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February 2013    Download the Entire Issue (PDF) Available to the Public Vol. 28, No. 2   RSS Feed for Undercurrent Issues
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Filling Cylinders In Water

perhaps a cause of a tank explosion

from the February, 2013 issue of Undercurrent   Subscribe Now

In my article last month about the death of a Belize dive shop employee due to an exploding tank, I noted that the tank was not filled in a water bath, as if that would matter. Several readers wrote that filling a cylinder in a water tank was not necessarily the right thing to do. So I turned to PSI-PCI, the experts in such matters, and received an article (which I have edited) written by its founder and former president, Bill High. He explains why a water bath may not be such a good idea, though a strong barrier between the tank being filled and people is a very good idea.

- - Ben Davison

* * * *

When just looking at physics, we can agree that heat transfer is far greater in water than in air. Cold water divers concluded that long ago without any physics classes. However, claiming one law of physics to justify placing dive cylinders into typical dive store water tubs during fill, as is still done at some air stations, simply isn't the right thing to do.

Chilled water tubs can contribute to overfilled cylinders, as can careless operators who think overfilling is all right because the water bath will drop pressure some. Structural damage during overfill is cumulative and irreversible. Hydro tests have been shown to be unreliable in detecting fatigue damage for overfill; therefore, even those overfilled cylinders, which have passed hydro tests, may suffer a leak or rupture.

A cylinder is full when an accurate gauge shows the marked service pressure at an ambient temperature of 70 degrees. Many air station gauges are seldom or never tested to assure accuracy. A cylinder filled slowly (as all cylinders should be) in chilled water will actually be overfilled when allowed to come to a higher ambient temperature. It is worth repeating: Water tubs often contribute to overfilling.

Reported Water Tub Benefits

Several perceived benefits to using a water bath during fill are offered by tub proponents. They include: 1) cooling allows more air in the cylinder; 2) cooling allows faster fills; 3) the water will absorb the energy of a ruptured cylinder; and 4) the tub itself provides explosion protection.

Regarding the first two benefits mentioned above, cylinders, when filled at the industry- recommended fill rate of 300-600 psi per minute, do not get hot. They may be warm but usually the temperature of the water is so close to the cylinder increase (about 100 to 110 degrees maximum) that the exchange rate is slow and low. We don't want more air in the cylinder than is allowed by law. We don't want fast fills beyond the industry standard, if for no other reason than cylinders will get warm, and such practice makes the air station undefendable.

The whole water tub thing began in the mid 1950's, when we knew little about cylinders and their care. Steel cylinders got warm during what we now know to be fast fills. We didn't know about prudent fill rates, and we often ignored the service pressure limit. Cylinders were filled quickly, removed from the water promptly, and very little actual in-water temperature reduction took place. Then along came aluminum cylinders with walls nearly half an inch thick. They didn't seem to get as warm. That was because although we still filled quickly, the heat generated within the cylinder took much longer to transfer to the outside. The water bath cylinder was removed from the water and sent on its way, long before the fast fill-generated heat could be dissipated into the water.

What about the perceived benefit that the water bath will absorb explosive energy? There simply is not enough water between the tub walls and the exploding cylinder to have any measurable effect whatsoever, unless of course the tub is a nearby swimming pool. But surely the tub itself will provide protection? Not necessarily true when you look at a great many of the water tubs in use today. Plastic garbage cans are used, as well as sheet metal buckets of one sort or another. The energy within a full, exploding cylinder is so great, well over one million pounds per foot of potential kinetic energy, that all these containers break up and contribute shrapnel to injury and property damage. Even concrete block barriers usually disintegrate.

Water Gets Inside

The greatest concern for water baths when filling cylinders is water entering into them. With water, metal and the ample oxygen in compressed air, cylinders can be damaged dramatically in a very short time. A study by the University of Rhode Island revealed that under adverse conditions, a steel cylinder with a small amount of saltwater (remember, the fill tub may contain contaminated water) could be in danger of exploding within as little as 100 days. In a perfect facility, tub water does not enter the cylinder, but in many tubs, water is allowed to enter the valve aperture, as well as the fill whip connector. Those water droplets are pushed into the cylinder.

Note that I referred to a perfect water tub system. A very few do exist, reinforced concrete and steel tubs serving both to hold water and act as a blast shield. Cylinders placed into the water cannot drop below the valve aperture, and fill whips can't reach water level. It has a drain that is used often. Of course, the fill station should be away from customers. Even this perfect water tub for filling is unneeded, although any true blast protection is a very wise investment. Nowhere else in the gas industry are cylinders routinely filled in a water tub.

P.S.: Fred Calhoun, writing in the November/December 1988 issue of NAUI News, addressed the dry fill/wet fill issue accurately and in detail. Fred's article is still distributed by PSI-CSI in its publication Scuba Cylinder Reprint File. The textbook Inspecting Cylinders also explains how the tub fails to achieve what its promoters desire.

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