Posted: Posted on January 22, 2018 Selection of Methods for Requalification of Compressed Gas Cylinders and Tubes

Q.  What does requalification mean?

A.  Requalification means the completion of a visual inspection and/or the test(s) required to be performed on a cylinder to determine its suitability for continued service.”  Code of Federal Regulations, Title 49 (Transportation) Part 180, Section 180.203, Definitions, Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20401.  www.gpo.gov

Q.  What tests do I need done to get my cylinders and tubes requalified?

A.  The choice of requalification method will be based on a variety of factors, including cylinder specification, gas service and purity level, authorized requalification cycle, location, turnaround time, and price.  First and foremost is the specification of the cylinder or tube that is being tested.  (NOTE:  Throughout this article the word cylinders includes tubes, unless specifically noted.)  For some specification cylinders, there will be only one approved method.  But, in most cases, DOT regulations or special permits issued by governing bodies will offer choices.  This article will explain the varied methods and reasons for selecting a particular method.

Regulatory approval of the test facility, personnel and equipment is required for cylinder requalification.  Whether domestic or international, persons requalifying U.S. DOT Specification cylinders must have a Requalifier Identification Number (RIN) issued by DOT upon the recommendation of an Independent Inspection Agency (IIA).

Visual Examination Method:

Visual examination of a cylinder is a part of any requalification process and should be performed before applying pressure.  In fact, most cylinders are rejected because of a finding during a visual examination.  The exam may be both internal and external or, in some cases, external only.  Some specification cylinders operating in certain gas services may be requalified exclusively by a visual examination.  Pertinent regulations for the specification of cylinder being requalified (e.g. DOT or TC) must be carefully reviewed for compliance and selection of the visual examination method appropriate for the cylinder undergoing requalification.  For example, the U.S. DOT references several Compressed Gas Association (CGA) standards for visual inspection of cylinders based on the material used to manufacture the cylinder (e.g. steel, aluminum or fiber).  Persons performing visual exams require a thorough understanding of the methods to use and potential defects to be found.

Visual examination of cylinder

Hydrostatic Methods:

1.       Water jacket method –

Historically, the most common method for requalification of cylinders is a hydrostatic method and, in most cases, this was the water jacket method, which permits measurement of volumetric expansion.  For most cylinders this is a method authorized by DOT.  But, it may not be the ideal method, particularly if you’re dealing with cylinders transporting high purity, flammable or toxic gases.  This method “consists of enclosing a cylinder filled with liquid [generally water] into a test jacket filled with water.  Internal pressure is then applied to the cylinder, causing the cylinder to expand.  The total and permanent volumetric expansions of the cylinder are determined by measuring the amount of water displaced by the expansion of the cylinder when [at test pressure] and after the pressure is released [and the cylinder is brought back to ambient pressure].  The elastic expansion is calculated by subtracting the permanent expansion from the total expansion.”  (CGA C-1, Methods for Pressure Testing Compressed Gas Cylinders, Compressed Gas Association, Inc., 14501 George Carter Way, Suite 103, Chantilly, VA 20151, 2016)

Generally, a satisfactory hydrostatic test authorizes DOT Specification cylinders for continued service for another 5 years.  Under specific conditions defined in 49 CFR § 180.209(b), DOT 3A or 3AA cylinders may be requalified every 10 years.  A star symbol confirms suitability for this extended cycle.  Irrespective of the conditions associated with a 3A or 3AA cylinder, a 10 year cycle is authorized for DOT UN cylinders and tubes operating in Division 2.2 (nonflammable) and 2.1 (flammable) gas services, with a few exceptions (e.g. carbon dioxide and hydrogen fluoride, anhydrous).  A 5 year cycle is mandatory for Division 2.3 toxic gases.

Water Jacket Test

2.       Direct expansion method –

This method “consists of forcing a measurable volume of liquid into a cylinder filled with a known weight of liquid at a known temperature, and measuring the volume of liquid expelled from the cylinder when the pressure is released.”  (CGA C-1)  A calibrated scale capable of weighing the vessel filled with water is necessary.  Adjustments for the compressibility of water must be factored into the calculation of expansion.  This method is most often used for welded, steel pressure vessels, such as 106A multi-unit tank car tanks.  DOT regulations prevent this method from being used to qualify a cylinder for filling to 10% above service pressure.

