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SEMI S5-0703 © SEMI 1993, 2003 2 5 Terminology 5.1 Definitions 5.1.1 equivalent orifice — a passage that will allow fluid flow e quivalent to a round h ole at 80% effici ency. The efficiency, known as the orifice coeffic…

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SEMI S5-0703 © SEMI 1993, 2003 1
SEMI S5-0703
SAFETY GUIDELINE FOR SIZING AND IDENTIFYING FLOW LIMITING
DEVICES FOR GAS CYLINDER VALVES
This safety guideline was technically approved by the Global Gases Committee and is the direct
responsibility of the North American Gases Committee. Current edition approved by the North American
Regional Standards Committee on April 11, 2003. Initially available at www.semi.org June 2003; to be
published July 2003. Originally published in 1993.
NOTICE: This document was completely rewritten in
2003.
1 Purpose
1.1 The purpose of this safety guideline is to provide a
method to size and identify flow limiting devices that
limit the rate of release of hazardous gases from the gas
cylinder valve during transportation, storage, and use.
2 Scope
2.1 This safety guideline provides a method to identify
flow limiting devices by size, and to calculate
maximum and minimum flow rates so that the size
required can be determined. Specific SAFETY
WARNINGS are contained in Section 3.
2.2 This safety guideline pertains to flow limiting
devices for valves on cylinders containing hazardous
gases. These devices are intended to limit the flow rate
of uncontrolled releases of gases at or downstream of
the valve outlet. This guideline does not pertain to gas
releases caused by failure of the cylinder, the valve
connection to the cylinder, actuation of the cylinder
pressure relief device, or some failures of the valve.
2.3 This safety guideline pertains to flow limiting
devices operating at critical flow rates that result from
cylinder pressures greater than 103 kPa (15 psig).
Generally,
flow limiting devices are not used for
cylinders pressurized less than 103 kPa (15psig)
because the size required to provide adequate flow for
the process is so large that flow reduction resulting
from the flow limiting device would be minimal.
NOTICE: This safety guideline does not purport to
address all of the safety issues associated with its use.
It is the responsibility of the user of this guideline to
establish appropriate safety and health practices, and
determine the applicability of regulatory limitations
prior to use.
3 SAFETY WARNINGS
3.1 Installation Only the gas supplier or cylinder
owner should install, remove, or otherwise service the
flow limiting device. Special equipment and
procedures may be required to perform any installation
or service operation safely. No other person should
install, remove, replace, clean, adjust, or otherwise alter
the flow limiting device unless authorized by the
cylinder owner.
3.2 Verification — When a flow limiting device is
required, the user should always verify that the size is
identified by tag, label or marking on the cylinder or
cylinder valve exterior, and that the device size
indicated matches the requirement. If a required flow
limiting device is missing, the cylinder should be
rejected prior to use.
3.3 Purging The addition of a flow limiting device
in the cylinder valve may restrict gas flow during
purging. The user should assure that adequate purging
methods are used to prevent plugging the flow limiting
device. As a minimum, the operator should purge the
valve outlet after use with the same procedure used for
purging the valve outlet just after installation of the
cylinder. This is important to prevent contamination in
the valve outlet and reduce the probability of creating a
hazard at the cylinder supplier’s plant.
3.4 Blockage Under certain conditions, some
hydrides such as diborane can polymerize and cause
obstruction or total blockage of flow limiting devices.
Care should be taken with cylinders that seem to be
empty, but may contain product.
4 Referenced Standards
4.1 SEMI Standard
SEMI S2 — Environmental, Health, and Safety
Guideline for Semiconductor Manufacturing Equipment
4.2 NFPA Document
1
NFPA 704 Standard System for the Identification of
Fire Hazards of Materials
NOTICE: As listed or revised, all documents cited
shall be the latest publications of adopted standards.
Regional standards may be substituted provided they
are equivalent. Equivalent international standards if
adopted, will be referenced in the next revision of this
guideline.
