Pipeline pigging: pig friction factors and pig velocities 1

[LaSalle1940]

This is really a two-part question:

FIRST: what are the general guidelines for velocities for pipe pigs for cleaning liquid lines? I would venture a guess that a pipe pig has to travel within a certain velocity range to ensure reasonable cleaning without bypassing deposits, or moving so slowly that there's not enough momentum transfer to knock depostis off walls.

SECOND: are there references in the open literature for friction factors for pipe pigs? And if anyone knows of such references, would they please provide exact citations? It seems to me that the velocity guidelines from the first part of this question (in conjunction with the line size itself) would determine the flowrate of the liquid providing the motive power for the pig--but the resistance to forward motion provided by the pipe and the stuff that you'd want to clean off would determine the pressure of that same motive fluid (well, that, plus the geometry of the pig).

Thanks very much.

 

[CRG]

The following link has some information; however, it may not as specific as you would like.

http://www.girardind.com/articles/pigging-pressures.htm

I would contact the folks that manufacture the type of pig you are planning to use and inquire about a specific application. If they are knowledgeable and helpful, they might be able to point you towards some technical papers or book on the subject.

 

[zdas04]

I was just doing some rough calculations, and I believe that I've been responsible for upwards of 10,000 pig runs in pipes from 3-inch to 20-inch. The velocity has been what the velocity wants to be. Your question assumes that the engineer and/or operator has some control over the pig velocity--they don't. I've calculated velocity after the fact many times and get numbers that vary by over an order of magnitude.

The definition of a "successful" pig run is that the pig comes home. If it comes home with bypassed liquid, then you need to increase the pigging frequenty to harvest that liquid over time. Every time I've seen someone try to apply science to pigging it has turned into a tail chasing exercise that was only of interest to the scientists.

The bottom line of pigging is that when there is enough force (represented by dP) behind the pig to move it, it will move at a velocity consistent with the ongoing dP (I started to say "steady state dP", but a pig run is a very unsteady undertaking and you never sustain any partiular velocity for any predictable period of time).

Think about the forces. 2 psi differential applied over a 20-inch diameter pig exerts 628 lbf. A 20-inch turbo pig weighs about 45 lbm. The coefficient of static friction is a non-zero quantity, but it is far from enough in most pipes to resist 583 lbf. I've seen it take 3-5 psid to start a pig moving when the fluids in the line are sticky, but once they start moving the upstream pressure drops to very near the pre-pig value.

I know that this is all anecdotal (and remember that the plural of "anecdote" is not "data"), but I got through a pretty successful career running pigs by the seat of my pants without much in the way of theory or arithmetic.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[SJones]

http://www.pigsource.com/default.cfm

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/