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SEMI F32-0998 © SEMI 1998 5 pressure ratio, and analyzes the data to compute th e choked f lo w pres sure drop ratio, x T . This method ha s been f ound, through exhaustive t esting, t o be a very accurate an d reliable …
SEMI F32-0998 © SEMI 1998 4
8.1.1 Pressure Taps — Pressure tap s should be welded
into the system per ANSI/ISA-S75.02.
8.1.2 Pressure Measurement — Pressure measurement
devices can be analog gauge, electronic transducer, or
both. They should be calibrated to maintain the
minimum accuracy necessary for the governing flow
equations.
8.1.3 Throttle Valve(s) — A throttle valve shall be
positioned downstream. There is no valve style
restriction, however they should be sized sufficiently
for the required flowrate.
8.1.4 Flow Measurement — Depending upon the test
media, several types of flow measurement devices may
be used. These devices may include: turbine meter,
orifice plate, bell prover, mass flow meter, and timed
volumetric capture. Flow measurement devices should
be calibrated to maintain the minimum accuracy
necessary for the governing flow equations. All flow
measurements should be normalized to standard
conditions per ANSI 2530. Install and calibrate per
manufacturer’s recommended procedure.
8.1.5 Temperature Measurement — Temperature
measurement devices should be calibrated to maintain
the minimum accuracy necessary for the governing
flow equations.
8.2 Incompressible Fluid
8.2.1 Procedure
8.2.1.1 The test procedure is specified in ANSI/ISA-
S75.02, Section 5.1.
8.2.2 Test Limitations
8.2.2.1 It is critical that the absolute upstream pressure
meets the criteria specified in ANSI/ISA-S75.02,
paragraph 5.1.3, as referenced below:
P
1
kPa
=
2 ×∆P
kPa
F
L
2
P
1psia
=
2 ×∆P
psid
F
L
2
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
Equation 1
Absolute Upstream Pressure
8.2.2.2 The liquid pressure recovery factor, F
L
is
calculated from the maximum attainable flowrate, Q
max
.
If Q
max
cannot be experimentally determined, the value
of F
L
can be estimated. See the referenced section for
details.
8.3 Compressible Fluid
8.3.1 Procedure
8.3.1.1 The test procedure is specified in ANSI/ISA-
S75.02, paragraph 7.1.
8.3.2 Test Limitations
8.3.2.1 If possible, at least 3 data points should be
taken at flowrates where the fluid approaches
incompressible behavior. To attain these conditions, the
pressure drop ratio, x, should be less than or equal to
0.02. Additional data points should be taken at
increasing flowrates up to the maximum possible for
the system.
9 Coefficients Calculation
9.1 Flow Coefficient, C
v
9.1.1 The flow coefficient, C
v
, can b e calculated using
both incompressible and compressible fluids. For
incompressible fluids, ISA provides a relatively simple
method for determining C
v
. For compressible fluids, the
ISA test method for calculating C
v
requires the
determination of the choked flow pressure drop ratio,
x
T
,, and ultimately the expansion factor, Y, which
characterizes the valve geometry and fluid properties at
sonic velocities in the vena contracta. It has been
determined through considerable testing that the
expansion factor is critical in calculating actual pressure
drops across a valve when flow rates cause the fluid
density to change due to pressure and velocity changes.
9.2 Pressure Drop Ratio Factor, x
T
9.2.1 The ISA test method requires t he experimental
determination of the flow coefficient C
v
and a second
coefficient x
T
, the pressure drop ratio factor.
9.2.2 x
T
is equal to the critical pressu re ratio of the ISA
flow equation, and is close but not necessarily identical
to the experimentally observed pressure ratio at choked
flow. x
T
is chosen to correlate the experimentally
observed choked flow. The ISA equation will then
predict choking at a pressure drop ratio close to, but not
identical with, the experimentally observed value.