1 National Fire Protection Association, Batterymarch Park, Quincy,
MA 02269 Website: www.nfpa.org
SEMI S5-0703 © SEMI 1993, 2003 2
5 Terminology
5.1 Definitions
5.1.1 equivalent orifice — a passage that will allow
fluid flow equivalent to a round hole at 80% efficiency.
The efficiency, known as the orifice coefficient, will
typically vary from 65% (.65) for a sharp entrance to
95% (.95) for a well-rounded entrance. A flow rate
tolerance of ± 20% is used in Table 1 to allow for
variations in the entrance geometry and passage size.
Figure 1 illustrates the flow rate variation resulting
from this tolerance for different equivalent orifice sizes.
5.1.2 flow limiting device a device installed in a gas
cylinder valve that will reduce maximum flow from the
valve under full flow conditions.
5.1.3 flow rate flow rates in this guideline are given
in standard liters per minute (slm) at the standard
conditions of 0°C (32°F) and 101 kPa absolute (14.7
psia). This corresponds to the standard conditions used
for calibration of mass flow controllers used in
semiconductor processing systems.
5.1.4 hazardous gases gases that have a degree of
hazard rating in health, flammability, or reactivity of
class 3 or 4 in accordance with NFPA 704, or
equivalent rating by a regional standard.
5.1.5 mixing gas — an inert gas used to dilute another
gas.
6 Flow Limiting Device Criteria
6.1 Where Used Flow limiting devices should be
installed or incorporated into cylinder valves that are
used for gas cylinders containing the hazardous gases
listed in Table 2. Table 2 lists hazardous gases for
which devices are known to be feasible by reason of
testing and experience. SEMI encourages testing by its
members of hazardous gases not listed in Table 2 so
that new information can be added.
6.2 Materials The flow limiting device should be
made of materials that are compatible with the gas in
the cylinder.
6.3 Installation The flow limiting device should be
installed by the gas supplier or cylinder owner. See
Safety Warnings.
6.4 Identification The preferred method of
identifying flow limiting devices is by equivalent
orifice size as shown in Table 1. Other means of
identification, requiring identification by the supplier as
to the device installed, may be used if unambiguous.
6.5 Service Life Removable flow limiting devices
may be reused, but should be tested by the gas supplier
or cylinder owner before each use to assure that the
flow rate is as listed in Table 1.
6.6 Sizing A flow limiting device that will not allow
nitrogen flows out of the valve outlet to exceed the flow
rates listed for its equivalent orifice size in Table 1.
The flow limiting device should be the smallest size
that satisfies the process requirements. The user should
make the choice of flow rate based on safety
philosophy contained in SEMI S2.
NOTE 1: Flow rates determined by the calculations in this
safety guideline are based on critical flow where the cylinder
pressure is equal or greater than 2 atmospheres and pressure at
the outlet of the flow limiting device is 1 atmosphere.
Consult the flow limiting device supplier to determine the
minimum size that will provide the required process flow rate
when cylinder absolute pressure is less than 2 times outlet
absolute pressure.
7 Flow Rate Calculations
7.1 The flow rates in Table 1 are specified for critical
flow of nitrogen through the flow limiting device at
several cylinder pressure levels. Densities are given in
Table 2 for some hazardous gases. First determine the
critical flow of nitrogen through the flow limiting
device from Table 1, or calculate using the method
specified in Section 7.2. Then, using the method
specified in Section 7.3, calculate the gas and pressure-
corrected flow rates to determine the proper flow
limiting device size for the application. For gas
mixtures, the density can be calculated using the
method specified in Section 7.4.
7.2 Nitrogen Flow Rate— Nitrogen flow rate
correction for cylinder pressures not listed in Table 1
follows:
NOTE 2: This equation is only valid for ideal gases, and
critical flow that results for most gases when P
cyl
/P
atm
≥ 2.
It is necessary to multiply the equation by a compressibility
factor (Z) for better accuracy, however for nitrogen at
temperatures between -54 to +60°C (-65 to +140°F) and at
cylinder pressures less than 21,000 kPa gauge (3045 psig), the
result will be accurate within a few percent.
