PSI/PCI Cylinders

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Technical Visual Inspections Enhance Cylinder Safety

Author's note: Although this article is based upon Canadian and US laws and standards, the general content may provide useful information about cylinder safety.

Visual cylinder inspection (VCI) does not mean the same thing to all who claim to conduct VCI nor to those who write laws or guidelines about VCI. For example, the U.S. Occupational Safety and Health Administration (OSHA) requires all respiratory equipment including SCUBA and SCBA (S/S) used by employees to be visually inspected at least every month and after each use.
Also, OSHA requires that all pressurized cylinders employees may be exposed to shall be safe as determined by visual inspection.  The U.S. Department of Transportation (DOT) requires hydrostatic retesters to conduct a visual inspection as part of the retest process.  The Compressed Gas Association (CGA) publishes cylinder related safety guidelines that are, in some cases, incorporated into law by reference.  Even one cylinder manufacturer (Luxfer Gas Cylinders) and the scuba instructor associations NAUI, PADI, NASDS and SSI have, at one time or another, prepared information about VCI.  Although each of those associations stress the importance of a VCI, none tell the technician clearly just what it is or how to accomplish it.

The PSI, Inc. visual cylinder inspector textbook, INSPECTING CYLINDERS is the best single volume source for technical VCI information about steel and aluminum S/S and composite SCBA cylinders. It was first published in 1987
and is now in its greatly expanded third (1997) edition.  PSI, Inc. publishes three additional cylinder inspector resource booklets, the REPRINT FILE for scuba inspectors, HAZMAT TRAINING INSTRUCTOR GUIDE and FILL STATION OPERATOR'S GUIDE.

Luxfer Gas Cylinders published an excellent guide in 1996 that clearly describes how to conduct a technical VCI on Luxfer aluminum scuba cylinders. Much of that information and procedures came from the program developed by
PSI, Inc. over the past 17 years.  A revised edition of the Luxfer SCUBA Guide will be released late in 1999.  Luxfer also published a guide for inspecting SCBA cylinders.  The technical SCBA inspecting guide is printed in two volumes.  Volume I deals with Luxfer aluminum and composite cylinders made before 1988 and volume II covers newer solid wall and several composite materials.

The INSPECTING CYLINDERS book and Luxfer's aluminum cylinder inspection guide contain the details necessary for a thorough visual inspection.  A technical visual cylinder inspection is a complete assessment inside and out against standards and damage limits.  Cylinders that meet those standards are returned to service. Many cylinders that do not meet the standards can be serviced, usually cleaned or hydro retested, and returned to service. Damage or other conditions that fail to meet the allowable limits cause the cylinder to be condemned.  Condemned S/S cylinders should not be reconditioned.

INSPECTING PROCEDURES

Technical VCI begins with a careful review of the code marks stamped into the cylinder shoulder or appearing on a composite SCBA side wall label.  In the U.S., the code must include DOT or ICC markings and in Canada, CTC or
TC.  The serial number MUST be legible.  There must be an original hydro tester's mark and a current mark, within 5 years for steel or aluminum cylinders and 3 years for composite SCBA according to North American regulations.  Composite SCBA must be removed from service at 15 years.  All other markings should be valid and compatible.  Specific information about correct and fraudulent marks is presented at PSI, Inc. workshops.  The
cylinder type must be correct for the intended service.  For example, composite SCBA cylinders MAY NOT be used for scuba or other underwater purposes.

The cylinder exterior surface should be free of bands, boots, stickers or other objects that might obscure damage.  Cuts, dings, gouges, dents, corrosion pitting, and bulges must be compared to specific damage limits for each cylinder type.  The inspector must be able to differentiate between a banana (bowed) cylinder, which is acceptable, and a bulged aluminum cylinder which is not.

The entire interior should be brightly illuminated to facilitate a critical examination of any contents or damage.  Corrosion indicates moisture has been allowed into the cylinder, usually during fills in water baths, or from
poorly maintained compressor moisture separators.  The technical inspector must estimate depth of any corrosion pitting and compare findings to the allowable limit for the cylinder type.  The PSI, Inc. developed Master Pit
Reference Plate along with pit limit tables provided to PSI, Inc. workshop participants simplifies this assessment.

Scuba and SCBA cylinder threads should be carefully checked for condition and minimum number.  The minimum number of threads allowed varies with the type of cylinder and may range from 6 to 12 complete and continuous threads measured from the top.  Thread chasers should not be routinely used to reshape cylinder threads.  Certain 3AL and composite cylinders with aluminum liners warrant a special examination just below and within the threads.  A
small magnifying mirror and bright, directed light will aid in detecting occasional cracks that may propagate from crown folds.  Training and experience are essential to locate and differentiate cracks from tool stops or other marks.  The recently developed Visual Plus eddy current apparatus is a valuable tool to augment the standard mirror and light inspection by locating sometimes difficult to see cracks.

Valve Inspection

Although valve repair is not a part of the visual inspector's duty, several items should be inspected. In North America, all valves used by divers and fire fighters MUST have a pressure relief device, commonly called a burst disc.  Both the leaded plug type and single port retainer nut type should be replaced with more effective designs.  Burst disc relief pressure must not exceed the hydrostatic test pressure.  Improper installation of the burst disc components may cause the device to release at much higher than the rated pressures.  Valves affixed with high disc pressure ratings must not be placed in lower pressure cylinders.  Several valve manufacturers recommend this safety device be replaced at specific intervals.  Many valves used in Europe have no relief device and have metric threads.  Some metric threaded
scuba valves have appeared in U.S. made cylinders and are both dangerous and illegal.  One death and one serious injury have resulted from such miss-matched components.

The cylinder to valve O-ring replacement policy varies among equipment manufacturers but most technicians automatically replace the O-ring and lightly coat the threads with a lubricant having dielectric qualities and no
off-gassing solvents.  Two dive equipment companies, U.S. Divers Co and Scubapro and SCBA maker, Survivair at one time placed plastic dip tubes on their valves.  None proved satisfactory and those plastic dip tubes have been recalled.  Many plastic dip tubes remain on both S/S valves even after more than 15 years since the first recall. Poorly trained and careless visual inspectors have failed to replace these recalled items.

Technical, high quality visual cylinder inspections are not difficult to perform.  However, without proper technical training, an inspector has little chance of knowing or accessing the specific information necessary to properly judge cylinder condition.  The rules, industry standards and other guidelines simply are too scattered for most individuals to locate.  PSI, Inc. has a single purpose and that is to search out high pressure cylinder information important to visual inspectors, fill station operators and cylinder handlers and make it readily available in our one day, hands -on
workshops.  Over 200 workshops are conducted annually in North America and at some international locations.

For more information about PSI and cylinder safety, visit our web site at www.psicylinders.com
e-mail at  This e-mail address is being protected from spambots. You need JavaScript enabled to view it. or call us in the USA at 425.398.4300.  We train fire fighters, fill station operators, equipment repair technicians, hydrostatic retesters, scuba instructors dive store staff and many more.