5 replies to this topic

[wojtar]

I would like to know some experienced person opinion about the following:

I have to determine the capacity of vacuum relief valve situated on atmospheric  storage tank.
The tank is equipped with three compressors collecting vapours during tank filling, boil-off etc. Two of them are for normal operation and the third is a reserve.
Is it reasonable to size vacuum relief valve for maximum capacity of those three compressors?

"Applied Process Design for Chemical and Petrochemical Plants, Vol.1" sais that "Withdrawal of vapor at maximum compressor sucttion rate should be considered".

However if I consider the maximum capacity of even two of my three compressors it appears that there is no valve which meets my requirements.
In other words there is no valve for specified capacity and set vacuum of -0,3 kPa G. The tank design vacuum is -0,5 kPa G. My idea was to lower the set vacuum and tank design vacuum to fit some safety relief valve flow curve. Unfortunately the people resposible for construction say that design vacuum lower than -0,5 kPa is unusual for such tanks.

To be clear: I'm not going to do any sizing calculations for such vacuum relief valve - leave the selection to the supplier. I'm only checking before sending inquiry to the suppliers.

[fallah]

Wojtar,

IMO, supposing there are 3*50% compressors to do the job, you shouldn't consider the maximum capacity of three compressors for vacuum relief valve sizing and maximum capacity of two compressors is proper and adequate value.

Indeed, you can not lower the design vacuum of the tank at all. You need to report maximum relief vacuum load (based on above) along with the design vacuum to vendors and leave the issue to be analysed and investigated by them. It might one vendor to be able to submit a vacuum relief valve to meet your requirement or present a solution for your high relief vacuum load.

Fallah

Edited by fallah, 02 February 2012 - 09:22 AM.

[wojtar]

Thank you for your fast response. That is exactly what I'm going to do.

Regards.

[Exclamation]

_{Good reply}

[Dacs]

IMHO if there's a possibility of operating three compressors at the same time (possible with faulty autostart provision on compressor motors), then I think the scenario of vacuum due to 3 compressors running is credible.

Well at least that's how I treat pumps with autostart provision, that they may inadvertedly work at the same time and I've sized breather valves with this scenario in mind. I dunno however if this applies to compressors as well.

[aju_1807]

I agree with Dacs comment, if there is any possibility of running 3rd compresor (during any mode of operation) then we need to consider that flow as well. Moreover, as far as I know the tank design vaccum can be as low as 2.5 mbar as per code.

During normal operation also we need to investigate the possibility of compressor running on end of curve(stone-wall point), if possible then that flow rate shall be considered as a basis for PVRV sizing.

Moreover, if flow rate is high than we can go for emergency vacuum relief manway for which maximum size available is as high as 36" (depending on your tank design code). You can refer Motherwell tank protection vendor link for size availability.

Thanks

AJ