Flare adequacy check and modification

[MHRG]

My requirement is to add a PSV discharge to an existing flare system.
My plan is to model the entire existing flare system and then check for the adequacy of the new one.
What is the best way to model the same, is it by Modeling using Flare net or using Hysis dynamics environment.

 

[EmmanuelTop]

Look at the governing scenario for flare sizing and see if the new PSV is supposed to pop open during the same scenario. If it doesn't, then you probably have nothing to worry about.
If it does, look at what is % of flow from the new PSV vs. total relief flow in the flare header. If less than 5%, again you have nothing to worry about.
If more than 5%, it is worth checking how the system interacts. If you have access to Flarenet or Flaretot, that is probably the fastest way to check system performance.
I'd say, based on my experience, that only as you approach 0.5 Mach at the flare tip (assuming you operate subsonic flare) things can get more complicated.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

A dynamic simulation of this PSV in operation installed as intended isnt required - any well known flare system hydraulics program will do. Add Flaresim also to ET's list. Some process engineers have their own spreadsheets for this too. You also need to confirm that the built up backpressure on the PSV exit is less than the maximum permissible.
Expressions for compressible flow behaviour can be found in the API 520 / 521 and in Perry's Chem Eng Handbook if you want to build up a spreadsheet.
Calculations of this sort ( which also includes the calcs related to the derivation of the controlling case relief load which sets the PSV orifice size) are process safety critical and must always checked and signed off by at least one other senior engineer familiar with how these calculations are done. This checker engineer must be authorised in writing by Plant Engineering Management to sign off such calcs. All process safety related studies / calcs etc should be maintained as part of the overall plant safety case documentation for audit / Plant Operations staff reference material / insurance purposes.

 

[MHRG]

Thank you Emmanuel,
Peak flow case of each Relief system connected to flare is taken as the governing case for Flare sizing.

In the second point, Is it evident from any standards, to convince the client ?

 

[MHRG]

Thank you George for your suggestions.
Is there any possibility to get the spread sheet you are mentioning.

 

[EmmanuelTop]

You haven't really supplied any data so how can we comment on the subject of convincing the client?

You didn't convince me, for example.

Dejan IVANOVIC
Process Engineer, MSChE

 

[MHRG]

In my case the ratio is well above 5%.

Existing flare capacity is 35 MMSCFD and the the rated flow of new PSV is 24MMSCFD (governing case: Tube rupture)

 

[EmmanuelTop]

It looks like it can be listed as a completely standalone scenario i.e. it does not seem connected to the existing relief scenarios for which the flare system was sized. I think only you can confirm that. List the events/releases for each scenario and confirm if tube rupture is an independent, unrelated event. That would be an evidence.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

If there are any continous flaring / venting events that may be coincident with this tube rupture case, then the sum total would be the rating check case. For example, if you have x mmscfd going continously to flare as a result a recovery compressor outage, then the max flow to be used would 24+x mmscfd. The max backpressure on the PSV should be found to be less that permissible for the type of PSV intended. Conventional PSV backpressure is 10% of set pressure, while that for bellows operated PSVs is usually 50% of set pressure. Mach number limitations also apply to both main header and feeder laterals.