Evaluation of 5000 PSI Vessel

It appears you are so far out on a limb, that an asthmatic wood pecker would cut you off in 1 peck.

This is a bomb, and you are trying to get rational justification or approval from strangers for this! It hasn't killed anyone in 7 years, it must be OK.

Can we....

No. Talk to an appropriate professional vessel testing or certifying company and ask them for advice as you have a potential bomb on your hands.

Just to be safe take it out of service until you do. Do you even have any records to indicate that it was designed and manufactured to be fit for purpose? Find all the records and contact the professionals. Safety first.

As one who is familiar with high pressure vessels, I have some suggestions.

1. A hydrostatic test at 1.5 times your 5500 psi will prove your vessel sound. This test should be performed by a professional, who will use a containment apparatus in case the vessel fails the test.

2. Assuming the vessel passes the test, weld a threaded hub somewhere on the vessel and install a 6000 psi pop-off valve.

3. A good rule of thumb is a detailed visual inspection of the vessel for corrosion annually, and a new hydrostatic test every 6 years.

If there are specific rules governing your particular situation/vessel that you did not disclose in your post, my post can be considered "off topic".

What surprises me is the possibility that this vessel is not on your organisation's general register for periodic insurance inspection. If it were, you wouldn't be asking a bunch of strangers for advice, you would be going to your insurance company directly.

It has lasted for 7 years under stated conditions, no one can state with assurance it will last for 8.