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Flare Tip Pressure Drop


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#1 benoyjohn

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Posted 05 September 2010 - 01:15 AM

Dear all,

We had an incident in one of our projects in which the vendor supplied a flare tip which was offerring a pressure drop of 1500 kpa against 15 Kpa which was the design parameter and guarunteed value. The design gas flow rate was 1400 ksm3/day gas. During actual plant operation it was observed that the back pressure value at flare stack bottom was 200 kpag at just 300 ksm3/day gas flow.

It was clear that the flare tip was incorrectly designed. However this was discovered a bit too late, in fact after 1 year of commisioning the flare tip. This has caught everyone by surprise as the plant integrity was doubted. During an emergency which requires the relief valves to operate, the relief header will be choked due to the high back pressure at the tip.

The tip pressure drop was confirmed to be less than 15 kpa all along the design phase and even now by the vendor. His supporting calculations are incorrect as we know now.

My querry to all of you is

1. Have you come across such gross errors in design by the flare vendor which can put the plant safety in jeopardy? How can this be found out and corrected in the design stage? (Apparently the pressure drop calculation is considered a propritery of the vendor and so is the tip design).

2. Are you aware of vendors who can carry out hydraulic capacity test (flow versus pressure drop) of the flare tips they are offering so that we do not end up with such difficultes?


Thanks and regards,

Benoy

Edited by benoyjohn, 05 September 2010 - 01:25 AM.


#2 ankur2061

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Posted 05 September 2010 - 03:43 AM

Benoy,

Why would the pressure drop for a flare tip be considerd proprietory unless the flare tip has some intricate internals with tortuous and twisted flow path for the flare gas.?

If it is a staright forward "pipe type" flare tip then the hydraulic capacity check (flow vs pressure drop) should be straight forward as of any pipe with a given diameter.

Is the flare vendor "Boskel" from Nigeria? If yes, then I had similar problems with their exaggerated claims of offering complete flare solutions for another job where they could not even provide the pressure drop for a reducer from the flare stack flange connection to the flare tip. I think the client (you know who) should seriously consider blacklisting this vendor.

The immediate and acceptable solution would be to replace the flare tip at the first available turn-around.

Regards,
Ankur.

#3 Zauberberg

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Posted 05 September 2010 - 08:29 AM

Flow vs Pressure drop (or Flow vs Backpressure) should be one of the vendor deliverables in any case. However, don't be confused with the fact that curve shows higher flows at higher pressures upstream of the flare tip - which is quite normal considering velocity limits at different pressure levels.

The pressure drop of 1,500 kPa indicates high-pressure or sonic flare, and that is OK from design point of view. The only thing you haven't told us is whether the flare system itself has been designed for 1,500 kPa + Max. Backpressure + Design margin. I would be very much surprised to see the flare system design pressure of 3.5 barg where the flare tip design pressure drop is 15 bar.

If this is the case then, As Ankur says, it is sufficient not only for blacklisting the vendor but rather for taking a plasma cannon and blasting him away.

#4 benoyjohn

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Posted 06 September 2010 - 05:35 AM

Benoy,

Why would the pressure drop for a flare tip be considerd proprietory unless the flare tip has some intricate internals with tortuous and twisted flow path for the flare gas.?

If it is a staright forward "pipe type" flare tip then the hydraulic capacity check (flow vs pressure drop) should be straight forward as of any pipe with a given diameter.

Is the flare vendor "Boskel" from Nigeria? If yes, then I had similar problems with their exaggerated claims of offering complete flare solutions for another job where they could not even provide the pressure drop for a reducer from the flare stack flange connection to the flare tip. I think the client (you know who) should seriously consider blacklisting this vendor.

The immediate and acceptable solution would be to replace the flare tip at the first available turn-around.

Regards,
Ankur.


Ankur,

I do not want to name the vendor in a public forum such as this.

The flare tip supplied is not a pipe type. It has got several nozzles on the top ; more like a shower head with multiple 1" short tubes fitted above the openings. Additionaly it has a fluidic seal also.

regards,
Benoy

#5 benoyjohn

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Posted 06 September 2010 - 05:45 AM

Flow vs Pressure drop (or Flow vs Backpressure) should be one of the vendor deliverables in any case. However, don't be confused with the fact that curve shows higher flows at higher pressures upstream of the flare tip - which is quite normal considering velocity limits at different pressure levels.

The pressure drop of 1,500 kPa indicates high-pressure or sonic flare, and that is OK from design point of view. The only thing you haven't told us is whether the flare system itself has been designed for 1,500 kPa + Max. Backpressure + Design margin. I would be very much surprised to see the flare system design pressure of 3.5 barg where the flare tip design pressure drop is 15 bar.

If this is the case then, As Ankur says, it is sufficient not only for blacklisting the vendor but rather for taking a plasma cannon and blasting him away.


Zauberberg,

The flare was NOT intended to be a sonic one with the flare KOD itself being designed for only 6 barg. The vendor had even confirmed Mach number below 0.5 at the tip in the evaluation stage.

regards,

Benoy

Edited by benoyjohn, 06 September 2010 - 05:47 AM.


#6 Zauberberg

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Posted 06 September 2010 - 09:48 AM

So you are getting 200 kPa pressure drop across the flare tip @ 20% of design relief rate? You may not encounter choked flow at maximum relief load if the corresponding back pressure is less than <6 barg (you have to calculate this) but you will have a high-pressure flare in such case, and that is completely different design scenario which your relief valves may not be capable to handle.

You can try to simulate 100% relief rate and see what conditions you will reach in the flare header/network. If it is excessively high, I would suggest the following:

- Confirm what is the minimum back-pressure from the flare tip you need in order not to exceed Mach criterion in the flare header at design relief load (according to your design input, it is equal to 15kPa = full relief load @ 116kPa results in 0.5 Mach in the flare header)

- Replace the flare tip with appropriate device, such that the maximum back-pressure in the system will not exceed design figures while still allowing for the full relief load.




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