Header Capacity

[bucs2047]

Hello All,

A new hot issue got thrown on the plate, having a tough start on how to tackle and would appreciate any help. I’m trying to calculate if the existing installation of a ‘surge header’ at a gas plant would handle a total plant upset condition – meaning if every valve lifted simultaneously.

Currently there’s 36 relief/safety valves piped to the surge header, which vents Methane, Ethane, & Propane to the stack. I have the design mass flow rates for each relief valve and of course could calculate the total volume of the piping, but not sure I’m on the right track.

Thanks

 

[MortenA]

seems so

However all PSV might not lift at the same time? There are differen design cases for the various PSVs. Try to identify these and see which happens at the same time. However iff all at the same time poses no problems then why bother?

Best regards

Morten

 

[stanier]

Sounds like you need AFT's Arrow program or an engineer that uses same.

http://www.aft.com.

AFT might take the job on as a consultant and provide an answer quicker and more conomically.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[bucs2047]

Morten/Stranier

Thanks for the tips.

 

[iainuts]

It sounds like a pain in the ass more than a difficult problem to solve. Start at the outlet of the stack where you have a known pressure. I assume this is going to be atmospheric pressure where the stuff is burned but that could be just a bit more complicated. Exactly what pressure is at what point in the vent near the outlet will be something you'll need to work out with the flare stack mfg, assuming it's going to a flare stack.

From the point of known pressure, you have a flow which is basically the flow of all 36 RV's open at once. If you follow that vent header back to the first T, you have some piece of pipe with all this gas flowing through it, and you should be able to determine the pressure drop in that section of line. Now you have a new known pressure at a new location (the T). There's gas coming in from each branch of the T and going out to the flare stack. Just continue to work back from that point. Every time you have a T, calculate the dP between the various points.

I'd agree with stainer that you could simply put everything into a program and let the program solve for you, so that's another way.

Also, I'd agree with Morten that you may not need to consider all RV's lifting at once. But at least if you do, you've covered the worst possible situation. Also, to rule out certain RV's lifting, you may need to go through a HAZOP with safety and the plant folks. Could take much longer just to do that than to simply do the calculation (even if you have to do it by hand/computer/spreadsheet).

 

[dcasto]

I look at it this way, If the 10MMSCFD going through a compressor, and all suction, discharge, interstage, fuel, ect PSV went off, how could you have more than 10MMSCFD of gas in the header (OK maybe you have a feed avaiable of 15MMSFD), so why would you need a headr for more than 10mmscfd (15mmscfd assuming a feed of 15)?

Also, the header doesn't have to be sized for 25 psig for this worst case scenerio. Sizing it for 50% of the lowest PSV set point would be acceptable, IMHO.

You'll have to join my club of "Engineers asked to do stupid thing by people who practice Engineering without a clue." . I still remember the day they made an Accountant the VP of Engineering.......

 

[bucs2047]

Jainuts - Make some first rate points.

Yes it is a flare stack and there is a ID fan as well, however I am still going under the assumption atm pressure. Honestly how would I determine/calculate the pressure drops on the header at all the various points?


Dcasto - Valid point as well, however there's four different stages throughout the process which include suction discharge, interstage etc.. All of which have safety reliefs devices and I very much understand what you are saying In=out, however I believe the header was designed over 30 years ago, this header was added to and modified and drains and stuff piped to.

But it thats the case, is there a way to just calculate the allowable volume within header and what in theory is in the process.

 

[dcasto]

An old engineer reminded me once of an engineering principal: If a piece of machinery has gone through a million cycles and not failed, why would you count on it failing at 1 million and 1.

As operator, you must select a scenerio that you believe is reasonable. Just like in RMP, EPA says a complete failure of everything must be filed with them, but the operator should select a resonable failure to prepare for.

I went to a community activest lecture in Houston one time. The community leader stated that she wanted the industry to be prepared for a 747 crashing into the largest refinery in the US located in Baytown Texas and that the death toll would be zero and the damage offsite be zero. Are you ready fro that scenerio at you little gas plant?

Back to my earlier point, wht should the flare header on one compressor (wit a jillion stages) need to be larger that what it can move? As soon as 1 CF leaves one relief valve all the others will stop associated with that compressor.

To chose your scenerio you need to look at your inlet ESD's, piping, plant layout, liquid inventory, control loops and decide what is a resonable rate. I disconnected some relief valves from the header at one plant because they were redundant sort of, they protected the bypass system on the edge of the plant when the ESD was activated, their set pressure was 10% higher than the plant PSV's. There are lots of thing you can do to bring the project back to reality. I could probabilly send you to several engineering consultants that would help document your assumptions.