# KISS

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Posted by Bill Johnson on January 30, 2001 at 11:45:04:

In Reply to: The + Rating Myth And Some Facts, For A Change posted by Eins on January 29, 2001 at 10:33:21:

I don't see why everyone is having trouble with this. I will explain it simply one more time:

Hydro for a non + cylinder consists of:
1. Elastic expansion shall be determined by the water jacket method. (This is the 5/3 test.)

Hydro for a + cylinder consists of:
1. Elastic expansion shall be determined by the water jacket method. (This is the same 5/3 test.)
2. That either the average wall stress or the maximum wall stress does not exceed the wall stress limitation shown in the table.

Let me summarize: a hydro is just an 'elastic expansion' test. a + hydro is an 'elastic expansion test AND a calculation.

The wall stress calculation uses elastic expansion (EE) in the formula below to determine Average Wall Stress. To calculate this, the K factor variable or REE for that cylinder must be known by the testing facility. If they do not know these, they could use the Max Wall Stress formula, but obtaining the variables for this formula is a lot of work and I'll bet most testing facilities will not do this for the price you want to pay for a + hydro.

I have posted the complete rule again below. Everyone please read it before posting more myths on overfilling cylinders.

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.

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