Posted by Bill Johnson on January 25, 2001 at 16:21:23:
In Reply to: + rating on LP tanks posted by Eins on January 24, 2001 at 12:56:52:
Eins,
First off, you could not have talked to the DOT or CGA about scuba "tanks" because there is no such thing. The DOT defines a tank as a vessel holding non-compressed stuff. A vessel that holds compressed stuff has always been defined a cylinder - since before SCUBA existed.
Below is the official cite regarding cylinder overfilling. When you send your cylinder in for a retest, it is normally only hydrostatically tested. In order to qualify for the + designation, "wall stress" must also be determined. In order to determine "wall stress", the
REE, "K" factor, or exact specifications for your cylinder must be known by the testing facility. If your testing facility does not have a copy of this documentation for your cylinder, "wall stress" becomes significantly difficult to determine.
A cylinder can get the + rating at anytime as long as it meets the qualifications for such rating.
The bottom line is that the + designation requires the testing facility to do more work and
will therefore cost more. This why when you drop your SCUBA cylinder off for a hydro, only a hydro gets done - and no + gets stamped on the cylinder.
Hope this helps,
Bill
49CFR173.302
(c) Special filling limits for Specifications 3A, 3AX, 3AA, 3AAX,
and 3T cylinders. Specifications 3A, 3AX, 3AA, 3AAX, and 3T
(Secs. 178.36, 178.37, 178.45 of this subchapter) cylinders may be
charged with compressed gases, other than liquefied, dissolved,
poisonous, or flammable gases to a pressure 10 percent in excess of
their marked service pressure, provided:
(1) That such cylinders are equipped with frangible disc safety
relief devices (without fusible metal backing) having a bursting
pressure not exceeding the minimum prescribed test pressure.
(2) That the elastic expansion shall have been determined at the
time of the last test or retest by the water jacket method.
(3) That either the average wall stress or the maximum wall stress
does not exceed the wall stress limitation shown in the following table
(see Notes 1, 2 and 3):
------------------------------------------------------------------------
Average Maximum
wall wall
Type of steel stress stress
limitation limitation
------------------------------------------------------------------------
Plain carbon steels over 0.35 carbon and medium 53,000 58,000
manganese steels...............................
Steels of analysis and heat-treatment specified 67,000 73,000
in spec. 3AA...................................
Steel of analysis and heat treatment specified 87,000 94,000
in Spec. DOT-3T................................
Plain carbon steels less than 0.35 carbon made 45,000 48,000
prior to 1920..................................
------------------------------------------------------------------------
Note 1: The average wall stress shall be computed from the elastic
expansion data using the following formula:
S = 1.7EE / KV - 0.4P
where:
S = wall stress, pounds per square inch;
EE = elastic expansion (total less permanent) in cubic centimeters;
K = factor x 10^-7, experimentally determined for the
particular type of cylinder being tested, or derived in accordance with
CGA Pamphlet C-5;
V = internal volume in cubic centimeter (1 cubic inch=16.387 cubic
centimeters);
P = test pressure, pounds per square inch.
Formula derived from formula of Note 2 and the following:
EE = (PKVD^2) / (D^2 - d^2)
Note 2: The maximum wall stress shall be computed from the formula:
S = (P(1.3D^2+0.4d^2)) / (D^2 - d^2)
where:
S = wall stress, pounds per square inch;
P = test pressure, pounds per square inch;
D = outside diameter, inches;
d = D - 2t, where t= minimum wall thickness determined by a suitable
method
Note 3: Compliance with average wall stress limitation may be
determined through computation of the elastic expansion rejection limit
in accordance with CGA Pamphlet C-5 or through the use of the
manufacturer's marked elastic expansion rejection limit (REE) on the
cylinder.
(4) That an external and internal visual examination made at the
time of test or retest shows the cylinder to be free from excessive
corrosion, pitting, or dangerous defects.
(5) That a plus sign (+) be added following the test date marking on
the cylinder to indicate compliance with paragraphs (c) (2), (3), and
(4) of this section.