News & Updates

July 2013

  • New cylinder shot blast system officially unveiled. Shot blasting services expanded to include a full range of cylinders from 20# to one ton.
  • Hydrostatic cylinder testing services expanded. Full hydrostatic testing of low pressure cylinders, by the direct expansion method, is available for a full range of cylinders from 20# to one ton.

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Hydrostatic Testing

The process of hydrostatic testing verifies the performance of a pressure vessel, e.g. cylinders. Using this test helps maintain the safety standards and durability of a vessel over time. Newly manufactured cylinders are initially qualified using the hydrostatic test. They are then continually re-qualified at regular intervals using the hydrostatic testing procedures. Testing is very important because such containers can explode if they fail when containing compressed gas. Cylinders may contain as high as 6000 PSI of pressure.

Hydrostatic tests are conducted under the constraints of both the Federal government, CFR 49 subpart 180, and customer's specific requirements. The vessel is filled with a nearly incompressible liquid, water, and examined for leaks and permanent change in shape, or expansion. The test pressure is always considerably higher than the operating pressure to give a margin of safety. This margin of safety is typically 150% or 5.3 of the design pressure, depending on the regulations that apply. For example, if a cylinder was rated to DOT 2015 PSI, it would be tested to a minimum pressure of 3360 PSI. This is to ensure maximum usage and to provide more safety. Water is commonly used because it is almost incompressible (compressible by weight, not air pressure); therefore, it will only expand by a very small amount should the vessel fail test and split. If high pressure gas were used, then the gas would expand to perhaps several times its compressed volume in an explosion, with the attendant risk of damage or injury. This is the risk which the testing is intended to mitigate.

Cylinders are normally tested using a water jacket test. The vessel is visually inspected for defects and then placed in a jacket filled with water, and in which the change in volume of the vessel can be measured by monitoring the water level. For best accuracy, a digital scale is used to measure the smallest amount of change, expansion. The cylinder is then pressurized for a specific period, usually 30 or more seconds, and then depressurized again. The water level in the jacket is then examined. The level will be greater if the vessel being tested has been distorted by the pressure change and did not return to its original volume, or some of the pressurize water inside leaked out. In both cases, this will normally signify that the vessel has failed the test. If the REE (Rejection Elastic Expansion) is more than 10%, or not up to DOT or customer standards the cylinder fails, and then goes through a process to condemn the cylinder as unsafe.

All the information the tester needs is stamped onto the cylinder. This includes the DOT information, serial number, manufacturer, and manufacture date. Other information may be stamped such as the REE or how the manufacturer specifies the cylinder should expand before it is considered unsafe. All the information is recorded on a computer prior to the testing process. All this information is necessary for keeping track of when the cylinder has been or needs to be hydro tested.

Water Jacket Method

Cylinders are commonly tested using a water jacket test. The vessel is visually inspected for defects and then placed in a jacket filled with water, in which the change in volume of the vessel can be measured by monitoring the water level. For best accuracy, a digital scale is used to measure the smallest amount of expansion. The cylinder is then pressurized for a specific time period, usually 30 or more seconds, and then depressurized. The water level in the jacket is then examined. The level will be greater if the vessel being tested has been distorted by the pressure change and did not return to its original volume, or some of the pressurized water leaked out. In both cases, this will normally signify that the vessel has failed the test. If the REE (Rejection Elastic Expansion) is more than 10%, or not up to DOT or customer standards the cylinder fails, and then goes through a process to condemn the cylinder as unsafe. The water jacket method is the only hydrostatic testing method that DOT regulations permit for qualifying a cylinder for charging to 10% in excess of service pressure.

Direct Expansion Method

The direct expansion cylinder testing method determines the total and permanent expansion of a cylinder by measuring the volume and weight of water forced into the cylinder to pressurize it to the given test pressure and the volume of water not expelled from the cylinder upon the release of pressure. The use of the direct expansion method for testing is incredibly crucial in its application, since very accurate measurements of the temperature, volume, and weight must be made when determining the compressibility of water. The compressibility of the water must be determined in order to differentiate the volume of water expressing the expansion of the cylinder from the volume of water representing the compression of water.

While the difference between the total and permanent expansion of the cylinder is the elastic expansion, DOT regulations prevent the direct expansion method from being used to qualify a cylinder marked for charging to 10% in excess of service pressure – plus (+) stamped cylinders. All of the information the tester needs is stamped onto the cylinder. This includes the DOT information, serial number, manufacturer, and manufacture date. Other information may be stamped such as the REE, or how the manufacturer specifies the cylinder should expand before it is considered unsafe. Prior to the testing process, all of the information is recorded on a computer. The information is necessary for keeping track of when the cylinder has been or needs to be hydro tested.