It's fairly normal to swap out or lock down safety relief vents on a vessel under test - because it's going to operate as a pressure vessel under the PED


For an vented cylinder that will never meet the criteria of a pressure vessel, then presumably the manufacturer is advocating a pressure test as a means of leak testing  - so you could swap out the vent for a pressure relief valve set above 7.5 bar - but you would absolutely want to see evidence that the vessel was designed to withstand those pressure (which would be unusual if it was always intended to operate as an atmospheric vessel) - if the elements have been installed properly using proper materials and good workmanship, any leak would be visible on first fill - as it's only atmospheric pressure acting on the vessel anyway


Ask the manufacturer again for the written methodology to undertake a 7.5 bar test on their equipment


Regards


OMS

It sounds to me as if the heater manufacturer is expecting the heaters to be in a pressurised tank (perhaps 3 bar tank so pressure tested at 2 1/2 times working pressure?) but as it is in a vented tank the pressure test may not be applicable and a leak test sufficient. In addition to asking the manufacturer for the methodology, and possibly ahead of that question, I would suggest explaining the situation to the manufacturer and checking whether they still recommend a leak test.

Alasdair