Chemical & Process Technology

Principle in Eliminating & Minimizing AIV Impact

2009-10-24T21:21:00.000-07:00

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High frequency acoustic excitation downstream of pressure reducing device potentially results downstream piping failure due to Acoustic Induced Vibration (AIV). Whenever there is pressure drop with mass passing through the valve, internal acoustic energy is generated and transmitted to downstream piping and potentially lead to severe piping excitation, vibration and stresses on downstream piping, in particular at discontinuity section i.e fabricated Tee, small bore connection, welded pipe and pipe support, etc. This acoustic excitation phenomena is generally involve high frequency (more than 1000Hz) acoustic energy. When high frequency acoustic energy is matches with mechanical natural frequency of piping and its component, excitation amplitude is at maximum and lead to increased stress level. AIV phenomenon and assessment methodology have been discussed in previous post. Sound Power Level (PWL) is commonly used in quantifying acoustic energy in AIV. Higher PWL, higher the potential of vibration level and higher the risk of AIV.

Principle in Eliminating & Minimizing AIV Impact
This post will focus in some common measures and techniques to eliminate and/or minimize PWL generation and transmission. The main principles in avoiding and minimizing the impact of AIV are :

Eliminate or reduce vibration level at pressure reduction device
Damper vibration level
Increase resistance to vibration
Minimizing vibration transmission

All shall measures are not exclusive between each and other but all measures shall be taken and apply all (if possible). However, It shall follow the sequence from principle no. 1 to principle no.4.

Application Example
A control valve discharge gas from high pressure vessel to flare header results high vibration level (or sound power level, PWL) and lead to AIV problem. The consideration shall be focused on reducing the PWL by using special trim control valve (principle no. 1). Splitting flow into several control valves in parallel may reduce the PWL (principle no.1). However one shall remember this sometime is not a good idea as common mode failure may worsen the situation. Next may consider a silencer insert downstream of control valve to damper PWL (principle no.2). One may also consider to increase piping resistance to vibration i.e. increase wall thickness (principle no.3). This high resistance piping may be extended to decrease the PWL to certain acceptable limit before it is tie in to downstream piping which is less resistance (principle no. 4).

Concluding Remark
In eliminating and minimizing AIV impact, all principles as mentioned above are not exclusive and shall be applied to maximum (if possible).

In coming post, the discussion will focus on
- type of AIV source
- measures in tackling each AIV problem

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FREE Chemical Engineering Digital Issue for Oct 2009

2009-10-13T08:45:00.000-07:00

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Chemical Engineering Magazine as just released Oct 2009 issue. If you are the subscriber of Chemical Engineering, you should have received similar notification.

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Interesting articles for this month :

Strategies For Water Reuse
Membrane technologies increase the sustainability of industrial processes by enabling large-scale water reuse

Estimating the Total Cost of Cartridge and Bag Filtration
When changeout and disposal costs are added to the purchase cost of filters, the total cost of disposable filters can more than quadruple . A proven method of reducing total lifecycle cost is larger surface area filters

FAYF - Chemical Resistance of Thermoplastics
This one-page guide outlines considerations for choosing thermoplastics in corrosion-resistant applications

Energy Efficiency : Tracking Natural Gas With Flowmeters
Thermal mass flowmeters provide advantages over other options for metering the consumption of natural gas by individual combustion units throughout the facility

Preventing Dust Explosions
Risk management programs are critical for safe handling and processing of combustible dust as well as for OSHA regulatory compliance

Compressed Gases: Managing Cylinders Safely
Follow these recommendations to ensure the safe handling, storage and use of gas cylinders

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Interesting Relationship Between Carbon number and LFL & UFL

2009-10-10T21:44:00.000-07:00

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A mixture is combustible / flammable if and only if the hydrocarbon composition is within mixture LFL/LEL and UFL/UEL as discussed in "Estimate Mixture Flammability & Explosivity At Reference P & T. As the operating pressure (P) and temperature (T) change (from reference P & T, the mixture LFL/LEL and UFL/UEL at P & T will change accordingly. In recent works, found in literature an interesting relationship between number of Carbon (in paraffin hydrocarbon) with LFL/LEL and UFL/UEL. This relationship is pretty useful especially when you have no information on hand.

A paraffin hydrocarbon with N_C of Carbon (C), the Upper Explosive Limit (UFL) and Lower Explosive Limit (LFL) can be established with following equations :

Example
A Methane (CH4) contains One (1) Carbon. From literature, the UFL = 15 vol% and LFL = 5%.
From above equations,

UFL = 1 / (0.01337 x 1 + 0.05151)
UFL = 5.6% (compare to 5%)

LFL = 1 / (0.1347 x 1 + 0.04343)
LFL = 15.4% (compare to 15%)

A Propane (C3H8) contains Three (3) Carbon. From literature, the UFL = 10.1 vol% and LFL = 2.1%. From above equations,

UFL = 1 / (0.01337 x 3 + 0.05151)
UFL = 10.9% (compare to 10.1%)

LFL = 1 / (0.1347 x 3 + 0.04343)
LFL = 2.2% (compare to 2.1%)

A Hexane (C6H14) contains Six (6) Carbon. From literature, the UFL = 7.0 vol% and LFL = 1.25%. From above equations,

UFL = 1 / (0.01337 x 6 + 0.05151)
UFL = 7.6% (compare to 7%)

LFL = 1 / (0.1347 x 6 + 0.04343)
LFL = 1.2% (compare to 1.25%)

Above equation is just equations for quick estimation. It may provide some idea of UFL and LFL when no information is available. The error could be large for certain component i.e. Octane. For design and practical use, an in depth method shall be employed.

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Visualize Spraying Nozzle Performance

2009-09-22T07:46:00.001-07:00

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Mixing of liquid in vapor using spraying nozzle is commonly used in reactor, column, mixing tank, etc. Mixing efficiency and effectiveness subject to liquid droplet size and contact time between droplet and vapor. In order to promote mixing efficiency and effectiveness, spraying nozzle is used to create small droplet size, well distribution of droplet and better spraying angle. Spraying liquid can be done hydraulically (high pressure liquid acting on a special design nozzle), gas assisted (gas kinetic energy acting on liquid for break-up) and rotary spraying (hydraulic pressure acting on moving device and results breakage of liquid).

