Valve Charecteristic(1)

8/22/2019 Valve Charecteristic(1)

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VALVE CHARECTERISTIC

Valve characteristic is the relation between the flowthrough the valve and the valve stem position.

Three types of valve characteristic in relation

between the stem position ( as % of full range) andflow rate (as a % of maximum).

Quick opening

Linear

Equal percentage

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2


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TYPES OF GLOBE VALVE

Three types of globe valve in relation between the

stem position ( as % of full range) and flow rate (as a

% of maximum).

Quick opening

Linear

Equal percentage

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1. Quick opening

It is used for ON/OFF control applications. A

relatively small motion of valve stem results a

maximum possible change in flow rate through

the valve.

It is also used for safety where quick opening isdesired.

It is known as the square root trim.

1)s(0valvetheofopenfractions sf

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2. Linear

Used when a relatively constant pressure drop is

maintained across the valve and the flow rate varieslinearly with the stem position according to the

following relationship:

3. Equal percentage

A given percentage change in stem position produces

an equivalent change in the flow.

5

maxmax S

S

Q

Q


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Rangeability is the ratio of maximum to minimum

controllable CV. This is also sometimes called CV Ratio orTurndown

The change in the flow rate depends on the flow rate

already occurring and the change in stem displacement.

max

min

max

minQ

Q SS

Q

Q

min

max

Q

QRtyRangeabili

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Q/Qmax

S/Smax

Valve Characteristics

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EXAMPLE

For example, an application may require a

maximum CVof 170. A 4 Equal Percentage Trimmay be selected that has a maximum CV of 195.

Using the rangeability value for this trim, the

minimum CVis 195/100 =19.5, not 170/100 =17.

DO EXAMPLES IN THE GUIDE

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VALVE SIZING

Valve sizing depends on the flow rates through the

control valve, stem position , fluid properties and thepressure drop through the valve.

The volumetric flow, Q, through the control valve can bemeasured by the flow equation given as :

, where P is the pressure drop across the

valve, SG, is relative density compared with water, and

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SG

PCQ vv


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VALVE COEFFICIENT(Cv) TABLE

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the Cvflow coefficient is the volume (in US gallons)

of water that will flow per minute through a valve

with a pressure drop of 1 psi across the valve.

Cv (gpm.psi-0.5)

VALVE

SIZEPORT DIAM Valve opening, percentage (%) of total travel Flow

characteristic(inches) (mm) 10 30 70 100

Equal percentageCV1 19 0.13 0.48 1.15 45/4 19 0.783 2.20 7.84 17.23/2 29 1.52 3.87 17.4 35.82 38 1.66 4.66 25.4 59.72-1/2 38 3.43 10.8 49.2 99.43 38 4.32 10.9 66.0 1364 51 5.85 18.3 125 2246 51 12.9 43.3 239 3948 76 27.0 105 605 818


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EXERCISE

Consider the diagram below, determine the size ofthe valve required to control the liquid water flow

when the maximum flow rate is 63 gpm and the

pressure of the heat exchanger & the total pressure

of the system are 6, and 49.4 psi, respectively.Size the valve to be used.

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Solution.

Complete the solution using the valve

coefficient table.

12

psigpmppsi

gpm

SG

PQCv

SG

PCQ

v

vv

56.9

1

4.4363


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EXERCISE

V = 3.08 GPM

P1 = 100 PSI

P2 = 40 PSISG = 1

Size the valve to be used.

Valve Charecteristic(1)

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