9 replies to this topic

[ChemEng01]

Should the relief valve materials and tailpipe materials e selected based on worst case low temperatures.

 

I have a relief valve for a fire case relief. Outlet temperatures during relief are between 30-45 degrees C.

 

However if my system was blocked in at CITHP and minimum operating temperatures and my relief valve leaked I could get very low temperatures down to -90 degrees C.

 

Should I select my tailpipe / RV materials based on this potential low temperature due to leakage? Or just base outlet temperatures on the relief case? Its the difference between a carbon steel valve / tailpipe and a stainless steel valve / tailpipe.

 

 

 

Thanks

[aroon]

hi,

 

What do you mean by leakage? RV should not leak at any cost.

 

If you mean to say relief valve is relieving in the event of Blocked-in case then for sure you have to select MOC based on -90°C temperature.

[ChemEng01]

Its based on a SHELL DEP philosophy that the RVs and tailpipes up to the 90 degree bend are heat traced such that if the relief valve leaks any hydrates formed will be melted.  

 

I agree the RV should not pass below its set pressure, however apparently its quite common if a rupture disk isn't installed upstream.  

 

These are relief valves on unmanned sites which relieve to atmosphere. If an RV were to start passing at a very small rate it would go unnoticed until gas detection picked it up.

[Bobby Strain]

So, where is this cold discharge going? And, I don't understand what your system is. But best to be careful and do the right thing.

 

Bobby

[ChemEng01]

Its hydrocarbon fluid relieving through a short tailpipe to atmosphere.

 

Reason for taking leakage into consideration is that maximum operating pressure (345 barg) is 90% of relief valve set pressure (380 barg) which is at the limit, and sand is expected.

 

Leakage tests are normally done at around 90% of relief valve set pressure, and there is an allowable leakage rate specified in API for conventional valves.

 

My concern is that a bit of sand could stop a tight seal, and the valve could leak unnoticeably and lead to low temperatures downstream.

 

As this is a relief valve for a fire case, obviously low temperature during the relief scenario is not an issue.

[aroon]

I don't know how sand is possible at the inlet of relief device as mention relief device is for fire and must be located at the top. Even if sand is expected, you must consider precautionary measure to avoid contact with seating surface during normal operation by placing rupture disc at the inlet one of the possible option.

 

Is your system is operated liquid full? If it is then there are several complex possibilities of liquid relief during fire scenario. So fire also may result in low discharge temperature during initial liquid relief.

 

90% is the typical value and it should be high for your case as operating pressure itself is 90%. In addition, allowable leakage rate is very low i guess it is 10 ml(cc) per hour. This will not cause any significant drop in temperature at the relief valve discharge. However, heat tracing can be applied to have precautionary measure.

[fallah]

ChemEng01,

 

There are several vague points to be clarified by OP rather than to guess the main issue through asking sequential questions:

 

How could a bit of sand travel to PSV inlet and stop its seat tightness?

 

Why would the PSV outlet be released to atmosphere through a short tail pipe while the fluid is HC and the scenario is fire case?

 

Why did you think the reason for taking leakage into consideration is maximum operating pressure equal to 90% of the PSV set pressure while such operating pressure would rarely cause the leakage rate passes the allowable value?

 

A simple sketch of the system might be a great help to find if presented issue could be logical...

[ChemEng01]

It's been resolved now.

 

Thanks for your inputs everyone.

 

cheers

[aroon]

It will be great if you could share your resolution .......

[ChemEng01]

It turned out that if a fire occurred at closed in conditions. Max P and min T the initial reliving temperature in the tailpipe would be lower than the min design pressure of LTCS. So stainless steel was required for tailpipe anyway.

 

Thanks for the responses guys

 

cheers