There are several well known spraying nozzle manufacturer such as BETE fog nozzle, Spraying Systems Co., etc Out of all, BETE Fog Nozzle, Inc., a leading supplier of engineered spray nozzle solutions, has posted high-resolution spray videos on their Web site to share with the public. BETE has published video in Youtube so that it can be viewed in public. Besides, BETE also provide embedding code so that publisher can display them in their website. Thanks to BETE.

The spray nozzle videos were created to help customers visualize the performance characteristics of the most common nozzle spray pattern types. The spray nozzle videos will assist customers in selecting nozzles for their application design and process improvements right from their own offices.

Spray pattern videos now available include examples of the following nozzles:

Spiral
Axial Whirl
Tangential Whirl
Fan
Misting
Air Atomizing
Tank Washing

This useful tool is the newest component of BETE Fog Nozzle, Inc.'s efforts to provide customers with solutions specially tailored to their unique spray challenges. BETE Fog Nozzle, utilizing our engineering resources and state-of-the-art Spray Laboratory, is fully prepared to collaborate with customers to supply nozzles that meet their most demanding industrial process conditions.

Following are 5 typical spraying videos.

Misting Spray Nozzle

Tangential Whirl, Hollow Cone Spray Nozzle

Full Cone Spiral Spray Nozzle

Extra-wide Full Cone Spiral Spray Nozzle

Air Atomizing, pressure-fed, internal mix, narrow angle, low flow

Quick Check if NPSHa is Sufficient Without Vendor Input

2009-09-18T07:02:00.000-07:00

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A pump with capacity of 100 m3/h and pump head of 75 m, base on suction drum elevation, fluid condition and suction piping condition, the calculated available Net Positive Suction Head (NPSHa) is 2m. This NPSHa shall be verified with NPSH required (NPSHr) by selected pump.

How would you know if NPSHa is sufficient before the pump vendor provide the information on pump required Net Positive Suction Head (NPSHr) ?

Thoma Cavitation curve has been used to check as discussed in "Quick Check if NPSHa is Sufficient Using Thoma Cavitation Curve". Present post will discuss the usage of Suction Specific Speed for checking.

Suction specific speed ( N_ss) of a pump is a dimensionless number expressed as

N_ss = ( N* Q ^0.5 ) / (NPSHr)^0.75

Where

N_ss : Suction Specific speed
Q : Flow rate (gpm) at the Best Efficiency Point
N : Pump rotational speed (rpm) *
NPSHr : Net Positive Suction Head required (ft)

Normal pump speed is 1450 rpm for low speed pump and 2950 rpm for high speed pump.

Soluation
Assumed pump curve is selected where pump flowrate is at best efficiency point

Pump Flow rate
Q = 100 m³/h
Q = 100 m³/h x ( 264.17 Gallon / m³) x (1 h / 60 min)
Q = 440.28 gpm

Pump Speed
Same as previous post, N = 2950 rpm for high speed pump

Pump Suction Specific Speed (N_ss)
Same as previous post, N_ss = 11000 is selected.

From above equation
N_ss = ( N* Q ^0.5 ) / (NPSHr)^0.75
(NPSHr)^0.75 = ( N* Q ^0.5 ) / N_ss

NPSHr = [( N* Q ^0.5 ) / N_ss]^(1/0.75)
NPSHr = [( 2950* 440.28 ^0.5 ) / 11000]^(1/0.75)
NPSHr = 10.009 ft = 3.05 m

Calculated NPSHr if 3.05 m is very close to NPSHr of 3.0 m as estimated using Thoma Curve. NPSHa of 2m is lower than NPSHr of 3.05m. Obviously the pump potential cavitate under normal operation as NPSHa less than NPSHr. Quick action to rectify the drum elevation & pump suction piping design may required to avoid potential future change.

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Rupture Disc Installation Guide Videos Reduce Premature Openning Risk

2009-09-14T06:02:00.000-07:00

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Rupture disc (RD) is one of the non-reclosable type pressure relief device for vessel overpressure protection. It has been used in many application due to it uniqueness and special characteristic as covered in "Why Rupture (RD) Upstream of Pressure Relief Valve (PRV) ?". In some application, two RDs are placed in series to minimize inventory lost cause by premature opening of RD. Premature opening of RD may be cause by RD quality during manufacturing. Besides, premature opening of RD may be caused by wrong handling and installation of RDs.

Elfab, a RD manufacturer offer a few RD installation videos which could very useful for most operator. you may download from the following links or visit Elfab.

Facts About NPSH - Cavitation Even NPSHa More than NPSHr ?

2009-09-12T23:47:00.000-07:00

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Earlier post "Relationship between NPSHa & NPSHr", Process engineer must always ensure the operating pressure along the pump (from suction to discharge) always higher than fluid vapor pressure. Net positive suction head (NPSH) is used to check if cavitation will occur. Process engineer must always ensure available Net positive suction head (NPSHa) is always higher than pump required Net positive suction head (NPSHr). In recent discussion with some engineers, there was some doubt or confusion on a simple statement. Should pump still cavitate eventhough Net Positive Suction Head available (NPSHa) higher than Net Positive Suction Head required (NPSHr) ?

Yes. Cavitation can exist eventhough NPSHa is above the NPSHr of a centrifugal pump. Based on Hydraulic institute definition of NPSHr, a NPSHr of a pump is the level of NPSHa that three percent (3%) reduction in total discharge head of the pump caused by flow blockage from cavitation vapor in the impeller eye. Pump manufacturers design their pumps based on this definition. There is still situation where head drop is below 3%. Nevertheless, it is believe this definition was based suction energy sufficient low (below 3% head drop) and will not results cavitation which is detrimental to pump internal i.e. impeller.

A few interesting facts about NPSHa and NPSHr

Cavitation occur when NPSHa is above NPSHr, however it is not reach detrimental level
Possible achieve 100% head when NPSHa = 1.05 to 2.5 times NPSHr
Zero cavitation when NPSHa = 2 to 20 times HPSHr, subject to suction energy, present of air, erosive & abrasive material, etc however
Common Zero cavitation when NPSHa = 4 times NPSHr
NPSHa in the range of 1.1-1.3 of NPSHr for low suction energy pump design
NPSHa in the range of 1.3-2.5 of NPSHr for high suction energy pump design
Minimum one (1) meter NPSHa above NPSHr

Above may be used as guildeline in determining margin between NPSHa and NPSHr. However, higher margin shall be included if air and abrasive material is present in the pumping fluid.

