Air Compressor - Hydrostatic Tank Testing
Posted: Sat Sep 20, 2008 2:05 pm
Air Receiver Hydrostatic Test
This is the way I test a tank, it is not a standard test procedure and you proceed at your own risk.
I position the tank so I have a port at the highest point for filling. All other ports plugged. Fill as full as possible with water (get all the air out). Install fittings as necessary (bushings) and install a nipple and cross fitting. One port for the test gage, another for pressurizing, and a bleed. Now fill the fittings also with water thru the bleed port if you can . I'll give you what I used years ago as I think you are more likely to have a grease gun than a portapower type pump and reservoir. I think a grease gun can be used to pump oil (be it automatic transmission fluid or all-trans hydraulic fluid) if held with the plunger end up but I haven't really tried it so I can't say for a fact. I have used general purpose grease and a standard grease zerk fitting in the pressurize port of the cross. The purge can be any kind of valve you have handy. Needle, ball, etc., I have a tire valve stem that threads into 1/8" pipe I have used. Need I say that all fittings should be rated for the test pressure you will be using. I remember once using a compression tester gage that had the little purge valve built right into it - I think that was an automobile tire valve stem also. I will leave it up to you what factor of safety you desire to test your tank to. If your compressor is rated for 135 psi maximum for instance a factor of safety of 2 would mean testing the tank at 2 times this max operating pressure, or 270 psi. The temperature of water and tank during the test is of lesser importance but I like to perform the test outside and exposed to a good hot sun - 100 degree day is fine. Remember compressed air pumped to the tank will be warmer than ambient temp. unless you are using a cooler/chiller. With the tank full of water you can now raise the pressure to the test pressure pumping grease (oil) in the zerk. A 60 gal. tank usually takes about one and 1/4 tubes of grease to reach a test pressure of 275 psi.. Sounds messy doesn't it. Pump until you reach the test pressure, wait an hour or so (assuming you didn't get wet already) and you will probably see the pressure has dropped a little. Tank expansion, grease compression I don't know but I usually see a few pounds drop and pump it back up to test pressure. I let it sit at pressure for 24 hours for all to come to equilibrium - nothing magic about 24 hours. First time I did a hydro test it was late in the day and I thought the test ought to have benefit of a good hot sun beating down on the tank and contents so I waited 24 hours for that to happen. I've been doing it that way every since. Once the pressure test is complete, crack the purge and expect a little squirt of water from it. Now you get to retrieve the GLOB of grease by opening the biggest bung on the tank to let the water out. I tried catching the grease using an old tee shirt rag as a strainer but water gushing out can push grease thru a rag at near 100 degrees F so I have since performed this tank draining in a sandy portion of what will someday be a driveway (maybe). The grease stays pretty much together in a big GOB or GLOB, sometimes 2 or 3 GOBS but that is as much as I've seen it separate. I chalk it up to, gob 1 the first tube pumped in, gob 2, change to the second cartridge of grease and gob 3, the final top off of pressure after the tank reaches steady-state. This seems to be supported by the size of the gobs that exit the tank. I scoop it up and smear the tops of fence post (rot protection) and/or save it for wiping down equipment to keep it from rusting (plow points and other ground engaging equipment). That's it, nothing fancy but it does give the best indication of the receivers ability to handle the job safely, at least for now. Hydrostatic testing is generally done on a schedule - every so many years. I won't say much about the frequency of testing as the only reference to that I've seen in print was for 4,000 psi air receivers we used in the Air Force to start Jet Aircraft. If you drain your tank often and monitor the condition of purge water from it you will have some idea of how much rust is free in the tank.
If you do use a non synthetic oil to pressurize, remember it floats on water and can be purged from the top of a tank if you leave yourself a means of supplying low pressure water to tank at some lower port. The idea is to catch the oil for proper disposal.
