWDC).

HWDC means that the reverse polarity or negative portion of the sine curve is eliminated as below.

With HWDC there are intervals of individual pulses of direct current and also intervals when no current is flowing.

Hysteresis loop:

The permeability of a material can be determined by increasing the magnetizing force (electric current strength) until the material reaches its saturation point. Each type of material has a point of maximum flux density (saturation). if we place a piece of steel in a coil. Through which alternating current is flowing. We can plot the relation between magnetizing current “H” and the flux density “B” the result is a hysteresis loop as shown below.

At each increase of the force “H” there is an increase in flux density “B” until the Saturation Point is reached. The dashed line above (points o-a) shows the maximum flux density and is often referred to as the virgin curve. As the magnetic force is increased. The flux in the material in creases quit rapidly at first, then more slowly until it reaches a point beyond which any increase in the magnetizing force does not increase the flux density (point A).

As the magnetizing force is reduced to zero (from point a to b), the flux density slowly drops until the magnetizing force (current) is zero (see below). The ability of the steel to retain a certain amount of residual magnetism is called retentivity, as shown between points o and b.

When the magnetizing force is reversed, as always happens with ac, the flux density is reduced to zero at point c as shown below.

Coercive force is the reverse magnetizing force required to remove residual magnetism from the material as shown above.

Hardened steel would require a stronger reverse magnetizing force to remove the residual magnetism. As the reverse magnetizing force is increased beyond point "c", the flux density increases to the saturation point in the reverse direction shown in point "d" below.

Point e below shows the residual magnetic field in the reverse direction.

The force required to remove this residual field is shown between points o and f and is again called coercive force (see below).

The hysteresis loop is completed as the magnetizing force is again increased to a maximum flux density at point a.

As shown on the previous, a hysteresis loop is formed with every complete cycle of 60 cps current. A hysteresis loop could also be used to describe the magnetization or demagnetization with dc where the current is either manually or automatically reversed between straight (+) polarity and reverse (-) polarity.

A wide hysteresis loop indicates a material that is difficult to magnetize (one with a high reluctance). The hysteresis loop gets its name from the lag between the applied magnetizing force and the actual flux density in the part. This lag is shown between points o and f below.

The distance between points o and f above will depend on the coercive force needed to overcome the reluctance of the steel.

A hard steel would have the following qualities and would produce a wide hysteresis loop.

1.low permeability- hard to magnetize.

2.High retentivity- retains a strong residual magnetic field.

3.High coercive force-requires a high reverse magnetizing force to remove the residual magnetism.

4.High reluctance- high resistance to magnetizing force.

5.High residual magnetism- retains a strong residual magnetic field.

A thin hysteresis loop indicates a material of low retentivity.

The loop below shows the qualities of a soft material such as low carbon steel. The coercive force is low because the material retains only a weak residual magnetic field.

A soft or low carbon steel would have the following qualities:

1.High permeability- easy to magnetize.

2.Low retentivity-retains a weak residual magnetic field.

3.Low coercive force- requires a low reverse magnetizing force to remove the residual magnetism.

4.Low reluctance- low resistance to magnetizing force.

5.Low residual magnetism- retains a weak residual magnetic field.

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Magnetic Particle Testing - I: Basic Concept & Theory of Magnetization
Magnetic Particle Testing -II : Circular & Longitudinal Magnetization

Magnetic Particle Testing -II : Circular & Longitudinal Magnetization

2020-07-07T21:44:00.013+05:30

Circular Magnetization

A circular magnetic field is induced into a specimen by:

Direct induction of a circular field into an article. This is accomplished by, passing a current through the article as shown in the head shot below.

Direct induction using prods-prod magnetization is used where the size or location of an article does not permit the use of a head shot or location of an article conductor. Current flow and circular field distributions are shown below.

Indirect induction of a circular field. This is accomplished by placing a current-carrying conductor into the specimen. This method is known as the central conductor technique and is illustrates below.

Circular magnetic field in a nonmagnetic material

In a nonmagnetic material the lines of force will not stay in the material for example, when a copper bar is used, the magnetic field is established around the bar as shown below.

Circular magnetic field in a ferromagnetic material

In a ferromagnetic material the lines of force are established within the material. Iron is permeable and readily conducts the magnetic field as shown below.

In both ferromagnetic and nonmagnetic materials. The lines of force are at right angles (90 Deg) to the direction of electric current flow. Iron particles will not be attracted to a magnetized part except where a flux leakage exists. A crack in the part as shown below. would cause a typical indication.

Circular magnetization will detect discontinuities that are between 45 and 90 degrees, to the lines of force. A crack that runs parallel to the lines of force will not attract particles because no poles or flux leakage exists at the crack (see below).