How to Increase Mass Flow Across RO When Choked Flow Already Occured ?

2009-09-10T07:30:00.000-07:00

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Restriction orifice (RO) is widely used to in Oil and Gas, Refinery and Petrochemical chemical plant. Simple search on internet lead you to plenty of articles related to functionality, calculation and specification of restriction orifice. "Restrcition Orifice Used in Many Applications in Different Manners" brought out a few applications of RO in the industries. Earlier post "A refresh to Process Engineer on few phenomenons in restriction orifice" has summarized a few concepts that a process engineer may needs to understand for restriction orifice :

Restriction orifice is used to limit flow to required or expected flowrate with the available differential pressure across.
Vena contracta (VC) present just short distance downstream of restriction orifice
Maximum velocity and minimum pressure at vena contracta (VC)
Choked flow occurred when velocity at vena contracta (VC) reach sonic velocity (Ma=1). The corresponding downstream pressure (P2) at choked condition called critical pressure (Pc).
Increase in upstream pressure (P1) will increase mass flow passing the restriction orifice but velocity at VC still maintaining at Ma=1

A young engineer asked... If compressible fluid composition and upstream pressure are fixed and choked flow already occurred, should there be other way to increase the mass flow passing the RO ?

For compressible fluid, mass flow passing through an fixed bore size RO is a function of

composition ( k, MW, ...)
driving force (Differential pressure) across RO (P1-P2)
density (function of P1, MW, z, T1)

If composition and upstream pressure (P1) are fixed,

composition (and therefore k, MW...) remain unchanged
reducing P2 lead to higher P1-P2 which will result higher mass flow passing RO. Mass flow increase will continue until until P2 reach critical pressure of fluid (Pc). Sonic velocity (Mach no =1) occurred at vena contrata, some distance downstream of RO. When P2 lower than Pc, further reduction of P2 will not result any mass flow increase.
reduce upstream temperature (T1) will results higher density and higher mass flow passing RO.

Thus, one of the way to increase mass flow through RO when gas composition and upstream pressure are fixed and choked flow already occurred is dropping upstream temperature (T1).

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FREE Chemical Engineering Digital Issue for Sept 2009

2009-09-09T06:02:00.000-07:00

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Chemical Engineering Magazine as just released Sept 2009 issue. If you are the subscriber of Chemical Engineering, you should have received similar notification.

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Interesting articles for this month :

Strategies For Water Reuse
Membrane technologies increase the sustainability of industrial processes by enabling large-scale water reuse

Measuring Dust and Fines in Polymer Pellets
The ability to carry out such measurements can help operators improve quality control, assess
equipment performance and optimize the process

FAYF - Heat Transfer - Design II
This one-page guide outlines considerations for designing a heat-transfer system

Multivariable Predictive Control : The Scope is Wider Than You Think
With tighter integration between process units and more aggressive optimization goals, this technique is gaining attention throughout the CPI as an alternative to PID control

CSTR Design for Reversible Reactions
A design approach for continuous stirred-tank reactorsis outlined for three cases of second order reactions

CPVC Piping in Chemical Environments : Evaluating the Safety Record
No torches, fewer burn hazards and outstanding fire characteristics make CPVC a safe, effective alternative for industrial piping

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Relate LFL to MOC

2009-09-05T02:56:00.000-07:00

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Minimum oxygen concentration (MOC) is minimum quantity of oxygen required present in hydrocarbon mixture so that a fire can be initiated and propagated. Below this limit, a fire will not form. More discussion in "Minimum Oxygen Concentration (MOC) for Flare Purge". A mixture is combustible / flammable if and only if the hydrocarbon composition is within mixture LFL/LEL and UFL/UEL as discussed in "Estimate Mixture Flammability & Explosivity At Reference P & T One shall remember, as the operating pressure (P) and temperature (T) change (from reference P & T, the mixture LFL/LEL and UFL/UEL at P & T will change accordingly. This post will discuss the relationship between MOC with LFL/LEL.

LFL Relate to MOC
LFL is minimum hydrocarbon (HC) concentration in air which a mixture will burn when an ignition source is present. LFL can be written as follow

MOC is minimum oxygen present in hydrcarbon mixture which a mixture will burn. MOC can be written as follow

Combining [1] & [2],

Combustion of Hydrocarbon

Above equation may be used to estimate MOC if you know the LFL of hydrocarbon.

Example
1) A Ethane (C₂) having LFL of 3.0 vol% (Refer to "Estimate Mixture Flammability & Explosivity At Reference P & T". Estimate MOC of Ethane.

Combustion of C₂H₆,

C₂H₆ + d.O₂ ==> 2CO₂ + 3H₂O

a = 2
b = 6
c = 0
d = 2 + 6 /4 - 0/2 = 3.5

MOC = 3.5 x 3.0 = 10.5 Vol%, close to 11.2 vol% in literature.

2) A n-butane (nC₄) having LFL of 1.86 vol% (Refer to "Estimate Mixture Flammability & Explosivity At Reference P & T". Estimate MOC of n-butane.

Combustion of C₄H₁₀,

C₄H₁₀ + d.O₂ ==> 4CO₂ + 5H₂O

a = 4
b = 10
c = 0
d = 4 + 10 /4 - 0/2 = 6.5

MOC = 6.5 x 1.86 = 12.1 Vol%. Close to 12.3 vol% in literature.

Related Topic

Estimate Mixture Flammability & Explosivity At Operating P & T

2009-09-02T08:14:00.000-07:00

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Earlier post "Estimate Mixture Flammability & Explosivity At Reference P & T" discussed about the Lower flammable limit (LFL) or Lower Explosive Limit (LEL) and Upper flammable limit (UFL) or Upper Explosive Limit (UEL) for single component fluid and mixture at reference pressure (P_ref) and temperature (T_ref). This post will discuss the way to correlate the LFL/LEL and UFL/UEL at P_ref and T_ref and operating pressure (P) and temperature (T) .

Temperature & Pressure Corrected LFL/LEL & UFL/UEL
Below are two equations may be used to correlate the LFL/LEL and UFL/UEL at P_ref and T_ref and operating pressure (P) and temperature (T).

