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Flow Characteristics of control valve
Flow characteristics Various types of flow characteristics are available. This tutorial discusses the three main types used in water and steam flow applications: fast opening, linear, and equal percentage flow; how they compare, and how (and why) they should be matched to the application in which they are used.All control valves have an inherent flow characteristic that defines the relationship between valve opening and flow rate under constant pressure conditions. 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. It does not refer to the orifice pass area. 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. For any valve, however, it is characterized, by the relationship between flow rate and orifice pass area is always directly proportional.
Valves of any size or inherent flow characteristic which are subjected to the same volumetric flow rate and differential pressure will have the same orifice pass area. However, different valve characteristics will give different ‘valve openings’ for the same pass area. Comparing linear and equal percentage valves, a linear valve might have a 25% valve opening for a certain pressure drop and flow rate, whilst an equal percentage valve might have a 65% valve opening for the same conditions. The orifice pass areas will be the same.
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. Typical trim shapes for spindle-operated globe valves are compared in Figure 6.5.1.
Linear Characteristics
A control valve is labeled LINEAR if its inherent flow characteristics can be represented by a straight line on a rectangular plot of flowrate F (or % F, or Cv or % Cv) versus % travel m (or Z for opening position) at the ideal condition of constant valve pressure drop (DPV). The flow rate can be varied with the percentage of the opening of the control valve. 10% as the valve opens to allow the flow rate is 10%, 15% or more by opening the valves would cause the flow rate to be increased to 15% as well, this is called Linear Characteristics.Equal Percentage Characteristics
A control valve is said to be EQUAL % if an equal increment of valve travel or opening m (or Z) produces an equal PERCENT (%) increment in flow F. If this occurs at the ideal condition of constant valve pressure drop (DPV), the valve is said to be inherently EQUAL %Quick Opening or Fast-Opening Characteristics
The Quick-Opening characteristic control valve has a flat disk instead of a contoured valve plug. Its flow (or Cv) increases rapidly to its maximum flow with minimum initial valve opening. At the initial or lower portion of the travel position, the valve gain Kv is too high for use in modulating control.Modified Parabolic Characteristics
Other less common types of characteristics include the “modified parabolic” which is approximately mid-way between the Linear and Equal % characteristics, giving fine throttling action at low valve travel (like Equal %) and approximately linear characteristic for the upper portion of travel.Ref.