control valve : the final control element · selection parameters for the valve : •size,...

Control Valve : The Final Control Element

Nimish Shah

VALVES

selection parameters for the valve : •size, •material of construction, •pressure and temperature ratings, and •End connections.

How to select Control Valve???

In addition, if the valve is for control purposes one must define additional parameters.


These parameters include :

– Method of operation,

– maximum and minimum flow capacity requirements,

– pressure drop during normal flowing conditions,

– pressure drop at shutoff, and

– Maximum and minimum inlet pressure at the valve.


Meeting these parameters happens through and by

• selecting body styles,

• material of construction,

• seats,

• packing,

• end connections,

• valve operators—actuators—and supports.


Cross-section of a Globe Valve

Control Valve Flow Characteristics

• The relationship between valve

capacity and valve travel is known as

the flow characteristic of the valve.


• There are main three types of Flow characteristics:

– Quick Opening

– Linear flow

– Equal Percentage

• Quick Opening-

• used for safety by-pass applications where quick opening is desired


• Equal Percentage-

• used for about 90% of control valve applications since it results in the most linear installed characteristics


• Linear-

• used when a relatively constant pressure drop is maintained across the valve


Inherrent Flow Characteristics For Common Valve Trim Designs

Control Valve Characteristics

• Quick opening

– Most of the flow variation takes place in the lower third of the valve (stem) travel

– Not ideal for regulating flow

– Appropriate for relief valves and on-off control (safety; avoid over-pressuring vessels)

Control Valve Characteristics

• Linear

– Produces flow proportional to valve position

– At 50% valve position, flow through valve is 50% of maximum flow

Control Valve Characteristics (cont.)• Equal Percentage

– Equal increments in valve position result in equal relative or percentage increments in flow

– When valve position increases 1% going from 20% to 21%, flow increases by 1% from value at 20%

– When valve position increases 1% going from 60% to 61%, flow increases by 1% from value at 60%

– Percentage change is equal, but the flowrate at 60% is greater than the flowrate at 20%

What happened if power failed?

What happened if power failed?

• It is a power-operated device

• Well, what happens if the power is cut off?

• "fail-safe mode" it is the key totroubleshooting it.

• In most applications (about 80%), it isdesirable for valves to fail closed.

• In other applications, you might want avalve to fail open or fail in place. Safetyconcerns and process requirements willmandate the fail mode of the valve.

Control Valve "Fail-Safe" Positions

• Cause of Fail-Safe Condition: Loss of Air Pressure

Fail Safe Mode in Rotary Valve

• When a valve is not sitting in its failposition, it is being told how and when tomove by some external signal.

• If a Control Valve is observed in anunstable condition or appears to not beresponding correctly to an input signal,remember that something is telling thevalve to behave that way.

How is works?

Two Components:

•Actuator

•Positioner

Actuators

• The actuator receives the conditionedsignal and changes it to some form ofmechanical energy or motion.

• Typical devices used as actuators includesolenoids, pneumatic valve positioners, ACand DC motors, stepper motors, hydraulicmotors, and hydraulic pistons.

• Many control valves include a pneumaticvalve positioner.

Actuators

Types of Actuators

• Spring and Diaphragm

• Pneumatic Piston

• Electric Motor

• Electro hydraulic

Spring & Diaphram.ppt

Pneumatic Piston.ppt

Electric Motor.ppt

Electrohydrolic.ppt

Valve Actuator Selection• Choose an air-to-open for

applications for which it is desired to have the valve fail closed.

• Choose an air-to-close for applications for which it is desired to have the valve fail open.

Cross-section of a Globe Valve

Optional Equipment

• Valve positioner- a controller that adjusts the instrument air in order to maintain the stem position at the specified position.

• Positioners are almost always used on valves serviced by a DCS.

Photo of a Valve Positioner

Valve Positioners

• This device increase or decrease theair pressure operate the actuatoruntil the valve stem reaches theposition.

• generally mounted on the side or topof the actuator.

• Connected mechanically to the valvestem.

Valve Positioners

Pneumatic Control Valves

Thank You

control valve : the final control element · selection parameters for the valve : •size,...

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