relieve valve outlet

[joonmin]

I have a PSV which is designed for both fire and thermal expansion case. Obviously the design basis is fire case as the vapour relieve loading is very high.

However when we designed the disposition, since there will be vapor or liquid relief, do we route the piping to Atmosphere as well as to grade? or Could we use the weep hole for liquid relief?

Another question:
We are using mineral oil in a petrochemical plant. A PSV is installed to safeguard the mineral oil filter. The set pressure is 35Barg. Since we don't have the composition as it is supplied by licensor, we cannot simulate the mineral oil for fire case. Could anybody help to calculate what will be the relieving temperature? will there be a vapor or liquid relief? we only have the flash point.

Last question:
We let down MP nitrogen from 8 barg to 2barg. To protect the LP N2 system from overpressurisation, a PSV is provided after the PCV and it is set at 2.2barg. When we prepared the process data for the PCV, do we specify discharge pressure of 2 barg at rated flow or normal flow? Normally variation occurs at the outlet of CV. PSV vendor do not provide the guarantee if the differential between set pressure and control valve outlet is less than 10%.

Thank you. Looking forward to your advise

 

[Montemayor]

joonmin:

I believe your queries are all answered by using common sense:

1) If you calculate the amount of hot liquid causing the PSV to relieve due to thermal expansion, you will obtain a few cubic centimeters. This will hardly begin to fill the outlet nozzle of the PSV. Why try to worry about the discharge routing when the weep hole will handle it? (here I'm assuming the fluid is hot enough to be free-flowing)

2) I assume that the mineral oil is capable of being vaporized without degrading into a charred mass or polymerized product. If so, use the latent heat value of a similar oil (or of water, if it is higher) or a conservative value. If the oil degrades, you've a more serious problem in that the tank or vessel will probably fuse or rupture due to the inability to transfer the fire heat into the heavy, viscous mass. At that point, pressure relief is of little concern compared to the destruction of the vessel and other safety steps -like sprinklers- should be employed.

3) A PSV set 10% over the operating pressure is too close! Why is this done? What is the problem that you can't set the PSV at a higher pressure? The control valve itself is probably already working in and out of that value. I would fix or change the constraints that limit you to the 2.2 barg and set the PSV to approximately 3.0 barg.

I hope this helps.

Art Montemayor
Spring, TX