impact of changing PSV operating pressure

[sager880]

Hi, i'm facing some difficulties on how to proceed and hope someone can extend their help. PSV was manufactured with maximum operating pressure 7 barg and during construction the PSV is unable to be located at less or equal 7 meters of hight(10%) and the only possibility is to relocate it to 17 meters hight. therefore, the operating pressure will certainly be effected due to new hydraulic calculation and the new maximum operating pressure will change to around 5.3 barg.

i'm not sure wether this will effect the basis of design for the PSV, i understand this will take longer time before PSV activate i.e. around 1 barg. also, this PSV is located downstream of HE and designed for external fire.

so my question: 1) is it possible to still use the same PSV on the new location even though operating pressure will is reduced. also, the set point won't be changed (MAWP of all equipment considered i.e. higher than set point.
2) to understand the situation how does the operating pressure effecting design of PSV (if applicable seat, size, flow or not related?)

the density of the fluid is 940Kg/m3

thanks and regards,

 

[MortenA]

Its a bit unclear what you are trying to ask (to me anyway), but yes, you shall correct the S/P for any hydrostatic difference between the equipment that your are protecting and your PSV. Its true that the PSV normally is allowed to have a max capacity at S/P+10% over pressure. This is to allow for the fact that a conventional PSV has a spring of of course this will lift more the higher the pressure. Since you choose the "next up" PSV size your actual max pressure will be a point between the S/P and S/P*1.1

Normally i would say that if your require s/p is 7 barg (for some reason lower than your MAWP then you should correct your S/P to 5.3 to allow for the lower pressure 17m above the vessel. Many PSV's sits in gas service where this will normally play no significant part, but in liquid service you need to consider this.

BTW time is normally not considered wrt PSV. Lift time can be assumed to be fast around 1/4 of a second seems to be a value often used where it matters (e.g. surge/water hammer calculations).

Best regards, Morten

 

[tickle]

Two things that should be considered during the design:
1. I am not sure about the piping arrangement, but the you need to ensure that the inlet pipework to the PSV is full of liquid
2. If you are considering the effect of external fire, the PSV is relieving vapour (typically). What impact does this have on hydraulic head.

I am not saying they are a problem, but should be considered during design.

 

[LittleInch]

I think you need to consider a pilot operated relief valve and place the pilot (small thing) close to the vessel. Then you can ignore the height of the actual valve.

Also don't forget to calculate the allowable pressure drop in the extended pipe if you're not going to do that for full relief flow. The inlet losses into the PRV can't be more than 3% of set pressure.

a complicating factor is going to be that for a fire case, the fluid is probably going to be initially liquid, then gas as the liquid boils. That's going to make life tough to calculate and get a correct set / re-seat pressure.

go for pilot operated.

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