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Control valves for steam and superheated steam

Calcultion of Cv and mass flow through the valve

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1. Cv and Kv definition
2. Calculation of Cv and flow through a valve - Case of liquids - SI units
3. Calculation of Cv and flow through a valve - Case of gases - SI unitsb
4. Control valves sizing
5. Calculation of flow through a valve - US units

1. Cv and Kv definition

What is the flow coefficient Cv and Kv - calculation of flow through a valve - SI units


Please refer to the valve flow coefficient page of Process Engineer's tools.


2. Calculation of Cv and flow through a valve - Case of steam - SI units

What is the flow of steam through a valve ?

The mass flow rate of steam through a valve can be calculated from the valve Cv thanks to the following formula :


Calculation and Cv and mass flowrate for a steam control valve

With

m=Flow rate (t/h)
FL = critical flow factor
Cv=valve flow coefficient (GPM)
P1 = upstream pressure (bar abs)
y = (1.63/FL)*√(ΔP/P1)

  • If y < 1.5, subcritical flow
  • if y > 1.5, then y is capped at y = ymax =1.5 for critical flow

3. Calculation of Cv and flow through a valve - Case of superheated steam - SI units

What is the flow of superheated steam through a valve ?

The mass flow rate of superheated steam through a valve can be calculated from the valve Cv thanks to the following formula :


Calculation and Cv and mass flowrate for a superheated steam control valve

With

m=Flow rate (t/h)
FL = critical flow factor
Cv=valve flow coefficient (GPM)
P1 = upstream pressure (bar abs)
Tsh = steam superheated temparature (c)
y = (1.63/FL)*√(ΔP/P1)

  • If y < 1.5, subcritical flow
  • if y > 1.5, then y is capped at y = ymax =1.5 for critical flow


4. Control valve sizing

What are the best practices to size control valves ?

In order to have a good control, the design of the control valve should be such that it represents around 1/3 of the total pressure drop of the line. The valve must also be designed to work at flows corresponding to 50 to 80 percent of the Cv of the valve.

5. Steam and superheated valve sizing - US units

The sizing of a control valve for steam (2 first equations) and superheated steam (2 last equations) can be calculated thanks to the following equations in  US units

Calculation and Cv and mass flowrate for a superheated steam control valve

With P1 in Psi, ΔP in Psi, m in lb/h, Cvin GPM, Tsh in F


Source
Masoneilan Control Valve Sizing Handbook, 2000