The circular magnetization created with prods will also only detect discontinuities that are essentially perpendicular to the lines of force.

Longitudinal magnetization

A longitudinal magnetic field is induced into a specimen by:

coil (solenoid)- when the length of the specimen is several times its diameter or cross section, the specimen can be successfully magnetized by placing it lengthwise in the field of the coil (see below).

Yoke- a yoke may be used to magnetize a specimen longitudinally as shown below. The yoke is essentially a temporary horseshoe magnet. It is made of soft. Low retentivity iron. Which is magnetized by a small coil wound around its horizontal bar.

Note: both illustrations above use the older convention (+to-using the right-hand rule) to demonstrate current flow. When copper wire is wound into a coil, the lines of flux around each turn of the coil

This increases the flux density in the longitudinal direction.

The flux density is the greatest at the surface of the copper wire; Therefore, flux density will be greatest at the inside surface of the coil.

Parts being magnetized should be placed near the inside surface of the coil as shown below.

Which of the discontinuities on the surface of the bar on the right would be detected if it were inspected in the coil on the left?

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Theories of failure
Magnetic Particle Testing: Basic Concept & Theory of Magnetization
The Principal Stresses

Magnetic Particle Testing - I: Basic Concept & Theory of Magnetization

2020-07-07T11:10:00.024+05:30

INTRODUCTION

Because magnetization of certain metals is possible. it is possible to reveal discontinuities by using a medium (iron powder) having magnetic attraction. The medium is applied to the surface of the test object after or during induction of a magnetic field.

The sketch below shows a build up of the magnetic particle medium over the discontinuity in the magnetized part.

Magnetic particle teat is a relatively easy and simple test method that can be applied at various stages of manufacture and processing operation.

The objective of magnetic particle testing is to insure product reliability by providing a means of:

Obtaining a visual image of an indication on the surface of a material.
Disclosing the nature of discontinuities without impairing the material.
Separating acceptable and unacceptable material in accordance with predetermined standards.

Theory of Magnetization

An object is magnetized when part or all of it magnetic domains have their north and south poles oriented as in the sketch below.

The ability of a magnet to attract or repel is concentrated at the local area called poles. The north and south poles exhibit attraction and repulsion as shown in the sketch below.

With all the magnetic domain’s lines up, the magnetic bar developed a total force equal to the sum of all the magnetic domains. These are the magnetic lines of force which form a closed loop or circuit. All the lines of force make up the magnetic field.

The force that attracts other magnetizable materials to the materials to the magnetic poles is known as magnetic flux.

Magnetic flux is made up of all the lines of force.

The horseshoe magnet will attract other magnetizable material only where the lines of force leave or enter the magnet.

If a magnet is bent into a complete loop as shown below. The magnetic field is entirely within. Thus no external force.

However, a crack in the circular magnet will disrupt the flow of lines of force and create a flux leakage.

Leakage fields (flux leakage) are magnetic lines of force that leave the part and pass through air from one pole to the other of opposite polarity.Whenever the leakage field is forced out of the part, iron particles would be attracted showing an indication of a discontinuity.Even some subsurface discontinuities may be detected if the leakage field is strong enough as shown below.

VECTOR FIELD

When two magnetizing forces are imposed simultaneously in the same part. The object is not magnetized in two directions at the same time. A vector field is formed which is the resultant direction and strength of the imposed fields. This is illustrated below. Where F_A is the first magnetizing force, F_B is the second force, and F_A+Bequals the resultant magnetizing force.

Magnetic Materials

If an object is placed in a magnetic field a force is exerted on it and it is said to become magnetized.

The intensity of magnetization depends upon the susceptibility of The material to become magnetized.

A. Diamagnetic metals: Havea small and negative susceptibility to magnetization (slightly repelled).

Copper, silver, and gold are examples of diamagnetic materials.

B. Paramagnetic metals: Have a small and positive susceptibility to magnetization (slightly attracted).

Magnesium, molybdenum, lithium, and tantalum are examples.

C. Ferromagnetic metals: Have a large and positive susceptibility to magnetization. They have a strong attraction and can retain their magnetization after the magnetizing field has been removed. iron, cobalt, and nickel are examples of ferromagnetic metals.

Ferromagnetic materials are the only metals commonly inspected with the magnetic particle testing method.

Magnetic Flux

The total number of magnetic lines of force existing in a magnetic circuit is called magnetic flux.

The lines of force in a magnetic circuit are always closed loops, therefore, a magnetic circuit are always closed loops, therefore, a magnetic circuit is. Always closed as shown below.

