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Hydrostatic testing involves pressurizing the vessel with water and verifying the vessel’s integrity. There are many different methods within hydrostatic testing. The common ones are proof testing, direct expansion and water jacket. CMW extensively employs water jacket method wherein a high pressure cylinder is filled with water and immersed into a water tank and pressurized from within. The internal pressurization causes the vessel to expand. The expansion could be elastic or plastic based on the stress experienced due to the test pressure. The expansion can be found by measuring the rise in the amount of water in the water tank. The water level is measured prior to start of pressurization (L1), at test pressure (L2) and after complete depressurization (L3).
Elastic Expansion = L2-L3;
Plastic Expansion = L3-L1
A great amount of data can be obtained by correlating the expansion with the test pressure, the wall thickness the strength of the vessel and the stresses experienced.
While hydrostatic testing is a good method to know the overall condition of a pressure vessel, it has certain limitation when it comes to localized flaws. If the vessel is put in service where the lading is corrosive or erosive, hydrostatic testing may be more appropriate.
Non Destructive Testing methods are indirect way of measuring certain properties and characteristics of a material. Each one focuses on a small set of variables and the results are based on the changes to those variables. When it comes to testing, there is no one method that is complete or fool proof, even if they are the touted to be the latest and the greatest.
Though the ocean of rules and regulations may permit multiple methods to certify a pressure vessel’s fitness, it is essential to understand the method, the assumptions, capabilities and more importantly their limitations prior to deciding on a particular test method for your pressure vessels. Users are encouraged to gain knowledge from the suggested reference material. If you have questions or require further clarification, please feel free to contact us.
Compressed Gas Association (CGA) Pamphlet C1 - Methods For Hydrostatic Testing of Compressed Gas Cylinders
Compressed Gas Association (CGA) Pamphlet C5 - Cylinder Service Life Seamless Steel High Pressure Cylinders