If anyone has another procedure for hydrostatically testing air receivers and suggested frequency of testing please don't keep it a secret, I for one would be interested to see how others do their testing.
This is the way I test a tank, it is not a standard test procedure and you proceed at your own risk.
I position the tank so I have a port at the highest point for filling. All other ports plugged. Fill as full as possible with water (get all the air out). Install fittings as necessary (bushings) and install a nipple and cross fitting. One port for the test gage, another for pressurizing, and a bleed. Now fill the fittings also with water thru the bleed port if you can . I'll give you what I used years ago as I think you are more likely to have a grease gun than a portapower type pump and reservoir. I think a grease gun can be used to pump oil (be it automatic transmission fluid or all-trans hydraulic fluid) if held with the plunger end up but I haven't really tried it so I can't say for a fact. I have used general purpose grease and a standard grease zerk fitting in the pressurize port of the cross. The purge can be any kind of valve you have handy. Needle, ball, etc., I have a tire valve stem that threads into 1/8" pipe I have used. Need I say that all fittings should be rated for the test pressure you will be using. I remember once using a compression tester gage that had the little purge valve built right into it - I think that was an automobile tire valve stem also. I will leave it up to you what factor of safety you desire to test your tank to. If your compressor is rated for 135 psi maximum for instance a factor of safety of 2 would mean testing the tank at 2 times this max operating pressure, or 270 psi. The temperature of water and tank during the test is of lesser importance but I like to perform the test outside and exposed to a good hot sun - 100 degree day is fine. Remember compressed air pumped to the tank will be warmer than ambient temp. unless you are using a cooler/chiller. With the tank full of water you can now raise the pressure to the test pressure pumping grease (oil) in the zerk. A 60 gal. tank usually takes about one and 1/4 tubes of grease to reach a test pressure of 275 psi.. Sounds messy doesn't it. Pump until you reach the test pressure, wait an hour or so (assuming you didn't get wet already) and you will probably see the pressure has dropped a little. Tank expansion, grease compression I don't know but I usually see a few pounds drop and pump it back up to test pressure. I let it sit at pressure for 24 hours for all to come to equilibrium - nothing magic about 24 hours. First time I did a hydro test it was late in the day and I thought the test ought to have benefit of a good hot sun beating down on the tank and contents so I waited 24 hours for that to happen. I've been doing it that way every since. Once the pressure test is complete, crack the purge and expect a little squirt of water from it. Now you get to retrieve the GLOB of grease by opening the biggest bung on the tank to let the water out. I tried catching the grease using an old tee shirt rag as a strainer but water gushing out can push grease thru a rag at near 100 degrees F so I have since performed this tank draining in a sandy portion of what will someday be a driveway (maybe). The grease stays pretty much together in a big GOB or GLOB, sometimes 2 or 3 GOBS but that is as much as I've seen it separate. I chalk it up to, gob 1 the first tube pumped in, gob 2, change to the second cartridge of grease and gob 3, the final top off of pressure after the tank reaches steady-state. This seems to be supported by the size of the gobs that exit the tank. I scoop it up and smear the tops of fence post (rot protection) and/or save it for wiping down equipment to keep it from rusting (plow points and other ground engaging equipment). That's it, nothing fancy but it does give the best indication of the receivers ability to handle the job safely, at least for now. Hydrostatic testing is generally done on a schedule - every so many years. I won't say much about the frequency of testing as the only reference to that I've seen in print was for 4,000 psi air receivers we used in the Air Force to start Jet Aircraft. If you drain your tank often and monitor the condition of purge water from it you will have some idea of how much rust is free in the tank.
If you do use a non synthetic oil to pressurize, remember it floats on water and can be purged from the top of a tank if you leave yourself a means of supplying low pressure water to tank at some lower port. The idea is to catch the oil for proper disposal.
If anyone has another procedure for hydrostatically testing air receivers and suggested frequency of testing please don't keep it a secret, I for one would be interested to see how others do their testing.