When setting up a gas distribution
system in a laboratory
or any other type of facility,
the safety of all personnel
must be taken into consideration. It
is important to meet all local and federal
codes, such as National Fire Protection
Association (NFPA), Uniform Fire Code
(UFC), Semiconductor Equipment and
Materials International (SEMI), and
Building Officials Code Administrators
International (BOCA). In addition, the
local fire marshal normally has the final say
on approval and startup of the system.
All compressed gases have the inherent
potential hazard of high pressure. Many
gases present chemical property hazards in
different categories, including:
Flammable gases (hydrogen, acetylene,
- Oxidizing gases (oxygen, nitrous oxide)
- Toxic gases (carbon monoxide, hydrogen
- Corrosive gases (chlorine, hydrogen
- Pyrophoric gases (silane).
Many gases have more than one hazardous
property. For example, carbon monoxide
is both flammable and toxic; hydrogen sulfide
is both corrosive and toxic.
The gas industry and equipment companies
have developed many different systems
to enable safe gas delivery. This article
describes gas cabinet enclosures, special
source systems with purge mechanisms, and
monitors (including alarms, valve shutoffs,
excess flow equipment, and gas detection).
Proper labeling of the gas cylinders and
equipment is also important in order to document
properly which hazards are associated
with the gas service, and to prevent imprudent
crossover of service (such as changing
a system used for oxidizers over to service
for flammables). This article focuses on gas
safety equipment as part of a site-wide gas
distribution system. Materials of construction,
selection criteria, general precautions,
and other issues are discussed in the context
of safety and gas purity.
Figure 1 - Typical gas cabinet.
Gas cabinets are designed to contain gas
cylinder(s) and their associated control
hardware (see Figure 1). The purpose of
the gas cabinet is to confine and remedy
(normally by venting) the hazard within
the cabinet, while preserving the safety
of the working environment outside of
the cabinet. Gas cabinets are continuously
vented at high airflow rates—from
the bottom intake up and out the top of
the cabinet through an exhaust stack. A
negative pressure is created within the
gas cabinet when the door is closed. In
the case of a leak, the leaking hazard will be
carried out the exhaust stack.
The most common hazardous area classification
is Class 1, Division 2. This classification
assumes that there will be no hazardous
gas in the area (inside the cabinet)
unless there is an equipment failure resulting
in a leak. The interior of the gas cabinet
is considered a Class 1, Division 2 area.
Storage of hazardous gas cylinders can
follow the usual guidelines, but when in
use, all cylinders containing hazardous gas
should be enclosed in a gas cabinet. Hazardous
gases should not have connections
made unless they are in the cabinet with
the door closed. All purging operations
and connections should be made through
an access hatch. This maintains the negative
pressure in the cabinet.
Gas cabinets are generally made of cold
rolled steel, with a minimum thickness of
12 gauge to provide the necessary fire rating,
and painted in a scratch-resistant paint,
such as epoxy or powder coat. Cabinets are
generally available in sizes for one, two, or
three cylinders, with common safety features
such as wire-reinforced safety windows,
doors, air intake filters, exhaust
stacks, sprinklers, and access hatches. The
self-closing door feature should never be
defeated, since this provides another measure
of safety in case of an accident.
Sprinklers are usually specified to actuate
in the vicinity of 155 °F and must be piped
to the facility water lines. They are useful
if there is a fire in the cabinet. Sprinklers
are wax-coated to resist corrosive gases
that may be present.
Restraints are mounted in the cabinet to
secure the cylinders. These may be nylon
straps or nylon straps and chains for flammable
Figure 2 - Bracket-mounted regulator.
Inside the cabinet, some type of equipment
is needed to deliver the gas to the
process equipment. The choice of source
systems will depend on the gas type and
- Bracket-mounted regulators with flex
hoses (see Figure 2)
- Switchover systems
- Analytical style panels—one-, three-,
- Ultrahigh-purity panels.
For simple flammable gases, with purity
levels of 5.0 or less (i.e., 99.999% pure), a
bracket-mounted regulator, configured with
a check valve in the CGA (Compressed
Gas Association) fitting and a high-pressure
vent, may be all that is necessary. The
check valve in the CGA fitting prevents
air from entering the pigtail during cylinder
changes. The high-pressure vent allows any
air that may have been in the system to be
vented out before the process gas is directed
downstream to the point of use.
Figure 3 - Switchover panel.
For higher usage of the same gases, a
panel may be appropriate
(see Figure 3). The switchover allows for
uninterrupted flow of gas; it automatically
switches from the depleted cylinder to the
reserve cylinder. The depleted cylinder can
then be changed while the reserve cylinder
is supplying the process; this supply/reserve
exchange can be continuous. Switchover
panels are available with check valves in
the pigtails and high-pressure vent valves,
as well as many other options. These are
suitable for situations in which an outage
of gas can ruin a production process or
delay the use of an instrument because it
must be reequilibrated.
