Since I can't seem to figure out how to attach a PDF file, I'm going to post an OCR version of the entire PDF file I got, the DOT's one page reply to a 5-page letter of enquiry on "plus" hydro's. I have another formal enquiry into them which might be answered in June. Comments in square brackets [ ] have been added for clarity. Bear in mind, this is OCR, there are some artifacts in there as well. Broken in two to meet post limits.
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[letterhead] [formal response]
U.S. Department of Transportation
Research and Special Programs Administration
[to]
Mr. John L. Hoffer
5104 - 97th Street, S.W.
Mukilteo, WA 98275
Dear Mr. Hoffer:
JUN 20 2003
400 Seventh St.. S.W.
Washington. D.C. 20590
Reference No. 03-0011
This is in reference to your inquiry concerning the requirements for marking a plus sign (+) on DOT 3A and 3AA cylinders as specified in §173.302a(b) of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180). I apologize for the delay in responding.
A DOT cylinder 3A or 3AA may be marked with a plus sign (+) and filled with a listed gas to 10 percent above its marked service pressure if all requirements for testing and evaluation contained in § 173.3 02a(b) are met. The plus sign is required to be marked on any cylinder, including a new cylinder, when charged to 110 percent. Only a plus sign after the last (most recent) test or retest date applies. That is, as stated in your Interpretation A, the cylinder must have the plus sign stamped after each requalification retest if it is be filled to 110 percent. The requalification retest must be performed using the [usual] water jacket method.
With regard to the differences in wall stress limits, the wall stress limits prescribed in § 178.37(£)(2) for the DOT 3AA cylinder must be met by the cylinder manufacturer at the time of construction. The wall stress limits prescribed in § 173.302a(b)(3) must be met at the time of each requalification retest. The HMR contain no limitations on the number of times a cylinder that meets the requirements in § 173.302a(b) may be marked with a plus sign.
I hope you find this information responsive and helpful. If you have further questions, please do not hesitate to contact this office.
Sincerely,
~<~JY!~
Hattie L. Mitchell, [former]Chief
Regulatory Review and Reinvention
Office of Hazardous Materials Standards
1111111111111111111
5104 - 97th Street S.W.
Mukilteo, WA 98275
[original letter of enquiry]
January 7, 2002
Mr. Edward T. MazzulIo, Director
Office of Hazardous Materials Standards
u.s. DOTIRSPA (DHM-lO)
400 Seventh Street S.W.
Washington, D.C. 20590-0001
Re: SCUBA tant<: pressure ratings
Dear Mr. MazzulIo:
I am a SCUBA diver with questions about the use of SCUBA diving tanks. I posed these questions on behalf of myself. However, I intend to widely disseminate your answers, as I believe the dive industry suffers from much misinformation on this topic. Toward that end, your response may be posted on the world-wide-web and reproduced or quoted in diving magazines or other periodicals.
1. Background.
Steel cylinders for use in SCUBA diving frequently have a "+" symbol stamped on the cylinder adjacent to the service pressure. For example, many cylinders are stamped with "2400 +" which in the dive industry are referred to as "low pressure steel tanks" to distinguish them from steel tanks that have higher service pressures such as 3000 or 3500 pounds per square inch (psi). -
These cylinders are subject to hydrostatic testing at least every five years. After the initial hydrostatic test, these cylinders are typically not stamped with the "+" symbol when they pass hydrostatic tests.
The "+" stamp permits these cylinders to be "overfilled" with compressed gas by 10% above the service pressure. 49 C.P.R. §173.302a(b).
It is "common knowledge" in the dive industry that tanks that do not have the "+" stamp may not be overfilled by 10%.
It is also "common knowledge" in the dive industry that tanks may not be stamped with the "+" symbol after the initial five-year period after purchase. According to this "common knowledge," a cylinder stamped "2400 +" may be filled to 2640 psi only during its first five years. Thereafter, when the cylinder passes another hydrostatic test it may be filled only to 24,00 psi.
On account of this "common knowledge," in the example above most dive shops are not willing to fill a six-year old tank to 2640 psi. (There are some exceptions to this practice, primarily in the state of Florida where cave diving is practiced. We need not concern ourselves with those practices for the purposes of this inquiry.)
