The Shop > Tools
Compressor safety, or how do I make sure an old compressor is safe?
hermetic:
Right! I am going to stick my oar in here, as I have had this argument on a few sites, and seen all the horror stories and pictures. If you look at the pics and THINK about it, you will notice that the tanks which have exploded or split wide open, are all modern tanks, and show no signs of rust damage whatsoever! the failures have been caused by over pressure where the pressure switch and/or safety valve have failed. Rust causes pinholing, which immediately releases the pressure. You will also see in the pics how frighteningly thin the modern tanks are. I have a British made Broomwade Tank with a 22 cu ft min compressor on it. The tank was made in 1944 and tested to 300psi giving it a safe working pressure of 150psi. The steel used to make the tank is about 3/8" thick. When I was in business in the 1970s this compressor was annually inspected by my insurer and the form showed
" internal rust pitting to a depth of 1/16" When I asked the engineer who did the testing if this was cause for concern he laughed, saying that if it had taken 30 years to corrode that far, there was still a lot of life in it! He also told me that "catastrophic failures are caused by overpressure, not rust" He always spent far more time checking the pressre switch and relief valve than he did on the inspection inside the tank.
Testing the tank is fairly simple, you need one of these
http://www.ebay.co.uk/itm/PRESSURE-TEST-PUMP-REMS-PUSH-EQUIVALENT-/231560548944, and a hosepipe, stand the tank on end, dome upwards, so it fills completely with water, attach the pump and pump up to twice the working pressure, and leave for one hour. Water is incompressible, so there is no danger involved in this test, as there is no stored energy (actually to be pedantic, there is no expansive energy beyond 300lbs ,assuming a working pressure of 150lbs), the tank cannot explode, it will merely start to leak if there is any weak areas. If it holds for an hour with no appreciable water loss it is safe. This is how most pressure vessels are tested. Actually using twice the working pressure is a more stringent test than is used nowadays, but it errs well on the safe side. If the test certificates issued with chinese compressor tanks are as accurate as the test certificates issued with their machine tools, the failure rate of imported compressor tanks is unsurprising.
Phil
petertheterrible:
Your a braver man than me bringing the Chinese into the story, but it this case it actually does hold water.
Metal used on old tanks was normal boiler plate. These modern tanks are made of thinner metal that is stronger but in my opinion corrode more easily when they are not treated with some sort of silicone preservative film at the factory.
What would your best advice be in using the compressor in the picture, hermetic? Use/not use/ test?
appletree:
A wonder how many people have bought, "cheap" compressors from the east and kept using them without a second thought as they bought them "new" and know the history . My parents used to have a compressor in their Hire fleet about 12 cfm which used the heavy duty 2 inch OD tube frame as the pressure vessel. It did not have any test plate etc it was my understanding that as it was made from a given class of steel tube/volume it was not considered a pressure vessel. This could be understandable as the distribution air pipework in industry is not tested or inspected.
appletree:
--- Quote from: hermetic on May 17, 2015, 10:59:21 AM ---Right! I am going to stick my oar in here, as I have had this argument on a few sites, and seen all the horror stories and pictures. If you look at the pics and THINK about it, you will notice that the tanks which have exploded or split wide open, are all modern tanks, and show no signs of rust damage whatsoever! the failures have been caused by over pressure where the pressure switch and/or safety valve have failed. Rust causes pinholing, which immediately releases the pressure. You will also see in the pics how frighteningly thin the modern tanks are. I have a British made Broomwade Tank with a 22 cu ft min compressor on it. The tank was made in 1944 and tested to 300psi giving it a safe working pressure of 150psi. The steel used to make the tank is about 3/8" thick. When I was in business in the 1970s this compressor was annually inspected by my insurer and the form showed
" internal rust pitting to a depth of 1/16" When I asked the engineer who did the testing if this was cause for concern he laughed, saying that if it had taken 30 years to corrode that far, there was still a lot of life in it! He also told me that "catastrophic failures are caused by overpressure, not rust" He always spent far more time checking the pressre switch and relief valve than he did on the inspection inside the tank.
Testing the tank is fairly simple, you need one of these
http://www.ebay.co.uk/itm/PRESSURE-TEST-PUMP-REMS-PUSH-EQUIVALENT-/231560548944, and a hosepipe, stand the tank on end, dome upwards, so it fills completely with water, attach the pump and pump up to twice the working pressure, and leave for one hour. Water is incompressible, so there is no danger involved in this test, as there is no stored energy (actually to be pedantic, there is no expansive energy beyond 300lbs ,assuming a working pressure of 150lbs), the tank cannot explode, it will merely start to leak if there is any weak areas. If it holds for an hour with no appreciable water loss it is safe. This is how most pressure vessels are tested. Actually using twice the working pressure is a more stringent test than is used nowadays, but it errs well on the safe side. If the test certificates issued with chinese compressor tanks are as accurate as the test certificates issued with their machine tools, the failure rate of imported compressor tanks is unsurprising.
Phil
--- End quote ---
Hi not being an a**e did you mean to post the pictures or have I missed something?
Lew_Merrick_PE:
Topic Drift Alert Whereas I am sure that there are good shops & suppliers in China, I have yet to run into one. A few years ago while doing work for a company that likes to claim that they are the largest American employer of Chinese workers we need material certificates to be supplied with parts. The supplier in question asked for a sample of such a material certificate. I sent them one for an entirely different material (rhenium bar for a rocket nozzle) and they sent it back to us with only the material identity changed out.
My customer on this project accepted it as genuine! That was the last time I did work for this particular company. They are a major, world-wide purveyor of consumer and industrial products...
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