Basics of Control Valve Sizing – Key Terms & concepts ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Basics of Control Valve Sizing – Key Terms & concepts

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Pressure Drop
The difference in pressure between upstream and downstream the control valve, caused by resistance to flow. Pressure drop is pressure loss across the valve created by system demand - NOT by the valve alone.

How to Determine Pressure Drop
To determine ΔP across a valve, subtract the outlet pressure (P2) from the inlet pressure (P1).
ΔP = P2 – P1

Importance of Pressure Drop in Valve Sizing
Pressure drop is a critical element in valve sizing and valve application. Pressure drop must be known by the engineer designing the system to ensure proper valve selection

What Determines Valve Pressure Drop?

The critical factors are orifice size and internal flow paths of the valve.

Relationship between the Flow Rate and Pressure Drop across a Control Valve
Pressure drop and flow rate are dependent on one another. The higher the flow rate through a restriction(control valve), the greater the pressure drop. Conversely, the lower the flow rate, the
lower the pressure drop.

What is Cv , Valve flow coefficient?
The definition of Cv factor is the number of GPM (gallons per minute) that will pass through a control valve with a pressure drop of one (1) psi. It is a unit of measure for comparing valve flows. See Valve Flow Coefficient, Cv .

Relationship between GPM, Cv, ΔP.
Any two of these variables are required to size a control valve. The set of equations below illustrate the basic relationship between GPM, Cv and ΔP:

Where :
GPM = Gallons per Minute
Cv   =  Valve flow coefficient
ΔP  =  Pressure drop across valve
S.G =  Specific Gravity of fluid

Valve Gain
Valve gain is the incremental change in flow rate produced by an incremental change in plug position. This gain is a function of valve size and type, plug configuration and system operating conditions. The gain at any point in the stroke of a valve is equal to the slope of the valve flow characteristic curve at that point.

Valve Plug or Trim
The shape of the valve plug determines the flow characteristic of the valve. Matching this plug flow characteristic to a particular control loop requires that valve gain change in such a way as to compensate for the gain changes of the other elements (coil, balancing valve, piping, etc.) in the control loop. The most efficient valve is one with a linear flow characteristic hence efforts are made during the valve sizing process to approximate as possible to a linear flow characteristic. The most common types of plugs are the equal percentage, linear, and quick opening plug. The typical designs of equal percentage, linear and quick opening valve plugs or trim is shown below:

Valve Rangeability
Rangeability is defined as the ratio between maximum and minimum controllable flow through the valve. Large values for rangeability are desirable because it will allow for control across a larger portion of the valve stroke:
          Valve Rangeability = Maximum Flow/Minimum Controllable Flow         

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