One shall take note that these equations are used for single component. For a mixtures, the LFL/LEL and UFL/UEL at operating pressure (P) and temperature (T) will be calculated using following equation (as discussed in "Estimate Mixture Flammability & Explosivity At Reference P & T":

Calculation Steps
2 steps in determining mixtures LFL/LEL and UFL/UEL at operating pressure (P) and temperature (T) .
(i) Estimate LFL/LEL and UFL/UEL at operating pressure (P) and temperature (T) for every component in a mixture
(ii) Estimate mixture LFL/LEL and UFL/UEL

Example
A mixture contains of Methane, Ethane and Propane with volume% of 20%, 20% and 60%. Estimate UEL at (i) 20 degC & 101.325 Pa, (ii) 70 degC & 3 MPa.

Data
Methane (C1)
UEL_C1,20C,1ATM = 15.0%, E_{C1,combustion} = 212.79 kcal/mole

Ethane (C2)
UEL_C2,20C,1ATM = 12.4%, E_{C2,combustion} = 372.81 kcal/mole

Propane (C3)
UEL_C3,20C,1ATM = 10.1%, E_{C3,combustion} = 526.74 kcal/mole

Output
(i) UEL_Mix at 20 degC & 101.325 Pa

UEL_Mix = 1 / [ 0.2/15 + 0.2 /12.4 + 0.6 / 10.1 ]

UEL_Mix = 11.25 vol% at 20 degC & 101.325 kPaA

(ii) UEL_Mix at 70 degC & 3 MPa
UEL_C1,7oC,3MPa
= 15 x [1+0.75(70-20)/212.79]
+ 20.6 x [Log10(3)+1]
= 48.07 vol%

UEL_C2,7oC,3MPa
= 12.4 x [1+0.75(70-20)/372.81]
+ 20.6 x [Log10(3)+1]
= 44.08 vol%

UEL_{C3,7oC, 3MPa}
= 10.1 x [1+0.75(70-20)/526.74]
+ 20.6 x [Log10(3)+1]
= 41.25 vol%

UEL_Mix,70C,3MPa = 1 / [ 0.2/48.07 + 0.2 /44.08 + 0.6 / 41.25 ]

UEL_Mix,70C,3MPa = 43.02 vol% at 70 degC & 3 MPaA

Estimate Mixture Flammability & Explosivity At Reference P & T

2009-08-31T02:16:00.000-07:00

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Lower flammable limit (LFL) or Lower Explosive Limit (LEL) is minimum vapor concentration in air which a mixture will burn when an ignition source is present. Upper flammable limit (UFL) or Upper Explosive Limit (UEL) is maximum vapor concentration in air which a mixture will burn when an ignition source is present. Concentration of mixture of vapor in air below LFL/LEL (too lean) or above UFL/UEL (too rich), mixture will not burn even an ignition source is present. Therefore, flammable range or explosive range is concentrations between LFL/UFL and UFL/UEL.

Component LEL & UEL
LFL/LEL and UFL/UEL for some common gases are indicated in table below. Some of the gases are commonly used as fuel in combustion processes.

Fuel Gas	(LFL/LEL) (%)	(UEL/UFL) (%)
Acetaldehyde	4	60
Acetone	2.6	12.8
Acetylene	2.5	81
Ammonia	15	28
Arsine	5.1	78
Benzene	1.35	6.65
n-Butane	1.86	8.41
iso-Butane	1.80	8.44
iso-Butene	1.8	9.0
Butylene	1.98	9.65
Carbon Disulfide	1.3	50
Carbon Monoxide	12	75
Cyclohexane	1.3	8
Cyclopropane	2.4	10.4
Dimethyl Ether	3.4	27
Diethyl Ether	1.9	36
Ethane	3	12.4
Ethylene	2.75	28.6
Ethylene Oxide	3.6	100
Ethyl Alcohol	3.3	19
Ethyl Chloride	3.8	15.4
Fuel Oil No.1	0.7	5
Hydrogen	4	75
Isobutane	1.8	9.6
Isopropyl Alcohol	2	12
Gasoline	1.4	7.6
Kerosine	0.7	5
Methane	5	15
Methyl Alcohol	6.7	36
Methyl Chloride	10.7	17.4
Methyl Ethyl Ketone	1.8	10
Naphthalene	0.9	5.9
n-Heptane	1.0	6.0
n-Hexane	1.25	7.0
n-Pentene	1.65	7.7
Neopentane	1.38	7.22
Neohexane	1.19	7.58
n-Octane	0.95	3.20
iso-Octane	0.79	5.94
n-Pentane	1.4	7.8
iso-Pentane	1.32	9.16
Propane	2.1	10.1
Propylene	2.0	11.1
Silane	1.5	98
Styrene	1.1	6.1
Toluene	1.27	6.75
Triptane	1.08	6.69
p-Xylene	1.0	6.0

Note : The limits indicated are for component and air at 20^oC and atmospheric pressure.

Mixture LFL/LEL & UFL/UEL

A mixture is combustible / flammable within mixture LFL/LEL and UFL/UEL. Common units for both limits is mole (or volume) percent fuel in air [moles fuel/(moles fuel + moles air)]. A mixture LFL/LEL and UFL/UEL limits can be calculated using the equations first proposed by Le Chatelier in 1891 :

Example
A vapor contains of 20 vol% of Methane (C1), 20 vol% of Ethane (C2) and 60 vol% of Propane (C3). Find LEL of this mixture at 20 degC and Atmospheric pressure (101325 kPaA).

LEL_C1 = 5 vol% at 20 degC & 101.325 kPaA
LEL_C2 = 3 vol% at 20 degC & 101.325 kPaA
LEL_C3 = 2.1 vol% at 20 degC & 101.325 kPaA

LEL_Mix = 1 / [ 0.2/5 + 0.2 / 3 + 0.6 / 2.1 ]

LEL_Mix = 2.55 vol% at 20 degC & 101.325 kPaA

Above LEL may be linked to MOC as discussed in "Minimum Oxygen Concentration (MOC) for Flare Purge". Vapor mixture flammability & explosivity at Operating P & T discussed in this post.

Minimum Oxygen Concentration (MOC) for Flare Purge

2009-08-29T21:40:00.000-07:00

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A fire is formed or a flame can sustains and propagate, three main elements shall present as defined in well known FIRE triangle. There are combustible material (or fuel), oxygen (O2) and heat. See following image. One additional characteristic shall also present is the potential of chain reaction to maintain continuous combustion before any of these three elements is removed. This speed of chain reactions define if a mixture is combustible (slow) or explosive (fast).

