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  • Getting a air tank inspected

    I have a older air tank that I am going to add to my compressor. I want to get it inspected and tested first. Do you guys have any suggestions of places that would do this? Thanks
    NorthridgeFab.com

  • #2
    I couldn't really find any practical options at all......Maybe check with a propane or CNG place? I know they phase out tanks, so they must have some way of deeming them safe or unsafe.
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    • #3
      Yellow pages or google boiler inspector they should also inspect LP air pressure vessels.

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      • #4
        http://www.ok.gov/odol/Safety_Standa...els/index.html

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        • #5
          Inspection of pressure vessels is a strange world to a lot of people. Each type of vessel has different requirements and the criteria vary, but for the most part inspection is done by either by an inspector provided by an insurance company, the jurisdiction, or the owner himself.

          Background:

          The majority of inspections and pressure tests are done by the owner. These are tests of tanks for compressed gas. There are specific rules and requirements, and there is usually periodic oversight and audits by the underwriter and a jurisdictional agency, but the owners themselves do the tests.



          Other types of pressure vessels have various requirements, but, in general, unless it is insured or there is a jurisdictional requirement (usually involving registration as well), the owner will do the inspections and tests here, as well. The procedures are the same in any case, so I will outline them.


          With an air tank, the most comon failure is pinholes rusting through along the bottom. Rarely catastrophic, but scrap the tank. Not worth fixing at this point. Failures that may be catastrophic come from cracks, usually near structural supports where stress it high. The most common catastrophic failure is overpressure from combustion of vapors in the tank (oil from a compressor and a source of heat, like a little hot shard of coked oil) Tanks must be kept clean.

          Outline of inspection and test procedures for an air tank:

          The first step is cleaning, inside and out. Remove all fittings and inspection plugs, flush thoroughly with mild detergent and water, and rinse completely. This is to remove any deposits and residue (oil in particular), as well as any loose scale. Dry the tank. DO NOT use compressed air unless you have filters, drier, and heater, or you will deposit moisture and oil residue. I usually provide heat with a couple 500W halogen lamps, and blow a hair drier into one of the inspection openings to add a little more heat and circulate the moisture out.


          Next is the visual inspection: Look at the entire outside surface. Look for dings, dents, cracks (especially near welds and attachments where stress may be high), pitting, etc. Small dings and dents are not necessarily a problem, but large ones may be. Cracks often show up near where supports attach, especially if the vessel had a compressor mounted on it (from vibration).

          Then check the inside. Use a mirror if need be, and us a flashlight. A bore scop is real handy if you have one available. You are looking for excessive scaling (which probably requires removal and recleaning to see what is underneath), deep pitting, lines of pits on the bottom, cracks, thin areas. A few pits arn't an issue. Horizontal tanks often get lines of pits along the bottom that can be an issue. vertical tanks often get a general thinning near the center of the bottom head with a lot of pits around the drain nozzle. If there are deep pits, measure the depth so you know how much metal is left. (this can take some creativity if you don't have access to an ultrasound thickness test unit. I have a ton of the 6" stainless rules cut, ground, welded into different configurations, and have been known to mount them on rods and wires to get to awkward spots when the UT probe won't get there.) A few pits, even deep ones, arn't an issue. Lots in a row can be, as they provide a line for a small failure to run.

          The original thickness should be on the data tag, and the minimum may be on the tag but probably isn't. A basic rule is at half thickness, reject, but some tanks are made with little corrosion allowance so the safety factor may be pretty low at that point. This is where calculations come in. If you have questions, the calculations arn't that hard, but you need the correct reference (ASME SECVIII is the starting point) A UT machine makes this a lot easier.


          If visual looks good- no cracks apparent, thickness looks good, etc- the next step is the pressure test. DO NOT PRESSURE TEST USING AIR OR ANOTHER GAS. Pressure test using water, and only water. This is very important for your safety. Water under pressure does not store much energy, so there isn;t much energy released in a failure. Air compresses a lot, and stores a lot of energy, so if a tank fails, it is quite unpleasant.

          The purpose of the test is to find leaks and to stress the tank more than it will see in service, so that any incipient failures will open up safely.

          To hydro test, you need two ports at a minimum, preferably three. You also need good support for the tank, and may need to be able to manipulate it when full of water. One port is for filling and pressurizing the tank, the other (which must be at the top) is for venting. I use a fitting with a pressure gauge and a ball valve on the vent nozzle. Plug all ports except the one for venting and the one for fill/pressurizing. One or the other MUST have a pressure gauge that reads to about 1.25 to 2 times the test pressure. A stop valve on the water inlet is needed as well.

          Fill the tank with water while the vent is open, allowing all air to escape. Don't trap any air. The water should be warm enough that moisture won't condense on the outside of the tank, but not so warm the you will be burnt or that water that leaks evaporated quickly.

          When the tank is full, shut off the water feed and close the vent. Initial pressurization can be done using water supply pressure. Pressurize above that point using a hand pump if possible. If using a powered pump, a SLOW, low volume pump is desirable, and a suitable relief valve is needed. With the hand pump, the relief valve is desirable, but not required. The safety valve for air use IS NOT suitable. Pressurization should be done in stages of about 5% of the MAWP (maximum allowable working pressure-it is on the data tag), starting from about 1/2 the MAWP. Final test pressure is often 1.5 times the MAWP (for a tank rated 120PSI, the test pressure is 180PSI in this case), but can be lower or higher.

          At each pressure increase, examine the outside carefully and thoroughly. Look for any bulging, stretching, or leaking.

          When you reach the final test pressure, shut the feed valve to seal the system and let it sit for a few minutes. Look for leaks, and watch for pressure drop. Pressure drop may be a leak past a valve or out a fitting, it may be a leak from a fitting, it may be a leak from the vessel itself (pinhole or crack) or it may be a stretching of the vessel. Leaks past the valves are ok, leaks from the vessel or stretching of the vessel is bad.


          Once you have completed the hydrotest, SLOWLY open the vent valve to relieve pressure. Relieve it slowly. When pressure is down to zero, open the vent wide and drain the tank (easiest if there is a plug at the bottom, but it can also be done by removing any plug or fitting and tilting the tank). You need the vent open so that air can get back in.

          Congratulations, You are done.


          Who to get to test it for you: some compressor shops will test for you. Otherwise, you are likely on your own. We go through this periodically in house with the inspector from the insurance company watching to recertify the tanks (National board comissioned inspector).

          see:
          http://www.nyc.gov/html/fdny/pdf/cof...y_material.pdf
          http://www.worksafe.vic.gov.au/wps/w...essor+explodes
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          • #6
            tank testing

            I know this is an old thread, but someone may benefit from this information. I used to have a friend who was the local fire chief. This is the way they tested small tanks, and one I have used several times. Fill the tank with water.( You will have to do this in a manner that lets air escape as you put the water in by making up a fitting for the fill port that has threaded openings for a grease zerk and a pressure gauge.) Snap a lever-type grease gun on the zerk and pump in enough grease to bring it up to to your desired test pressure. This method takes a little time and ingenuity, but is absolutely safe as water is incompressable. If you have any leaks or the tank won't take the pressure, all you will have is a little water on the floor. The main thing is to be sure you have absolutely all the air out, otherwise it will take a lot grease to compress any remaining air to the desired pressure.

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