A control valve must, as its name suggests, have a controlling influence on the process. Whilst details such as connection sizes and materials of construction are vitally important, they do not give any indication of the control exerted by the valve.
Control valves adjust processes by altering:
• Flowrate - For example, the amount of steam or water that enters the process equipment.
With a two-port valve for example, as the valve moves to the closed position, less steam flows, and less heat is added to the process.
With a three-port valve for example, as the valve plug moves to a new position, it diverts hot water away from the process.
• Differential pressure - This is defined as the difference between the pressure at the valve inlet and the pressure at the valve outlet (see Figure 6.2.1).
For any given valve orifice size, the greater the differential pressure the greater the flowrate, within certain limitations.
With saturated steam, the lower its pressure, the lower its temperature, and less heat transfer will occur in the heat exchanger.
These two factors (a) Flowrate and (b) Differential pressure are brought together as a flow coefficient or ‘capacity index’ as it is sometimes termed.
The flow coefficient allows:
• The performance of valves to be compared.
• The differential pressure across a valve to be determined from any flowrate.
• The flowrate through a control valve to be determined for a given differential pressure.
Because many different units of measurement are used around the world, a number of flow coefficients are available, and it is worthwhile understanding their definitions. Table 6.2.1 identifies and defines the most commonly encountered capacity indices.
Cv (Imperial) = Kv x 0.962 658
Cv (US) = Kv x 1.156 099
Av = 2.88 x 10-5 Cv (Imperial)
The flow coefficient, Kvs for a control valve is essential information, and is usually stated, along with its other data, on the manufacturer’s technical data sheets.
Control valve manufacturers will usually offer a number of trim sizes (combination of valve seat and valve plug) for a particular valve size. This may be to simplify the pipework by eliminating the need for reducers, or to reduce noise.
A typical range of Kvs flow coefficients available for a selection of valves is shown in Table 6.2.2.
The relationship between flowrates, differential pressures, and the flow coefficients will vary depending upon the type of fluid flowing through the valve. These relationships are predictable and satisfied by equations, and are discussed in further detail in:
• Module 6.3 - Control Valve Sizing for Water Systems.
• Module 6.4 - Control Valve Sizing for Steam Systems.