4 Dec 03 16:59
Note: There is a difference between bellows type and balanced relief valves. Anderson Greenwood 81P and Swagelok R31 do not use a bellows (see http://www.andersongreenwood.com/direct_spring.asp
for data on 81P). However, there is a seal between the stem and atmosphere with a balanced valve; as such, a potential for a leak is possible. To put this in perspective, the block valve which is creating the pressure barrier necessitating the TRV (thermal relief valve) has a seal separating the fluid from the atmosphere as well. For information on one type of ASME BPV listed bellows TRV, look at Hydroseals section H, http://www.hydroseal.com/Products/h.html
Check with supplier/manufacturer to verify that their TRV models are fully balanced if your design requires this.
As I recall, B31.4 does not require ASME BPV listed PRVs and B31.3 does. Not positive about this without looking it up so make sure you follow the design/code requirements. Consequently, many owners prefer to use only the listed PRVs for B31.4 designs. I agree with them in most cases.
Back pressure is relevant for sizing the orifice size in a fully balanced TRV.
Provide supplier with set pressure, downstream pressure, fluid type, temperature range of fluid, relief flow requirements, and type of connection. Most suppliers of TRVs have information for sizing their valves on the Web. Use this information to ensure that their recommendations for TRVs meet your requirements.
Make sure all scenarios are evaluated and discussed with owner, because the installations of one blind flange installed downstream for service work may disable the required thermal protection for the entire pipeline.