Minimum Oxygen Concentration (MOC)
Oxygen is common obtained from atmosphere. Standard air at Mean Sea Level (MSL) contains 20.95 vol% Oxygen (O2), 78.08 vol% Nitrogen (N2), 0.038 vol% Carbin Dioxide (CO2) and others inert gas i.e. Neon, Xenon, etc. Although Oxygen is the major component in generating fire, there is still a minimum oxygen concentration (MOC) required present in combustible mixture so that a fire can be initiated and propagated. Below this limit, a fire will not form.

Following MOC for some hydrocarbon common found in oil and gas plant.

Component	MOC (Vol% O2)
Hydrogen	4.0
Acetylene	6.2
Methane (C1)	12.0
Ethane (C2)	11.2
Ethylene (C2=)	9.9
Propane (C3)	11.6
Propylene (C3=)	11.5
Butane (C4)	12.3
1-Butene (C4=)	11.0
Pentane (C5)	11.8
Hexane (C6)	11.8
Benzene (Bz)	11.5
Carbon Disulfide	5.0

Base on this principle, a flare header is purged with hydrocarbon (i.e. fuel gas ) to evacuate air that ingressed via flare tip and stack in order to ensure quantity of oxygen level in the flare system is always below minimum oxygen concentration (MOC).

MOC For Flare Purge
From above table, you may noticed that MOC for majority of components are equal to or more than 10 vol% except Hydrogen (H2), Acethylene and Carbon Disulfide (CS2). It is recommended MOC of 6 vol% for flare purging design with 4 vol% as design margin. One shall remember plant releasing large amount of Hydrogen shall use lower MOC with margin i.e. 2 vol%. One of the example is Hydrogenation unit in Refinery plant.

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Emerging Technologies to Monetize Small Natural Gas Resources

2009-08-24T06:34:00.000-07:00

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Simple update to IEM Members or those who are practicing Engineering in Malaysia...

The number of large natural gas fields for mankind’s exploitation is reducing as we go into the future. Future natural gas fields are relatively smaller, scattered in various geographical locations and contains higher acidic components. As the world’s energy demand continues, there is an urgent need to develop emerging technologies to monetize small natural gas resources around the globe which could not be economically carried out now.

The emerging technologies have to be safe, economical and more robust than presently available technologies for successful and meaningful gas production. This presentation will share several emerging technologies currently being studied and developed by the oil and gas industry for such purpose.

A talk on "Emerging Technologies to Monetize Small Natural Gas Resources", organized by Chemical Engineering Technical Division, IEM has been scheduled.

Date : 10 October 2009 (Saturday)
Time : 9.00 am – 11.00 am (Refreshment would be served at 11.00 am)
Venue : C&S Lecture Room, 2nd Floor, Wisma IEM, Petaling Jaya

Speaker : Engr. Dr. Chan Tuck Leong

*Any queries, please contact sec@iem.org.my.

Inert Gas or Fuel Gas For Flare Purge ?

2009-08-22T20:36:00.000-07:00

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Flare collection header is normally "NO flow" as most (if not all) devices connected to flare header are non-discharging fluid into flare system. Among all are pressure relief valve (PRV), blowdown valve (BDV), overpressure dump valve (PCV), etc. All these devices are kept as close position during normal plant operation and will only open in the event of overpressure, emergency situation i.e. fire, runaway reaction, plant shutdown/blowdown for maintenance.

On the flare tip end, it is open to atmosphere. it is very likely that atmosphere air contain oxygen ingress and stay into flare collection header. PRV/BDV/PCV passing and open on demand will discharge large quantity of hydrocarbon gas into flare collector header filled with air and create combustible mixture, as this combustible mixture travel along flare header and reach flare tip which equipped with flare pilot, combustible mixture will be ignited and potentially created flash back to the flare header and flare knock-out drum (KOD). Subject to flare header capacity and mechanical integrity, large flash back lead to severe internal pressure act on the piping & vessel and vapor wave results severe vibration and movement of structure, this potential lead to catastrophe failure of flare collection system. Therefore a flare header is sweep or purge with fuel gas or inert gas i.e. Nitrogen.

Advantages using inert gas compare to fuel gas as purge gas

i) Environment & Green House Effect (GHE)
IG : Inert gas has NO impact to environment
FG : Burn fuel gas in atmosphere generate Carbon Dioxide (CO2) which contributes to increase of Co2 content in atmosphere and increases Green House Effect (GHE)

ii) Burn back damage flare tip - reduce life span of flare tip
IG : Inert gas do not burn. NO burn back and potential damage of flare tip.
FG : Potential FG burn back damage flare and shorten flare tip life span.

iii) High OPEX avoid Burn back
IG : NO burn back. Minimum purge rate and low OPEX.
FG : Potential burn back lead to high purge rate (potential 10 times higher than purge rate of IG) and high OPEX

iv) Visible Flame
IG : Inert gas do not burn. No flame present.
FG : FG continuous burn and continuous visible flare at flare tip. Potential create uneasy situation in environment sensitive area.

v) Smoke Flaring
IG : Inert gas do not burn. No smoke flaring issue.
FG : Burning heavy FG lead to smoke flaring. Potential create uneasy situation in environment sensitive area. Increase likelihood of unburnt component and impact on environment.

vi) Steam injection for smokeless flaring
IG : Inert gas do not burn. No smoke flaring issue.
FG : Burning heavy FG lead to smoke flaring. Steam injection to reduce/eliminate smoke flaring. This increases CAPEX (additional steam injection facilities) and OPEX (steam loss).

vii) Radiation
IG : Inert gas do not burn. No additional radiation.
FG : Fuel gas burn lead to increase of radiation level (on top of solar radiation) to personnel working near flare stack.