9.2.3 There are two methods to determine the pressure
drop ratio factor. The procedure of ANSI/ISA-S75.02,
Section 7.2 requires that C
v
be determined at values of x
less than 0.02, and that choking actually be achieved to
determine x
T
. The alternate procedure of paragraph 7.3
does not actually require choked flow and uses all the
data points to determine C
v
, not just a few points at a
small value of x.
9.2.4 In practice, valves used in the semiconductor
manufacturing facility, when connected as described
above, may not be able to achieve choked flow across
the pressure taps provided by this procedure. This will
happen when choked flow occurs first in some other
part of the test setup or connections. However, the
alternative test procedure of ANSI/ISA-S75.02, Section
7.3 will give the correct values of C
v
and x
T
in this case.
9.2.5 The ISA alternative method de termines the
pressure drop ratio factor by measuring flowrate vs.
SEMI F32-0998 © SEMI 19985
pressure ratio, and analyzes the data to compute the
choked flow pressure drop ratio, x
T
. This method has
been found, through exhaustive testing, to be a very
accurate and reliable method to determine the
expansion factor, without having to achieve choked
flow in the test valve itself.
10 Pressure Drop Calculation
10.1 The determination of both flow coefficient and
expansion factor are critical in calculating the pressure
drop across a valve flowing compressible fluid. Use of
the flow coefficient C
v
only, will yield a pressure drop
which is lower than the actual value.
10.1.1 Incompressible Flow
10.1.1.1 To calculate pressure drop across a valve
flowing an incompressible media, only the C
v
coefficient must be provided by the valve manufacturer.
The following equation has been derived from
ANSI/ISA-S75.01, Section 4.1, and should be used for
pressure drop computations:
∆P
kPa
= 0.0865*
Q
m
3
hr
C
v
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
× S
f
∆P
psid
=
Q
gpm
C
v
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
× S
f
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
Equation 2
Pressure Drop for Incompressible Flow
10.1.2 Compressible Flow
10.1.2.1 To calculate pressure drop across a valve
flowing a compressible media, both C
v
and x
T
coefficients may be used to determine more accurately
the pressure drop. Contact the valve manufacturer for
values of C
V
and x
T
coefficients. Equation 3 has been
derived from ANSI/ISA-S75.01, Section 6.1, and
should be used for pressure drop computations.
Equation 4 is a close approximation of Equation 3 that
computes pressure drop directly.
∆P
kPa
=
Q
m
3
hr
4.17 × C
v
× Y × P
1kPa
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
× S
g
× T
°K
× P
1
KPa
∆P
psid
=
Q
scfh
1360 × C
v
× Y × P
1
psia
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
× S
g
× T
° R
× P
1
psia
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
Equation 3
Pressure Drop for Compressible Flow – ISA
Iterative Solution
∆
P
=
P
1
kPa
×
x
T
1.125
1 − 1 −
1.125
x
T
S
g
×
Q
m
3
hr
3905.6 ×
C
v
×
P
1
kPa
T
°
K
S
g
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
∆
P
=
P
1
p
sia
×
x
T
1.125
1 − 1−
1.125
x
T
S
g
×
Q
scfm
16.04 ×
C
v
×
P
1
p
sia
T
°
R
S
g
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
Equation 4
Pressure Drop for Compressible Flow – Non-
Iterative Solution
NOTE: The numerical value of x used in these equations must
not exceed the choking limit (x
T
) regardless of the actual
value of x.
11 Related Documents
11.1 ISO Document
2
ISO 6358 — Pneumatic fluid power — Components
using compressible fluids — Determination of flow-rate
characteristics
11.2 SAE Documents
3
ARP 24B — Determination of Hydraulic Pressure Drop
ARP 868 — Pressure Drop Test for Fuel System
Components
2 International Organization for Standardization, Casa Posatale 56,
CH-1211, Geneve 20 , Switzerland
3 Society of Automotive Engineers, World Headquarters, 400
Commonwealth Dr., Warrendale, PA 15096-0001, Telephone:
724.776.4841, Fax: 724.776.5760
SEMI F32-0998 © SEMI 1998 6
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