They say nothing last forever in this world. Well, that passage is applicable to most objects we find, including the thick and strong material we have in scuba tanks and BA cylinders. Speaking of material, scuba tanks are commonly made of aluminum steel while BA cylinders can either be made up of a fully-wrapped fiberglass, Kevlar or carbon fiber. When compared to other materials, we all know that these materials have superior strength and longer service life. But it will become a different scenario when they are used to hold and contain high pressure breathing air.
At rest, these materials are in a normal state. However, as they are filled with high pressure air, these materials expand to a certain degree. It will expand microscopically that their expansion cannot be seen by our naked eyes. Material expansion alone posses no problem. But if you frequently use and fill your cylinders, it makes the material expands (during filling) and goes back to its natural state (when depressurized) causing metal stress. If you can’t visualize this, just imagine a piece of wire being bent back and forward several times until it will break off. And we do not want a material break off in our high pressure vessel (also called as a failed cylinder) as the consequences are often fatal.
Read a passage from a related article to know the degree of damage a failed cylinder can do.
Why do we need to test our cylinders?
Subjecting our cylinders to a hydrotest is a safety test and precaution against unwarranted accidents and it is the only logical test available. Some may think: why not use a hammer, hit different sections of a cylinder and assess the material condition based on the sound it produces. This is big No No. Otherwise, you will further contribute to metal stress and fatigue.
So what is a hydrotest?
Hydrotest is a safety test for cylinders used to contain high pressure air. Instead of air, the cylinder is filled with an incompressible liquid like water then pressurized exceeding its normal service pressure. Usually during a hydrotest, cylinders will be over-pressurize up to 160% or 5/3 exceeding its normal service pressure. In this case, scuba cylinders with a service pressure of 3,000 psi (206 bars) will be pressurized up to 4,800 psi (330 bars) and BA cylinders with a service pressure at 4,800 (330 bars) will be pressurized up to 7,680 psi (530 bars).
Once the cylinders reached the controlled pressure over-rating, it will be depressurized until it will be completely drained out of pressure. After which, it will check for any metal deformities, cracks and leaks.
ALL CYLINDERS THAT FAIL THE HYDROTEST (MEANING IT HAD SHOWN SIGNS OF CRACKS, LEAKS OR DEFORMITIES) SHALL BE CONDEMNED IMMEDIATELY AND TAKEN OUT OF SERVICE. OTHERWISE, USING SUCH FAILED CYLINDER IS LIKE CARRYING A BOMB WAITING TO EXPLODE . . . ANYTIME ANYWHERE
The Process of Hydrotesting
After filling-up the necessary documents, your cylinder will initially be inspected for early signs of metal stress, fatigue, deformities and if found to have one, will immediately condemned and taken out of service.
If no physical manifestation for metal stress, fatigue and deformity, your cylinder will be prepared and installed in a pressure port , which afterwards, will be immersed in a pressurized water jacket. But this is not just any ordinary water jacket that you can customize one or even make a concrete version of it. Please don’t customize your own water jacket as this is a highly specialize water vessel that can withstand ultra high pressure.
Once inside the water jacket, your cylinder will then be brought to an over-pressure rating equivalent to 160% of its service pressure using water. Once it reaches its maximum pressure, it will be depressurized, totally drained of remaining pressure and then taken out of the water jacket. The servicing technician will then examine for any metal stress, leaks and deformities.
As we have been saying, all cylinders that failed the hydrotest will immediately be condemned. For cylinders that pass the test, markings indicating month and year of the hydrotest will be engraved on the cylinder, particularly on the tank neck for purposes of visibility and reference for the next conduct of hydrotesting. Unlike the cylinder visual inspection that requires to be conducted once a year, a hydrotest is recommended to be done once every five years.
The bottom line here is that doing a hydrotest will eliminate the possibility of an exploding scuba or BA cylinder that can happen anytime and anywhere. The cost of having your cylinders hydrotested is just equivalent to a couple of beers. With this and on a final note, we will give the ball to you and decide which path you choose: (1) Disregard this caution and go to a bar of your choice, or (2) skip at least one binge drinking weekend and use the saved funds for a hydrotest where you are guaranteed to be safe near your cylinders anytime anywhere, regardless if you’re holding an iced-cold beer or not.
Video courtesy fromLakeHickoryScuba