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Hi Greg, Many people will tell you all that will happen is the tank will begin leaking. No one, I repeat - no one, can predict with any certainty that such a benign failure would occur. Anyone who does tell you such is doing you a servere disservice, even if all he has personally experienced is just a leak. He was lucky indeed. The risk is real that a catastophic explosion can occur because of the vast amout of stored energy in a compressible fluid, in this case air. The personal observations of those who responded to the previous thread prove this to be true. That is a reason why air storage tanks have to be manufactured per the ASME Code by just about all state or local laws. In commercial applications, those same state laws also usually require an annual safety inspection and certification process by an "authorized inpsector", usually a specialized employee of a state safety agency or an insurance company. In most cases, it is illegal to operate an air storage tank, of the size and pressure you are talking, by a commercial business without an annual, signed safety certificate by such an inspector. That inspector may inspect the interior of the tank for deterioration with a borescope or may, at his discretion, require a hydrostatic pressure test be conducted at the owners expense, prior to issuing a safety operating certificate. These types of safety requirements for manufacture and continuing in-service inspections and certifications would not be necessary if all that could be expected was a routine leak from a tank. The requirements were developed over many years of industry experience with failures where significant property damage and deaths have occurred due to corrosion in old tanks used for compressible fluids such as air. The best way to prove the current safety of your older tank is by hydrostatic pressure testing as stated elsewhere. A tank that size should have been manufactured to ASME Code criteria, even in 1955. There should be an ASME plate tackwelded to the tank somewhere. It may even be the same tag that you got the date from. On that plate should be the ASME cloverleaf symbol and the letters MAWP, among other things. MAWP stands for "Maximum Allowable Working Pressure". Stamped adjacent to the MAWP lettering should be a psig numerical rating. For a typical storage tank of that size used with a two stage air compressor, a MAWP rating would likely be 200 psig. There should also be a relief valve somewhere on the tank/compressor assembly. A two stage compressor usually has a maximum shutoff pressure of 175 psig so the relief (safety) valve setting would be a little higher than that, perhaps 185 psig. It would absolutely have to be equal or less than the MAWP stated on the ASME plate. Since you have been using yours at a shutoff of 150 psig, it obviously is higher than that. Assuming your tank was manufactured per the ASME Code and labeled with an as-new MAWP, you would want to pressurize the tank, using water as the medium, to a pressure of 1.5 times that MAWP. If it happened to be the 200 psig typical MAWP, you'd pressurize the tank to 300 psig. This is a safe practice as long as there is no appreciable amount of air remaining in the tank during the testing. The worst that could happen is that you might get a little wet should a failure occur. You'll have to cobble together some means to slowly pressurize the tank to that pressure. A hydraulic handpump comes to mind, provided a hydraulic reservoir of sufficent capacity is available. The tank will expand as the pressure goes up so additional fluid - either water or oil - will be needed. Perhaps another quart or so. Use a known good pressure gage of about 400 psig capacity and make sure the tank is initially filled "to the brim" with water. Inspect the tank carefully while under pressure to observe any indication of leakage, failure or obvious distortion. The above is kind of a pain to do but is an excellent safety precaution to take where you cannot visually observe the interior condition of an old tank. If you had access to a borescope, you could utilize the tank discharge opening and visually inspect the interior bottom area of the tank for general condition, pitting etc. Not knowing how much pitting is safe however, would suggest a pressure test anyway to verify safety. I have an old (35 years) 20 gal tank/compressor combination that I have not used in several years. If I ever have the desire to put that unit back in service, I will definitely put my mind to ease by testing the tank as noted above.  Rod
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