CONTROL VALVES – SELECTION, SIZING AND SPECIFICATION
The correct sizing and selection of a control valve must be based on the full knowledge of the process.
1 – PROCESS DATA(The following data should at least be known)
a – Type of fluid and its chemical-physical and thermodynamic characteristics, such as pressure “p”, temperature “T”, vapour pressure “pv”, thermodynamic critical pressure “pc”,specific mass “r”, kinematic viscosity “n” or dynamic viscosity “m”, specific heat at constant pressure “Cp”, specific heat at constant volume “Cv”, specific heat ratio “g”, molecular mass “M”, compressibility factor “Z”, ratio of vapour to its liquid, presence of solid particles, inflammability, toxicity.
b – Maximum operating range of flow rate related to pressure and temperature of fluid at valve inlet and to Dp across the valve.
c – Operating conditions (normal, max., min.etc.).
d – Ratio of pressure differential available across the valve to total head loss along the process line at various operating conditions.
e – Operational data, such as:
– maximum differential pressure with closed valve
– stroking time
– plug position in case of supply failure
– maximum allowable leakage of valve in closed position
– fire resistance
– max. outwards leakage
– noise limitations
f – Interface information, such as:
– sizing of downstream safety valves
– accessibility of the valve
– materials and type of piping connections
– overall dimensions, including the necessary space for disassembling and maintenance
– design pressure and temperature
– available supplies and their characteristics
On the ground of the above data it is possible to finalise the detailed specification of the valve (data sheet), i.e. to select:
The Manufacturer shall:
1.check the calculation of the selected Cy value for the given data;
2.quote/supply a Cy value for each valve in the fully open position;
3.complete the attachments where requested;
4.quote for control valves, meeting the requirements of the relevant requisition;
5.inform the purchaser of any irregularities found in the relevant requisition;
6.perform noise prediction calculations on all valves under all given process conditions.
The Manufacturer is responsible for the design and construction of the supplied control valves for the services and conditions specified in the requisition.
If a control valve is provided with an actuator and/or accessories from an other source, but supplied as part of the control valve requisition, the responsibility of the overall valve assembly (valve complete with actuator and/or accessories) shall be with one party. The responsible party (valve Manufacturer or actuator Manufacturer) shall be as indicated on the requisition.
All accessories shall be as specified on the site vendor list or the project specification vendor list,whichever is applicable the variety of types and sizes of control valves and supplied related materials ordered under the requisition shall be minimised.
For economic reasons and for control of variety, Manufacturers’ standard control valves shall be requisitioned wherever possible. In general, standard control valves should be used unless the process conditions， pressure, temperature, etc. are so severe that special control valves are needed.
The first sizing step is to determine the required CV value for the application. Next determine if there are unusual conditions that may affect valve selection such as cavitation, flashing, high flow velocities or high flow noise. The valve sizing process will determine the proper valve size, valve trim size , valve trim style and actuator size. Warren’s Valve Sizing Program (ValveWorks) will accurately calculate the CV, flow velocity and flow noise. The program will also show messages when unusual conditions occur such as cavitation, flashing, high velocity or high noise. The results from Warren’s Valve Sizing Program are only one element of the valve selection process. Knowledge and judgment are also required. This overview will give the user some of the sizing basics. The liquid, gas and steam CV calculation methods, in this manual, are in accordance with ISA S75.01 and the gas and steam flow noise calculations are in accordance with ISA S75.17. These two ISA Standards are in agreement with IEC-534. These standards have worldwide acceptance as the state of the art in CV and Flow Noise determination.