Filter
Top Products
245
APPENDIX
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 100
gpm full open and a rangeability of 30 to 1,
can accurately controls flow accurately as
low as 3 gpm.
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 controllable
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 66 gpm maximum
flow through the valve and since the
minimum accurately controllable flow is
3 gpm, the turndown is 22.
Valve Ratings
Flow coefficient (capacity index): Used to state the flow
capacity of a control valve for specified conditions. In
the control valve industry currently one of three flow
coefficients is used depending upon the location and
system of units; British Av, European k
vs
, or United
States C
v
. The flow coefficients have the following
relationships:
A
v
= 0.0000278 k
vs
A
v
= 0.0000240 C
v
k
vs
= 0.865 C
v
The flow coefficient Av is in cubic meters per second
and can be determined from the formula:
Where:
Q = volumetric flow in cubic meters per
second.
ρ=fluid density in kilograms per cubic meter.
∆p = static pressure loss across the valve in
pascals.
The flow coefficient k
vs
is water flow in cubic meters
per hour with a static pressure loss across the valve of
10
5
pascals (1 bar) within the temperature range of 5
to 40°C and can be determined from the formula:
Where:
Q = volumetric flow in cubic meters per hour.
ρ=fluid density in kilograms per cubic meter.
ρ
w
= density of water in kilograms per cubic
meter.
∆p k
vs
= static pressure loss of 10
5
pascals.
∆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 100F and can be determined for other conditions
from the formula:
Where:
Q = volumetric flow in US gallons per minute.
ρ=fluid density in pounds per cubic foot.
ρ
w
= density of water in pounds per cubic foot
within the temperature range of 40
to 100F
∆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,
by structural parts such as the stem can be the limiting
factor. The construction of gate-style valves, such as
ball valves, often allows them to hold back high head
pressures in the closed position, although the actuator
may not be powerful enough to operate the valve
against such forces.
EXAMPLE:
A valve with a close-off rating of 10 psi
could have 40 psi upstream pressure and
30 psi 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.
The valve close-off rating is independent of the actual
valve body rating. See definition of BODY RATING
(ACTUAL).
A
V
p
∆p
-------
Q
=
k
vs
Q
∆ρk
vs
∆ρ
-----------------
ρ
ρ
w
-------•=
C
V
Q
1
∆ρ
-------
ρ
ρ
w
-------•=
Appendix A: Valve Selection and Sizing