Due to a mishap from improper testing of a vapor safety relief valve on a compressed air system, the Coast Guard Office of Commercial Vessel Compliance (CG-CVC) reminds all those concern of the risks and best practices in conducting or witnessing the testing of relief valves. While this information is based on testing compressed air systems using vapor relief valves, the guidance is also relevant to safety valves in other pressure systems, except boilers.
In summary, Marine Inspectors should be aware of the following:
1) A Marine Inspector should not allow removal or alteration of a secondary safety device to facilitate a test of the intended safety device2;
2) All systems are different, and the attending Marine Inspector should become familiar with each system and the valve settings before testing;
3) The attending Marine Inspector is observing the test only, and the appropriate vessel representative should perform all functional tests; and
4) The attending Marine Inspector should verify that all relief valves meet the design, installation, and performance criteria in 46 CFR Subchapter F.
Mishap summary: While preparing for a test, the relief valve on the associated compressor was removed because it had a lower set-point than the relief valve on the air receiver being tested. Unfortunately, the isolation valve between the compressor and the air receiver was mistakenly left closed during the test, resulting in a dead-head situation, causing the compressor to rupture, and sending shrapnel throughout the space. Fortunately, no one was injured.
Section 61.10-5(i) of 46 CFR requires checking the settings of safety or relief valves periodically based on Certificate of Inspection (COI) issuance3. The most common way to meet this requirement is witnessing the valve operate under pressure by bench testing (that is, with the valve removed, witnessing a pneumatic pressure test using a test rig in a controlled manner) or in-situ testing (that is, witnessing an operational test of the valve under pressure as installed). .
However, if not operationally pressure testing the valve, the Marine Inspector should verify the following:
1) The valve set-point, as shown on the name plate or other documentation, is set to relieve at a pressure which does not exceed the “maximum allowable working pressure” (MAWP) of the pressure vessel or piping system in accordance with 46 CFR 54.15-10(a);
2) There is no apparent defect, deterioration, or damage of the valve;
3) The valve is of the proper type (for example, vapor type valves for vapor systems); and
4) The hand relief mechanism is exercised by an appropriate vessel representative to ensure the valve functions.
If a pressure test is called for, Marine Inspectors may require bench testing if performing an in-situ pressure test is considered unsafe or as otherwise agreed to by the vessel representative. Requiring a bench test in lieu of in-situ test should be determined on a case-by-case basis and should not be required as a blanket policy. Instead, the risks and any mitigating factors should be considered when deciding on the type of test. Nonetheless, the Marine Inspector shall communicate with the vessel and company representative well ahead of the inspection to prepare for all contingencies (for example, the ability to remove and bench test valves or install new valves if determined necessary during the inspection). Valves undergoing bench testing should be witnessed by a Marine Inspector, class surveyor or performed at a testing.
This MSIB does not apply to testing boiler safety valves which require gagging in order to properly test; see VVNCOE MI Notice 01-13, dated June 3, 2013.
A Marine Inspector should not remove, isolate, or gag a relief valve or alter a system that disables a safety device – except for adjusting the set-point of a compressor limit switch. Or any time a valve is suspect because of apparent deterioration, damage, defect, lack of recent testing, or un-suitability, the valve should be tested or replaced.