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Guidelines For Standard Gas Cylinder and Pressure Regulator Use Pressure Gauge
Standard Gas Cylinder:
Trace
gas standards used for measuring CO2, CO, N2O, SF6, and CH4
by the Carbon Cycle-Greenhouse Gases group (CCGG) of NOAA
CMDL are contained in aluminum cylinders purchased from Scott-
Marrin, Riverside, California. The cylinders are treated with
a proprietary passivation treatment. CCGG uses three different
size cylinders but most of our standards are contained in
30 liter (internal volume) cylinders. The cylinders are ordered
with brass Ceodeux cylinder valves (CGA590) containing all-metal
seats and nickel stems. The cylinders are shipped to CMDL
with 1380 kPa (200 psig) of dry, ultrapure air. It is important
for the cylinders to be dry (and remain dry) during filling
and use. Brass cylinder valves rather than stainless steel,
are recommended for all trace gas species measured by CCGG.
Note: Read and
follow all safety procedures for handling high pressure gas
cylinders.
Pressure Regulators:
The
proper regulator to use for a trace gas standard may depend
on the particular trace gas being measured. The regulators
CMDL uses for carbon cycle trace gas standards do not always
perform well for trace gas stable isotopic measurements. Standards
for SF6 measured by GC-ECD may require special high-purity
electron capture sensitive regulators. CCGG uses two-stage,
high purity, low flow, compact regulators on most standards.
The regulators are nickel-plated brass with stainless steel
diaphragms and low internal volume. The desired delivery flow
rates, purity levels, and corrosiveness of the standard gas
should be determined before purchasing a regulator. It is
important to note that delivery pressure range and gauge delivery
pressure are different. Order a regulator that is within the
required delivery pressure range. (For example, a 0-700 kPa
(0-100 psig) delivery pressure gauge may have a maximum delivery
pressure of 350 kPa (50 psig).)
Standard Gas Cylinder Handling:
After
purchasing a calibrated standard gas from CCGG, the following
guidelines are suggested.
1.
Dedicate a regulator for each standard gas cylinder. The
regulator should be installed, leak checked, flushed, pressure
checked, conditioned, and remain on the cylinder at all
times if possible.
2. Regulator Installation: Follow basic
cylinder/regulator installation procedures as indicated
in the instructions sent with each regulator.
3. Leak Check: After the regulator is attached
to the cylinder a leak check should be performed. The first
step in this procedure differs from gas company guidelines.
We allow a small amount of gas to escape the regulator when
first opening the cylinder valve to prevent contamination
of the standard.
•
Turn the pressure delivery handle clockwise to allow a
small amount of gas to escape the regulator when the cylinder
valve is opened.
• Slowly open the cylinder valve until gas starts
to escape from the regulator.
• Close the regulator by turning the pressure delivery
handle fully counterclockwise.
• Fully open the cylinder valve.
• Leak check the regulator/cylinder connection (CGA
nut) using Snoop or another leak detector.
• If a leak is detected, close the cylinder valve
and depressurize the regulator before attempting to fix
the leak.
• After tightening the CGA nut or adjusting the
position of the regulator, open the cylinder valve and
recheck for leaks.
• Once the cylinder/regulator connection is leak
free we recommend an initial regulator flushing and pressure
check to detect small leaks and malfunctioning regulators.
4.
Flushing: Both the high and low sides of
a two stage regulator should be flushed (pressurized and
vented) after initial installation and before each use.
•
Close the cylinder valve and turn the pressure delivery
handle clockwise to allow a small amount of gas to escape
the regulator outlet after the cylinder valve is opened.
• Fully open and fully close the cylinder valve
so that the low and high side gauges are pressurized and
then drained to zero. (The low side pressure can be increased
for faster flushing.)
• Pressurize and vent the regulator four times.
• Fully open the cylinder valve and turn the pressure
delivery handle fully counterclockwise to close the regulator
to prepare for the pressure check.
5.
Pressure Check: A pressure check consists
of pressurizing both the low and high gauges of the regulator,
letting it sit overnight, and then rechecking the pressures.
•
Plug the outlet side of the regulator at the regulator
outlet, shutoff valve, or somewhere downstream of the
shut off valve. Do not use the pressure delivery handle
to plug the outlet.
• Pressurize the high side by fully opening the
cylinder valve.
• Set the delivery pressure to a setting about one-half
of the maximum delivery pressure by turning the pressure
delivery handle clockwise.
• Shut off the regulator by turning the pressure
delivery handle fully counterclockwise.
• Close the cylinder valve. (This is important:
If there is a leak, the cylinder could be empty on the
morning.)
• Write down the initial pressure for each gauge
A
pressure check is used to isolate different
parts of the cylinder and regulator system and look for leaks.
If, after closing the cylinder valve and regulator, the pressure
on the high side gauge increases, then the regulator is defective
and it should not be used. If the high side pressure gauge
decreases, then there is a leak between the cylinder and the
high side of the regulator, most likely the CGA fitting. If
the high side gauge decreases and the low gauge increases,
there is a leak in the regulator across the diaphragm and
this regulator should not be used. If the low side gauge decreases,
then there is a leak between that gauge and the downstrem
shutoff.
6.
Conditioning: Conditioning the regulator
is important to ensure that the standard air equilibrates
with the regulator. After the regulator is conditioned,
standard air should pass through the regulator with no gain
or loss of the species of interest. The regulator should
be conditioned with standard air for at least one week before
use. Failure to condition the regulator may result in incorrect
measurements.
• After pressure checking
the regulator leave it pressurized (high and low side)
with the cylinder valve closed for 2 or 3 days.
• Open the system downstream and flush the regulator
two times using the pressurize and vent method described
in #4.
• Close the downstream outlet, open the cylinder
valve, pressurize the regulator by turning the pressure
delivery handle clockwise, then close the regulator by
turning the pressure delivery fully counterclockwise,
and then close the cylinder valve.
• Leave the regulator pressurized for two more days.
• On day 3, vent and pressurize two more times.
• Repeat #6c and leave the regulator pressurized
for the remainder of the week.
7. Daily Use: Before each
use (each day) the regulator should be flushed (vented and
pressurized) four times as in #4. Allow the regulator to
fully depressurize before opening the cylinder valve. At
the end of the day the cylinder valve should be closed,
leaving the regulator pressurized (high side). The low side
can also remain pressurized if there is a shut off downstream.
8. Notes:
• Cylinder valves can be difficult to
open or close, CCGG uses gloves or some other gripping
device to close hard-to-turn cylinder valves. Failure
to fully close a cylinder valve may result in loss of
standard air.
• If a regulator is removed from the cylinder, it
is important to retighten the cylinder valve after the
regulator is depressurized and removed.
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