Flux density-or induction is usually designated in “Gauss” units and refers to the flux-per-unit area at right angles to the direction of the flux.

Right-hand rule vs Left-hand rule

To find the direction of an electrically induced magnetic field, place your thumb on the conductor in the direction of “current flow” and your fingers will then point in the direction of the lines of force. A circular magnetic field is produced in the sketch below.

The general dynamics. Handbook uses the “current flow” theory which utilizes the right-hand rule. This older convention electricity is considered to flow from + to-.

If the more commonly accepted “electron flow” theory were used, it would be necessary to use the left-hand rule. The “electron flow”theory considers electricity to flow from-to +.

Important

The two methods of determining the flow of electricity should not become confusing. They both result in a magnetic field flowing in the same direction.

The sketch below shows how a magnetic field is produced utilizing a coil. The field is circular around the cable but produces a longitudinal field in the specimen.

Using figure “A” below. Try the right-hand rule (current flow+ to -) to demonstrate the direction of the magnetic field with the “current flow” theory.

Also, try the left-hand rule on figure "B" below to demonstrate that the “electron flow” theory (-to+) will produce a magnetic field in the same direction.

The following properties of a metal can determine how effective the magnetic particle method will be in evaluating a part. These properties will be discussed in greater detail in the next lesson.

1.Permeability- this refers to the ease with which a magnetic flux is established in the article being inspected.

2.Reluctance- this is the opposition of a magnetic material to the establishment of a magnetic flux.

3. Residual magnetism- this refers to the amount of magnetism retained after the magnetizing force is removed.

4. Retentivity- refers to the ability of the material to retain a certain amount of residual magnetism.

5.Coercive force-refers to the reverse magnetizing force necessary to remove the residual magnetism from the part.

For example: if a piece of high carbon steel were placed in a magnetizing field it would exhibit the following:

It would- have low permeability because it would be hard to magnetize.
It would be highly reluctant to accept magnetic flux because of the high carbon content.
It would have a high residual magnetic field. The high carbon steel is reluctant to accept a magnetic flux but is also reluctant to give it up once it has been accepted.
It would be highly retentive of the magnetic field that it has accepted.
It would take a high coercive force to remove the residual magnetism from the high carbon steel part.

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Magnetic Particle Testing

Pipe Stress Analysis: Theories of failure

2020-04-04T09:19:00.008+05:30

Pipe Stress Analysis: Theories of failure

Introduction:

Piping parts fail when the stresses induced by external forces exceed their strength. The external loads cause internal stresses in the elements and the component size depend on the stresses developed. Loading may be due to:

a) Thermal loads

b) Sustain Loads

c) Inertial forces

d) Displacement loads

e) Occasional Loads

As per our last articles, the system behavior and failure are dependent on the type of loading imposed. These are mainly classified as Primary vs. Secondary or Static vs. Dynamic. We have already understood the classification of load base on Primary Loads vs. Secondary Loads.

Now Load maybe classified as:

a) Static load- Load does not change in magnitude and direction and normally increases gradually to a steady value.

b) Dynamic load- Load may change in magnitude, for example, Vibration and shock are types of dynamic loading.

The following figure shows load vs. time characteristics for both static and dynamic loading of machine elements.

load vs. time characteristics

Theories of failure

In design an element is said to have failed if it ceases to perform its function. When a Piping element is subjected to a system of the complex stress system, it is important to predict the mode of failure so that the design methodology may be based on a particular failure criterion. Theories of failure are essentially a set of failure criteria developed for the ease of design. Important Theories of failure commonly used as follows;

Maximum principal stress theory (Rankine theory)
Maximum shear stress theory (Tresca theory)

Maximum principal stress theory (Rankine theory)

According to this, if one of the principal stresses S1 (maximumprincipal stress), S2 (minimum principal stress) or S3 exceeds the yield stress, yielding would occur. In a two-dimensional loading situation for a ductile material where tensile and compressive yield stress are nearly of same magnitude

S1 = ± Sy

S2 = ±Sy

Using this, a yield surface may be drawn, as shown in the following figure. Yielding occurs when the state of stress is at the boundary of the rectangle. Consider, for example, the state of stress of a thin-walled pressurized pipe. Here S1= 2 x S2, S1 being the circumferential or hoop stress and S2 the axial stress. As the pressure in the pipe increases the stress follows the dotted line. At a point (say) the stresses are still within the elastic limit but at b, S1 reaches Sy although S2 is still less than Sy. Yielding will then begin at point b. This theory of yielding has very poor agreement with experiment. However, the theory has been used successfully for brittle materials.

