Control Valve Selection and Sizing
The control valve is the most important single element in any fluid handling system because it regulates the flow of fluid to the process. To property select a control valve, a general knowledge of the process and components is usually necessary. The following information provides a guide to help select and size the control valve that most closely matches the process requirements.
Control Valve Sizing – Valve Capacity
The sizing of a valve is very important if it is to render good service. If it is undersized, it will not have sufficient capacity. If it is oversized the controlled variable may cycle, and the seat and disc will be subjected to wire drawing because of the restricted opening, Systems are designed for the most adverse conditions expected (i.e. coldest weather, greatest load, etc.). In addition, system components (boiler, chiller, pumps, coils, etc.) are limited to sizes available and frequently have a greater capacity than system requirements. Correct sizing of the control valve for actual expected conditions is considered essential for good control.
The most important variables which must be considered are:
- What medium will the valve control? Water? Air? Steam? What effects will specific gravity and viscosity have on the valve size?
- What will the inlet pressure be under maximum load demand? What is the inlet temperature?
- What pressure drop (differential) will exist across the valve under maximum load demand?
- What maximum capacity should the valve handle?
- What is the maximum pressure differential the valve top must close against?
When these are known, a valve can be selected by formula (Cv method) or water and steam tables, In any case, the valve size should not exceed the line size and it should preferably be one or two sizes smaller.
Pressure Drop for Water Flow
A pressure drop must exist across a control valve it flow is to occur. The greater the drop, the greater the flow at any fixed opening. But the pressure drop across a valve also varies with the disc position-from minimum when fully open, to 100% of the system drop when fully closed. To size a valve properly, it is necessary to know the full flow pressure drop across it. The pressure drop across a valve is the difference in pressure between the inlet and outlet under flow conditions. When it is specified by the engineer and the required flow is known, the selection of a valve is simplified. But when this pressure drop is not known, it must be computed or assumed, A basic rule of control valve sizing is: 'The higher the percentage of drop that can be made to occur across the wide open valve in relation to the percentage of pressure drop through the line and process, the better will be the control obtained.'
If the pressure drop across the valve when fully open is not a large enough percentage of the total system drop, there will be little change in fluid flow until the valve actually closes, forcing the valve's characteristic toward a quick opening form. Figure 1 shows flow-lift curves for a linear valve with various percentages of design