Testing Interval For Air Storage Banks

How often must large cylinders used as air storage banks at dive businesses and fire departments be inspected and re-qualified is a question asked of the PSI technical team at least weekly.  Some people believe no inspection or hydro test is required.  Many people believe that storage cylinders are to be hydro tested every 10 years while still others believe the test period is 5 years.  A few people believe a wide variety of other choices.  To accurately answer the question, we need to know more about the cylinders and whether they are used in commerce.

In the US, the Department of Transportation (DOT) administers specific federal regulations pertaining to DOT designated cylinders.  Cylinders made before 1970 for transporting gases were administered by the Interstate Commerce Commission (ICC).  ICC cylinders are subject to all DOT regulations.  Cylinders intended for permanent installations are made under specifications established by the American Society of Mechanical Engineers (ASME).

ASME cylinders are designed for permanent and semi-permanent installations or where removal may be difficult once installed.  They are not subject to DOT regulations but that does not mean ASME cylinders don’t require inspection.  ASME cylinders are much stronger than DOT cylinders, have moisture drainage ports and may be mounted horizontally or vertically.  While these are the proper cylinders for use by most air stations, their higher cost results in few air stations being so equipped.

The vast majority of dive and paintball businesses, fire departments and others who maintain compressed air storage systems use DOT 3AA cylinders that are about 54 inches high and 9 inches in diameter.  Since the air or other dive related gases DOT cylinders store may be sold or transported on federal highways, all DOT regulations apply.  These same cylinders placed on vehicles or trailers must also comply.

The Ten Year Rule

The ten-year hydro cycle is referred to as STAR service and is listed in the US Code of Federal Regulations Title 49 CFR 173.34(e)(15).  That rule states: A cylinder made in compliance with specification DOT 3A or DOT 3AA not exceeding 125 pounds water capacity and removed from any cluster, bank, group, rack or vehicle each time it is filled may be retested every 10 years instead of every 5 years, provided the cylinder complies with ALL of the following-


Was manufactured after December 31,1945


Used exclusively for air or certain other gases (argon, helium, nitrogen, oxygen, or permitted mixtures)


Passes the CGA C-6 hammer test prior to each refill


Is stamped with a five pointed star following the test date


Is dried inside immediately following hydrostatic testing


The cylinder is not used for underwater breathing.

The ten-year hydrostatic interval does not apply to storage cylinders used in most air banks.  The reason is that the cylinders are not removed from the cluster each time they are filled and do not receive a hammer test.  When the compressor is connected directly to the air bank, the cylinders are refilled frequently, often several times each day and of course they never receive a hammer test.  This rule was confirmed by Hattie L. Mitchell, Chief, Exemptions and regulations Termination, DOT in a May 15, 1995 letter.  Therefore fill station operators are left with the five-year hydrostatic cycle for their air banks.

Visual Inspection

Federal DOT regulations specify that a cylinder visual inspection must be conducted at the time of each hydrostatic retest.  Experience tells us that over 80% of the cylinders condemned by retesters failed the visual inspection rather than the pressure test.  Is an internal inspection once in 5 years adequate?  That all depends on whether moisture gets into the cylinder because of compressor filter or moisture separator problems.  Also, how old are the cylinders, how have they been treated over their life and have they been setting on a concrete or moist surface and exposed to weather?  At the very minimum, the first cylinder in line, the one first and most often receiving refill, should be internally inspected at least annually.

Most of the relevant federal Title 49 DOT regulations have been consolidated into the quick reference booklet titled: Scuba Cylinders and Title 49 of the Code of Federal Regulations by Fred Calhoun. This informative booklet is available from PSI, Inc. for $15.00 plus shipping.  Order by calling 425.398.4300 or email This e-mail address is being protected from spambots. You need JavaScript enabled to view it. .

Comments on this article may be addressed to the author at the above email address.

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