3.       Proof pressure method –

This method “is permitted when regulations do not require the determination of total and permanent expansion.  It consists of pressurizing a cylinder to test pressure and examining it under pressure for leaks, bulges, and any visible defects.”  (CGA C-1)  It is also used to pressure test ASME-coded pressure vessels, piping systems and UN portable tanks.  Pressure may be applied by either a hydrostatic or pneumatic method.  When a pneumatic method is used (e.g. air or nitrogen) great care must be taken to insure the safety of test personnel and equipment.

Acoustic Emission Methods:

1.       Acoustic Emission (AE) –

Merriam-Webster defines acoustic emission as “the production of sound waves by a material when it is subjected to stress”.  (www.merriam-webster.com)  When requalifying a seamless cylinder using AE, the cylinder is subjected to internal pressurization (by a gas or liquid) to a specified test pressure (based on the cylinder service pressure) while a transducer is affixed to each end.  If the cylinder material (e.g. steel or aluminum) contains a growing flaw a release of stress energy from an event source will occur, resulting in elastic waves that propagate within the cylinder wall.  AE equipment will monitor and record these events and, most importantly, determine the AE source location where the event occurred.  Events deemed “significant” (based on predetermined criteria) and potentially detrimental to the integrity of the cylinder are later inspected, measured, and characterized by ultrasonic examination (UE).  A comparison of UE findings to maximum allowable flaw sizes will determine if the cylinder is suitable for continued service or must be rejected and condemned.

AE is a desirable method for tubes when disassembly is not required to inspect tube neck mounting surfaces.  AE without disassembly can reduce costs and downtime and often permit requalification to be done at the owner’s site.  There are DOT special permits for both 5-year and 10-year requalification cycles.  A 10-year cycle will require adherence to certain conditions spelled out in the special permit (e.g. moisture and gas content and fill cycle counting) as well as a visual examination of the tubes at the end of the 5^TH year.

Acoustic emission testing of tubes in process

 2.       Modal Acoustic Emission (MAE) –

MAE is starting to be applied to the requalification of a composite overwrapped pressure vessel (COPV) and wrapped cylinders using methods authorized by DOT special permits.  The test method uses an analysis of waveform and wave propagation generated by a source event to determine if a cylinder has any indications of weakness in its structure that would result in rejection of the cylinder for continued service.  Some examples of flaws in a COPV are:  delamination, debonding and fiber fractures.

Ultrasonic Methods:

1.       Automated ultrasonic examination (UE) –

An automated UE system examines the entire cylindrical surface of a cylinder with multiple transducers that simultaneously measure thickness with a straight beam and detect discontinuities using several angle beams inspecting the cylinder wall from different directions.  Automated UE as a requalification method for 3-series, seamless steel cylinders and tubes is authorized by DOT under special permits.  Automated UE is a mandatory requalification method for DOT UN cylinders specifically (not UN tubes) with a tensile strength greater than or equal to 950 MPa.  Increasingly, UE is being used as the preferred requalification method, particularly for cylinders transporting high purity, flammable or toxic gases.  UE is a direct measurement and assessment of cylinder integrity, which is not the case for hydrostatic methods.  For 3-series, DOT Specification cylinders, UE is authorized by DOT under special permits.  Like AE, UE permits are written for both 5-year and 10-year requalification cycles.

Automated UE equipment for cylinders

2.       Hand-held ultrasonic examination –

Hand-held UE is not a requalification method by itself.  It must be accompanied by either AE or automated UE, which is explained above.  Hand-held UE is paired with AE to characterize and size flaws that emitted acoustic emissions during pressurization and, ultimately, determine whether or not the cylinder is suitable for continued service.  Hand-held UE can use both straight beam and angle beam testing techniques to measure wall thickness or detect discontinuities.

Hand held UE of tube in process

Other Test Methods:

While not used exclusively to requalify a cylinder, there are techniques employed by requalification facilities to help assess the integrity of a cylinder as a whole or a specific portion of the cylinder.  Radiography may be used to examine welds.  Magnetic particle testing and liquid penetrant testing may be used to examine cylinder necks or further evaluate surface flaws found by any of the above methodologies.  Electromagnetic Eddy current is frequently used to examine neck threads in aluminum cylinders to check for cracks, particularly in certain cylinders known to be susceptible to sustained-load cracking in this area.

For more information regarding several of the test methodologies reviewed in this article, readers are encouraged to contact the Compressed Gas Association (CGA).  CGA has several documents relating to test methods reviewed, including standards that are referenced by the U.S. Department of Transportation and Transport Canada in regulations, special permits and/or equivalency certificates pertaining to cylinder requalification.  Readers should also review definitions found in 49 CFR Section 180.203.