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    Extinguishers: Hydrostatic Testing Ensures Safety

    By Mark Conroy


    Hydrostatic testing is pressure testing of an extinguisher to make sure it has the strength to prevent rupture. This test is standard practice for pressure vessels and is required by NFPA 10, Standard for Portable Fire Extinguishers. Periodic testing ensures safety, because a catastrophic failure in the field could cause severe injuries. Here are the insights needed to make sure extinguishers remain safe.

    Prior to performing a hydrostatic test, an extinguisher is visually examined both externally and internally. It is then filled with water or another approved liquid and is pressurized for a period of time. A cylinder fails the test if it leaks or does not return to its original condition after it is depressurized. For safety reasons, air or nitrogen are never used as the sole medium for cylinder pressure testing.

    Extinguishers tested with water must be dried after testing (except water-type extinguishers). Special dryers are used and the temperature must never exceed 150°F inside the shell. Alternative fluids for testing will dry quicker, so dryers are not needed in most cases. Extinguisher manufacturers should be consulted prior to using alternative fluids for hydrostatic testing.

    Cylinders fall into two categories; high-pressure cylinders or low-pressure cylinders. High-pressure cylinders are at a pressure higher than 500 psi at 70°F, such as carbon dioxide extinguishers. Low-pressure cylinders contain extinguishing agents and expellant gases, such as nitrogen and compressed air at a service pressure of 500 psi, or lower, at 70°F.

    Special equipment meeting the specifications of CGA C-1, Methods of Hydrostatic Testing of Compressed Gas Cylinders is used for testing high pressure cylinders. Low-pressure cylinders typically undergo a “proof pressure test.” A hand or power-operated pump is used to pressurize cylinders, and most often a cage surrounds the cylinders being tested for protection of workers in the vicinity. The extinguisher is visually observed while under pressure for indications of leaks, deformations, or any indication of possible failure.

    Hydrostatic Test Interval of 5 Years
    Stored-pressure water, water mist, and loaded stream (antifreeze)
    Stored-pressure foam (AFFF and FFFP)
    Stored-pressure wet chemical
    Carbon dioxide
    Hydrostatic Test Interval of 12 Years
    Stored-pressure dry chemical and dry powder
    Cartridge or cylinder operated dry chemical and dry powder
    Halogenated agents

    A hydrostatic test is required within the calendar year of the specified test interval. If an extinguisher is discharged, a hydrostatic test should be conducted if the extinguisher is due within the current calendar year. In no case is an extinguisher to be recharged if it is beyond the specified retest date.

    Hydrostatic testing is performed to avoid possible catastrophic failure of the cylinder and prevent severe injuries. Your knowledge of this topic will help your customers to maintain a safe environment where the extinguishers you have serviced are installed and used.


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    Mark Conroy is an engineer in our Boston, MA office and a principal member of the NFPA Technical Committee on Portable Fire Extinguishers. © 2018 Brooks Equipment.

    Disclaimer: The opinions expressed in the above Tech Series article are the author’s only and provide limited information. Although the information is believed to be reliable, Brooks Equipment Company, LLC expressly disclaims any warranty with respect to the information and any liability for errors or omissions. The user of this article or the product(s) is responsible for verifying the information’s accuracy from all available sources, including the product manufacturer. The authority having jurisdiction should be contacted for code interpretations.