Disadvantages using inert gas compare to fuel gas as purge gas

a) Fuel Gas Readily Available in Plant
IG : Required Nitrogen generator or use of Liquid Nitrogen and evaporator. Additional CAPEX and OPEX.
FG : Fuel gas readily available in plant. Minimum CAPEX and OPEX. Some plant generate hydrocarbon gas which shall be disposed off. This gas is readily serve as purge gas and inccur NO cost.

b) Inert Gas Cloud
IG : Flare system purge with inert gas, entire flare system is filled with IG gas (which potential heavier than air). Once any PSV/BDV open and release large amount of gas into flare header, it will "push" IG release through the flare tip. Heavy IG (compare to air) will sink create a IG gas cloud near plant. This is potential fatal thread (suffocation) to personnel on site.
FG : Continuous flaring lead to no or nearly no potential of gas present in atmosphere

c) Unburnt hydrocarbon gas emission
IG : PRV/BDV/PCV leak or passing lead to low heating value mixure (less than 200 btu/ft3) which is non-combustible. Release of hydrocarbon gas into atmosphere directly has more GHE impact than burning it. For example 1 mol of methane create 1 mol of CO2 if it is burnt. 1 mol of methane create 20-21 mol of equivalent CO2 if it unburnt.
FG : Continuous flaring lead to no or nearly no unburnt gas in atmosphere

d) Combustible Cloud lead to Instant Ignition
IG : Slowly hydrocarbon gas emission to atmosphere and built-up of combustible mixture in the plant, once the heating value for auto-ignition is reached, the combustible mixture potentially ignited. Its impact is just like a explosion and potential thread to personnel and surrounding facilities.
FG : Continuous flaring lead to no or nearly no unburnt gas in atmosphere

Concluding remark
Inert gas purging is normally understood as clean, low CAPEX, low OPEX, etc and regards as most likely candidate for flare purging. However, the associated SAFETY related issue may needs additional attention and focus. All...use inert gas wisely...

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Survey on Making CE More Interesting

2009-08-20T08:14:00.000-07:00

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If you have successfully subscribed to FREE Chemical Engineering (CE) magazine, you should have received a note from the publisher requesting you to participate a survey in order to help in making Chemical Engineering an even more interesting and informative publication for readers . The survey contain 21 simple questions and should only take a few minutes (less than 3 minutes) to complete.

Chemical & Process Technology webblog member (FREE CE subscriber) is encouraged to participate this survey.

Why ?
Reason being
i) to provide your inputs to make Chemical Engineering and so that information can be delivered in effective manners
ii) to serve your commitment to Chemical Engineering Magazine as a free magazine subscriber
iii) to increase your "points" so that you qualify again for free Chemical Engineering during next renewal

What to do ?
You as valid FREE CE subscriber, you will receive a message similar to above image. Just click the "Start the survey" button and it will direct you to complete the 3 minutes survey.

FREE Chemical Engineering Subscription Released every month (almost) but limited ! Quick act before they are fully subscribed. Before proceed, get some tips in "Tips on Succession in FREE Subscription". If FREE CE is temporarily unavailable, please try again beginning of the month (tips). Click here to qualify your FREE Chemical Engineering !

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Estimating Heat Loss from Buried Pipe

2009-08-15T20:38:00.000-07:00

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Plant in country experience subzero ambient temperature, one of the common natural phenomenon is increased heat loss from piping to ambient lead to low temperature fluid. Low temperature fluid will results fluid characteristic change i.e. low reactivity, increased viscosity, promote freezing, crystallization, etc and subsequently lead to several problem such as low productivity, high pressure drop, blockage, scaling, fouling, etc. It is important to design plant system to minimize heat loss to ambient.

Water system at subzero may freeze and lead to blockage and potentially cause pipe damage. One of the effective and non-costly method is utilize low conductivity of soil to reduce heat to ambient by burying water pipe. This may involve a typical heat loss calculation.

Above image is a typical buried pipe in soil.
Heat loss,

Q = S x Km x (T1 - T2)

with Shape factor S

S = 2 x PI x L / Ln [ 4 z / D ]

where
Q = heat loss from pipe, W
S = Shape factor, m
Km = Average soil conductivity, W/m-K
T1 = Pipe skin temperature, degC (or K)
T2 = Soil surface temperature, degC (or K)
PI = 3.141592654
L = Buried pipe length, m
z = Distance between the ground surface and the center of the buried pipe, m
D = Outside diameter, m

Above equations are applicable when L is large compare to D and z is larger than 1.5D.

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Example

A 30m long 100mm diameter hot water pipe of district heating system is buried in the soil 500mm below the ground surface (from pipe center line). Ground surface exposing wind chilling is 10 degC whilst pipe pipe skin temperature is expected to be 80 degC. Determine heat loss from pipe if soil average conductivity is about 0.9 W/m-K.

Km = 0.9 W/m-K
T1 = 80 degC
T2 = 10 degC
PI = 3.141592654
L = 30 m
z = o.5 m
D = 0.1 m

L is much larger than D and z = 0.5 larger than 1.5(0.1) = 0.15
Above equations may be used.

Shape factor,

S = 2 x PI x L / Ln [ 4 z / D ]
S = 2 x PI x 30 / Ln [ 4 x 0.5 / o.1 ]
S = 62.9 m

Heat loss,

Q = S x Km x (T1 - T2)
Q = 62.9 x 0.9 x (80 - 10)
Q = 3963 W

Ref : Section 3-7, Heat Transfer A Practical Approach by Yunus A. Cengel, 2nd Ed.

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FREE Chemical Engineering Digital Issue for Aug 2009

2009-08-12T06:55:00.000-07:00

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FREE Chemical Engineering Digital Issue for August 2009 has been released !

Chemical Engineering has just released FREE August 2009 issue. If you are subscriber of Chemical Engineering, you should have received similar notification.

***********************

Interesting articles for this month :

FAYF - Adsorption
This one-page guide explains various adsorbent properties and provides the corresponding
equations

Viscosity: The Basics
An important concept, sometimes forgotten, is that viscosity is not a single-point measurement.
Basic concepts and terminology are reviewed and techniques for quantifying viscosity are addressed

REACH: Looking Beyond Pre-registration
The next steps to ensuring compliance are significantly more complex

Keeping Cooling Water Clean
Proper monitoring and control of water chemistry is essential

Today’s SIS: Integrated Yet Independent
Bringing process control and instrumented safety systems together while still keeping them separate provides benefits beyond risk reduction

Monitoring Air Emissions
New requirements are spurring strong demand in air pollution monitoring

***********************

Relate Heat Radiation Level and Temperature

2009-08-09T01:40:00.000-07:00

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Earlier post "Heat Radiation For Pain & Blistering Threshold" and "Personnel Exposure Time For Heat Radiation" have discussed the heat radiation level lead to plant personnel pain and blistering threshold within an exposure time. Heat radiation design criteria for personnel exposure has been proposed for continuous and emergency exposure. Flare radiation run using software i.e. FLARESIM or calculation using Brzustowski and Sommer method, heat radiation level will be estimated and presented. For equipment, instrument, piping, painting, steel structure, etc expose to flare radiation will experienced increase temperature.

