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Flare System Design

flare tailpipe pressure header api521

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#1 Prashant Process

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Posted 18 November 2011 - 02:19 PM

Hi,

The objective is to calculate tailpipe velocity.

Case Description:

Hydrocarbon system pressure is 50 barg. System temperature is 20C. There is a restriction orifice downstream of the blowdown valve.The peak blowdown rate is 25,000 kg/hr. The critical pressure is 26 barg. The header pressure is 1 barg. The velocity in the tail pipe is to be calculated.

Approach considered-
Assume flare header pressure to be 1 barg.
Calculate the pressure drop in a 10m tailpipe.
This is the pressure at the downstream of the restriction orifice.
Now using this pressure calculate the volumetric flowrate and then the velocity.

Few of my colleagues suggested to consider 1 barg pressure at the downstream of the RO but this results in a highly conservative tail pipe sizing.

Is there any standard or calculation for tail pipe velocity.

Your valuable comments will help me immensely.

#2 ankur2061

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Posted 18 November 2011 - 10:54 PM

Prashant Process,

Get hold of API STD 521 and refer Section 6.3 for guidelines related to tail pipe velocity.

Regards,
Ankur.

#3 Prashant Process

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Posted 19 November 2011 - 04:14 AM

Ankur ji,

API 521 section 5.4.1.3 Design of Relief Device Discharging Piping mentions that the discharge piping - tailpipe should be designed for rated flow of the device and more conservative design approach needs to be followed for header design.

API is not very specific about the allowable pressure in the tail pipe section, In this case the inlet pressure is 50 barg (P1) the header pressure (P3) is known 1 barg. The question is what should be the pressure in the tail pipe(P2). For example currently the pressure in the tailpipe is 7 barg for a 4 inch line size. The mach no. is 0.3 which is acceptable. Is there any guidance which specifies that the tail pipe pressure or least standard P2/P1 ratios. There is P2/P1 table in API 521 but no recommendation on allowable ratios.

Await your guidance and directions

#4 ankur2061

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Posted 19 November 2011 - 05:45 AM

Prashant Process,

Negative feedback on replies for posts is provided when the reply is either offensive or it is totally irrelevant to the subject and not at the whims and fancies of an individual. My reply providing reference of API STD 521 is not irrelevant to the subject nor is it offensive so your negative feedback is not justifiable.

For further reference in tail pipe design NORSOK standard P-001 provides information for Mach Nos in tail pipes. It can be downloaded from the following link:

http://www.standard....ue&preview=true

Ankur.

#5 Prashant Process

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Posted 19 November 2011 - 05:59 AM

Ankurji,

I apologise for my accidental clicking on the reputation icon of your reply.

I can't change it - Is there a way to undo it.

Your answer was relevant to my question and you have been kind to reply to my query.

Sir - I mean no irreverence.

Thank you for the NORSOK link.

I will surely refer to it.

Thanks and Regards

Prashant Kurle

#6 Prashant Process

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Posted 19 November 2011 - 06:19 AM

Ankurji,

I think I am not asking the question lucidly.

Norsok P-001 gives a general guidance to mach number in flare headers and sub headers.

My question is - if inlet pressure is 50 barg- and header pressure is 1 barg

With 7 barg pressure in a 4" tail pipe I am arriving at a mach no. of 0.3 which is acceptable.
My counter part has challenged this - they deem that the pressure in the tail pipe should be same as the pressure in the header that is 1 barg. With this 1 barg pressure and 4" tail pipe the mach no. is 1.5 which is not acceptable, this is due the fact that the volumetric flow rate at 1 barg is incredibly high and tail pipes are not designed for this flow. I need to upsize this tail pipe to 6" or more to conform to 0.7 mach no.

Alternatively, I was looking for a standard which mentions that the pressure in the tail pipe need not be same as the header. With help of this standard I could make a case for a allowable pressure drop in the tail pipe.

Look forward to your comments.

#7 breizh

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Posted 19 November 2011 - 08:14 AM

http://www.cyberax.e...ocess-design-ma

let you consider this resource from Fluor daniel.
Hope this helps
Breizh

#8 ankur2061

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Posted 19 November 2011 - 09:44 AM

Prashant,

You cannot have a Mach No of 1.5 for a PSV tail pipe. We are talking of choked flow at Mach No: 1 and above which is not practical. You need to increase the tail pipe size to reduce the Mach No to 0.7 or below. The other thing which is important for the sizing of the tail pipe is the backpressure (superimposed + variable) at the relief valve outlet. It cannot exceed the folowing:

Conventional Relief Valves: 10% of the set pressure

Balanced-Bellows: 50% of the set pressure

Pilot-Operated Relief Valve: as high as 70 to 90% of the set pressure

API STD 520 Part 1 provides the above mentioned values for conventional and balanced bellows valve.

