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Cylinder Testing - Various Methods

Various Methods for Requalifying DOT Cylinders


Water Jacket Method


The water jacket method for hydrostatic testing consist of loading a water filled cylinder into a sealed chamber (test jacket) filled with water and connected to a calibrated glass tube (burette) Expansion Measuring System (the expansion bowl was invented to replace the burette). The burette or  Expansion Bowl is first zeroed and the cylinder is then pressurised to its specified test pressure.

This test pressure is held for a minimum of thirty seconds.
 As pressure is applied to "inflate" the cylinder, the cylinder expands and forces water out of the test jacket and up into the Expansion Bowl or burette. After the thirty second test time has elapsed, the Expansion Bowl or burette is read to determine the Total Expansion (in cubic centimetre) of the cylinder under test pressure. The test pressure is released and the cylinder "deflates". As the cylinder shrinks to its approximate original size,  water is allowed to drain back into the test jacket from the burette or Expansion Bowl. In most cases the cylinder  will not return to its original size, having been slightly stretched by the pressurisation process. This stretching is called the Permanent Expansion. The difference between the "Total Expansion" and the "Permanent Expansion" is called the "Elastic Expansion". The Percent Permanent Expansion of the cylinder is determined by the following formula:
Percent Permanent Expansion = (Permanent Expansion Total Expansion) X100

When the Percent Permanent Expansion exceeds the predetermined limits for the cylinder being tested, (REE), the cylinder must be condemned and removed from service. A high percent permanent expansion value is an indication that the cylinder metal has lost it's elasticity, or that there has been excessive thinning of the cylinder wall and that the cylinder is no longer safe for use. All test records must be saved and maintained for the duration of the re-qualification Plus (+) stamped cylinders may be filled to an additional 10 percent beyond the service pressure that is stamped on the cylinder shoulder for certain specified gases. The Water Jacket Method of testing compressed gas cylinders is the only hydrostatic test method that qualifies for filling to 10% over service pressure. Star (*) stamping makes the cylinder eligible for an extended ten year re-test interval. The procedures of requirements for plus stamping and star stamping are found in 49 CFR 180.209 (c) for plus stamping and 173.34 (e) (16) for the star. REE values for common cylinder can be found in compressed Gas Association Pamphlet C-5, "Cylinder Service Life, Seamless High Pressure Cylinders".



Proof Pressure Method


The Proof Pressure Test involves pressurising a cylinder to the appropriated test pressure and then thoroughly inspecting the cylinder, while under pressure, for indications of leaks, deformations, or other indication of possible failure.



Ultrasonic Testing (UT) Method


This method of cylinder testing was introduced in the U.S. on exemption basis in 1994. UT differs from other test methods in that the cylinder valve and contents remain intact as no water is used to pressurise the cylinder in this testing procedure.
The UT test involves positioning the cylinder on a rack of rollers that rotate the cylinder, or full immersion of the cylinder in water. Each cylinder is rotated where the entire cylinder sidewall is examined by the inspection probe sensor. This transmits Ultrasonic energy into the cylinder in the form of 'ping-like- sonar soundings from multiple transducers.
A longitudinal beam transducer sends a pulse that echoes straight off the back wall of the cylinder, measuring  the time it takes the echo to return, thus measuring the thickness of the cylinder wall. Shear wave or angle beam transducers send sound waves diagonally through the cylinder wall detecting  any cracks, pits, or flaws.  These 'soundings' are recorded electronically and reviewed to ensure that each cylinder is safe for continued use.
Aluminium cylinders manufactured from 6351 aluminium are required to have an ultrasonic or eddy current neck test every two years to inspect for neck cracks that may have developed over time relating to that grade of aluminium.
This is indicated with UT stamped as part of the test stamp. It is sometimes called a "Eddy Current" test after the instrument used to conduct the test.



Direct Expansion Method



This method is the most commonly used with in Australia.
Cylinder Hydrostatic pressure testing is carried out in accordance with Australian Standard 2030.1 & 2337.1 by a Certified Test Station.
The cylinder is then stamped around the neck area indicating the month and year it was tested along with the test station number.

During the direct expansion test, the cylinder is completely filled with water and the test connection is then screwed into the cylinder neck. Water is pumped into the cylinder until the desired test pressure is achieved. 

The Volume of water that is pumped into the cylinder to reach the test pressure is measured to determine the Total Expansion. The Volume of the water that is expelled from the cylinder when pressure is released is measured to determine the Permanent Expansion.

Because air has a different compressibility factor than water, air trapped inside the cylinder will cause inaccurate test results so it is very important that the cylinder is completely filled with water to eliminate trapped pockets of air. The weight of the water contained in the cylinder,
the test pressure, test volume (s) and temperature are used to determine the compressibility factor for calculation of the expansion values.