Hydrostatic Requalification (also known as a hydrotest) is the most common way to check a cylinder for leaks or flaws. During a hydrostatic requalification test, the cylinder is examined to ensure it can safely hold its rated pressure. Cylinder hydrostatic requalification is crucial as such containers can rupture if they lose structural integrity when containing compressed gas.
A hydrostatic requalification consists of filling the cylinder with a nearly incompressible liquid, in most cases, this is water, pressurizing the cylinder, and examining it for leaks or permanent changes in shape. The test pressure is always considerably more than the operating pressure to give a margin for safety. Typically, either 5/3’s or 3/2’s of the cylinders rated pressure is used. Most compressed gas cylinders require periodic hydrostatic requalification as required by 49 CFR 180.205. The frequency of the testing depends upon the cylinder material.
• Steel cylinders should be tested every five years and have an indefinite service life until they fail a hydrotest and visual inspection
• Aluminum cylinders (not including hoop-wrapped composite cylinders i.e. FRP-2) should be tested every five years and have an indefinite service life until they fail a hydrotest and visual inspection
• Hoop-wrapped cylinders should be tested every three years and have a 15-year service life.
• Fully wrapped fiberglass, Kevlar, and Amarid cylinders should be tested either every three years or every five years and have a 15-year service life. Refer to the Special Permit/Exemption for required requalification intervals.
• Fully wrapped carbon fiber cylinders should be tested every five years and have a 15, 20 or 30-year service life. Refer to the Special Permit/Exemption for required requalification intervals and service life
Cylinders should not be filled if they have exceeded their valid service life or re-test dates. Cylinders which show evidence of exposure to high heat or flames (paint turned to a brown or black color, decals missing or gauge lens melted) need to be removed from service and requalified prior to filling. If there is any doubt about the suitability of the cylinder for filling, it should be returned to a certified hydrostatic test facility for examination and retesting. Any evidence of a crack, defect or damage requires the cylinder to be removed from service.
Visual inspections should also be performed on a regular basis as recommended by the Department of Transportation (DOT), the Compressed Gas Association (CGA) and the cylinder manufacturers. The visual inspection should include, but is not limited to, removing the cylinder valve, inserting a high-intensity light probe and angled mirror into the cylinder and examining the inner surfaces of the cylinder. This inspection is necessary to aide in identifying defects in the inner surfaces of the neck, threads, shoulder area, sidewalls, and base of the cylinder. An excellent Inspection training class is taught by Professional Scuba Inspectors-Professional Cylinder Inspectors INC (PSI-PCI).
In addition to the mandatory cylinder hydrostatic requalification and visual inspections, it is also suggested to submit cylinders for non-destructive testing in between the required hydrostatic testing. These types of tests are usually ultrasonic tests or eddy current tests. For those cylinders manufactured from 6351-T6 Aluminum Alloy, a eddy current test is required at time of hydrostatic requalification.
The DOT requires that hydrostatic retesting and re-qualification be conducted by registered agents who have been certified by the DOT and who have been issued a valid Re-testers Identification Number (RIN). The recommended visual inspections outside of those conducted during the hydrotest, do not have to be conducted by a DOT-certified RIN holder. However, the visual inspection should be performed by an individual who has the proper training and is competent in performing visual inspections.