Maximum principal stress theory

The Maximum Principal Stress Theory (Rankine theory) forms the basis of a piping system governed by ASME B31 and subsection NC & ND (class 2 & 3) of Section III of ASME Boiler & Pressure Vessel Code.

Maximum shear stress theory (Tresca theory)

According to this theory, yielding would occur when the maximum shear stress just exceeds the shear stress at the tensile yield point. At the tensile yield point S2= S3 = 0 and thus maximum shear stress is Sy/2. This gives us six conditions for a three-dimensional stress situation:

S1 – S2 = ± Sy

S2 – S3 = ±Sy

S3 – S1 = ±Sy

Maximum shear stress theory

In a biaxial stress situation ( As shown in above figure) case, S3 = 0 and this gives

S1 – S2 = +Sy if S1>0, S2<0

S1 – S2 = -Sy if S1<0, S2>0

S2 = +Sy if S2>S1>0

S2 = -Sy if S2<S1<0

S1 = +Sy if S1>S2>0

S1 = -Sy if S1<S2<0

This criterion agrees well with the experiment.

In the case of pure shear,

S1 = - S2 = k (say), S3 = 0 and this gives

S1- S2 = 2k= Sy

This indicates that yield stress in pure shear is half the tensile yield stress and this is also seen in the Mohr’s circle for pure shear.

The Mohr’s circle for pure shear

The maximum shear stress theory is more accurate than The Maximum Principal stress theory for predicting both Yielding and Fatigue failure in a ductile material. This theory forms the bases of Piping of subsection, NB(class 1) of ASME Section III.

Note: For Complex loading, Principal Stress S1(Maximum/Major Normalstress) and S2 (Minimum/Minor Normal Stress) & Maximum Shear Stress are calculated by the following formula;

Complex Stresses in Piping System

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Pipe Stress Analysis: Classification of Loads

2020-03-29T14:10:00.007+05:30

Introduction

The system behavior and failure are dependent on the type of loading imposed. These are mainly classified as Primary vs. Secondary or Static vs. dynamic or Sustained vs. Occasional...

The loads on the piping system are broadly classified into

Primary Loads
Secondary Loads

Primary Loads or Primary Stresses.

Primary loads have their origin in some force acting on the pipe causing tension, compression, torsion, etc. leading to normal & shear stress. The failure of the piping system may be a sudden failure due to onetime loading or fatigue failure due to cyclic loading. The sudden failure is attributed to primary loading. Following are some important points about Primary Loads

Primary loads are usually force driven (gravity pressure, spring forces, relief Valve, fluid hammer, etc.)
Primary loads are not self-limiting. Once plastic deformation begins it continues till the failure of the cross-section results.
Allowable limits of primary stresses are related to ultimate tensile strength.
Primary loads are not cyclic in nature.
Design requirements due to primary loads are encompassed in minimum wall Thickness requirements

Primary loads are usually classified further, according to their duration of loading as follows;

1. Sustained Load:

Those primary loads which are nearly always present thought out the operation are called Sustained loads. Example. Pipe weight, Insulation weight, etc.

2. Occasional Loads:

Those primary loads which occur less frequently are called Occasional loads. Example. Pressure wave generated due to water hammer.

The deformation due to these loads is limited only if the material shows strain hardening characteristics. If it has no stain hardening property or if the load is so excessive that the plastic instability set in, the system would continue to deform till rupture.

One says that, primary loads are not self limiting. It means that the stresses continue to exist as long as load persists & deformation does not stop because the system has deformed into a no-stress condition but because strain hardening has come into play.

Secondary Loads or Secondary Stresses

Secondary loads are caused by the displacement of some kind. The failure of the piping system may be a sudden failure due to onetime loading or fatigue failure due to cyclic loading. Fatigue failure is attributed to secondary loading. Thermal loads are “secondary loads” because they are self-limiting. That is, yielding or deformation of the part reduces the stress. Following are some important points about Secondary Loads;

Secondary loads are usually displacement driven (Thermal expansion, Settlement, Vibration, etc.)
Secondary loads are self-limiting i.e. the loads tend to dissipate as the system deforms through yielding.
Allowable loads for secondary stresses are based upon fatigue failure modes.
Secondary loads are cyclic in nature (expect settlement).
A secondary application of load never produces sudden failure and sudden failure occurs after a number of applications of loads.

The secondary loads are often cyclic but not always. E.g. loads due to tank settlement are not cyclic. Whereas load due to vessel nozzle moving up is cyclic. Failure due to such loads are often due to fatigue & not catastrophic in nature.

