![]() Rotary valves (for example, ball and butterfly) each have a basic characteristic curve, but altering the details of the ball or butterfly plug may modify this. In this Module, the term ‘valve lift’ is used to define valve opening, whether the valve is a globe valve (up and down movement of the plug relative to the seat) or a rotary valve (lateral movement of the plug relative to the seat). Typical trim shapes for spindle operated globe valves are compared in Figure 6.5.1. ![]() The physical shape of the plug and seat arrangement, sometimes referred to as the valve ‘trim’, causes the difference in valve opening between these valves. Comparing linear and equal percentage valves, a linear valve might have a 25% valve opening for a certain pressure drop and flowrate, whilst an equal percentage valve might have a 65% valve opening for exactly the same conditions. However, different valve characteristics will give different ‘valve openings’ for the same pass area. Valves of any size or inherent flow characteristic which are subjected to the same volumetric flowrate and differential pressure will have exactly the same orifice pass area. For any valve, however it is characterised, the relationship between flowrate and orifice pass area is always directly proportional. The orifice pass area is sometimes called the ‘valve throat’ and is the narrowest point between the valve plug and seat through which the fluid passes at any time. It does not refer to the orifice pass area. Please note that ‘valve opening’ in this context refers to the relative position of the valve plug to its closed position against the valve seat. ![]() ![]() All control valves have an inherent flow characteristic that defines the relationship between ‘valve opening’ and flowrate under constant pressure conditions. ![]()
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