Pressure Vessels Reliefs or System Design Proctection

[cjames17]

Hi

I have an installation with 5 shell and tube steam (@85 psi) to hot water heat exchangers used to heat a building. I had an insurance inspector come by for an annual inspection and identify that each individual exhanger was not protected by a safety relief valve on the steam/shell side. I indicated that these were protected by a saftey relief upstream at my boiler which was set at 125 psi. My exchanger MAWP is 150 psi so I thought it was protected by code.

I was informed later by another inspector that this is only acceptable when there are no isolation valves present between the system safety relief and my exchangers.

It will be quite difficult to install properly sized safeties on each exchanger, is there any variance in the ASME to this code?

 

[psafety]

I've known of inspectors that would bend if the isolation valves were "locked open" allowing a free path between the safety valve and vessel.

 

[cjames17]

Thanks psafety

I will contact the insurance agent to see if there is any way to get around this. However, I am only the maintenance contractor for the building I will probably report this to the owner so they can make a proper decision

 

[davefitz]

Every PV is required to be protected by a relief valve, even if the relief is a small whistle sometimes referred to as a "thernal relief valve".

If there is an isolation valve installed in a line, sooner or later it will be closed, otherwise it would never have been installed in the first place.

It sounds as if you will need to both lock open the valves and add a 3/4" thermal relief valve directly to a PV nozzle.

"Nobody expects the Spanish Inquisition! "

 

[cjames17]

An engineer I work with pointed out to me ASME Boiler and Pressure Code Section VIII Appendix M Installation and Operation M-5(b):Basically it states that a PV with an outside pressure source may have isolation valves between the PV and the relief provided that the isolation valve closes off the energy source of pressure.

This would help my case, but I will probably need to install something for the fire case to make sure I'm code compliant there. Also, some contractors have quoted larger reliefs to evacuate the entire running capacity of the unit which is significantly more expensive and will require a lot more work.

Has anyone ran into this case in M-5?

Thanks

 

[davefitz]

In addition to isolating the energy source, one also has to address thermal expansion of the fluid in the tank in those cases where the tank is isolated. To address that case,the 3/4" thermal remleif valve is needed in direct communication with the PV, or one might rationalize installing a sealed bladder in direct communication.

"Nobody expects the Spanish Inquisition! "

 

[puigi]

I don't believe the exceptions in non-mandatory appendix M apply to the configuration you describe. Before you go down this path I recommend you read paragraphs UG-125 & UG-140 of ASME section VIII. UG-125 states that all PV regardless of size under the scope of the division shall have overpressure protection in accordance with the req. in UG-125 through UG-138 and/or overpressure protection by system design (UG-140).
Under the system design scenario (which is what you appear to be proposing) it is the user's responsibility to request a manufacturer's data report indicating that overpressure protection is provided by system design and to conduct a detailed analysis examining all potential over pressure scenarios described in ANSI/API Std. 521. At this point you may be better off following your AI's recommendations and installing the individual PRVs on each PV.

 

[chicopee]

If your HX'swere built to ASME/NB code, you should examine their shells carefully for threaded plugs designed for PRV installation.

 

[cjames17]

Unfortunately no threaded plugs for reliefs. There were some for a pressure gauge but that's it.

I'll have to continue reviewing the code, there seems to be many interpretations and contradictions within the asme. Not sure why it was written to be so complicated.