Pigging Forces 4

[sigma1]

What are the forces, if any, applied to the piping system during the pigging process.
Any information will be appreciated.

 

[rconner]

I would think you would need to get this information from the manufacturer of/specific experts in the specific pigging device/operation and for the specific purpose. If one pig design develops more friction or resistance to motion against the pipe/lining wall than another, I would think there would be more axial? force applied to the piping. Likewise if a pig were to for some reason (a partially closed valve, other stubborn obstruction etc.) abruptly become stuck/"jammed", I wouldn't be surprised if the pressure/motion against same might exert more force/impact than a normally moving pig.

 

[BigInch]

Figure a pressure differential across a pig of 200-400 psi for larger diameters going up to as high, in some cases, as much as 800-1000 psi for smaller diameters.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[zdas04]

BigInch,
Those are very big numbers. An 80 lbm, 16-inch turbo pig will move with less than 1 psi dP (1 lbf applied over 201-in^2 is about 200 lbf). Putting 400 psi across the same pig is 80,000 lbf which would move Volkswagons let alone 80 lbm pigs.

I generally see about 5 psi dP plus the weight of water on the longest uphill on 6-inch and smaller pigs (i.e., if you have 100 ft uphill elevation change your dP available needs to be 5+100*0.44=49 psig).

The coeffecient of sliding friction is a pretty small number when you're pushing liquid so you really only have to provide enough dP to shift the mass plus a small amount.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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[BigInch]

Those were reported from a series of gas system runs. I was not present. I thought the forces were quite large too, but they supposedly came from the "pigs mouth" so to speak. Perhaps they are instantaneous spikes, and I can't argue as I agree still seem to be on the high side.

If you have some direct observations I'd appreciate as much detail as you can spare.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[zdas04]

I've watched upstream pressures really closely a bunch of times (I really don't have a life), and I've never seen a spike much higher than the highest hill full of water.

Once I put a Barton pressure chart on a launcher and the biggest dP I saw (about a 5 psi spike in a 20-inch line) was when the pig was trying to get out of the launcher dry. After it started moving, you could tell where it was by the pressure reading (it was pretty cool data). The pressure at the end of the line never varied from 100.0 psig during the run (I had people stationed at the Compressor Station to make darn sure of that) and the pressure at the launcher was about 1 psi higher than that on the flat, up to 50 psi higher on the up hill and up to 20 psig lower on the down hill (I'm assuming that was from the water running away from the pig and pulling it along). The annotated chart was in a file that I left in my old office when I retired, its probably been discarded on a records-retention binge like most of my other paper.

David

 

[BigInch]

David, buy a scanner.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[zdas04]

Got a scanner, just don't have access to the files that I left behind when I retired.

David

 

[acedude]

Significant forces can be imparted to a pipeline due to a "runnaway" pig in gas lines as the pig moves through a bend. When a gas line pig gets "stuck" for any reason, the gas behind the pig compresses, and the gas pressure in front of the pig literally drops to nothing quite rapidly. When the pressure in back of the pig sufficiently reaches a value to "un-stick" the pig, the stored energy in the compressed gas behind the pig is released. Stuck pigs have been reported to reach 200 km/hr or more, sometimes resulting in the gas line jumping off above ground supports where there is a bend in the line. The forces at a specific bend can be calculated in accordance with classical mechanics knowing the pig mass and estimate of pig velocity. This is very similar to calculating slugging forces at bends in mutiphase lines.

For liquids lines, cleaning pigs normally run at 10 ft/sec or less, and instrumented pigs need to be run at 5 ft/sec or less. Therefore there is generally no significant inertial pig forces created at bends at these velocities in liquids lines.

 

[zdas04]

Acedude,
Interesting combination of units there. Just so that everyone doesn't need to get out a calculator, 200 km/hr is about 82 ft/sec which is a high but not outrageous gas velocity in a low pressure flow line. I've seen pigs run a lot faster than that, and yes, it can get exciting.

When you calculate the forces that these chunks of liquid and pig apply to the lines you rarely reach a significant fraction of SMYS--steel pipe has considerable elasticiity.

David

 

[BigInch]

Fortunately.

As long as the forces remain axial, axial stresses won't be much, however going around a 90 may be another story when some rather large displacements can be developed with that elasticity.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Ashereng]

Wouldn't the type/design of pig also matter?

Most of the pigs I am familiar with ride in the process fluid and flow with the flow as it were. I would have thought it wouldn't really impose much more pressure/stress to anything beyond what the process fluid already does.

Then again, I am no expert - just my thoughts.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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