14-06-2012, 03:56 PM
CONTROL VALVE
control valve.pptx (Size: 1.28 MB / Downloads: 51)
Flow Characteristic:
Relationship Between flow through the valve and percent rated travel as the latter is varied from 0 to 100 percent. This term should always be designated as either inherent flow characteristic or installed flow characteristic.
Flow Coefficient (Cv):
A constant (Cv) related to the geometry of a valve, for a given travel, that can be used to establish flow capacity.
It is the number of U.S. gallons per minute of 60F water that will flow through a valve with a one pound per square inch pressure drop.
Capacity of control valve
The Cv is defined as the number of U. S. gallons of water per minutes that will flow through the wide-open valve when there is a 1-psi(kPa) pressure drop across it.
Rangeability:
The ratio of the largest flow coefficient (Cv) to the smallest flow coefficient (Cv) within which the deviation from the specified flow characteristic does not exceed the stated limits. A control valve that still does a good job of controlling when flow increases
to 100 times the minimum controllable flow has a rangeability of 100 to 1. Rangeability can also be expressedn as the ratio of the maximum to minimum controllable flow rates.
Push-Down-to-Close Construction:
A globe-style valve construction in which the closure member is located between the actuator and the seat ring, such that extension of the actuator stem moves the closure member toward the seat ring, finally closing the valve (figure 1-3). The term can also be applied to rotary-shaft valve constructions where linear extension of the actuator stem moves the ball or disk toward the closed position. (Also called direct acting.)
Inherent Flow Characteristic:
The relationship between the flow rate and the closure member travel as it is moved from the closed position to rated travel with constant pressure drop across the valve.
Installed Flow Characteristic:
The relationship between the flow rate and the closure member travel as it is moved from the closed position to rated travel as the pressure drop across the valve is influenced by the varying process conditions.
Dead Band
Dead band is a general phenomenon where a range or band of controller output (CO) values fails to produce a change in the measured process variable (PV) when the input signal reverses direction.
When a load disturbance occurs, the process variable (PV) deviates from the set point. This deviation initiates a corrective action through the controller and back through the process.