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Quick-opening: A valve which provides maximum possible flow
as soon as the stem lifts the disc from the valve seat.
Valve flow characteristic: The relationship between the stem
travel of a valve, expressed in percent of travel, and
the fluid flow through the valve, expressed in percent
of full flow.
VALVE FLOW TERMS
Rangeability: The ratio of maximum flow to minimum
controllable flow. Approximate rangeability ratios are
50 to 1 for V-port globe valves and 30 to 1 for
contoured plug valves.
EXAMPLE:
A valve with a total flow capacity of 10 L/s
full open and a rangeability of 30 to 1, can
accurately controls flow accurately as low
as 0.3 L/s.
Tight shut-off/close-off: A valve condition in which virtually
no leakage of the controlled fluid occurs in the closed
position. Generally, only single-seated valves provide
tight shut-off. Double-seated valves typically have a
one to three percent leakage in the closed position.
Turndown: The ratio of maximum flow to minimum con-
trollable flow of a valve installed in a system.
Turndown is equal to or less than rangeability.
EXAMPLE:
For the valve in the rangeability example,
if the system requires a 6.6 L/s maximum
flow through the valve and since the
minimum accurately controllable flow is
0.3 L/s, the turndown is 22.
VALVE RATINGS
Flow coefficient (capacity index): Used to state the flow
capacity of a control valve for specified conditions.
Currently, in the control valve industry, one of three
flow coefficients British A v , North American K v , or
United States C v is used depending upon the location
and system of units. The flow coefficients have the
following relationships:
A v = 0.0000278 K v
A v = 0.0000240 C v
K v = 0.865 C v
The flow coefficient Av is in cubic meters per second
and can be determined from the formula:
A v = Q
P
ENGINEERING MANUAL OF AUTOMATIC CONTROL
VALVE SELECTION AND SIZING
Where:
Q = volumetric flow in cubic meters per
second.
fluid density in kilograms per cubic meter.
p = static pressure loss across the valve in
pascals.
The flow coefficient Kv is water flow in cubic meters per
hour with a static pressure loss across the valve of 10
pascals (1 bar) within the temperature range of 5 to
40˚C and can be determined from the formula:
K v = Q
Where:
Q = volumetric flow in cubic meters per hour.
fluid density in kilograms per cubic
meter.
density of water in kilograms per cubic
w
meter.
p
= static pressure loss of 10
Kv
p = static pressure loss across the valve in
pascals.
The flow coefficient C v is water flow in gallons per
minute with a pressure loss across the valve of one
pound per square inch within the temperature range
of 40 to 100˚F and can be determined for other
conditions from the formula:
C v = Q
Where:
Q = volumetric flow in US gallons per
minute.
fluid density in pounds per cubic foot.
density of water in pounds per cubic foot
w
within the temperature range of 40 to
100˚F.
p = static pressure loss across the valve in
pounds per square inch.
Close-off rating: The maximum pressure drop that a valve can
withstand without leakage while in the full closed
position. The close-off rating is a function of actuator
power to hold the valve closed against pressure drop,
but structural parts such as the stem can be the limiting
factor.
EXAMPLE:
A valve with a close-off rating of 70 kPa
could have 270 kPa upstream pressure and
200 kPa downstream pressure. Note that in
applications where failure of the valve to
close is hazardous, the maximum upstream
pressure must not exceed the valve close-
off rating, regardless of the downstream
pressure.
429
K v
•
P
w
5
pascals.
•
P
w
5
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