American Paintball, SCBA and SCUBA Hydrotesting

HYDROSTATIC TESTING

Test stand, test vessel, vessel cap and pneumatic clamps.

Tank attached to test vessel cap, with quick release and tank attachment.

Yodaworks, Inc. is the proud owner and operator of a Cylinder Requalification facility.  Certified by the Department of  Transportation (DOT), Sound Dive Center/Hydro Test NW performs "Hydrostatic testing" on PAINTBALL, SCUBA, SCBA (fire) and other "small" bottles that require cylinder requalifications (a hydrotest).  To get your paintball tank tested go to this site: www.hydrotester.com

The United States Department of Transportation (DOT) licenses companies to test cylinders in order to insure that they are safe to use by divers, firefighters, homeowners, etc.

This is the beginning of testing a cylinder for structural integrity. The red tank is a 90 cubic-foot, aluminum scuba cylinder .  It has a working pressure of 3300 psi, the test pressure is 5500 psi.

After insuring the tank is empty, we remove the tank valve.  The tank is then given a thorough visual inspection (VIP). The tank's interior is checked for corrosion, particulate, and any other abnormalities. The threads are checked for integrity and imperfections. On aluminum tanks, a special electronic device is used to check the neck threads for cracks.  See the pictures on the right.  We use Visual Plus to test for stress cracking.

      
If the tank passes the VIP, it is filled with water for the hydrostatic test. If a tank were to rupture the results would be minor because it is filled with water instead of gas!  Remember, water does not compress, unlike air.  So if the tank were to rupture, even at 5000psi, only a couple of gallons of water would escape and be far less destructive than 90 cubic feet of air.  Water has little kenetic energy.

A special valve is inserted in the top of  the cylinder that has been filled with water. The valve is sealed with Teflon tape or an O-ring. In this case, since this is an aluminum cylinder with an O-ring gland at the top of the threads, the valve you see has an O-ring just at the top of the threads.

It is tightened so the seating is firm. This should insure that the water does not leak out of the neck of the tank when it is subjected to the testing pressure. Valves requiring Teflon tape for a seal are tightened with a  wrench to insure there is no leak when pressure is applied to the tank.

The quick-release disconnect at the top of the valve will be mated to the top of the hydrostatic test tank. This is where the water will enter to insure the tank is full and to apply the pressure to the tank for the test.

The entire test chamber is kept full of  water as well as the tank to be tested. As the tank is lowered into the chamber the water rise and possibly spill over into the waste well that surrounds the chamber.

The tank is being lowered into the test chamber. Notice the O-ring used to make the seal between the lid (holding the tank) and the rest of the chamber. As the tank is lowered, the water will rise and possibly spill over. If not, there is a pipe running into the vessel to insure it is filled completely eliminating the possibility of compressing gas instead of fluid.

You are looking at the tank attached to the test vessel top. The tank is suspended from this and is lowered into the  water. So, the tank is completely filled with water and the chamber outside the tank is filled as well. The top is attached to the vessel lip with the pneumatic clamps. There is an O-ring between the them to insure that water cannot leak out between the two.

Any air is purged from the test vessel.  There is a hose to the back left. It supplies water to the interior of the cylinder.   It is also the source of the high pressure that will be applied to the tank.

The scuba tank is enclosed in this vessel. Both are filled with water and pressure is about to be applied to the scuba cylinder.  The air that might be in the SCUBA tank is being purged.


The burettes to the left are for measuring the  expansion of the scuba cylinder when pressure is applied to it. The cylinder is going to be pressurized to 5/3 of the DOT pressure stamped on the neck of the tank. If the stamp read 2250 psi, then the tank would be pressurized to 3750 psi. Likewise, for a standard 3000 psi scuba tank, it would be pressurized to 5000 psi. The cylinder being tested is a 3300 psi tank, so it will be pressurized to 5500 psi!   
       

As this test pressure is applied the tank will expand. Remembering the tank is in a chamber of water, the expanding tank will push against the water surrounding it. That water will be forced through a small hose and up into one of the burettes. The burette that is chosen for each cylinder would be the one that would have the water go near the top when the final pressure is attained. But, only 1 burette is used at a time. In this case, the scuba cylinder is large and pushes a lot of water from the chamber so the 3rd burette from the right will be used.

The water pressure is applied to the interior of the water-filled, scuba tank. The pressure is at 5500 psi on this super-calibrated gauge. It took about 30 seconds to go from atmospheric pressure to 5500 psi! The tank has expanded and being held at this pressure for at least 30 seconds(we use 40 seconds for good measure). The amount of expansion is read on the burette.

The second burette from the right now reads 65ml. The water has been pushed up the burette by the tank expansion due to the pressure on it of 5000 psi. This must be a stable reading. In other words, the water level in the burette cannot change for at least 30 seconds. This could be due to a continual expansion of the metal of the tank which would be reasonable cause to fail the cylinder.

After the burette level is recorded. The pressure is released from the cylinder.  The cylinder will not return to the size it was before the pressure was applied because it was permanently stretched. The burette to the left reads 2.4 ml. That 2.4 ml represents the amount of permanent stretch. If the tank stretched and did not return close to its original size, it would mean the metal of the tank was not resilient enough to be safely used to contain high pressure. This SCUBA tank must return to at least 10% of the upper reading.   SCBA bottles must return to 5% usually.

This scuba tank has passed the hydrostatic test. It is removed from the chamber. The water is drained from the interior and it is put on a drying stand to remove all moisture. A final VIP is performed to insure no damage was done by the high pressure the  tank was subjected to. The hydro test date and federal hydro station number are stamped on the crown of the tank.


Hydrostatic testing should be done on scuba cylinders every 5 years unless something has happened to the tank in the meantime that would indicate a need to check the tank sooner. Since the hydro test stretches the metal of the tank beyond what a normal fill would do, it is not recommended that the test be done more often. In any case, most scuba tanks are guaranteed for 100,000 fills if they have been visually inspected once per year and hydrostatically tested once every five years.  Paintball and SCBA bottles are tested in a different manor and have a totally different marking procedure.

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Visual inspection tools include a bright light, reflective tools and good eyes.

"Visual Plus" used for eddy current/non distructive inspecting