The flow coefficient, also known as the Cv value, is a measure of the efficiency of a valve in controlling fluid flow. The Cv value depends on various factors such as the fluid type, pressure, temperature, and flow rate.
Cv for Saturated steam
Saturated steam for Critical pressure drop
The outlet pressure (Po) after the control valve is approximately 58% of the inlet pressure (pi) before to the control valve, resulting in a critical pressure drop. The formula for the flow coefficient at choked, or critical, flow is :
Cv = m / 1.61 Pi
m refers to steam flow rate (lb/hr).
Pi refers to inlet steam absolute pressure (psia)
Saturated steam for non critical pressure drop
The outlet pressure (Po) after the control valve is greater than 58% of the inlet pressure (Pi) before to the control valve in cases of non-critical pressure decrease. The formula for the flow coefficient for non-critical flow is
Cv = m / (2.1 ( (Pi + Po) dp)1/2) (or)Cv= m/(2.1 √((Pi+Po)dp))
Where Po refers to outlet steam absolute pressure (psia)
Cv for Superheated steam
For superheated steam, the calculation of flow coefficient is more complex due to the variable properties of the steam. The most common method used is to approximate the steam properties at the average temperature and pressure of the valve, and then use the same equation as for saturated steam.
Super-heated Steam Flow Coefficient (Cv)
A correction factor should be multiplied by the flow coefficient for superheated steam:
Cv = Cv_saturated (1 + 0.00065 dt)
where dt is the difference between the steam temperature and the actual pressure in degrees Fahrenheit.
Alternatively, provide Cv values for superheated steam at specific inlet and outlet pressures and temperatures. These values can be used directly in flow calculations.
Example: Superheated Steam Flow Coefficient
The steam’s superheated flow coefficient at saturated steam’s (pure steam) temperature 298°F, and assumption of the inlet temperature is 500°F.
Cv = Cv_saturated (1 + 0.00065 (500-298) = 1.1313 Cv_saturated
It’s important to note that the Cv value is a relative measure of valve performance and should only be used for comparing valves of the same type and size