7 replies to this topic

[furio]

Dear all,
I'm sizing the PSV on a LP gas/oil separator considering the scenario in which the liquid control valve that regulates the level of the upstream HP separator is in a fully opened position. As psv discharge flow rate I usually consider the maximum gas flow rate that can pass through the valve fully open without taking into account the liquid contribution. Is this approach correct? Or would it be better considering the bi-phase flow through the control valve? Which are the considerations that should be made in this case?
Many thanks for your time.

[Zauberberg]

Normally, I would expect the full gas blow-through to be the worst case for overpressuring the LP separator.

There was one interesting thread in the forum, where we discussed about the orifice installed downstream of the level control valve. I see this type of orifice in the plant where I work at the moment. It is there to act particularly in case of a gas blowthrough, so as to prevent excessive gas flow (from HP to LP separator in your case). This limits the quantity of gas that can pass into the LP separator, and therefore you can define the PSV size based on the gas flow through the orifice.

Thing to be remembered, is to have the level control valve sized by taking into account presence of the orifice, as it will be acting in the way of limiting liquid flow during normal operation. Pressure drop across the orifice needs to be taken into account when sizing the level control valve.

[fallah]

I'm sizing the PSV on a LP gas/oil separator considering the scenario in which the liquid control valve that regulates the level of the upstream HP separator is in a fully opened position. As psv discharge flow rate I usually consider the maximum gas flow rate that can pass through the valve fully open without taking into account the liquid contribution. Is this approach correct?
Yes,because mentioned PSV is to be sized for gas relief in the gas blow-by case as a result of LV full open
Or would it be better considering the bi-phase flow through the control valve? Which are the considerations that should be made in this case?
If mechanical stop is considered in LV or RO is installed after LV,maximum gas flow rate should be calculated based on percentage of LV opening at mechanical stop setting or RO size,respectively.

[ogpprocessing]

I have seen safety valves are sized for two phase relief due to upstream control valve failure. So whay gas blowby should the worst case?

[ogpprocessing]

I have seen safety valves are sized for two phase relief due to upstream control valve failure. So whay gas blowby should the worst case?

[furio]

Thanks all for your replies.

I have this doubt because I found some old calculations relevant the mentioned psv considering, as the flow rate passing through the control valve fully open, the minimum design liquid flow rate (= 50% of the normal flow rate) plus the maximum gas flow rate that can be handled in that condition. This gas flow rate minus the normal gas flow rate going out from the gas outlet of the lp separator has been considered as psv relieving rate considering that the liquid coming from the hp separator is easily discharged from the liquid outlet.

I can't understand that criterion...but considering just all the gas that can flow through the control valve fully open, as I did, I'm worried to oversize the psv: my gas flow rate is much bigger!

[bernath]

Normally, I would expect the full gas blow-through to be the worst case for overpressuring the LP separator.

There was one interesting thread in the forum, where we discussed about the orifice installed downstream of the level control valve. I see this type of orifice in the plant where I work at the moment. It is there to act particularly in case of a gas blowthrough, so as to prevent excessive gas flow (from HP to LP separator in your case). This limits the quantity of gas that can pass into the LP separator, and therefore you can define the PSV size based on the gas flow through the orifice.

Thing to be remembered, is to have the level control valve sized by taking into account presence of the orifice, as it will be acting in the way of limiting liquid flow during normal operation. Pressure drop across the orifice needs to be taken into account when sizing the level control valve.

Dear Zauberberg,

If I may, could you please give me the link to the 'interesting thread' that discussed about the orifice installed downstream of the level control valve you mentioned above? Do you still remember the title of that thread?

many thanks
regards,
bernath

[lokesh_09]

Hi
It is not necessary to size the PSV for total gas capacity. First you calculate the size of the level control valve for normal operating conditions and normal difference pressure or get it from control valve vendor. From the rated valve coefficient(cv) , calculate the maximum gas flow for differential pressure of upstream vessel PAHH and downstream Psv set pressure. In case if u get high gas capacity use the mechanical stop or RO to limit/restrict the gas flow rate.

Regards
lokesh.