Snubber. 4

[Frank1344]

Dear Friends,

we are working on a blow down piping system which is prone to slug accumulation and may see a huge slug force in the magnitude of 50 to 75 kN.

We will make sure that the line will be adequately supported with guides and anchors and will design the structural steel based on the slug force.

I applied the load to the elbows one at a time to see its effect, in some we are fine and in a few we are failing.

Is installing an hydraulic or mechanical snubber to the elbows (change of direction spots) helping to mitigate the slug flow effects?

My apology in advance as my background in Dyanmics Analysis is very low.

Any help and comment is appreciated.

Kind Regards,
Frank

 

[LittleInch]

frank 1344,

Personally I think you're doing this completly the wrong way around. Get rid of the slugs then design the piping. B31.3 and all other codes and practices require vent piping to drain liquids away or catch then in knock out pots before they enter vent piping, precisely because of the risk of the vent piping rupturing in operation.

The texaco pembroke refinery blew up some years ago because liquid got into the vent piping and blew a huge hole in it which then covered the plant in the gas it was trying to vent, which then exploded.

If you insist on this action then I think the answer is no - a snubber or other flow reducing device won't work. It would also be interesting to see how you came up with the slug velocity and force.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Frank1344]

This plant is existing and there is no proper drain/ vent system.

We already asked the Client to modify their system but no success.

The velocity and density are coming from Process engineering. The one parameter that process has a problem to provide us is the slug length.

We used F=p.A.V^2 to calculate the slug force.

 

[LittleInch]

Frank 1344,

I don't know where this plant is, but in my opinion you need to advise the client in wiritng that what he is doing goes against all known codes and safe operating procedures and possibly violates local safety laws and then walk away. To do otherwise is to risk being implicated in designing somehting which is inherently dangerous and unsafe. I don't know how your process designers have come up with a figure for slugs, but it is likelly ot be inaccurate, ditto the max V - what is it by the way?

See also the thread below which gves you the references to quote.

http://www.eng-tips.com/viewthread.cfm?qid=342205

I hope others who post here agree with me and I wish you luck in your discussions with your client, but sometimes writing somehting down in clear unambiguos language and keeping a copy can make even the most obstinate client think again.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Frank1344]

Thanks a lot for your feedbacka.

V=50 m/s
Slug Density= 984 Kg/m^3
Slug Length= 118 meter

 

[LittleInch]

Frank,

Apart from everything else at 50 m/sec the issue isn't force but shock loading. The pipework would just leap off your piperack or you'd need to make it out of such thick pipe it would cost you a fortune. Slug length is irrelevant. Good luck.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Frank1344]

Hello LittleInch,

I elevated the concern to higher management and they are all agreed with your comments.

After showing these numbers to the Client, we are going to change the design to a system with proper vent and drain.

Thanks a lot,
Frank

 

[LittleInch]

Wow, thanks for feedback and glad to be of assistance. Glad that you're now going to design a proper system.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[C2it]

Littleinch seems to have solved your problems.

IMHO your design parameters are not feasible. Littleinch is correct in that the slug length is irrelevant since only a given mass can turn around an elbow in a given time. Thus once the elbow is filled, length doesn't matter.

Velocity and density however do matter. The kneejerk response from a Process engineer will be to give max values for both. But liquid will not stay that way when propelled at 50m/s in a pipe. It will become mist or annular and thus the density will be far lower, maybe similar to gas, thus the slug force impact will be way down. A decent Process simulation would prove this.

 

[BigInch]

Frank Congratulations. You did exactly what you needed to do.
Glad you took the advice and achieved positive results.
I'm sure it took a bit of courage.
Great advice Littleinch.
Stars for both of you.


Independent events are seldomly independent.