Understanding Re-qualification Testing Methods For Portable Cylinders Part 2

Travis Jolliff

The choice of re-qualification testing method for portable cylinders is based on a variety of factors.  In most cases, DOT regulations offer choices.

This 5 part article discusses the different choices when it comes to re-qualification testing methods for portable cylinders. Each part will identify the type of re-qualification testing method and provide a brief description of each. These choices include such methodologies as:

  • Visual Examination Method
  • Hydrostatic Testing Methods
    • Water jacket method
    • Direct expansion method
    • Proof pressure method
  • Acoustic Emission Method
  • Ultrasonic Examination Methods
    • Automated ultrasonic examination
    • Hand-held ultrasonic examination
  • Other Test Methods

Part 1 addressed the Visual Examination Method, Part 2 addresses Hydrostatic Testing Methods, Part 3 will address Acoustic Emission Method, Part 4 will address Ultrasonic Examination Methods and Part 5 will address Other Test Methods.

Hydrostatic Testing Methods for re-qualifying portable cylinders

Three different hydrostatic testing methods are approved for re-qualifying portable cylinders. These methodologies are: water jacket method, direct expansion method and proof pressure method.

Water jacket method – Historically, the most common method for re-qualification of cylinders is a hydrostatic method and, in most cases, this was the water jacket method.  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 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 and the elastic expansion is calculated.  Generally, a satisfactory hydrostatic test authorizes DOT cylinders for continued service for another 5 years.  Under specific conditions defined in 49 CFR, certain cylinders may be re-qualified every 10 years.

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.  A calibrated scale capable of weighing the vessel filled with water is necessary.  This method is most often used for welded steel pressure vessels.

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.  Pressure may be applied by either a hydrostatic or pneumatic method.

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Jay L Harman’s DOT Hydrostatic Testing and Ultrasonic Examination facility and equipment meet all NFPA requirements for the hydrostatic testing and Ultrasonic Examination of compressed gas cylinders and portable fire extinguisher cylinders.

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