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.

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 instrument.

Conclusions

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 lcgcedit@lcgcmag.com
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References

(1) J.V. Hinshaw, LCGC North Amer. 30(11) 978–981 (2012).

(2) J.V. Hinshaw, personal observations.

John V. Hinshaw