Figure 4 - Analytical-grade panels.
Analytical-grade panels can also be used, and
they come in a variety of configurations (see
Figure 4). A panel with a single valve is typically utilized for inert gases, which can also be
used as the purge supply. A three-valve panel
is commonly used for simple flammable gases,
and is equipped with a high-pressure vent for
removing pressure and air from the panel.
In cases in which a corrosive or highly toxic
gas is being used, or where a higher degree
of purity is required, a five-valve panel is
appropriate. These panels offer multiple
functions, such as high-pressure venting,
purging, low-pressure venting, and a venturi
to assist in the purge function.
Using a five-valve panel with venturi, it is
possible to deliver 5.0–6.0 purity gas (i.e.,
99.999%–99.9999%), while extending the
life of the equipment in corrosive gases service.
Proper purging is extremely important
for corrosive gases and for UHP purity levels,
and will be covered in future articles.
Alarm monitors are available with a variety
of features. The simplest annunciators
will alert the user that a given source
is depleted. Taken to the next level, gas
alarms can provide some sort of response,
such as shutting down a process gas in the
event of a system failure.
Annunciators are typically set up for one or
two inputs (single source or a switchover),
with a green light to indicate full or in service
status of the cylinder(s) and a red light
to indicate a depleted cylinder(s). Many
also include some type of audible alarm and
output contacts for remote indication. In
this case, an alarm is a good thing.
Figure 6 - Cylinder scale.
Figure 5 - Indicating pressure switch.
Annunciators can be triggered by input
- Pressure switches
- Indicating pressure switches (IPS) (see
- Electronic cylinder scales (for monitoring
liquefied gases) (see Figure 6).
Regulators, switchovers, and analytical
panels can all be configured with pressure
switches and indicating pressure switches
to monitor the cylinder contents. Cylinder
scales can be installed inside the cabinet
(see Figure 6). In most cases, the alarm setpoint
can be adjusted by the user to suit the
requirement of the gas and the application.
Figure 7 - Pneumatically activated valve.
When the trigger is used to signal that a
hazardous condition may exist, an emergency
shutoff (ESO) valve is required
to shut down the hazardous gas. This requires an alarm monitor with pneumatic
control to operate the ESO valve
(see Figure 7).
In situations involving hazardous gas
shutdown, not all the triggering events
will be gas-related. Monitors will typically
have inputs for most, if not all, of
the various alarm trigger devices listed
below, with lights, horn, and outputs for
external notification. Shutdown alarm
monitor triggers include:
- Gas cabinet exhaust failure switches
- Fire detectors
- Excess flow switches
- Gas detection equipment.
An exhaust failure switch monitors the
negative pressure in the cabinet generated
by the exhaust blower. Since the gas cabinet
exhaust maintains the Class 1, Division
2 area in the cabinet, it is very important
to maintain the exhaust flow and to
shut off the flow of gas in the event of an
Fire detectors are typically infrared,
and can detect a flame inside the cabinet
and trigger an alarm. Excess flow
switches are designed for a catastrophic
failure, such as a break in the piping
system. The switch monitors the flow
through the system and triggers an
alarm if the flow exceeds the set point
of the switch. The switches are available
in various flow rate trip points, are
not adjustable, and are affected by the
gas type and system pressure.
Figure 8 - Gas monitor.
Gas detection equipment is used both
in the cabinet itself and in various
points around the facility (see Figure 8).
Sensors are available in various styles,
types, and sensitivity ranges, depending
on the gas. They are used for detection
of very toxic and corrosive gases, flammable
gases, and oxygen deficiency.
Typically, the sensors are connected to
a control box that displays the real-time
reading of the sensor and includes relays
to be used for alarms. Specialized sensors
may also be used as external triggers;
for instance, seismic sensors are
useful in areas in which earthquakes
In any laboratory or production setting,
there are numerous circumstances
that can lead to an unsafe condition,
resulting in injury or even loss of life.
There is, of course, no substitute for
the “human factor”—personal preparation,
training, and vigilance. However,
safety equipment is designed to add
mechanization and automation to safety
programs. Careful attention to gases,
applications, and equipment choices
will help to ensure safe gas delivery to
the point of use.
Mr. Marone is Senior Project Manager, and Mr.
Geib is Product and Technology Marketing Manager,
MATHESON, 166 Keystone Dr., Montgomeryville,
PA 18936, U.S.A.; tel.: 215-641-2700; e-mail: MMarone@Mathesongas.com.