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Pipe Stress Analysis: The Principal Stresses

2020-03-25T13:22:00.005+05:30

Basics of Pipe Stress Analysis: The Principal Stresses

The Principal Stresses

When we calculate the stresses, we choose a set of orthogonal directions and define the stresses in this coordinate system. In a pipe subjected to internal pressure or any other loads, the most used choice of the coordinate system is one comprising of

Axial Or Longitudinal direction stress, SL
Circumferential Or Hoop Stress, SH
Radial Directional Stress, SR

These Stress which stretch or compress a grain/crystal are called normal stress, because of act normally to the surface of the crystal. But all grains are not oriented as the grain is shown in the diagram. In fact, the grain would have been oriented in the pipe wall in all possible orientations. The above stress would also have stress components in the direction normal to the face of such randomly oriented crystals. Each crystal thus does face normal stress. One of these orientations must be such that it maximizes one of the normal stresses. It also called Major Principal Stress. The mechanics of solid-state that, it would also be an orientation, which minimizes some other normal stress. It also called Minor Principal Stress.

Normal stress for such orientation (Maximum/Major Normal stress and Minimum/Minor Normal Stress) are called principle stress and are designated as S1 (Maximum), S2 & S3 (Minimum).

The Sum of the three Normal Stress for all orientation is always the same for any given external load.

SL+SH+SR=S1+S2+S3

Importance of principal stresses

Assume that a material can be deemed to fail of any normal stress exceed some threshold value. If a Conventional coordinate system is used, one may find for certain stress that SL, SH & SR are within this threshold limit. The design then would appear to be safe. However, grins that are oriented in maximum normal stress orientation may have one of the stresses more than this threshold. The pipe would thus fail as far as these grains are concerned. The design has to be safe for such a worst-case. Principal stresses are thus a way of defining worst-case scenarios as far as normal stress is concerned.

Shear Stresses

In addition the normal stresses, grain can be subjected to shear stresses which act parallel to the crystal surface as against the perpendicular direction applicable for normal stresses. Shear stresses occur if the pipe is subjected to Torsion, Bending, etc. Just as there is an orientation on which normal stresses are maximum, there is an orientation that maximizes shear stresses. The maximum shear stress in the 3-D state of stress can be shown as

I.e. half of the difference between the maximum (S1) & minimum (S3) principal stress.

The maximum shear stress is important to calculate because failure may occur due to shear stress. Hence, it is necessary to take the worst-case scenario for shear Stress also as above and ensure against failure.

Hence,

It is easy to defined stress in the coordinate system such as Axial-Hoop-Radial (L-H-R) that we defined for the pipe. The load-bearing cross-section is then well defined. The stress components are calculated as the ratio of the load to the bearing cross-section.
Similarly, it is possible to calculate shear stress in a particular plan for the given torsion or bending loads
The principal stresses and strain can be calculated from the normal stress and shear stress in an orthogonal coordinate system.

Complex Stresses in Piping System

In most pipe design cases, the radial component of normal stress is negligible as compared to the other two components. The 3-D state of this can be simplified to the 2-D state of the stress. Use of Mohr’s Circle then allows calculating the Principal Stresses & Maximum shear stresses as follows.

The third principal stress (S3) is zero. (minimum/negligible)

All failure theories state that these principals or maximum shear stress or some combination of them should be within allowable limits for the MOC under consideration.

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Pipe Stress Analysis: Introduction & Objective

2020-03-21T14:41:00.006+05:30

Basics of Pipe Stress Analysis: Introduction & Objective

Introduction

Pipes are the most dedicate component in any process plant. They are also the busiest entities. They are subjected to almost all kid of loads – intentional/unintentional. It is very important to take note of all potential loads that the piping system would encounter during operation as well as during other stages in the life cycle of a process plant.

Ignoring any such load while designing, erection, hydro testing, start-up, shutdown, normal operation, maintenance, etc can lead to an inadequate design & engineering of the piping system.

The step involved in stress analysis

The approximate step involved in stress analysis can be listed below.

Identify potential loads that would come onto the pipe or piping system during the entire life.
Relate each of these loads to the stress and strains that would be developed in the crystal grain of MOC of the piping system.
Decide the worst three-dimensional stress states that MOC can withstand without failure.
Get the Cumulative effect of all the potential loads on the 3D stress scenario in the piping system under consideration.
Alter the piping system design to ensure that, the stress pattern is within the failure limit.

The Goal of analysis and qualification of the piping system is to provide safe design through the erection, hydro testing, start-up, shutdown, normal operation, maintenance, etc. A piping designer would have a number of alternatives to choose from giving the most suitable & economical solution.

Pipe Stress work process diagram.

