ch 5 control valves.ppt

Control Valves

Ted Huddleston

Dept of Chemical Engineering

University of South Alabama

Fig C-7.2 page 747

Reciprocating Stem Sliding Stem

vp

Control Valve ActionPage 201 - see Figure C-7.2 on page 747, In this figure supply air enters the case above the diaphragm. For an air pressure of 3 psig, the diaphragm and valve stem are at their topmost position and the valve is wide open. When the air pressure is 9 psig, the diaphragm and valve stem are at their mid point and the valve is half open. When the air pressure is 15 psig, the diaphragm and valve stem are at their bottom position and the valve is closed.

This is an air-to-close (ATC) valve. If control air pressure were to fail, the spring would push the stem up and open the valve - called a fail-open (FO valve.

ATO and ATC Actuators

Control Valve Actions

Which to select? ATC or ATO ? Ask “Which is safer if there is a malfunction

which includes loss of control air?” On fuel and steam valves, use FC. On cooling water, and inert gas blanket lines

use FO. On all valves perform a safety audit.

P&ID Example

Control Valve Characteristics

Describes how flow through valve varies with stem position.

Results from the shape of the valve’s seat and plug, which determines how valve resistance changes with stem position.

Inherent valve characteristic Installed valve characteristic

Control Valve Action

Actuators are built to either open a valve with increasing air pressure (ATO) or to close a valve with increasing air pressure (ATC).

See Figure C-8.1 page 750.

Plugs and Seats

Parabolic Characteristic Quick-Opening Characteristic (Equal Percentage)

Valve Equation

f

vv G

ΔpCf

f = liquid flow, U.S. gpm pv = pressure drop, psi Gf = specific gravity of liquid Cv = valve coefficient Cv = Cv(vp)

Eqn (5-2.1) page 203

LIQUID SERVICE

Inherent Characteristic

constant)vp(Cf v

Valve is mounted on a test stand such that vp can be changed, changing Cv and f, but pressures are adjusted such that pv is held constant.

f

vv G

Δp)vp(Cf

Inherent Valve Characteristic Curves

% vp

Cv,maxfmax100

vmax

Cf100f

f

0

constant)vp(Cf v

Page 211

Inherent Valve Characteristic Curves

Linear Characteristic:

Cv = Cv,max (vp) 0 < vp < 1

Equal Percentage Characteristic

Cv = Cv,max (vp-1) 0 < vp < 1

25 < < 100

Page 211

Equal Percentage Valves

Note the shape of the inherent characteristic.

When the valve is at vp = 0.2, flow is small, and the slope f/vp is small.

When the valve is at vp = 0.8, flow is large, and the slope f/vp is large.


The quantity

100Δvp

fΔf

100fΔvp

Δf

represents the percent change of flow per unit change in valve position.

The percentage change of flow per unit change

in valve position is the same or equal for all vp.


constantαCf 1)(vpmaxv,

)1)(ln(αconstantC 1)(vpmaxv,

dvp

df


)ln(

αconstantC

)1)(ln(αconstantC11)(vp

maxv,

1)(vpmaxv,

dvp

df

f

Installed Valve Characteristics

Section starts on page 212. See diagram on page 213. A control valve is installed in series with a heat exchanger. Water is supplied from the left at a constant pressure, and exhausts to the atmosphere at the right. Valve position can vary, flow through the process can vary, pressure drop through the exchanger, through the pipe and fittings, and through the valve can vary. But the total pressure drop po is constant.


Assume flow is fully turbulent. Pressure drop through line, fittings, and heat exchanger is

2fLL fGkΔp

Pressure drop across valve is obtained from valve equation

2v

2

fv C

fGΔp


The total pressure drop is

2fL2

vo fGk

C

1Δp

Solving for the flow f yields


f

o

2vL

v

G

Δp

Ck1

Cf(vp)

Linear

=%1)(vp

maxv,v

maxv,v

αCC

(vp)CC


What installed valve characteristic is desirable?

Flow rate should change by the same amount per vp change throughout valve stroke.

Gain (flow rate change/vp change) should be constant for all values of vp.

This is a linear installed valve characteristic.

Inherent vs Installed Characteristics

vp

flow

When a valve is installed in series with process equipment, the installed characteristic flow curve is shifted up and to the left of the inherent flow curve.

See Figure 5-2.7 on page 217.

Installed

Inherent

So – which to choose ?

When valve takes up most ( > 80 %) of the pressure drop – use a linear valve. (Ex: Valve on a fuel gas line between the header and the burner.)

When valve is installed in series with other equipment and valve takes up 20 – 60% of the pressure drop – use =% valve.

When valve takes up 60-80% of pressure drop – let boss decide.

Valve Rangeability

f(0.05)

f(0.95)

positionvalve5%atFlow

positionvalve95%atFlowtyRangeabili

Defined on page 212. Also known as the turn-down ratio, it is the ratio of the maximum controllable flow to the minimum controllable flow FOR THE INSTALLED VALVE. Flow f(vp) is computed using EQN (5-2.14) on page 214.

Control Valve Cv

The size of a control valve is described by its “Cv”.

f

vmaxv, G

ΔpCf

The valve is fully open.

pv = 1 psi

Gf = 1 (fluid is water)

f = Cv,max gpm

Control Valve Cv

The Cv of a control valve is the flow in gpm of water that would flow through the fully open valve caused by a pressure drop of 1 psi.

Control Valve Sizing

Linear Installed Characteristic Small Pressure Drop High Rangeability Operates approximately half open at design

flow


Method One: The Overcapacity Method in which the valve is specified so that when the valve is fully open with specified pv, the flow is double the design flow.

1) flow = 2 x Design Flow

2

ΔpΔp

4

Δp)2 o

vo


4) From the manufacturer’s literature select the smallest Cv that exceeds the value of Cv,max found in part (3).

f

v

maxv,

GΔp

FlowDesign2C)3


Method Two: The Design Flow Method in which the valve will pass the design flow at valve position vp = 0.6 with the specified pv.

1) flow = Design Flow

2

ΔpΔp

4

Δp)2 o

vo


f

v

maxv,

GΔp

vp

FlowDesignC

)3

ValveLinear


f

v1vpmaxv,

GΔp

α

FlowDesignC

%)3

Valve


4) From the manufacturer’s literature select the smallest Cv that exceeds the value of Cv,max found in part (3).


We have not considered cavitation, flashing, sonic flow, the pressure drop recovery factor, the Reynolds Number recovery factor, piping geometry factor, noise reduction, and other important items. The equation used here is for liquid service. Equations for gases and vapors are more complicated.

When buying valves from a particular manufacturer use that manufacturer’s equations and technical support.

ch 5 control valves.ppt

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