Q
N
= Q
NT
x
atmNT
atmcyl
PP
PP
+
+
Equation 1.
Q
N
is the critical flow rate in slm of nitrogen at
pressures not listed in Table 1.
Q
NT
is the critical flow rate in slm of nitrogen from
Table 1 that corresponds to a given equivalent orifice
size at pressure.
P
cyl is the gauge pressure kPa gauge (psig) in the gas
cylinder for which the flow rate is being calculated.
SEMI S5-0703 © SEMI 1993, 2003 3
P
atm is standard atmospheric pressure 101 kPa
absolute (14.7 psia) at sea level.
P
NT
is the pressure in kPa gauge (psig) from Table 1
that produces critical flow rate Q
NT
for a given
equivalent orifice size.
EXAMPLE 1: What is the minimum flow rate of
nitrogen through a 0.25 mm (0.01 inch) equivalent
orifice at 862 kPa gauge (125 psig)?
Q
N
= 2.90 ×
101700
101862
+
+
= 3.5 slm nitrogen (Q
NT
= 2.90 is
the minimum flow rate at 700 kPa gauge, the minimum
flow rate for a 0.25 mm orifice at any pressure listed in
Table 1 will provide the same result.)
EXAMPLE 2: What is the maximum flow rate of
nitrogen through a 0.25 mm (0.01 inch) equivalent
orifice at 12,414 kPa gauge (1,800 psig)?
Q
N
= 4.35 ×
101700
10112414
+
+
= 68 slm nitrogen (Q
NT
= 4.35
is the maximum flow rate at 700 kPa gauge.)
7.3 Hazardous Gas Flow Rate— Flow rate correction
for gases listed in Table 2 other than nitrogen follows:
NOTE 3: This formula is only valid for critical flow,
typically where P
cyl
/ P
atm
> 2
Q
g
= Q
N
×
g
DZ)(
25.1
Equation 2. (Use Q
NT
in place of
Q
N
when the applicable cylinder pressure is listed in
Table 2.)
Q
g
is the critical flow rate in slm of the gas listed in
Table 2.
D
g
is the standard density of the gas in kg/m
3
at one
atmosphere and 0°C (32°F) listed in Table 2.
Z is the compressibility factor at P
cyl
for the gas listed
in Table 2. For pressures not listed, interpolation of Z
with Z assumed proportional to pressure and equal to
1.0 at zero pressure should provide adequate accuracy
in the pressure and temperature ranges found in this
document. For gas mixtures with the mixing gas 95%
by volume or greater, use of Z for the mixing gas
should provide adequate accuracy, for lower
percentages consult the gas supplier.
NOTE 4: Density is the reciprocal of specific volume.
Density or specific volume is frequently given at a
temperature of 21.1°C (70°F). To convert 21.1°C (70°F) to
0°C (32°F) standard density, multiply by 1.077.
EXAMPLE 3: What is the minimum flow for Nitrogen
Trifluoride through a 0.75 mm (0.03 inch) equivalent
orifice at 700 kPa gauge?
Q = 26.1 ×
)20.3)(97.0(
25.1
= 16.6 slm Nitrogen
Trifluoride (Z is interpolated at 700 kPa)
7.4 Density For Gas Mixtures — Density correction
for mixtures of gases listed in Table 2, or Table 2 and
Table 3 follows:
D
mix
=
∑
V
g
× D
g
Equation 3. (For gas mixtures, use
D
mix
in place of D
g
in Equation 2.)
V
g
is the volume fraction of each gas in the mixture.
EXAMPLE 4: What is the density for a mixture of 5%
arsine and 95% nitrogen?
D
mix
= (.05 × 3.45) + (.95 × 1.25) = 1.36 kg/m
3
8 Related Documents
8.1 CGA Standard
2
CGA V-9 Compressed Gas Association Standard for
Compressed Gas Cylinder Valves
2 Compressed Gas Association, Inc., 1725 Jefferson Davis Highway,
Suite 1004, Arlington, VA 22202