The objective of Stress Analysis

The objective of piping stress analysis is to ensure safety against the failure of the piping system by verifying the structural integrity against the loading condition, both external and internal, expected to occur during the lifetime of the system in the plant. Thus, the Objective of the Stress analysis could be listed below.

Ensure that, the stresses in the piping components in the system are within the limits.
Limiting the sagging of the piping system within allowable limits
Directing the line movements so as protect sensitive equipment against overloading
Locating supports, guides, anchors, compressor hold downs, and hanger rods
Resisting pipe system to collapse in case of earthquake, wind or shock loadings.
Providing pre-spring, cold spring, clearance required for line expansion, and additional line flexibility.
Solve dynamic problems developed due to mechanical vibration, acoustic vibration, fluid hammer, pulsation, relief valve, etc.
Transmitting and handling stress sketches in complete compliance with procedures outlined in Piping Engineering Practices.
Solve the problem associated due to higher or lower operating temperature such as

· Displacement stress range

· Nozzle loading on connecting equipment.

· Pipe Displacements

· Load & Moments on supporting structures.

Basically, Piping stress analysis is a discipline that is highly interrelated with the piping layout and support design. The layout of the piping system should be performed with the requirement of piping stress & pipe support in mind. i.e.

Sufficient flexibility for thermal expansion.
Proper pipe routing so that simple & economical supports can be constructed.
Piping material & section properties commensurate with the intended services, temperature, pressure & anticipated loading.
If necessary, the layout solution should be iterated until a satisfactory balance between stress & layout efficiency is achieved.
Once the piping layout is finalized, the piping support system must be determined.
Possible support location & type must be iterated until all stress requirements are satisfied & other piping allowable are meet.
Piping support is then designed base on the selected location and type and the applied loads.

Basically, Piping Designer is not responsible for the flexibility calculations nor the validity of such calculations made by the Stress Engineer. Designer is responsible for any calculations or flexibility analysis that was undertaken and resolved without Stress Supervisor's approval

Causes of pipe stress

The two common causes of pipe stress are weight and thermal loads which cause loads on equipment nozzles.

1.Weight

Weight causes the pipe to sag, which puts stress into the piping material

and forces onto equipment nozzle. It includes the weight of the pipe, the weight of the insulation, the weight of valves, instruments, etc.

2.Thermal

When the temperature of the pipe is higher the size of the pipe increases which causes the nozzle loads to increase and the nozzle loads are further increased when the supports restrain the pipe from moving. Thus improperly stress the analyzed system will cause very high loads on connecting equipment nozzles.

3.Occasional Loads

The other causes of the pipe stress are the occasional loads caused due to Wind, earthquakes, dynamic loads due to equipment operation like Reciprocating Compressor, Pilot safety valve reaction force, Slug flow, etc.

Governing Codes and Standards for Stress Analysis

Codes and Standards specify minimum requirements for safe design and construction (i. e. provides material, design, fabrication, installation and inspection requirements.)

Following are the codes and standards used for Refinery Piping:

· ASME B31.3: Process piping Code

· ASME B31.1: Power Piping Code

· Centrifugal Pumps: API 610

· Positive Displacement Pumps: API 676

· Centrifugal Compressors: API 617

· Reciprocating Compressors: API 618

· Steam Turbines: NEMA SM23/ API 612

· Air Cooled Heat Exchanger: API 661

· Fired Heaters: API 560

· Flat Bottom Welded Storage Tanks: API 650

· Heat Exchangers: TEMA/ Vendor Specific.

· Vessel/Column: Vendor Specific

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Stress Analysis Of Column Piping System - 01
Stress Analysis Of Column Piping System - 02
Stress Analysis Of Column Piping System: PIPING &COLUMN
Modeling IN CAESAR-II
Skirt Temperature/Expansion Calculation

Stress Analysis Of Column Piping System:PIPING & COLUMN MODELLING IN CAESAR-II

2020-01-28T13:18:00.010+05:30

Fig.1:PIPING & COLUMN MODELLING IN CAESAR-II

In Last Article we have studied column profile by Average Method & Division Method Piping connected to columns is having first rest support taken from column itself. This support is also knowing as vessel clit support. Clit support is designed by mechanical group based on piping load calculated by pipe stress engineer. As column will have thermal growth due to temperature, first support should be modeled properly, and correct temperature must be given. Improper modelling may result in lifting of this support and nozzle load will exceed allowable limits. The procedure to model clit support is also discussed in subsequent topic. Procedure for top nozzle connection is discussed. Same can be implemented for side and bottom nozzle.