How shall a heat radiation level relates to temperature ?

Heat level (Et) at recepting location can be related to the sum of radiation heat (Er) plus convection heat (Ec).

Et = Er + Ec

For unit area (m2),

qt = qr + qc

where
qt = total heat flux at receptor (kW/m2)
qr = radiation heat flux at receptor (kW/m2)
qc = convection heat flux at receptor (kW/m2)

Radiation heat flux,

qr = sigma x (T⁴ - Ta⁴)

Convection heat flux

qc = (h / e) x (T - Ta)

where
sigma = stefan-Boltzman constant = 5.67 e -11 (kW/m2K)
h = convective heat transfer co-efficient = 0.007 (kW/m2K) at zero wind speed
e = Emissivity (0.8-0.9 for copper or rusted CS pipe, 0.1-0.2 for polished SS pipe)
T = Surface temperature (K)
Ta = Ambient temperature (K)

Therefore

qt = sigma x (T⁴ - Ta⁴) + (h / e) x (T - Ta)

If a heat radiation level is known at particular location, the corresponding temperature may be estimated using above equation.

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Personnel Exposure Time For Heat Radiation

2009-08-03T08:10:00.000-07:00

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Earlier post "Heat Radiation For Pain & Blistering Threshold" has briefing discussed about the exposure time for heat radiation level which lead pain and blister threshold. Equations have been proposed in order to link between exposure time for dedicated heat radiation. Example, with heat radiation of 6.3 kW/m2 (2 000 Btu/h·ft2), the pain threshold is reached in 8 s and blistering occurs in 20 s.

Maximum allowable personnel exposure time to particular heat radiation level proposed in API RP 521 (previous revision - 1997) and API Std 521 (latest revision - May 2008) are marginally different. Detail list out as follow

Heat Radiation (kw/m2)	API Rp 521 (March 1997)	API Std 521 (May 2008)
9.45	Exposure must be limited to a few (approx. six) seconds, sufficient for escape only. May consider tower or structure provide some degree of shielding.	Required urgent emergency action. Radiation shielding and/or special protective apparel (e.g. a fire approach suit) required
6.31	Emergency actions lasting up to 1 minutes without shielding but with appropriate clothing.	Emergency actions lasting up to 30s without shielding but with appropriate clothing.
4.73	Emergency actions lasting up to several minutes without shielding but with appropriate clothing.	Emergency actions lasting 2 min to 3 min without shielding but with appropriate clothing.
1.58	personnel with appropriate clothing can be continuously exposed	personnel with appropriate clothing can be continuously exposed

Above listing shows that latest API Std 521 has more stringent requirement compare to previous revision (1997), specifically for 6.31 and 4.73 kW/m2. The maximum allowable exposure time limit have been reduced. Continuous exposure limit remain unchanged.

Emergency Exposure
Maximum allowable exposure heat radiation of 6.31 and 4.73 kW/m2 (during emergency scenario) have been used as criteria in determining sterile area, an area around flare stack which no personnel shall be around without any personnel protective apparel. For conservative design and/or expect potential increase in flaring rate in future, a limit of 4.73 kW/m2 may be used. For common design, heat radiation limit of 6.31 kW/m2 is widely used. In general, maximum allowable heat radiation of 9.46 kW/m2 will be limited at the flare stack base where personnel may access to sterile area with proper personnel protective apparel.

Continuous Exposure
For continuous exposure, operator is assumed wearing appropriate clothing in a plant. Heat radiation at any location shall be limited to 1.58 kW/m2.

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Heat Radiation For Pain & Blistering Threshold

2009-07-26T01:38:00.000-07:00

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Flare is commonly installed in oil and gas process plant to burn hydrocarbon and/or toxic gas to avoid formation of combustible mixture, to minimize green house effect (GHE), to minimize health hazards to personnel on site, etc. Flaring hydrocarbon gas may generate carbon dioxide & water for complete combustion and soot (contribute to smokeless level) & unburnt components (contributes to toxic environment) for incomplete combustion. Besides, heat and noise are generated and radiated and transmitted around the flare tip.

Heat radiated from flare may transmitted in sphere form around the flare tip. Along the transmission, the energy is distributed in sphere form and this lead to reduction in heat radiation level (heat flux, kW/m2). Personnel or equipment along the transmission path will expose to this heat radiation. Personnel or equipment closer to flare tip will experience higher heat radiation level.

With studies conducted by Stoll and Greene (1958), following graph relate heat radiation versus time for Pain threshold and Blister threshold. With heat radiation of 6.3 kW/m2 (2 000 Btu/h·ft2), the pain threshold is reached in 8 s and blistering occurs in 20 s.

The following equation derived from above graph and can be used to relates heat radiation with time for pain and blister thresholds.

Pain threshold :

q = 25.544 x t ^-0.6742

Blister threshold :

q = 75.691 x t ^-0.8399

where :
q = heat radiation (kW/m2)
t = time (seconds)

Ref :
(i) A. M. STOLL and L. C. GREEN, The Production of Burns by Thermal Radiation of Medium Intensity, Paper Number 58-A-219, American Society of Mechanical Engineers, New York, 1958

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Pneumatic Test Failure in Mississippi Pipeline Project

2009-07-25T07:45:00.000-07:00

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An explosion caused by pneumatic test failure has occurred in Shanghai LNG Terminal on Feb 06 2009. This accident has resulted one worker killed and fifteen others injured.

Again it was heard that another pneumatic test failure has occurred in Mississippi in July 2009 and lead to at least one fatality and three others injured during a pneumatic test failure. The following photos show how severe is the destruction of pneumatic test failure.

Overall scene

Air-lifted pipeline

Twisted metal pipe

For those who intended to conduct pneumatic test, really have to ensure that proper procedure is followed. Ensure personnel who are not directly involved in the testing process are removed from the area and not allowed in the area for any reason. If you not involved directly in pneumatic test, please be alert and move away from the testing location.

