##### 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 | = valve coefficient |

| = flow rate, gpm for liquid, ft |

| = specific gravity at flowing temperature (water |

? | = pressure drop across valve, psi |

| = flow rate, lb/h |

| = specific gravity of gas relative to air |

= the molecular weight of gas | |

| = |

| = flow temperature, |

| = upstream pressure, psia |

| = downstream pressure, psia |

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

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