CO2 Fire Extinguisher Hydrostatic Testing

by

Ed Robisheaux, NAFED President


Back to Basics

There occasionally comes a time when the value of old practices and procedures comes under question. It then becomes useful to 'go back to basics,' to examine the reasons that underlay the old assumptions, and to re-examine things with a fresh perspective. The requirement to hydrostatically test Carbon Dioxide (CO2) fire extinguishers affords a good example of such a basic re-thinking.

With this in mind NAFED in the fall of 1993 conducted a statistical survey of CO2 hydrostatic tests conducted by a number of different distributors located in various parts of the United States. The intent was to determine ‘what is really going on’ in the world of CO2 hydrotesting. The results are interesting.

About the Survey

The survey compiled data from 86,096 hydrostatic tests completed by participating NAFED members from 1988-1993. The participants were located in the eastern, southern, and midwestern parts of the United States. Those submitting data to the survey group their facts by cylinder construction (steel or aluminum), by size (2 1/2-100 lb. cylinders), by year of test, and by cause of failure.

The data were gleaned from hydrostatic testing logs maintained by all the participating companies.

Failures come in three categories. Some fail by rupture of the cylinder wall; others exhibit excessive permanent expansion; still other cylinders exhibit sufficient internal or external corrosion to warrant a visual failure. Not all participants in the NAFED survey recorded visual failures in all testing years. Hence the failure rate for CO2 cylinders is understated in this survey. It is not possible to estimate the degree of understatement, only to note that it did occur. We will leave to others the task of speculating on the degree of understatement.

How High Pressure Cylinders Were Tested

Testing was conducted in accordance with 49CFR and Pamphlet C-1 of the Compressed Gas Association (CGA,C-1), Methods for Hydrostatic Testing of Compressed Gas Cylinders. We assume the reader is familiar with the requirements of CGA-1, or has some access to the standard. Copies can be purchased from the Compressed Gas Association, 703-412-0900.

How and Why Cylinders Fail Hydrotesting

Chart 1 details the total failures by all type of cylinder construction. 566 such cylinders of 86,096, or 0.65% of the total, failed hydrostatic re-test.

Visual Failures

Of those cylinders failing hydrostatic re-test, the largest single cause of failure is visual rejection (73%).
See chart 2. A cylinder fails visual examination when the re-tester, applying his professional judgment, deems the cylinder condition likely to fail pressure test, or whose safety in irretrievably compromised. The causes of visual failure include excessive rust or corrosion, evidence of external welding, cutting, or brazing on the cylinder shell, or mechanical damage to the cylinder wall. Standard 10 of the National Fire Protection Association (NFPA 10), and pamphlet C-6 of the Compressed Gas Association (CGA,C-6), indicate a more complete list of qualifying conditions warranting visual rejection.

Excessive Permanent Expansion

The second greatest cause of failure is by excessive permanent expansion. When a high pressure cylinder is subjected to pressure, the interior volume of the cylinder expands. When the pressure is released, the volume contracts. The difference between the total volume (under pressure) minus the final volume (no pressure) is the amount of permanent expansion caused by the hydrostatic testing pressure. Without going into a lot of exceptions and qualifiers, let us say that this permanent expansion in most cases cannot exceed 10% of the total expansion. Cylinders with a permanent expansion exceeding 10% fail hydrostatic test. For additional detail on testing, see CGA,C-1.

About 26% of all cylinders failing hydrostatic test did so because of excessive permanent expansion. Chart 3 indicates the distribution of failure.

Cylinder Rupture

When a cylinder ruptures the cylinder is propelled at high velocity along a destructive path that often destroys property and life. Shrapnel can sever limps, cause blinding injuries, and even death. The prevention of cylinder rupture is one of the basic justifications for hydrostatic testing.

The hydrostatic testing method used during the NAFED survey was the water-jacket method of testing. Cylinders subjected to test are immersed in a sub-surface water-jacketed enclosure to minimize danger in the event of a catastrophic breakdown under pressure.

Fortunately, such failures are relatively few. Only 9 of 86,096 tested cylinders, or 0.00116% ruptured during testing. The survey indicates that 1.06% of the 566 cylinders failing hydrostatic test did so because of cylinder rupture.

Steel vs. Aluminum

The survey produced data on the testing of 34,599 steel cylinders and 7,564 aluminum cylinders. (These do not total 86,096 because some of the reporting companies did not distinguish between steel and aluminum.) Often one wonders which cylinder construction is most likely to fail hydrostatic test.

The data are clear: steel cylinders are 5 times more likely to fail hydrostatic test than aluminum cylinders. Chart 4 presents the data in graphical format.

The Larger Significance

Nobody knows precisely how many carbon dioxide fire extinguishers are in circulation throughout the United States. Various estimates by the CGA put the estimate at 95 million returnable cylinders with some 40 million being schedule 3A and 3AA in use.

If we project from our statistical sample of 86,096 hydrostatic restests we could assume the following experience nationwide:

Chart 5 illustrates the above projected failure rates.

Changing the Hydrostatic Testing Interval

Occasionally one hears the idea that the testing interval for carbon dioxide fire extinguishers should be increased from 5 years to 12 years. The basis behind such an idea is the well recognized fact that there are relatively few cylinder ruptures in the course of any given year.

But how would such a change alter the number of annual cylinder failures circulating among the public at large? The answer is not reassuring.

In the event the testing interval is changed from 5 to 12 years, no extinguisher tested in the previous 5 years would require testing for an additional 7 years. We know, however, that annual testing removes approximately 6,574 cylinders per million from circulation. For 7 years this figure would increase by approximately 6,574 per million of such potential failures per year, until we could expect there to be some 46,019 cylinders per million of potential failures in circulation among the public. Chart 6 provides such projections for all types of failures.

The inescapable conclusion is that the number of potential cylinder failures would increase by approximately 700%. The number of cylinders subject to possible rupture would increase by some 488 per million of circulating extinguishers. Increasing the testing interval from 5 to 12 years seems hardly in the public interest.

Final Remarks

Hydrostatic testing of carbon dioxide fire extinguishers removes thousands of potential safety hazards from circulation annually. The effects of mechanical damage, corrosion, and metal fatigue become cumulative without some process to cull them from service. Hydrostatic testing at 5 year intervals seems to have served the purpose well.

Posted December 14, 1994.

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