Valve vary the flow of fluid through it by means of the change of pressure to the valve top. The relation between valve stem position and flow through the valve is known as valve characteristics.
The amount of fluid passing through a valve at any time depends upon the opening between the plug and seat. Hence there is a relationship between stem position, plug position and the rate of flow, which is described in terms of flow characteristics of a valve. The two types of valve characteristics are inherent and installed flow characteristics.
The inherent flow characteristic of the control valve is the relationship between the flow and travel of the valve to a constant pressure drop across the valve.
The inherent flow characteristics of the valve are determined by the valve orifice and plug geometry. There are three types of inherent flow characteristics, namely linear, fast opening and equal percentage
Valves of any size or inherent flow characteristic which are subjected to the same volumetric flow rate and differential pressure will have exactly the same orifice pass area. However, different valve characteristics will give different ‘valve openings’ for the same pass area.
Linear flow Characteristics:
The linear flow characteristics show that the flow rate is directly proportional to the path of the valve. Control valves with linear flow characteristics are commonly specified for liquid level control. The sensitivity of a control valve with linear flow characteristic is always constant.
Equal percentage characteristic:
These valves have a valve plug shaped so that each increase in valve lift increases the flow rate in a certain percentage of the previous flow. The relationship between the elevation of the valve and the size of the hole (and, therefore, the flow) is not linear but logarithmic.
Fast opening characteristic :
The valve plug characteristic of quick opening will give a large change in the flow rate for a small valve lift from the closed position. For example, a 50% valve lift can result in an orifice passage area and a flow rate of up to 90% of its maximum potential.
Quick opening valves tend to be electrically or pneumatically driven and used for on / off control. The type of automatic action control valve tends to have a plug shape similar to the quick opening plug.
The inherent characteristics of the valves described are subject to distortion due to variations in the pressure drop with the flow. The resistance of the line distorts the linear characteristics towards that of the valve of rapid opening and equal percentage to that of the linear valve
Each application will have a unique installation feature that relates the fluid flow to the heat demand. The pressure differential across the valve that controls the flow of the heating fluid can also vary:
- In water systems, the pump characteristic curve means that as flow is reduced, the upstream valve pressure is increased.
- In steam temperature control systems, the pressure drop over the control valve is deliberately varied to satisfy the required heat load.
The characteristic of the control valve chosen for an application should result in a direct relationship between valve opening and flow, over as much of the travel of the valve as possible.