Thermal relief valve outlet connection
Thermal relief valve outlet connection
(OP)
What are the options for disposing discharge fluid of a thermal relief valve in a piping system between production units and and storage tank farm? Ideally the TRV discharge piping should be connected to the other side of the block valve where it is free from blockage. But in case it requires a relative long piping to find such a spot for discharge connection, is it better to connect to a nearby closed drain? Is it permissible to have an open discharge if it is a non-toxic hydrocarbon fluid?





RE: Thermal relief valve outlet connection
RE: Thermal relief valve outlet connection
Suppose that one has a block valve at the offsite battery limit (B/L) and one at an onsite B/L block valve. They will place a thermal relief across the onsite block valve and another at the offsite block valve. The problem becomes a bit hairy because one has to keep setting the downstream valves at a lower setting. In addition if one were to take the downstream tank out of service then the upstream block valves have no place to discharge to.
It is done but it creates a lot of additional controls.
RE: Thermal relief valve outlet connection
RE: Thermal relief valve outlet connection
Correct and good point.
But bellows type valves bring on their own set of problems even when delta P is not a problem.
Bellows wear out.
The bonnet has an opening to the atmosphere.
One still has to manage the downstream blinds in an offsite area.
In an offsite area a leaking bellows is less likely to be noticed than in an onsite area. But they are a means of removing the variable downstream pressure from the set pressure.
RE: Thermal relief valve outlet connection
Do you mean that using a balanced bellow type of thermal relief valve in a cascading relief system, you only have to specify set pressure and there is no need to specify back pressure (variable or constant)? Do you need to specify spring setting (differential set pressure)? A cacading relief system is exactly what we need for the OSBL piping system and I have a problem to properly prepare a releif valve datasheet. Could you list the required data for a complete thermal relief valve? Is it better to use a pilot-operated thermal relief valve in a cascading relief system? I will appreciate it very much for your guidance.
RE: Thermal relief valve outlet connection
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.