If the test pressure for the vessel is 8.58 barg, and it is connected to the column, testing the assembly at 10 barg will over-pressurise the vessel, which is only rated to 8.58 barg; this should be obvious.

Has the Engineer/Surveyor for the company supplying burst indemnity insurance cover for the equipment got anything different to add?

If the two are connected, why was one designed for 10 barg and the other for only 8.58 barg?

Both equipment designed on their different operating pressure. Hence different Hydrotest pressure

If the maximum pressure the assembly will see is less than 8.58 barg, what is the merit in testing any part of it to 10 barg?

Surendra,

You wrote: "Can someone pls tell me , How to calculate combined Hydro-test pressure for this assembly?"

You cannot combine hydro-tests unless the test pressures for both are the same.

In your case it sounds like you are trying to save money by cutting corners. This is not recommended. The proper solution is to run two tests.

First test for 8.5 bar rated item,isolate by valve & test other for a higher pressure.

Why?

The test pressure for 8.58bar & 10 bar rated items will be different & the lower pressure rated item need not be tested for more than its test pressure(if tested together)

The original poster stated that the two items were interlinked. So why test to the higher pressure if the equipment is not designed to be used at that pressure? It doesn't make sense.

read my posts 4 & 6,self explanatory

Neither addresses the need identified in the original posting.