Flare with multiple inlet 1

[engr2GW]

Hello,

I have a question about flare design on a tank battery oil/gas facility
Situation: we have waste gas from atmospheric tanks 0-10 oz/si, a pressure vessel of 1-4 psi, and another vessel of 10-20 psi, all going to the same flare that has only 2 inlets. when we tie the low pressure vessel and tanks together, we see gas leak from the tank due to back pressure from the vessel at 1-4 ounces of pressure. both inlets to the flare have separate piping going to the flare

the two solutions I'm thinking about are
1. put a third inlet to the flare and have all three sources have different inlets so that there'll be no interaction of pressure OR
2. size the flare line for all three lines as one, but have enough pipe size to where there might not be any back pressure on any of the vessel

anyone with this experience before? any thoughts?

As much as possible, do it right the first time...

 

[EmmanuelTop]

A precise sketch would help very much in this thread.

Regards

Dejan IVANOVIC
Process Engineer, MSChE

 

[engr2GW]

sketch attached
thanks

As much as possible, do it right the first time...

 

[EmmanuelTop]

Thank you. What is the logic behind connecting pressure vessels and atmospheric tank(s) to the same flare header? This is definitely not a good practice, which may result in overpressure of the tank. If the calculated backpressure under the maximum simultaneous relief rate is higher than the tank design pressure, you have a big problem there.

Dejan IVANOVIC
Process Engineer, MSChE

 

[EmmanuelTop]

Maybe the previous reply was too short and lacking of description, so I will try to elaborate a bit further on the subject.

Each system connected to the flare, including the flare itself, is commonly designed for 50 psig pressure (roughly equivalent to 3.5 barg). There are certain variations on the subject (i.e. equipment with design pressure >10 barg connected to HP Flare, equipment with design pressure <10 barg connected to LP Flare), but this is more or less generally accepted process design criteria. The key reason is to ensure integrity of the connected system in case of high relief or process loads resulting in significant backpressure from the Flare tip.

Atmospheric tanks are a completely different category. If a flare system is required for storage facilities, then a dedicated storage flare must be provided, and it must be completely segregated from the process flare. Tapping atmospheric storage vents into process flare headers/piping is normally not allowed, for the reasons mentioned in the paragraph above. I believe the facility/storage depicted in your sketch is suffering from this problem - although not (yet?) in catastrophic proportions that would result in rupture of the tank due to excessive backpressure.

There is no engineering logic which could justify connecting two systems with different design pressures without a specification break and appropriate overpressure protection system on the lower pressure side. Ultimately, if both systems discharge to the same relief header and flowing up to the same flare tip, they essentially become parts of one single system. And that, obviously, cannot be accepted.

Dejan IVANOVIC
Process Engineer, MSChE

 

[Longinthetooth]

Emmanueltop is exactly right. This arrangement is DANGEROUS.

 

[golpesar88]

Hi guys, Actually I am more practical than a designer but as I remember in our projects during commissioning and start-up, there was 2 system for low pressure gases, one as normal low pressure design and another for Low low pressure systems like offspec storage tank with high content of H2S! they connected all these lines to a small tiny KO drum and then the outlet goes to Low pressure flare stack in parallel with Low pressure system.
Low low flare pressure system: dedicated KO drum, dedicated flare network
Low pressure flare system: dedicated KO drum, dedicated flare network
Flare: common between LLP & LP system.
we had MP & HP flare system also, pressure varies from 0 up to 130 barg in our plant.

and at the end, never let a atmospheric fix roof tank goes over pressure, I saw the disaster by my eyes! lifted up by air and water (two different case!)

 

[georgeverghese]

Time to stop your plant for some urgent modifications before anything worse happens