Topic 7

Control Valves

What We Will Cover

Topic 1

Introduction To Process Control

Topic 2

Introduction To Process Dynamics

Topic 3

Plant Testing And Data Analysis

Topic 5Enhanced

Regulatory Control Strategies

Topic 7

Process Control Hardware Systems

Topic 4

Controller Actions And Tuning

Topic 7

Control Valves

Topic 8

Process Control Troubleshooting

Control Valves

Control valve construction

Valve types

Control valve characteristics

Valve failure characteristics

Control valve sizing

Control valve performance– Common problems– Testing

Control Valve Construction

The control valve is roughly divided into 2 parts– Actuator Assembly– Valve Body

Actuator Assembly

Valve Body

Valve types

Globe valve

Ball valve

Butterfly valve

Control Valve Characteristics Three of the most common characteristics are

– Linear– Equal-percentage– Quick-opening

Assumptions:– Valve travel is proportional to controller OP– Pressure difference across the valve is constant– Process fluid is not flashing, cavitating or approaching sonic flow

Selection is based on applications– For quick pressure relief, we may want to use quick-opening– For controlling flow at low rates, we may want to use an equal

percentage– For control of flow across a wide-range of flow rates, use linear

Control Valve Characteristics

Failure Characteristics

Not to be confused with Control Valve Characteristics

Also known as “failure position”

“Fail” refers to instrument air or signal failure

When instrument air fails, there is no air to move the control valve

When signal fails, there is no signal to make more or less air go into the control valve

Failure Characteristics

Control valves can fail-open or fail-close

Fail-open means it will open when there is a failure

Open is its default position

It needs air to close– 3 psig means open, 15 psig means close

It is also called an air-to-close control valve

What about a fail-close valve?

Failure Characteristics

Control valve failure position selection is generally based on safety considerations:– Is it safer for the valve to go full open or close in the

event of failure?

In some control systems, failure position has impact on controller action – be careful!– For such systems, in fail-open valves, increasing OP

means closing valve (0% OP 3 psig; 100% OP 15 psig)

– In other systems, 0% OP 15 psig; 100% OP 3 psig can be configured

Control Valve Sizing

Control valves are sized according to their Valve Sizing Coefficient, Cv

Need to know how to calculate Cv

Many equations are available for control valve sizing, but most equations will come up with roughly the same answers

Each control vendor makes a range of control valves with different Cv’s

Excess Capacity

Flow-control loops– Size the control valve for 150 percent of normal flow

rate at the normal flow pressure drop or 120 percent of maximum flow rate at the maximum flow pressure drop, whichever results in the larger Cv

Level, pressure and temperature-control loops – Size for 180 percent of normal flow at the normal flow

pressure drop or 120 percent of maximum flow at the maximum flow pressure drop, whichever results in the larger Cv

Sizing Equations - Liquid

Nomenclature– Q (l/s) = Liquid flow rate through valve– Delta P (kPa) = Pressure drop across valve– G (dimensionless) = Specific gravity of liquid at its

flowing temperature– SG is defined as density of liquid / density of water @

4C

GP

QCv

/

67.41

Sizing Equations - Steam

Nomenclature– W (kg/s) = mass flow rate– Pi (kPa abs) = Inlet pressure– ρ (kg/m3) = Inlet density– C1 is a valve type-specific factor (See next slide)– Kc = Valve capacity correction factor for steam or gas flows when ΔP is

less than critical– ΔP (kPa) = Pressure drop across valve– Value in parentheses is in degrees, not radians

ic

vPK

C1

3

C

W108.78

ic P

P

CK

1

3417sin

Sizing Equations – C1

Sizing Equations - Gas

Nomenclature– W (kg/s) = mass flow rate– Pi (kPa abs) = Inlet pressure– M (g/mol) = Molecular mass– T (K) = Inlet temperature– C1 is a valve type-specific factor (See next slide)– Kc = Valve capacity correction factor for steam or gas flows when ΔP is

less than critical– ΔP (kPa) = Pressure drop across valve– Value in parentheses is in degrees, not radians

ic P

P

CK

1

3417sin

TMPK

Cic

v

1

3

C

W10226

Example

Size a control valve to control the flow rate of water in a pipe. This control valve is part of a flow-control loop. These are the information given:– Normal flow rate = 3000 kg/hr– Normal flow temperature = 30ºC– Upstream pressure, Pi = 400 kPa abs– Downstream pressure, Po = 240 kPa abs– Specific gravity of water = 0.99 @ 30ºC

Control Valve Performance

Control valve is usually the weakest link in a process control loop– Measurement devices can measure with very

high accuracy– A 32-bit processor used as a controller can

calculate up to many decimal places of accuracy

– The control valve, being a mechanical device is subjected to wear and tear

Factors Affecting Performance

Deadband– A region in which the control valve does not

respond to controller output

Backlash– Due to looseness in mechanical fittings

Hysteresis– Different behaviours which opening or closing

Process control engineers will only say that the control valve is “sticky”

Deadband

Small OP changes do not affect the PV. Only when the OP has changed more than the deadband will the PV change

Usually due to friction– Air pressure cannot overcome the frictional force

resulting in no control valve movement

So more air is introduced into the control valve

When the air pressure finally overcomes the friction, the valve has moved too much

Control Valve Performance

Good valve

Backlash

Due to wear and tear

Actuator moves but there is some slack that must be overcome before there is movement

Results in deadtime – results in conservative controller tuning

Hysteresis

The control valve responds to the controller output

But it moves more in one way than the other

Controller gets confused because it has only one set of tuning parameters

20

25

30

35

40

45

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60

65

70

0 200 400 600 800 1000 1200

Time (s)

Co

ntr

olle

r O

utp

ut

(%)

0

10

20

30

40

50

60

Flo

w R

ate

(m3

/hr)

Controller Output Flow Rate

Good Control Valve Performance

11

11.5

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0 500 1000 1500 2000 2500 3000 3500 4000

Co

ntr

oll

er O

utp

ut

(%)

0

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10000

15000

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Flo

w (

Nm

3/h

r)

FIC-13052.OP FIC-13052.PV

Response even at 0.2% output change

FIC-13052 data with spikes removed.

Terrible Control Valve Performance

Testing Control Valve Performance

Put loop in MAN, do OP step change by small amounts, observe PV

If no change in PV, then continue to change OP in the same direction until you observe PV change

If a PV only responses only after the OP is changed by a total of 1%, the control valve probably has a deadband of 1%

Backlash shows up as deadtime, that is why it is good to use flow loops to determine backlash as flow loops has almost zero deadtime

Hysteresis is easily seen from the process gains you get in the “up” compared to the “down” directions

Assessing Deadband

SP

PV

OP

% Deadband

Another way to assess deadband:

Tune loop sluggishly;

Put loop in AUTO;

Make a change in the SP;

When you observe a change in the PV, change the SP in the opposite direction;

Assess deadband using data from 2nd step

Plant Testing