Hydrostatic Cylinder Testing 101

Hydrostatic Cylinder Testing 101by Bob SheridanAlthough all divers know that they need to have their cylinders visuallyinspected once a year, many do not understand how or why their cylindersare required to be pressure tested every five years, used or not.The simple explanation, for testing, is safety for the public, the cylinderfilling technician and the transportation of potentially explosive high pressurevessels over our highways.Although cylinders are made of non-shatterable metal, they still can tearapart with explosive force, fire off the valve, spin or take off at high speedsthrough walls, causing severe bodily harm, death and destruction.The hydrostatic test actually measures the elastic expansion of the metaland its ability to return to its original shape within 10% of the originalvolume. The water test is safe since it cannot compress or expand like air.If the cylinder leaks or blows out, it leaks a stream of water and the worstcase scenario is a water jacket's copper gasket breaks allowing water tospray everywhere.An overview of hydrostatic testing:• Document the cylinder.• Calibrate the hydro testing machine.• A visual inspection and/or Eddy Current neck thread test.• Fill the cylinder with water.• Install a spud connection in the cylinder and then attach it to the testtank lid.• Lift tank into the test jacket.• Clamp down the lid.• Add water to the test jacket to level it off.• Check for equalization of water temperature in the tank and waterjacket. Set burette water scale or digital scale to zero.• Start the pump to increase the pressure in the tank to 90% of the testpressure. Wait 30 seconds and check for leaks.• Run pump to minimum test pressure and hold the pressure for 30seconds. The expanded tank displaces water in the jacket and ismeasured by burette tubes or digital scale which indicates the (TE)Total Expansion value.• Open the bleed valve on the tank and measure the (PE) PermanentExpansion value by checking the burette or the electronic device forthe % of contraction when the pressure returns to zero (within 10 %).• Record the value of (EE) Elastic Expansion (TE-PE=EE).

• Determine PASS or FAIL (condemn) and record Disposition on datasheet.• Unclamp the lid and move the cylinder over for draining.• Place cylinder on an air drying system.• Place cylinder in vice for stamping test dates and RIN number (testingcenters Federal DOT Requalification Identification Number) for Passedtanks. Failed tanks have Xs stamped over the DOT specifications, orthe word CONDEMNED stamped above the DOT specifications.• Install valve with new neck o-ring and lubricant.• Fill out paper work on the test for the DOT.• Put boot on if necessary.• Fill cylinder with gas of choice if requested.• Make sure the cylinder inspection (VIP) sticker is still on or needs anew one.• Store for customer pickup.David Kimball, UDT instructor #127 and manager of Palm Beach Hydro, Inc.in Lantana, said he likes to visually inspect and Eddy Current test each6351-T6 cylinder before and after the hydrostatic test, since cracks can befound in the neck area that were not there before testing.In order to protect the public and reduce the risk of high pressure cylinderfailures, the US Department of Transportation (DOT) has Federal regulations(CFR section 49) for manufacturing, testing and transporting high pressurevessels. These CFR regulations are strictly followed by hydro testingfacilities under the penalty of severe fines and imprisonment.Since 1913, the Compressed Gas Association (CGA) has been developingand promoting safety standards and practices in the industrial and medicalgas industry including guidelines for the inspections of high pressurecylinders.Individuals tampering with cylinders by faking tests or stamping false codescan get up to five years in prison and/or a $25,000 fine. This is a Federaloffense and represents a hazmat danger to the public.Hydro testing facilities failing to keep complete and accurate records ofcylinder reinspections and retesting, follow testing protocol, stamp cylindersproperly, not having accurate equipment or having uncertified personnelconducting tests, can be fined from $500 to $11,000 for each infraction.Foreign cylinders cannot be filled in the US unless they have an original DOTstamp on them. Foreign cylinders can be hydro tested for private ownersbut cannot have a DOT stamp placed on them and must be transporteddirectly to the export company for shipping (CFR 49 10.209.k1).

Catastrophic cylinder failures have destroyed dive shops, cars, trucks, sunkboats and have caused airline crashes. Unfortunately a diver in Japan had ascuba tank explode on his back, just prior to entering the water, promptingstricter and more frequent cylinder inspections and testing. While filling andservicing scuba cylinders for Argonne National Labs in Chicago during thelate 1970s, they required us to visually inspect their tanks twice a year andhave them hydro-tested every two years.The CGA recommends that the frequency of filling, use and abuse ofcylinders in various environments may require more frequent inspections.Divers using cylinders daily should inspect them monthly and immediately ifthey run dry, near dry or neck o-rings leak. High oxygen content cylindersshould be inspected more frequently, especially if they are high pressuresteel cylinders. Internal damage to high O 2 cylinders, with moisture orsaltwater, can cause failure within 90 days.Rick Nickels, manager of Hydrostat, in Deerfield Beach, FL., states that,"Most steel or aluminum scuba cylinder test failures are caused by externalcorrosion under the tank boot and internally when breathing a tank empty orleaving the valve open." Hydrostat tests over 4,500 diving cylinders a yearand only about one out of 500 fail the hydro, while about 50 fail visualinspections.Most diving cylinders vary in working pressure from 2400 psi, 3000 psi to3500 psi. The hydrostatic testing pressure can be five-thirds or three-halfsof the working pressure depending on the cylinder. This can vary from4000, 5000 and 5250 psi. The surprising fact to most divers is the actualpressure test lasts a minimum of 30 seconds to one minute.Over filling cylinders, based on the hydrostatic test or burst disk pressurerating, is totally unsafe. It causes undue stress to the molecular structureand elastic quality of the cylinder and can cause first stage leaks, rapid seatfailure in regulators and high pressure hose leaks.Note: The working pressure of your tank is stamped in the neck so do notexceed it. Several years ago I suggested, to OMS, that they should promotenever to overfill their cylinders to technical divers. OMS went one stepfurther and had Faber stamp their cylinders with "Do Not Over Pressurize"for safety. If you need more gas volume buy a larger tank, get in bettershape and reduce your breathing rate.Hydrostatic testing is very serious business. This process maybe more thanyou wanted to know or think about, but this is what it takes to meet theFederal safety guidelines to recertify your diving cylinder for safe filling andhandling.

For more information contact:David Kimball www.palmbeachhydro.com 561-586-2887Rick Nickels www.hydrostat.com or rnickels@hydrostat.com 954-428-7677Bob Sheridan, PSI and UDT-TCI cylinder inspector instructor 954-548-4457www.andoniancryogenics.comwww.hydro-test.comwww.cga.comwww.c-f-c.com/gaslink/docs/dot_cylinders.htm