Fig.2:PIPING & COLUMN MODELLING IN CAESAR-II

Modeling for top nozzle connection

As shown in Fig.2, piping is connected at column top nozzle. Column can be modeled by either rigid modeling or by using pipe element with actual dimensions. Let’s start modeling from line to column. Rest support from column is modeled at last.

1. Enter all pipe properties in CAESAR-II spread sheet. Model node 10 to 20 as pipe element in +Y direction. At node 20 trunnion support is located, which can be modeled as described in later step.

2. Model node 20 to 30 in “+Y “direction. Define bend at node 30.

3. Model node 30 to 40 in “–X “direction. Define bend at node 40.

4. Model node 40 to 50 in “-Y “direction. Model flange connection from node 50 to as rigid element and enter weight of flanges.

5. Model node 60 to 70 in “-Y” direction. This is nozzle stand out distance from column. At the location of column and nozzle junction, it requires to check nozzle load (axial force and moment) due to piping reaction. To obtain this, enter “ANCHOR” restraint at node 70 with “C-NODE” as 71.

6. Column modeling start after node 71. Model node 71 to 80 with diameter and thickness of column. Node 80 and node 20 should have same elevation. This node is required to model rest support which is taken from vessel. Enter correct column material, operating and design temperature from the vendor drawing. Column can also be modeled as rigid element.

7. Model node 80 to 90 in “-Y” direction with length up to column bottom tan line. This element should be divided in number of elements to enter temperature gradient profile and to consider effect of varying thickness at different zones. Column is having different operating temperature at different zones. To simulate actual condition, column must be modeled with “OPERATING TEMPERATURE GRADIENT PROFILE”.

8. Model node 90 to 100 in “-Y” direction with length of column skirt. Calculate the skirt temperature by using REAL excel sheet of “SKIRT TEMP CALCULATION”.For skirt modeling use skirt diameter and wall thickness. This finish the column modeling.

9. Modeling of clit support taken from vessel: Model trunnion support from node 20 to 110 as pipe element with diameter and thickness of trunnion. Provide ambient temperature for this element. Enter “+Y” restraint and friction factor 0.3 at node 110 with “C-NODE” 111. Now model element from 111 to 120 with ambient temperature as rigid element. Length and orientation should be such that it can touch to column periphery. Model element from 120 to 80 as rigid element with diameter and thickness equal to column diameter and thickness value. It should have same temperature of column.

10.Similarly, model other trunnion with node no 20-130-131-140-80.

Column in CAESAR-II Model and 3D Model

Links For Reference;

Stress Analysis Of Column Piping System : Skirt Temperature/Expansion Calculation

2020-01-19T19:56:00.001+05:30

Stress Analysis Of Column Piping System

Skirt Temperature/Expansion Calculation:
To Calculate the thermal expansion of skirt, Following formula is generally used by many organization. This calculation is base on equation mentioned in Bechtel Spec.3PS-PL008, Appendix A-A.8.

Average Skirt Temperature=(T-Ta) F+ Ta

Where, Ta=Ambient Temperature

T=Temperature at the top of the Skirt;

h = Skirt Height

T = Skirt Wall Thickness. &

Generally, organization maintain on Excel sheet format to avoid manual calculation.

Skirt Temperature/Expansion

In above attached picture, Column temperature profile is calculated by using Average Method Or By Division Method
But in both this case, Calculation of temperature profile is by same above mention question.

Stress Analysis Of Column Piping System - 02

2020-01-06T18:42:00.014+05:30

In Last Article we have studied column profile by Average Method. In this article we are going to discuss about second method of column temperatureprofile. Many organizations use this method too. This method helps to reduce nozzle load and stress in piping system during column piping stress analysis.

Division Method:

Following Procedure is used for this method;

Identify and Mark the nozzle size from P&ID which will make considerable impact in column temperature. Generally, big size process nozzle is considered in it. Instrument Nozzle, utility Nozzles, Small Bore Nozzle and Manhole are not considered.
Identify the operating temperate of pipes connected to two beside nozzle.
Divide Column portion in between this two nozzles in two equal parts
Assigned first half column portion with first nozzle temperature and second part with next beside nozzle.
Repeated the same step for other temperature case Egg. DEG Case.
For Skirt temperature is calculation, please follows link : Skirt Temperature/Expansion Calculation

Let’s assigned temperature profile with this method in same example used in Average method. (Refer Stress Analysisof Column Piping System - 01)

So, temperature profile for that column can be created as shown in Fig.

Operating Case Temperature Profile is as following;

OPE Case Temperature Profile by Division Method

After considering all process line temperature, A master column C-II model is generating which can be used in all different stress system connected to same column.

