What About Tanks Kept Outdoors for Gas Distribution Into the Lab?
Many laboratories use an outdoor shed to hold both in-use and spare gas tanks. This is an excellent solution to concerns over
housing compressed gas tanks in the laboratory, and this arrangement liberates some laboratory space otherwise occupied by
numerous tanks. However, such a setup comes with extra requirements.
First of all, outdoor tanks are subject to extreme temperature swings both daily and as seasons change. Daily temperature
fluctuations can drive the internal tank pressure up and down by about 5–7%, while pressure changes could swing as much as
two to three times more annually in areas that experience large seasonal temperature changes.
The combination of widely fluctuating tank pressure and temperature affects pressure regulator output levels. Data from outdoor
single-stage regulators with outlet pressure monitors that experienced ambient temperatures from -15 °C to +40 °C over the
course of six months showed an inverse outlet pressure variation from 86 psig (593 kPa) to 78 psig (538 kPa) in one case,
about ±5%. A similar effect was noted for all the observed regulators (2).
A tank heating blanket is another solution for temperature fluctuations at the tank, but usually this is used only on gas-mixture
tanks in which a component might liquefy or stratify at cold temperatures. Dual-stage regulators help maintain a more constant
output pressure, too, but tank heaters or dual-stage regulators are not necessary for outdoor GC gas tanks, as explained below.
Tank regulators and associated valves and fittings outdoors can be subject to high levels of dust, moisture, and chemical
pollution. Brass regulators and conventional pressure gauges — even with nickel flashing on the outer surfaces — can undergo
significant degradation in aggressive environments such as near sea water, in a desert location, or in proximity to a chemical
plant. Unsealed regulators and gauges are subject to dust and insect intrusion. Any of these can cause the regulator mechanisms
to exhibit bias, stick, or even fail.
Beyond regular inspection and maintenance operations to keep regulators in good operating condition, I have encountered a
few solutions to the above environmental problems. Installation of outdoor-rated stainless steel regulators and sealed stainless
steel gauges is a simple solution, but such devices can be difficult to acquire. A sealed regulator enclosure, with a short
flexible high-pressure hose that connects to the gas tank, effectively isolates the regulator from long-term exposure to environmental
pollutants. Enclosures specifically designed for this service will have an external safety vent in addition to the inlet and
outlet connections. I have also seen sites where the tanks and regulators were located in an enclosed heated gas shed. It
is also possible to route high-pressure hoses and piping through outdoor walls to a protected location, but this arrangement
requires special materials and skills to properly contain the associated high pressures coming directly from the gas tanks.
The pressure fluctuations in the output of a single-stage outdoors pressure regulator are large enough to cause some supply
pressure problems at the gas chromatograph: Why not use a dual-stage regulator? A better solution consists of two single-stage
regulators, with one at the tank (outdoors) followed by in-line regulators between the incoming gas line and the backs of
the instruments. Set the tank regulator to 150–200 psig (1–1.4 MPa) and set the in-line regulators to deliver 80–100 psig
(500–700 kPa) or as required by the instruments.
One feature that is invaluable in outdoor installations is a lock-nut knob-free arrangement for the outlet pressure settings
on all regulators. This prevents the inevitable knob-twiddling from no doubt well-intentioned persons. I personally witnessed
one such situation where the twiddler said, "Hey, more pressure is better, right?" Another time, someone turned the pressure
knob all the way clockwise in a mistaken attempt to turn it off; the result was a broken regulator.
New, cleaned ¼-in. (6-mm ) o.d. copper tubing is a good choice for plumbing between the tank area and the laboratory, if the
tubing will not be flexed or disturbed much during its service lifetime. (The metric diameters given here are the commonly
available sizes, not the exact equivalent of the fractional sizes.) Larger diameter ⅜-inch (10-mm) tubing is appropriate for
runs longer than 50 ft (15 m). Stainless steel tubing is better, but it is also more expensive and difficult to cut and bend.
For either tubing material, it's a good idea to connect 6-ft (2-m) lengths of convoluted stainless steel–core hose with a
stainless braided outer covering between the tank regulators and the start of the tubing that runs to the laboratory. The
flexible links will prevent repeated stress and eventual failure of rigid tubing because of normal tank changes when connected
directly to the pressure regulators.
Remember to include a valve and purge-tee arrangement close to the regulator. The purge tee will speed up tank changes by
accommodating purging of only the pressure regulator and short tubing in front of the tee. Everyone purges the regulator upon
tank changes, don't they? And yes, this is another knob for the twiddlers. Install a lock-out style valve with a padlock if
you are seriously concerned.
For high-flow gases such as carrier gas with split injection or air for flame-type detectors, where the total gas flow delivery
is greater than a few liters per minute, consider using a tank regulator with a higher flow rating and perhaps even two tanks
with an automatic or manual switch over manifold.
Include a pressure gauge, shut-off valve, and another purge-tee arrangement where the incoming gas supply tubing enters the
laboratory from the outdoor tanks; this will facilitate purging the long tubing runs from the tanks when necessary. Connect
the incoming tubing to a series of tee fittings to distribute the gases to in-line regulators in parallel. Install suitable
pressure gauges at the outlets of the in-line regulators if the connected gas chromatographs cannot display incoming gas-line
pressures. Additional shut-off or isolation valves at each in-line regulator or, even better, at each instrument will make
it easier to install or remove instruments when necessary.
A single large in-line filter is unnecessary if the gas delivery system is well designed, clean, and leak-tight. In fact,
a low-grade high-capacity filter might introduce more contamination than it removes at first. However, be sure to include
appropriate filters after each in-line regulator, or position individual filters or filter banks close to the back of each
These two installments on gas Q & A have covered a range of issues about carrier and detector gases, gas distribution, gas
generators, filter selection, regulators, tubing, fittings, and connections. Chromatographers should pay careful attention
to these areas when setting up or expanding a laboratory, when installing instruments, and of course during routine use. Readers
are encouraged to send their comments and additional questions about these or any other GC-related topics to firstname.lastname@example.org
(1) J.V. Hinshaw, LCGC North Amer.
30(11) 978–981 (2012).
(2) J.V. Hinshaw, personal observations.
John V. Hinshaw
"GC Connections" editor John V. Hinshaw is a Senior Scientist at BPL Global, Ltd., in Hillsboro, Oregon, and a member of
LCGC's editorial advisory board. Direct correspondence about this column to the author via e-mail:
John V. Hinshaw