Why is it needed for control valve sizing?
Valves must be sized correctly to perform the control as desired. Undersized valves cannot deliver sufficient quantities for maximum load conditions, and oversized valves attempt to perform correctly but must do so at the very end of their strokes where hunting or cycling is hard to avoid.
Sizing is setting the specification setting of valve size for required flow and pressure drop of the valve. if the valve is oversized or undersized.
Valve Flow Coefficient:
The sizing of a control valve is simplified by using the ‘valve flow coefficient’. The valve flow coefficient is a measure of the capacity for a control valve in the fully open position.
The first step in finding the size of a valve is to determine the flow coefficient (Cv) that is required for the system. Cv factor is defined as “the number of US, gallons per minute of 60F water that will flow through a fully open valve with a 1 psi drop across it”.
This empirically determined factor is the flow coefficient and is designated as follows:
Cv - when expressed in imperial units Kv - when in European units Av - when in SI metric units
Flow Coefficient, Cv:
The valve flow coefficient is defined as the number of US gallons of water per minute (at 60°F) that will flow through a wide-open valve with a pressure drop of 1psi.
The valve sizing formulas (also known as Cv formulas) are based on the basic equation of liquid flow Liquids:
QL = Cv √ DP/GL
Q = flow rate in US gallons per minute G = specific gravity of liquid (water = 1.0) DP = pressure drop across the valve in psi
From this formula the value of the valve coefficient (Cv) can be calculated.
Flow Coefficient, Kv
The valve flow coefficient, Kv is defined as the number of cubic metres per hour (at 15°C) that will flow through a wide-open valve with a pressure drop of 1bar (100kPa).
The valve sizing formula is based on the basic equation of liquid flow:
QL = Kv √ DP/GL
Q = flow rate in cubic metres per minute G = specific gravity of liquid (water = 1.0) DP = pressure drop across the valve in bar
From this formula the value of the valve coefficient (Kv) can be calculated.
Flow Coefficient, Av:
The control valve capacity can be evaluated using the flow coefficient Av, and is described in the British standard.
The valve sizing formula is based on liquid flow: Instrumentation for Automation and Process Control Control Valves
QL = Av √ DP/P
Q = flow rate in cubic metres per second G = density of liquid in kg/m3 (water = 1000) DP = pressure drop across the valve in pascals
From this formula the value of the valve coefficient (Av) can be calculated.
check out the Working of control valve