4 replies to this topic

[deerun75]

Hey guys, I need to lower the set point for one of our compressor PSVs from 1300 to 1150 psi. It's a 2" by 2" valve with 1" orifice. The compressor through put is 17 MMSCFD (natural gas, specific gravity of 0.65). What steps do I need to take to decide if this will be feasible? Also, I will appreciate if someone has a copy of API 520. Thanks.

[jrtailor09]

Deerun 75,


I presume that you are not carrying out any changes in compressor. As per operation requirement you are lowering the set pressure from 1300 psi to 1150 psi.

You can reset the same by replace the spring for the different set pressure. You may use existing PSV also. I request you to consult with PSV vendor as this is a critical application.

Regards,

Jatin

[fallah]

Hey guys, I need to lower the set point for one of our compressor PSVs from 1300 to 1150 psi. It's a 2" by 2" valve with 1" orifice. The compressor through put is 17 MMSCFD (natural gas, specific gravity of 0.65). What steps do I need to take to decide if this will be feasible? Also, I will appreciate if someone has a copy of API 520. Thanks.

deerun75,

PSV spring normally has a reserve capacity adequate to change the set pressure +/-5% from its nameplate set, but in your case set pressure reduction is out of this range and probably you should change the PSV spring. Consulting with relevant vendor would be so effective in solving your problem.

One more thing should be kept in mind; reduction of the PSV set pressure down to such value (11.5%) along with keeping the same orifice area will lead to decrease the PSV capacity for new set pressure.

Fallah

Edited by fallah, 14 December 2011 - 02:48 AM.

[ankur2061]

Fallah,

I am somewhat confused about the statement you have made in regard to the reduction of the capacity of the PSV if the PSV setpoint is reduced. Can you please calrify.

I am quoting from the famous article on "Relief Valve Set Pressures" published on "Cheresources" itself:

This is not true; for a certain MAWP, the capacity of the relief device is not a function of its set point, but of MAWP alone. For example, for a MAWP of 100 psig, the relief valve capacity will be same whether it is set at 80 psig or 100 psig. In both cases, the maximum relieving pressure for the ASME non-fire case (or BS 5500 fire case) is 124.7 psia and the discharge capacity will remain identical. The only difference is that if the set point is 80 psig, the allowable overpressure will be 37.5%, while, at the same for a set point of 100 psig, it will be 10%. For the ASME fire case, the values will be 51.25% and 21% respectively. These are defined very clearly in API 520, Tables 2 to 6.

The article is at the link::

http://www.cheresour...e-set-pressures

Regards,
Ankur.

[fallah]

Ankur,

Thanks for your accurate attention. The statement you quoted from that article is absolutely right because it has assumed the relieving pressure in all values of the set pressures remained unchanged on 110% of MAWP (121% infire case) by adjusting the relevant overpressures.

The statement i have made in my previous post is based on keeping overpressure percent constant and equal to 10% of the set pressure for each set pressure value. It will lead to decreasing the relief pressure of the PSV due to lowering the set pressure that results in PSV capacity change.Of course, this point should have been referred in my statement.

Fallah