Hope this helps.

Regards,
Ankur.

#9 paulhorth

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Posted 19 November 2011 - 11:58 AM

Prashant,

pproach considered-
Assume flare header pressure to be 1 barg.
Calculate the pressure drop in a 10m tailpipe.
This is the pressure at the downstream of the restriction orifice.
Now using this pressure calculate the volumetric flowrate and then the velocity.

Few of my colleagues suggested to consider 1 barg pressure at the downstream of the RO but this results in a highly conservative tail pipe sizing.


Your approach described here is correct, and your colleagues are wrong. The pressure can only be the same at both ends of the tailpipe if there is no flow. As soon as you open the blowdown valve, there will be a pressure drop along the tailpipe, which will produce a pressure at the RO which is higher than 1 barg. Your job is to calculate what the pressure drop is, using a compressible flow equation.But you can check the tailpipe size without even doing this.

The highest velocity will be at the outlet end, where the pressure should be about 1 barg, and this velocity should be below 0.7 Mach. If you get a velocity higher than 0.7 Mach at this point you need to make the tailpipe larger. If you get a velocity higher than 1.0 Mach, that is not possible and it means that the pressure there is actually higher than 1 barg, with a sonic shock as the gas goes into the header.

You haven't given the mol weight of the gas, but let's try a calculation with a few estimates.

If the mol weight is say 25, then the density at 1 barg (2 bara) and say 20 C will be about
2 x 25/(0.08314 x (273+25)) = 2.0 kg/m3.
Volume flowrate at 2 bara = 25000/(3600 x 2) = 3.47 m3/s

4 inch tailpipe c/s area = 0.0079 m2
Hence, velocity of this flow at 1 barg is 3.47/0.0079 = 442 m/s. This is just about Mach 1 or maybe above. Therefore you need a larger tailpipe.

Notice that I haven't introduced the length or the upstream pressure in this calculation at all.

Now, do the same for a 6 inch tailpipe: Velocity for 1 barg will be about 196 m/s which is much better. Now you can calculate the pressure drop along the line using a compressible flow equation. This will tell you the pressure at the RO.

Paul

#10 kkala

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Posted 19 November 2011 - 02:33 PM

My understanding suggests following view, subject to critisism (see attached Fconfig.doc).
1. Said blow down valve (BDV) is not a PSV due to downstream restriction orifice (RO); it could be a depressuring valve (http://webwormcpt.bl...ce-is-some.html).
2. Due to high upstream pressure (50 barg) flow is choked, at BDV for small flows or at RO for higher flows (when BDV is more open). Moreover max flow downstream RO is given at 25000 kg/h, apparently corresponding to choked flow at the RO.
3. Approach considered by Prashant Process is principally right. Starting from flare header pressure and proceeding upwards in the tail pipe (flow=25000 kg/h), pressure P downstream RO can be found. Assuming P low enough to verify the assumption of choked flow at RO (it is expected so), developed velocities have to be looked into.
4. Velocity in tail pipe should not exceed 0.7 Mach at point of lowest pressure (*), which is the point A of its connection to flare header. Choked flow not occurring at A, tail pipe pressure at A will not have discontinuity, being practically same as flare header pressure Ph at A.
For Ph=1 barg~2 bara, 4" tail pipe velocity at A is probably ~8/2*0.3 = 1.2 Mach (unacceptably high**).
(*) Note: Mentioned Mach criterion is routinely applied when sizing flare header and its branch lines using Flarenet. It must comply with API 520 / 521, but I do not have these available at hand.
5. Ph=1 barg is understood to be max operating flare header pressure at A, when numerous discharges go to flare simultaneously (e.g. in power failure, or general fire). If only referred flow is sent to flare header, Ph will be less than 1 barg, probably close to 0 barg.
In this case 4" tail pipe velocity at A is probably ~8/1*0.3=2.4 Mach (unacceptably high**).
(**) Note: "Probable" Mach numbers are based on the (uncertain) assumption that 0.3 Mach corresponds to 25000 kg/h at 7 barg. Please advise.
6. Consequently tail pipe diameter has to satisfy that gas velocity at A should not exceed 0.7 Mach under any possible operating pressure at point A of flare header. Comments on the above would be appreciated.

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#11 kkala

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Posted 19 November 2011 - 03:03 PM

After having sent the previous post, post be paulhorth was noticed, clearly and practically explaining what I tried to express in points 3 and 4.
When reply window is open, a new post can escape attention.




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