There is a minority view which holds that a SCUBA tank may receive a "+" stamp after passing hydrostatic testing, even if the test is not the initial test. Because dive shops will not permit any subsequent "+" rating after the initial five years, divers must take their tanks to some place other than a dive shop to have their tanks hydrostatic tested and restamped with the "+" symbol.
There is a second minority view, a much smaller minority view, which holds that once a SCUBA tank has been stamped with the "+" symbol, it may be overfilled by 10% so long as it continues to pass the same visual inspection and hydrostatic testing required of all SCUBA tanks. No special testing or stamping is required.
I have called US Department of Transportation (DOT) Info Center Hotline 1-800-467- 4922 and was informed that DOT's position is essentially that of the second minority view. Overfills are permitted without regard to subsequent "+" stamping, so long as hydrostatic test is current. I have read on the Internet of others who have received advice from DOT consistent with this interpretation.
2. First Question.
First, assume facts that give your agency jurisdiction to regulate the filling of the cylinder. That is to say, interstate transportation by a common carrier is involved. Dive shops do not know how tanks will be used. As a practical matter they must assume that your agency has jurisdiction to regulate every fill, even if that conclusion is valid only intermittently.
Second, assume that the cylinder is stamped "2400+" because that is the most common tank that presents the issue.
Third, assume that the cylinder has passed visual inspection and hydrostatic testing at the time of its initial retail sale to the end user, a SCUBA diver. Also, assume that it passed such tests again in the fifth year after the retail sale, but no "+" symbol was stamped at that time.
May the cylinder described above legally be filled with compress air or oxygen enriched air to 2640 psi? .
Or, to state the question in more generic terms, once a cylinder has been stamped with the "+" symbol, is anything special needed (in the way of inspecting, testing, stamping, or otherwise) to retain the ability to legally overfill the cylinder by 10%7
3. Analysis.
'Page 2 of5
In reviewing the regulations, I note that 49 C.F.R §173.302a(b) permits the 10% overfill.
There are five requirements in 49 C.F.R §173.302a(b):
(1) A frangible disc pressure relief device must be installed on the cylinder, which in the dive industry is mown as a burst disc. No sane diver uses SCUBA
tanks without one.
(2) ''The cylinder's elastic expansion was determined at the time of the last test or retest by the water jacket method." (Emphasis added.) I believe from
information available on the Internet that the water jacket method is used
industry wide for hydrostatic testing of SCUBA tanks.
(3) Average or maximum wall stress cannot exceed certain limitations.
Compliance with the average wall stress computation may be determined
through a variety of methods, including the manufacturer's marked elastic
expansion rejection limit (REE) on the cylinder or in accordance with CGA
Pamphlet C-5.
(4) The cylinder must pass visual inspection.
(5). A plus sign (+) "is added following the test date marking on the cylinder to indicate compliance with" the other four requirements above. I note that 49
C.F.R §173.34(e)(7) requires date markings on the cylinder after it
successfully passes a hydrostatic retest.
Interpretation A: If "the test date marking on the cylinder" in requirement (5) refers to the test date for the instant hydrostatic test, then clearly SCUBA tanks must have the "+" stamped after every hydrostatic test in ord~r to be overfilled 10%.
Interpretation B: If the "test date marking on the cylinder" in (5) refers to the test date of the cylinder's initial hydrostatic test, then SCUBA tanks need not have the "+" stamp after every hydrostatic test in order to be overfilled by 10%.
The use of the word "last" in requirement (2) lends support to Interpretation A.
Requirement (3), regarding wall stress, does not clarify when the limitation applies: at the time of the initial hydrostatic test or subsequent retests. Presumably it applies at the time of each hydrostatic test and retest.
49 C.F.R §173.34(e), regarding periodic qualification and marking of cylinders, does not contain limitation concerning wall stress. 49 C.F.R. 178.37(f)(2) contain wall stress limits that differ with those in 49 C.F.R § 173 .302a(b )(3). Thus, the "+" symbol appears to signify that the different requirements in 49 C.F.R §173.302a(b) concerning wall stress have been met.
Page 3 of5
I conclude that the "+" symbol must be stamped on the cylinder after every hydrostatic test in order to be overfill the cylinder by 1 0% because of the different wall stress limitations imposed by 49 C.F.R §173.302a(b)(3). Unfortunately, my conclusion conflicts with the verbal conclusion given to me by your agency via the US Department of Transportation (DOT) Info Center Hotline, 1-800-467-4922.
