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Lecture 3 Control valves - PowerPoint PPT Presentation

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Lecture 3 Control valves. Elements of the final control. Automatic Valve actuator. I/P transducer. Valve body. Current Signal 4-20 ma. Flapper. Nozzle. Back pressure. Pneumatic Signal 3-15 psi. Diaphragm control valves. Globe valve. Rotary valve. Rotary valve. Globe valve.

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Lecture 3 Control valves

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Lecture 3 control valves

Lecture 3

Control valves

Lecture 3 control valves

Elements of the final control


Valve actuator

I/P transducer

Valve body

Lecture 3 control valves

Current Signal

4-20 ma







3-15 psi

Lecture 3 control valves

Diaphragm control valves

Lecture 3 control valves

Globe valve

Lecture 3 control valves

Rotary valve

Lecture 3 control valves

Rotary valve

Lecture 3 control valves

Globe valve

Most common control

valve style

Can be single- or double-


Single seated valves

Single-seated valves

  • Usually are employed when

    • Tight shut-off is required,

    • In sizes of 1 inch or smaller where unbalance forces acting on the valve stem is unimportant.

  • Usually have a top guided construction

Double seated valves

Double-seated valves

  • Usually is top and bottom guided.

  • Practical leakage approaches 0.5% of the rated CV.

  • Advantage lies in reduction of required actuator forces.

  • Have upper and lower ports of different diameters---allow to withdraw smaller plug through the larger port.

Angle valve

Angle valve

  • Single seated valves with special body configuration to suit specific piping or flow measurements.

  • May be used in case where the piping layout does not allow installing a globe valve.

Three way valves

Three way valves

  • A design extension of a typical double-seated valve.

  • Can be used for diverting service and for mixing service.

Actuators for control valves

Actuators for control valves:

  • Pneumatically operated diaphragm actuators;

  • Piston actuators;

  • Electro-hydraulic actuators;

  • Electro-mechanical actuators;

    More than 90% in use are pneumatically operated piston or diaphragm type

Safety consideration

Safety consideration

  • Air-to-open (AO) Failure close

  • Air-to-close (AC) Failure open

Lecture 3 control valves


Lecture 3 control valves

Defined at maximum

Valve opening

Related to valve inherent characteristics

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Over sized plug to provide

Additional Cv

Special seat machined

Into the body

Lecture 3 control valves

Linear valves:

A (X) =X= x/xo

Equal percentage valves:

A (X) = e -k(x/xo-1)

= a-(X-1)


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Distortion after installation

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Valve hysteresis

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Valve positioner

What accomplishment a positioner can have

What accomplishment a positioner can have?

  • Provide precise positioning of the valve

  • Provide adequate power on high-pressure applications

  • Increase control valve speed of response

  • Reverse valve action

  • Provide split range operation

Typical positioner performance

Typical positioner performance

  • Pneumatic signal ranges: 3-9, 3-15, 9-15, 3-27, 6-30 psig

  • Air supply pressure: 20 to 100 psig

  • Repeatability: within 0.1% of stroke

  • Hysteresis: within 0.3% of stroke

  • Linearity: 0.5% of stroke

Lecture 3 control valves

Split range control valves

Lecture 3 control valves

Control valve sizing

Given expected pressure conditions, select

throttling control valve to pass the required

flow rate. It is a key step In ensuring that the

process can be properly Controlled.

Basic sizing practices have been standardized

Upon (e.g., ISA S75.01) and are implemented as

PC-based program by manufactures.

Allocating pressure drop

Allocating pressure Drop

The value of b is important to the installed valve characteristic curve

The pressure drop is an economic loss to the process operation

Low pressure drop result in larger valve sizes and in decrease in a range of control

Rules of thumb: “20% to 50% of total dynamic pressure drop”, or, “25% or 10 psi”

Determine valve capacity to meet required flow rate computing c v

Determine valve capacity to meet required flow rate--- Computing CV

For liquid service:

Lecture 3 control valves

For gas and vapor:

For steam:

Cf: critical flow factor

Tf: degree of superheat in oF

P1:upstream pressure

Gf: specific gravity at 14.7psi and 60oF

Lecture 3 control valves

There are still many other formulas in

Provided by control valve manufacturers.

The formulas have different forms but give

similar results.

Lecture 3 control valves

To meet working flexibility, CV, DPV and

DPt are to satisfy the following:

Lecture 3 control valves

Given either three of the following five variables

( Q ,qo, CV, DPt , DPV ), the other two can be


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Rangeability of a control valve:

After being installed:

Lecture 3 control valves

There are standard procedures to size a

control valves, besides computing CV. In

most sizing problem, the size of pipe is

Known. It is usually to solve for CV and

reducers combined. Thus, a geometric

factor, Fp, is required in the flow formula:

The details are out of the scope of this lecture.

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