Control Valve

Sliding-Stem Control Valve Terminology (3)

2008-08-25T15:05:00.001+07:00

Seal Bushing :
Top and bottom bushings that provide a means of sealing the yoke,to adjust the spring compression.

Spring Seat :
A plate to hold the spring in position and to provide a flat surface for the spring adjustor to contact.

Static Unbalance :
The ned produced on the valve stem by the fluid pressure acting on the closure member and stem with the fluid at rest and with stated pressure condition.

Stem Connector :
The device that connects the actuator stem to the valve stem.

Trim :
The internal components of a valve that modulate the flow of the controlled fluid. In a globe valve body,trim would typically ionclude closure member,seat ring,cage,stem,and stem pin.

Trim Soft-Seated :
Valve trim with an elastromeric,plastic or other readily deformable material used either in the closure component or seat ring to provide tight shutoff with minimal actuator force.

Upper Valve Body :
A half housing for internal valve parts and having one flow connection. It usually includes a means for sealing agains leakage along the stem and provides a means for mounting the actuator on yhe split valve body.

Valve Body :
The main pressure boundary of the valve that also provides the pipe connecting ends,the fluid flow passageway,and supports the seating surface and valve closure member. Among the most common valve body construction are :
a) Single-ported valve bodies having one port and one valve plug.
b) Double-ported valve body having two port and one valve plug.
c) Two way valve bodies having two flow connections,one inlet and one outlet.
d) three-way valve bodies having three flow connections,two of which can be inlet with one outlet (for converging or mixing flows),or one inlet and two outlet (for diverging or diverting flows). The term valve body,or even just body,frequently is used in refering to the valve body together with its bonnet assembly and included trim parts. More properly,this goup of components should be called the valve body assembly.

Valve body assembly (commonly valve body or valve,more properly valve body assembly):
An assembly of a valve,bonnet assembly,bottom flange (if used),and trim elements. The trim includes the closure member,which opens,closes,or partially obstructsone or more ports.

Valve plug :
A term frequently interchanged with plug in reference to the closure member.

Valve stem :
In a linear motion valve,the part that connects the actuator stem with the closure member.

Yoke :
The structure that rigidly connects the actuator power unit to the valve.

Sliding-Stem Control Valve Terminology (2)

2008-08-25T15:01:00.000+07:00

Cylinder :
The chamber of a piston actuator in which the piston moves.

Diaphragm :
A flexible,pressure responsive element that transmits force to the Diaphragm plate and actuator stem.

Diaphragm Actuator :
A fluid powered device in which the fluid acts upon a flexible component,the Diaphragm.

Diaphragm Case :
A housing,consisting of top and bottom section,used for supporting a Diaphragm and establishing one or two chapter chambers.

Diaphragm Plate :
A plate concentric with the Diaphragm for transmitting force to the actuator stem.

Direct Actuator :
A Diaphragm actuator in which the actuator stem extends with increasing Diaphragm pressure.

Extention Bonnet :
A bonnet with greater dimention between the packing box and bonnet flange for hot or cold service.

Lower Valve Body :
A half housing for internal valve parts having one flow connection. The seat ring is normally clamped between the upper valve body and the lower valve body in split valve connection.

Offset Valve :
A valve construction having inlet and outlet line connection on different planes but 180 degrees opposite each other.

Piston :
A moveable pressure responsive element that transmits force to the piston actuator stem.

Piston Type Actuator :
A fluid powered device in which the fluid acts upon a moveable piston to provide motion to the actuator stem. Piston type actuator are classified as either double-acting,so that full power can be developed in either direction,or as spring-fail so that upon loss of supply power,the actuator moves the valve in the required direction of travel.

Plug :
A term frequenly used to refer to the closser member.

Port :
The flow control orifice of a control valve.

Retaining Ring :
A split ring that is used to retain a separable flange on a valve body.

Reverse Actuator :
A diaphragm actuator in which the actuator stem retracts with increasing diaphragm pressure. Reverse actuators have a seal bushing installed in the upper end of the yoke to prevent leak age of the diaphragm pressure along the actuator stem.

Rubber Boot :
A protective device to prevent entrance of damaging foreign material into the piston actuator seal bushing.

Sliding-Stem Control Valve Terminology (1)

2008-08-25T14:59:00.001+07:00

The following terminology applies to the phisical and operating characteristic of standart sliding-stem control valves with diaghram or piston actuator. Some of the terms,particularly those pertaining to actuators,are also appropriate for rotary-shaft control valves. Many of the definitions presented are in accordance with ISA S75.05,control valve terminology,althought other popular terms are also included. Additional explanation is provided for some of the more complex terms. Separate sections follow that define specific rotary-shaft control valve terminology,control valve function and characteristic terminology,and other proces control terminology.

