Calculate typical gas and liquid pipe control valve sizing and minor loss factors (K, Cd, Cv*, Av, Cv-uk, Cv-us, Cv-met and Kv).
The control valve sizing is calculated in two steps using the ISA-75.01.01 iteration method for Kv flow coefficient. The other flow factors (Av, Cv-uk, Cv-us, Cv-met, Cv*, K, and Cd) are calculated from Kv.
Step 1 : Calculate the required valve flow coefficient (Av, Cv-uk, Cv-us, Cv-met and Kv) assuming that the valve ID is equal to the pipe ID. Use the required flow coefficient to select a suitable valve.
Step 2 : Select a suitable valve size, type and flow coefficient based on manufacturers data. If a full bore valve is too large, a smaller valve should be selected, with assumed concentric reducers. Calculate the required flow coefficient for the selected valve. The required flow coefficient should be less than or equal to the valve flow coefficient. A trial and error process may be required to determine the appropriate valve. It is recommended that the valve diameter is not less than half the pipe diameter. The calculation is not valid if the valve diameter is greater than the pipe diameter. The calculation might not converge if the valve size is too small.
For viscous fluids or very low flow velocity flow, with low Reynolds number (Rev < 10,000) use the Reynolds number factor option. For most flow cases the Reynolds number can be ignored (Fr = 1).
Check for choked conditions. If the outlet pressure for step 1 or step 2 is greater than the minimum (choked) outlet pressure, set the outlet pressure equal to the choked outlet pressure. The maximum (choked) flowrate, maximum (choked) delta pressure and minimum (choked) outlet pressure are calculated from the fluid vapour pressure, and the fluid critical point pressure. Specially designed valves are required to operate at choked conditions.
The K factors should include fittings located with 2D upstream and 6D downstream. The fluid velocity is calculated from the valve ID. The piping is assumed to be constant diameter upstream and downstream of the valve. The liquid pressure recovery factor Fl, and the valve design factor Fd depend on the valve type and geometry. Typical values are included in the data tables. Manufacturers data should be used if it is available. Check that the convergence is close to or equal to one. Convergence problems can indicate that the selected valve size is too small.
The dimensionless flow coefficient Cv* equals Cv-us / IDin^2, where IDin is the valve inside diameter in inches. For control valves, a maximum Cv* value of 30 is recommended, equivalent to a minimum K factor of 1.
Minor loss factors are calculated for:
- Av (SI) flow coefficient - the flow in cubic meters per second fluid density 1 kilogram per cubic meter which gives a pressure drop of 1 Pa
- Cv-uk (UK) flow coefficient - the flow in UK gallons per minute of water at 60 degrees F which gives a pressure drop of 1 psi
- Cv-us (US) flow coefficient - the flow in US gallons per minute of water at 60 degrees F which gives a pressure drop of 1 psi
- Cv-met (Metric) flow coefficient - the flow in liters per minute of water at 16 degrees C which gives a pressure drop of 1 bar
- Kv (EU) flow coefficient - the flow in cubic meters per hour of water at 16 degrees C which gives a pressure drop of 1 bar
- Cv* the dimensionless US flow factor = Cv-us / din^2 (din is the inside diameter in inches)
- K factor - the ratio of pressure loss over the dynamic pressure
- Cd or discharge coefficient - the ratio of the actual flow rate of the fluid through the fitting over the frictionless flow rate.
Reference : ISA-75.01.01 Industrial Process Control Valves Part 2-1 Flow Capacity Sizing Equations For Fluid Flow Under Installed Conditions
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