Following are collection of article that helps you to educate yourself or your operators related to Pneumatic test :

Pneumatic Test During Pre-commissioning
This article has brief introduction about pneumatic test related procedure, safety concern, test pressure level, etc

Pneumatic Test Accident in Singapore
On May 22, 2002, a fatal accident occurred in Singapore involving the failure of a refrigerant receiver during a pneumatic test. In light of this incident, it is appropriate to again remind our readers of the hazards involved in pneumatic tests and to review the precautions that must be taken in conducting such tests.

Hazards of Trapped Pressure and Vacuum
A leak test on a heat exchanger was being conducted using low pressure gas when the tube bundle was ejected with great force striking two employees. One of them died on massive internal injuries...

Pipeline fails under air pressure test - Kills worker
Two workers had completed laying a 30 metre length of 300 mm diameter PVC pipe, in order to connect it to an existing steel pipe, along a suburban roadside. The pipe was then to be pneumatically tested up to a pressure of 690 kPa (100 psi)...

Pneumatic Test Operation Maintenance
This tank is intended for use vented to atmosphere. For outdoor applications, install a weatherproof vent hood or cap on the vent riser pipe and on the interstitial space vent of double wall tanks.

Pneumatic Test - Incident in Brazil
Incident happened in a non-ExxonMobil facility in Brazil during a pneumatic test of the tank associated piping. A blind was NOT installed to isolate the ...
Pdf

Pneumatic Test - IncidentASTM A1047 / A1047M - 05
ASTM A1047 / A1047M - 05 Standard Test Method for Pneumatic Leak Testing of Tubing

Read others incident...Click HERE

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FREE Chemical Engineering Digital Issue for July 2009

2009-07-19T07:16:00.000-07:00

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FREE Chemical Engineering Digital Issue for July 2009 has been released !

Chemical Engineering has just released FREE July 2009 issue. If you are subscriber of Chemical Engineering, you should have received similar notification.

***********************

Interesting articles for this month :

FAYF - Flowmeter Selection
This one-page guide details important facts for selecting a flowmeter

Revamps - Strategies for A Smooth Turnaround
Tie-in opportunities are few and far between. These rules of thumb will help make sure everything and everyone line up in time
Removal of Fouling Deposits on Heat Transfer Surfaces in Coal-Fired Process Heaters and Boilers
When conventional soot blowers are inadequate, an automated shot-blasting system offers a powerful solution
* If you encounter problem in reading this article, you may check this article via this link (for FREE subscriber only).

Disperse Difficult Solids
Recent advances in mixing technology offer increased efficiency in dispersing powdered additives into liquids for both low- and high-viscosity applications

***********************

Energy Input or E-method In Assessing AIV

2009-07-18T19:32:00.000-07:00

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High frequency acoustic Excitation downstream of pressure reducing device and potential of downstream piping failure on Acoustic Induced Vibration (AIV) has raised concern in many process plant

Earlier post "Extra Attention to Common Point and Similarity on AIV Failure" has discussed the common points and similarity of AIV such as typical failure location, system experienced failure in the past, failure time & period and Mach no. An engineer assessing AIV shall pay extra attention in these factors.

There are several methods have been discussed earlier in assessing AIV problem. There are "D-method" and "D/t-method". These two methods are widely used by many engineering contractor and consultants in many previous projects. There is another method, the Energy Input method or "E-method" which has been introduced in mid 1990.

Energy Input or E-method In Assessing AIV
The "E-method" is focus on the acoustic energy transmitted along the pipe, downstream of pressure reducing devices. The acoustic energy can be measured by Ma2.dP where Ma2 is the Mach number at downstream pipe and dP is the pressure drop across pressure reducing devices. The higher the acoustic energy or Ma2.dP, higher the risk for AIV failure.

Similar to the other two methods, based on data available in Carucci & Mueller (1982) studies, "No failure" and "Failure" points have been plotted in following chart with Ma2.dP versus Pipe Diameter / Wall thickness (D/t).

Note :
1) A "failure" point (54.8, 0.35) is below curve. It is failed on bad welding. No further failure after good welding.

2) Red point are "failure" point and Blue point are "No failure" point

From above chart a clear distinctive limit line can be established. This line may be used to assess potential failure of piping downstream of pressure reducing device. Any point above the line potentially fail on AIV, piping treatment or redesign required to minimise the risk of AIV failure.

*As limit line is straight or following any pattern, a representative equation is yet to be developed.

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FAYF - Fluid Flow

2009-06-24T16:14:00.000-07:00

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Chemical Engineering has shared a FAYF related FLUID FLOW. This FAYF is infact has been released in previous month, however, CE share with CE FREE subscriber again this month.

Fluid Type
In this FAYF, it starts with definition of fluid type such as :
i) Newtonian
ii) Power law
iii) Bingham plastic

where

Newtonian fluid
A fluid is known to be Newtonian when shear stresses associated with flow are directly proportional to the shear rate of the fluid.

Power law fluid
A structural fluid has a structure that forms in the undeformed state, but then breaks down as shear rate increases. Such a fluid exhibits “power law” behavior at intermediate shear rates

Bingham plastic fluid
A plastic is a material that exhibits a yield stress, meaning that it behaves as a solid below the stress level and as a fluid above the stress level

This one-page fact sheet summarizes information pertinent to laminar and turbulent pipe flow for the various types of fluids commonly encountered in the CPI...

Ref. :
1. Darby, R., Take the Mystery Out of Non-Newtonian Fluids, Chem. Eng., March 2001, pp. 66–73.
2. Churchil, S. W., Friction Factor Equation Spans all Fluid- Flow Regimes, Chem. Eng., November 1997, p. 91.
3. Darby, R., and Chang, H. D., A Generalized Correlation for Friction Loss in Drag-reducing Polymer Solutions, AIChE J., 30, p. 274, 1984.
4. Darby, R., and Chang, H. D., A Friction Factor Equation for Bingham Plastics, Slurries and Suspensions for all Fluid Flow Regimes, Chem. Eng., December 28, 1981, pp. 59–61.
5. Darby, R., “Fluid Mechanics for Chemical Engineers,” Vol. 2, Marcel Dekker, New York, N.Y., 2001.

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*This FAYF is only available FREE to Chemical Engineering Magazine registered user. Login required. Subscribe FREE CE, click here.
** Download immediately as article available FREE within short period only. Do not wait.
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