Actuator Spring :
A spring,or group of springs,enclosed in the yoke or actuator cashing that moves the actuator stem in a direction opposite to that created by diaphgram preesure.

Actuator Stem :
The part that conects the actuator to the valve stem and transmits motions (force) from the actuator to the valve.

Actuator Stem Extension :
An extension of the piston actuator stem to provide a means of transmitting piston motion to the valve positioner.

Actuator Stem Force :
The net force from an actuator that is available for actual positioning of the valve plug.

Angle Valve :
A valve design in which one port is co-linear with the valve stem or actuator,and the other port is at a right angle to the valve stem.

Bonnet :
The portion of the valve that contains the packing box and stem seal and can guide the stem. It provides the principal opening to the body cavity for assembly of internal parts or it can be an integral parts of the valve body. It can also provide for the attachment of the actuator to the valve body. Typical bonnets are bolted,threaded,welded,pressure seals,or integral of the body.

Bottom Flange :
A part that closes a valve body opening opposite the bonnet opening. It can include a guide bushing and/or serve to allow reversal of the valve action.

Bushing :
A device that supports and/or guides moving parts such as valve stems.

Closure Member :
The moveable part of the valve that is positioned in the flow path to modify the rate of flow through the valve.

Process Control Terminology

2008-07-26T22:10:00.000+07:00

Accessory : A device that is mounted on the actuator to complement the actuator's function and make it a complete operating unit. Examples include positioner,supply pressure regulator,solenoids and limit switches.

Actuator : A pneumatic,hydraulic or electrically powered device that supplies force and motion to open or close a valve.

Actuator Assembly : An actuator,including all the partinent accessories that make it a complete operating unit.

Backlash : The general name given to a form of deadband that result from a temporary discontinuity between the input and output of a device when the input of the device changes direction. Slack,or looseness of a mechanical connection is a typical example.

Clossed Loop : The interconnection of process control components such that information regerding the process variable is continuously feed back to the controller set point to provide continuous,automatic corrections to the process variable.

Controller : A device that operates automatically by use of some established algorythm to regulate a controled variable. The controller input receivesinformation about the status of the process variable and then provides an appropriate outputsignal to the final control element.

Control Range : The range of valve travel over which a control valve can maintain the installed valve gain between the normalized values of 0.5 and 2.0.

Control valve Assembly : Includes all components normally mounted on the valve. The valve body assembly,actuator,positioner,air sets,transducers,limit switches,etc.

Dead Band : The range throught which an input signal can be varied,upon reversal of direction,without initiating an ofservable change in the output signal. Dead band is the name given to a general phenomenon that can apply to any device.

Dead Time : The time interval (Td) in which no response of the system is detected following a small (usually 0.25 - 5%) step input. It is measured from the time the step input is initiated to the first detectable response of the system being tested. Dead time can apply to a valve assemblyor to the entire process.

Disk : A valve trim element used to modulate the flow rate with either linear or rotary motion. Can also be referred to as a valve plug or clossure member.

Introduction to Control Valves

2008-07-21T11:09:00.000+07:00

Control Valves is a Process plants consist of hundreds,or even thousand,of control loops all networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variable such as pressure,flow,level,temperature,etc. Within a required operating range to ensure the quality of the end product. Each of this loops receives and internally creates disturbances that detrimentally affect the process variable,and interaction from other loops in the network provides disturbances that influence the process variable.

To reduce the effect of these load disturbances,sensors and transmitters collect infor,ation about the process variable and its relationship to some desired set point. A controller then processes this information and decides what must be done to get the process variable back to where it should be after a load disturbance occurs. When all the measuring,comparing and calculating are done,some type of final control element must implement the strategy selected by the controller.
The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid,such as gas,steam,water,or chemical compounds,to compensate for the load disturbances and keep the regulated process variable as close as posible to the desired set point.

Many people who talk about control valves are really referring to a control valve assembly. The control valve assembly typically consists of the valve body,the internal trim parts,an actuator to provide the motive power to operate the valve,and a variety of additionalvalve accessories,whice can include positioners,tranducers,supply pressure regulators,manual operators,snubbers,or limit switches.

Whether it is called a valve,control valve or a control valve assembly,is not as important as recognizing that the control valve is a critical part of the control loop. It is not accurate to say that the control valve is the most important part of the loop. It is useful to think of a acontrol loop as an instrumentation chain. Like any other chain,the whole chain is only as good as its weakest link. It is important to ensure that the control valve is not the weakest link.

welcome

2008-07-17T18:58:00.000+07:00

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