It is not unreasonable to expect a "hydrostatic test" of the valve housing, i.e. a pressure-test of the housing to ensure that it not leaking.
(Insufficient time to research this at length.)

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Note added at 7 days (2018-06-04 14:08:54 GMT) Post-grading
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Notes on other answerers' submissions:
--> Valve cover is not idiomatic in English. Also, the cover is only a small part of the housing. http://www.wermac.org/valves/valves_general.html, …Valve bonnet (defined as the cover of a valve housing).
--> The bonnet is only a small part of the housing. To some extent, "bonnet" makes no sense as the basis for a hydrostatic test because a bonnet itself would never leak. http://www.wermac.org/valves/valves_general.html, …Valve bonnet.