6 replies to this topic

[Amin Va]

Hi Folks,

 

There is tank connected to LP flare (both the vent and PVRV) with the design pressure of 6.9 kPag. The PVRV pressure set point is 5.00 kPag and the back pressure at the relieving condition is 1.8 kPag. The becakpressure is 36% of the set pressure but still summation of the PVRV set pressure and backpressure is less than the design pressure of the tank.

 

Do we need to go to conventional PVRV or balanced bellow PVRV?

Edited by Amin Va, 14 November 2014 - 01:48 PM.

[proinwv]

Let me ask you some questions.

 

The PVRV seems to be an operating vent, then will you require an emergency vent for fire protection? If you do there is not any pressure capabability remaining.

 

Even so, can you not reduce the set point so as not to get so close to your MAWP? Can you use a larger vent to reduce the backpressure at relief?

 

One thing about a piloted PVRV is that they are sensitive to dirt in the controls and might not be as reliable as a simple weight loaded vent. Would this be an issue with the contents of your vessel?

[Amin Va]

Hi Proin,

Thanks for your comment. I should have given a more clear picture of this.

In this case actually PVRV is not for operating venting but as an emergency venting in case the back pressure regulator on the tank got blocked. Both pressure valve of PVRV and vent are routed to flare because here we are dealing with a sour service.  There is a thief hatch also on the vessel that acts as the last level of protection and is connected to atmosphere.

If I want to reduce the back pressure on the PVRV, I should increase the flare size from 4" to 6" which is a huge cost. Also if I reduce the PVRV set point too much it might interfere with the function of back pressure control valve on the tank vent to flare (I can still reduce it a little tough). All the set points are shown on the attached sketch.


The first question that I have is "do we really need to go to balanced bellows PVRV in this case" which are very expensive. Although we have 36% back pressure, even at the relieving condition we are still below the design pressure of the tank. In case of PSV's, we usually go for balanced bellow option when we have a back pressure of more than 10% percent, but based on the code we can't go higher than 10% of the vessel design pressure (PSV set point). But in here we have 36% back pressure and we can still handle 36% extra pressure over the PVRV setpoint, because even with 36% percent overpressure we are still below the design pressure of the tank.


We can look at it this way too.  When we are at 5 kPag the PVRV lifts and relieves the flow. When the flow passes through the PVRV orifice and goes to flare, a back pressure of 1.8 kPag pressure develops behind the PVRV.  To keep the PVRV open, the pressure in the tank should increase to 5+1.8=6.8. This is a simple force balance on both side of the PVRV. At this condition we are still below the DP of the tank and the PVRV is fully open and relieves the flow.  So my question is can we just use a conventional PVRV?

Thanks,

Amin





 

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Edited by Amin Va, 14 November 2014 - 08:33 PM.

[fallah]

Dear Amin,

 

Lack of adequate info to evaluate the possibility of using conventional PVRV for mentioned situation:

 

-Superimposed back pressure in the LP flare network...

-Overpressure percent (based on the set pressure) needed for PVRV full lift based on relevant vendor data sheet...

[Amin Va]

Dear Mr. Fallah,

 

Thanks  a lot for your comment and guidance.

 

The superimposed back pressure in the flare network is negligible. The only thing that flows to the flare at normal condition is a very small purge gas flow rate (~7 SCFH, there is a purge reducer) and hence superimpised back pressure is almost 0. There are only two storage tanks (1000 bbl) connected to flare and the value that I provided for the back pressure is based on the full flow from all wells (outbreathing+flashed gas) and so we don't need to consider the effect of second tank.

 

I don't have the vendor datasheet but let me ask this in a more general way. I just would like to know if the overpressure is higher than 10% of the set point but the total pressure (set pressure + overpressure) is still below the design pressure of the tank/vessel do we need to necessarily pick balanced below relief valve style? I would like to know the answer for both PSVs and PVRVs.

 

(For PSVs for example, we know that back pressure of 21% is accetable for fire case because we can go to 121% of the vessel design pressure (21% back pressure => 21% accumulation). But lets  assume an imaginary case that we set the PSV at the 80% of the vessel design pressure and the back pressure is 25% of the set point. Now the summation of back pressure and set point is exactly same as design pressure (100% of the design pressure and we should be OK! (25% back pressure => 25% accumulation).  In other words, although the back pressure is 25% of the setpoint but the total pressure at relieving condition is less than 110% of the vessel design pressue)

 

It is appreciated if you shed some light on this.

 

Thanks,

 

Amin

Edited by Amin Va, 15 November 2014 - 04:26 AM.

[fallah]

Dear Amin,

 

Then all back pressure relates to built-up one...

 

In your case, in general, because the built-up pressure doesn't exceed the maximum allowable overpressure (%38) you can apply conventional PVRV for the application...but when I mentioned the overpressure of PVRV full lift in vendor data sheet should be checked I mean: suppose vendor stated it's needed %50 overpressure for full lift then either the tank design pressure would be passed or PVRV wouldn't be able to pass required flow rate due to reduced capacity...

[Amin Va]

Thanks a lot Mr. Fallah for your guidance.