Asme section viii boilers and pressure vessels ug 135 d-1

[cjames17]

My understanding of ug 125 to 137 is that every PV needs a relief with no block devices between it and the pv except in 135 d-1. I don't completely understand this clause.

Can someone provide an example where block devices can be installed. Not counting appendix m

 

[gr2vessels]

If you have several PRV's installed on a manifold for redundancy, each PRV having full relief capacity, these can be fitted with individual isolation valves (for maintenance, test, etc), provided you have a foolproof procedure or mechanical interlock preventing the closure of all isolation valves at once.
Cheers,
gr2vessels

 

[cjames17]

Ok understood

Second question:

Can you have a isolation valve between the relief and supply of the vessel since it isolates the exchanger thus not affecting the relieving capacity.

 

[gr2vessels]

Yes, you must have a PRV between the exchanger and the piping isolation valve, unless you can prove the exchanger is protected with piping relief. I am talking about the main piping, not an isolation valve for the PRV alone.
Cheers,
gr2vessels

 

[cjames17]

How could you prove the exchanger is protected by piping relief?

My understanding is that if the exchanger is isolated from the the PRV and energy source, there still could be a fire, unexplained source of heat that could cause the exchanger to explode.

 

[gr2vessels]

Perhaps my choice of words is confusing. The exchanger must be protected by PRV, weather is a dedicated nozzle on the exchanger or is a nozzle on the first piece of pipe attached to the exchanger, before the outlet pipe isolation valve and after the inlet pipe isolation valve. If the piping is welded, not flanged to exchanger, the PRV should be a close as practical to the exchanger.