Control Valves

• Purpose• Terminologies• Construction• Selection criteria• Sizing calculation• Issues on control valve

Cheer up. It is not as

difficult as you

thought!

• Purposeto regulate flow of any phase (liquid, gas, fluid and mixed phases) that could pass through it. At the moment, solid phase (such as sand, flour, sugar) is not possible.

• Chemical process applications>80% of final control element found in chem. proc. ind.

• Symbols

=

Introduction

Construction

Seat Plug

Actuator

Body

Selection Criteria

Air-to-open (ATO) or Fail closed (FC). Use to prevent flow when no pneumatic air supply. This action may prevent hazardous chemical or flammable fluid from endangering lives.

Air-to-closed (ATC) or Fail open (FO). Use to allow flow of fluid during loss of pneumatic air supply. This action may save overheating of temperature related processes such as catalyst heating or heating of polimerization process.

Valve action: ATO vs. ATC

compare

compare

compare

compare

Selection CriteriaValve action: ATO vs. ATC

The selection of control valve action would depend upon safety of personnel, action time and process hazard upon loss of pneumatic signal.

Prevent pressure build-up

Preventing reaction upstream.

Prevent material from over-heating.

(Smith & Corripio, 3rd. Ed., p.158)

Selection CriteriaCapacity and sizing

..GS

PCQ v

P in psi

Fluid S.G. QFlowrate In gpm

Example

Calculate the Cv of a control valve when a 12.5 gpm fluid having a SG of 0.8 pass through it. The pressure drop across the control valve is 14 psi.

3

8014

512

.

.

..GSP

QCv

Selection CriteriaValve characteristics: Linear, equal percentage and quick opening.

Selection CriteriaValve characteristics: Linear, equal percentage and quick opening.

0 gpm

8 gpm

4 gpm

6 gpm

2 gpm

ExampleWhat will be the flowrate when the indicator shows 0.75?

Quick openingFlow coef. = 0.96Flowrate = 0.96 x 8 = 7.7 gpm

LinearFlow coef. = 0.75Flowrate = 0.75 x 8 = 6 gpm

Equal percentageFlow coef. = 0.3Flowrate = 0.3 x 8 = 2.4 gpm

Selection CriteriaControl valve size selection

Select an equal percentage control valve for the following condition.

Flow condition Flowrate, gpm Pressure drop, psi Valve lift, %

Maximum 120 10 80

Normal 42 15 50

Minimum 13 20 20

Flow condition Flowrate,gpm

Pressure drop, psi

Valve lift, %

Maximum 120 10 80

Normal 42 15 50

Maximum 13 20 20

Cv calculated

37.9

10.8

2.9

Cv actual

37.3

10.8

2.93

Contiunue on next page for valve rangeability calculation…

Selection CriteriaControl valve rangeablity

061392

937

013

..

.

C

CR

.P

P

Q

QR

min,v

max,vshelf

ratemax

ratemin

min

maxinstalled

Check for valve rangeability

Small difference. Selection 2” okay to use.

Issues on Control Valves

• What make a control valve becomes linear, equal percentage or quick opening control valve?

• Why flashing and cavitation are serious problem in control valve?

• What causing flashing and cavitation in a control valve?

• What is the current solution to flashing and cavitation in control valve?

What make a control valve becomes linear, equal percentage or quick opening control

valve?

The amount of flow produced with respect to steady

increment in stem travel.

Why flashing and cavitation are serious problem in control valve?

Erosion

What causing flashing and cavitation in a control valve?

Hear sound of cavitation and flashing.

What is the current solution to flashing and cavitation in control valve?

Read p.135 of Fisher Control Valve Handbook