Here at J & R Welding Supply we use the "Water Jacket Test Method" for Hydro Testing of tanks.  This method is applicable to all hydrostatic tests where determination of volumetric expansions are required.  It consists of enclosing a cylinder filled with water inside a test jacket filled with water.  Pressure is then applied internally to the cyclinder, causing the cylinder to expand.  The total and permanent volumetric expansion of the cylinder is determined by measuring the amount of water displaced by the expansion of the cyclinder when under pressure and after the pressure has been released.  This is the only testing method that will requalify a cyclinder for charging to 10% in excess of the marked service pressure.
Eddy Current Testing: Hydrostatic testing alone does not reveal cracking in the neck/shoulder area of aluminum cylinders unless they are leaking. Visual Plus 3 identifies 100% of these defective cylinders.  Visual Plus 3 is the only eddy current inspection device both used and recommended by Luxfer Gas Cylinders - ensuring the same results upon cylinders return to the manufacturer.
Visual Plus 3 is a non-destructive testing (NDT) device which utilizes eddy currents to detect neck and shoulder cracks, folds or other similar imperfections in aluminum cylinders such as scuba, SCBA (all metal, hoop-wrap or full wrap), beverage, medical, industrial gas cylinders, fire extinguishers, etc.
An electromagnetic wave is introduced into the metal of the vessels near the area which is desirable to inspect for imperfections (the threaded area).  The wave causes currents to flow in a circular manner in the metal.  They are called eddy currents because the flow is circular.  An imperfection can increase the path that the currents have to flow and so make the material appear higher in resistance in that region where the imperfection is present.  This generates a spike on the computer generated report which alerts the technician to a problem that requires further investigation.
A visual inspection using the AIT OPTICALPLUS 4X magnification device on the neck threads is the last step in the process.
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When a cylinder comes into the shop for Hydrostatic Testing the first step is to vent the cylinder.  After the cylinder has been vented the valve body is removed at the "De-valving Station".  The cylinder is then filled with water and allowed to stand for 24 hours.
The next step is to attach a test valve body and lower the cylinder part way into the water jacket. A high pressure hose is attached to the test valve body then the cylinder is lowered the rest of the way into the water jacket.  The water jacket is then clamped and sealed shut.  Our water jacket has the ability to hold three cylinders at one time, thereby speeding up the testing procedure.
All of the testing is preformed here and logged into our records.  On most tanks the pressure is raised to 5/3 rds of the rated tank pressure.  If the water displacement is 10% or greater the tank is rejected.  If the cylinder passes the new date is stamped onto the top near the neck.  Depending on the tank most dates are good for 10 years.
After the tank has passed the "Hydro Testing" the water is emptied and the cylinder is brought to the "Drying Station".  This station is capable of drying 6 cylinders.  The tubes you see hanging are disconnected and inserted into the cylinders and reattached.  Forced hot air is then blown into the tank to dry them.
After drying the cylinder is loaded into the steel shot blaster.  This machine rotates the cylinder while its being blasted and is set on a automatic timer.  Aluminum tanks are not blasted.  After blasting the valve body plug is removed and the valve body is inserted.  The tanks are then repainted and returned to the customer.