Ejector in case of block outlet

[songkarn]

Dear sir,
I have project about ejector installation at low pressure waste gas unit send gas to recycle unit.
Low pressure waste gas have pressure .09 barg and recycle unit have vessel design pressure 2.5 barg. Vendor's ejector recommand to take motive drive pressure of ejector 40 barg. . motive pressure is higher recycle unit , would expert tell me how can we protect downstream of ejector? Or give me some example installation same my condition.

Best regards,
SK.

 

[EmmanuelTop]

What would likely happen first in that case (blocked ejector outlet) is the reverse flow towards the low pressure waste gas unit, leading to overpressurization of the equipment upstream of the ejectors. That is a safety scenario that you must consider in the design.

Assuming that the section upstream of the ejectors is protected against reverse flow, the PSV on the discharge vessel (in the "recycle unit") would have to be sized for the blocked discharge scenario, considering maximum steam supply pressure and the corresponding flow of steam through the motive nozzle.

Process safety time might be considerable in this particular unit, depending on the maximum steam flow and the size (volume) of the equipment upstream and downstream of the ejectors. You might want to consider a safety instrumented function to close the (new) SDV at the motive steam supply line, as an additional safeguard.

Dejan IVANOVIC
Process Engineer, MSChE

 

[songkarn]

Dear Dejan IVANOVIC ,
Thanks lot for your kindly reply.
Please find attached is conceptual sketch , would you have any suggestion?
Best regards,
SK.

 

[georgeverghese]

Accomodating for supply valve failure on C2= supply line for 7barg setpoint RV load derivation sounds good.

Also, 40barg C2= temp drop to 2.5barg can result in low temps and hydrates also - check if a preheater is required on HP C2= supply line to keep out of hydrates and account for low temp materials of construction also if relevant.

 

[songkarn]

Dear georgeverghese ,
Thanks lot for your kindly take concern point about temp drop to low temp.
Best regards,
SK.

 

[EmmanuelTop]

The sketch seems OK. Depending on actual lengths, you may want to consider fully rating the ejector suction and discharge piping, which will eliminate the two PSV's on the piping segments and leave only the vessel PSV's.

Note that the loss of motive fluid (Ethylene) will always cause reverse flow from ejector discharge to ejector suction, i.e. the SDV on Ethylene supply line will prevent overpressurization but it will not prevent the reverse flow. See if this causes any additional concerns.

Dejan IVANOVIC
Process Engineer, MSChE

 

[songkarn]

Dear EmmanuelTop ,
Thanks lot for your kindly take more concern.
Best regards,
SK