Thermal Relief Valves
Thermal Relief Valves
(OP)
All:
I am sizing thermal relief valves for liquid LNG. Typically we do not size those and use default, but the client wants to see the calcs. These valves are located on 26" diameter long and short run of piping.
The pipe length is approx 1000ft and is insulated.
Do we have to consider external fire case as a possible scenario for relief valves? If that is the case are we considering double jeapordy along w/ the blocked in the condition.
Also during fire all the liquid flow is stopped to the fire risk area and it would essentially be blocked in.
If I have to take a quick shot at the fire case, what pipe length should I assume to be exposed to fire?
Your comments would be appreciated.
Regards,
npf
I am sizing thermal relief valves for liquid LNG. Typically we do not size those and use default, but the client wants to see the calcs. These valves are located on 26" diameter long and short run of piping.
The pipe length is approx 1000ft and is insulated.
Do we have to consider external fire case as a possible scenario for relief valves? If that is the case are we considering double jeapordy along w/ the blocked in the condition.
Also during fire all the liquid flow is stopped to the fire risk area and it would essentially be blocked in.
If I have to take a quick shot at the fire case, what pipe length should I assume to be exposed to fire?
Your comments would be appreciated.
Regards,
npf





RE: Thermal Relief Valves
Therefore calculate the diameter of a 2500 ft2 - 5000 ft2 circle and use that length of pipe for the fire case. Just a thought.
Note that at our refinery we don't consider piping fire case scenarios and I don't think many others would do so either.
Also, if you needed to calculate the relief rate for the thermal expansion case, you'd need to calculate the volume of the pipe line in question and assume some sort of heat absorption to the pipe from some hot ambient condition and then apply the coefficient of thermal expansion to see what the rate of volume increase would be.
RE: Thermal Relief Valves
RE: Thermal Relief Valves
As for solar thermal, you can use the solar thermal calculation on the line and then calculate boil off rate just like any liquid line. The thermal relief valve will be a 1/2" to 3/4" valve OR you can provide a check valve around one of the block valves that would relieve the boiled up liquids back to a tank or vessel that has a thermal relief valve on it.
RE: Thermal Relief Valves
You need to evaluate your system and make a determination.
RE: Thermal Relief Valves
RE: Thermal Relief Valves
RE: Thermal Relief Valves
Double jeopardy
Don't think it is double jeopardy. As fire is occurred, all shutdown system is activated (as planned) and result inventory trapping in the system. All are in sequence and expected.
Piping to be included in fire load calculation
Piping within plant would normally be included as it is concentrated, piping inventory and associated risk could be big Discussed in Requirement of overpressure protection devices on system design to PIPING code and relief load can be managed in many occasion. Nevertheless, cross country pipeline which involved plant BL, it is always ended up a long discussion as some may argue that piping/pipeline code do not demand for pressure relief, BL definition, ...
Area to be considered
If fire scenario is credible, idea brought up by molecularman (Chemical) is one quick, normal & code "compliant" way to "limit" the area. If turn out to be excessive, qualitative risk assessment (QRA), fire modeling could be employed.
RE: Thermal Relief Valves
Thnaks for the comments. I am planning to take a quick shot at the fire case and look at the relief load.
SeanB: The pipe is insulated and the operating temp is around -260F and this relief valve is placed between 2 fail closed control valves.
Regards,
npf
RE: Thermal Relief Valves
RE: Thermal Relief Valves
Please refer to API Std 521, Jan 2007, Fifth edition, section 5.15.1.1 & 5.20.2.2. Read more...