For particulate system, no need to attached Nozzle and Pipe Model which are not the part of same system. Hence, final column model for our stress system is show in following picture.

Few organizations use the operating and design temperatures mentioned in equipment GA as the equipment operating and design temperature. However, the above two methods mentioned will result in thermal growth close to reality

From following picture; we can find the difference in the temperature profile by this two methods.

Links for Reference articles:

Stress Analysis Of Column Piping System - 01

2020-01-03T13:00:00.013+05:30

Columns are one of the principal equipment of any processing plant. Columns are cylindrical steel vessels that are used for distilling raw materials in the production of such products as gasoline, diesel, and heating oil. Looking to the construction of column, it has number of trays at different elevation. Temperature at each tray location differs based on process. This article highlights column modeling in CAESAR-II and stress analysis procedure for column connected lines. Same procedure can be adopted for Reactor line, Stripper line stress analysis.

REQUIRED DOCUMENTS FOR COLUMN LINE ANALYSIS

♦ Column G.A. drawing with all dimensions, nozzle orientation, materials etc.

♦ Column temperature profile.

♦ Line Designation Table/ Line list/Line Parameters and P&ID.

♦ Column line ISO.

♦ Allowable nozzle load for column as specified in project.

Column nozzle is checked for axial force, circumferential moment and longitudinal moment. Nozzle in skewed direction is checked for axial force and resultant moment.

Column Temperature Profile.

Different organization uses different methods for creating column temperature profile.Generally, Single OPE and DEG temperature case is considered from Mechanical Data Sheet for Column. This is the most conservative approach.

But sometime, in large bore piping, huge nozzle load is created by this conservative method, hence we can use following any of these two methods for Stress Analysis by creating temperature profile of column. Different organization uses different methods for creating column temperature profile. Here I will describe two methods which are most widely used.

Average Method:

Following Procedure is used for this method;

Identify and Mark the nozzle size from P&ID which will make considerable impact in column temperature. Generally, big size process nozzle is considered in it. Instrument Nozzle, utility Nozzles, Small Bore Nozzle and Manhole are not considered.
Identify the operating temperate of pipes connected to two beside nozzle.
Take average operating temperature of these two lines.
Assigned same average temperature to the column portion between this two Nozzle.
Repeated the same step for other temperature case Egg. DEG Case.
For Skirt temperature is calculation, please follows link : Skirt Temperature/Expansion Calculation

Let’s assume that there are five big size (N1, N2, N3, N4 and N5 as shown in Fig.) outlet nozzles in a typical tower. So, temperature profile for that column can be created as shown in Fig.

Operating Case Temperature Profile is as following;

OPE Case Temperature Profile by Average Method

Design Case Temperature Profile is given as following;

DEG Case Temperature Profile by Averaging Method

After considering all process line temperature, A master column C-II model is generate which can be used in all different stress system connected to same column.
For particulate system, No need to attached Nozzle and Pipe Model which are not the part of same system. Hence, final column model for our stress system is show in following picture.

Links for Reference articles:

Circular Motion Notes : Short Notes on radial vector,angular displacement & angular velocity

2019-12-31T14:57:00.007+05:30

Circular Motion

Definition of Circular Motion:

1. Motion of a particle along the circumference of a circle is called circular motion.

2. Examples:

I. The motion of a cyclist along a circular path.

II. Motion of the moon around the earth.

Definition of radius vector:

1. A vector drawn from the center of a circle to position of a particle on circumference of circle is called as ‘radius vector’.

2. It is given by,

where, δs = small linear distance

δθ = small angular displacement

3. It is directed radially outwards.

4. Unit: meter (m) in SI system and centimeter (cm) in CGS system.

5. Dimensions: [ M⁰L¹ T⁰ ]

radius vector

Definition of angular displacement:

1. Angle traced by a radius vector in a given time, at the centre of the circular path is called as angular displacement.

2. Consider a particle performing circular motion in anticlockwise sense. Let,

A = initial position of particle at t = 0

B = final position of particle after time t

θ = angular displacement in time t

r = radius of the circle

s = length of arc AB

Angulardisplacement is given by,

3. Unit: radian (rad)

4. Direction of angular displacement is given by right hand thumb rule or right handed screw rule

angular displacement

Notes on angular velocity:

1. Definition: Angular velocity of a particle performing circular motion is defined as the rate of change of limiting angular displacement with respect to time

2. Instantaneous angular velocity is given by,

3. Finite angular velocity is a vector quantity and it is given by,

4. Direction: The direction of angular velocity is given by right hand rule and is in the direction of angular displacement.

5. Unit: rad s⁻¹

6. Dimensions:

angular velocity