From Chemical Engineering License Review, Second Edition

Control Valves

A control valve is a variable opening used to regulate the flow of a process fluid as required by the process. The three important aspects of a control valve are capacity, characteristics, and rangeability.

Capacity This is expressed in terms of C v, the flow coefficient, which is the flow of water at 60 F in gallons per minute at a pressure drop of 1 psi across the valve when it is completely open.

Characteristics This is the relationship between the change in the valve opening and the change in the flow through the valve, viz., equal percentage, linear and nonlinear.

Rangeability This is the ratio of the maximum controllable flow to the minimum controllable flow. In terms of C v, the rangeability R is given by

The formulas used for control valve sizing are as follows:

Volumetric flow:

Mass flow:

where C v

= valve coefficient

Q

= flow rate, gpm for liquid, ft 3/min for gases and vapors

s

= specific gravity at flowing temperature (water s at 60 F = 1)

? P

= pressure drop across valve, psi

W

= flow rate, lb/h

s g

= specific gravity of gas relative to air

= the molecular weight of gas

s gf

= s g (520/T)

T

= flow temperature, R

P 1

= upstream pressure, psia

P 2

= downstream pressure, psia

In terms of ? P and Q (gpm) the rangeability...

Copyright Engineering Press 1999 under license agreement with Books24x7

Products & Services
Globe Valves
Globe valves are linear motion valves with rounded bodies, from which their name is derived.  They are widely used in industry to regulate fluid flow in both on/off and throttling service. 
Industrial Valves
Industrial valves are classified in many different ways. They can be distinguished by material of construction, media handled, and/or application.
Diaphragm Valves
Diaphragm valves close by means of a flexible diaphragm attached to a compressor.
Control Valves
Control valves or proportional valves are power-operated devices used to modify fluid flow or pressure rate in a process system.
Pinch Valves
Pinch valves include any valve with a flexible elastomer body that can be pinched closed, cutting off flow, using a mechanism or fluid pressure.

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