Hazard from object in motion due to differential pressures (forces)

[frogcurry]

This is a tricky question to define as its not a specific item or system, but a generic concept. While reading the Loss Prevention Bulletin article this month on pigs, I read a comment about "20 tons of force" driving a pig out of a pipeline which had its pig reciever open causing the pig to be launched through the air. This got me thinking, i.e. when dealing with residual pressures in purged vessels and opening manways, and other such situations there are a number of possible situations where we can deal with forces driving objects "out" where they can be a hazard. But converting that "20 tons" or a "150 kN" force into an assessable hazard is hard for me to grasp.

It seems to me that it should be assumed that an object won't be slowed down much by friction etc if its been launched or pushed out into atmosphere, in which case the concern isn't usually going to be "how fast" which was my initial thought but "how much force" it delivers in any impacts on discharge. Is this correct or is velocity a factor in judging potential hazards of this sort (in which case the velocity that the pig in the LPB article would need to have been esimated)?

But is this a kinetic energy issue or momentum issue when calculating possible hazards (my initial thought is to use the law ofconservation of momentum but disregarding KE doesn't seem quite right)? Also at what level does a force have the ability to become dangerous (i.e. if I'm hit by a 1 kg object propelled by a force of 400 kN how problematic is this? What about a 10 kg object propelled by a force of 400 kN?)

Apologies if this is a bit vague, but as I said its a very generic query. Thanks to anyone who replies.

 

[zdas04]

Normally, heavy things like pigs are contained within sealed receivers. The big exception is running drying pigs after a hydrotest on new construction. These are often launched without a proper receiver so the pigs exit the line with a lot of force. A couple of examples.

An 8-inch line open ended pointing up. With 10 psi air pressure we pushed the water out with a pig (very flat line). When the pig exited it went about 50 ft into the air and was lost in the nearby woods.

A 20 inch mandrel pig (weight just over 500 lb) pushed by 50 psi air down a 10 mile line. We couldn't get the pig to move with the receiver closed and a vent open so we opened the closure, but positioned a track hoe bucket in front of the exit. When the pig came out and hit the hoe the near track came off the ground about 18 inches and the steel mandrel was bent enough that we could never run it again.

There is an amazing amount of force associated with a small dP applied over a large surface area. Increase the dP slightly and the forces can become very dangerous. I just don't know when you would ever expose people or external equipment to those forces in normal activities. The exact scenario would have to be considered to determine the magnitude of forces in a particular case.

I think that the governing equation would be the same arithmetic you would use to calculate the force of an auto collision.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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[frogcurry]

Thanks. What you describe is exactly the sort of scenario that the Loss Prevention Bulletin describes save that the pressure differential driving the pig was on the order of 400 psig for a 12 in pig...

When you're doing this do you have any estimation techniques you use for assessing how fast the pig could be travelling on exit and the force it has, or is it just a matter of letting it fly out and making sure no-one is downstream?

 

[zdas04]

With dP and surface area you can calculate force on the pig and then f=ma and the integral of acceleration is velocity. Now you just have to guess what the resistance to the pig moving--that is a WAG (not even a "scientific" wild ass guess). If you ignore friction, then the velocities can reach astronomical numbers pretty quickly.

I do that math, but it doesn't mean much. Mostly you just open it up and try to keep from getting wet/killed.

David

 

[epoisses]

...and even that is a simple calculation, because it's a linear motion, compared to any potential velocity that a manhole cover (as you mentioned) may be subject to because of a blob of overpressured nitrogen that escapes as you loosen the bolts.
You probably get a more accurate answer and spend less time if you just ask a couple of your mechanics how fast they ever saw a manhole cover fly.

 

[BigInch]

Who said "Pigs can't fly"?

If I can add some more of the same.

When running pigs in new line segments, crossing high points can cause them to "run away" and reach such high velocities that, even if caught in temporary pig receivers, they can hit a blind flange and leave a nice big fingerprint. A 12 inch line with 400 psi behind a pig has about 40,000 lbs of force, which must be close to an initial acceleration of 10,000 ft/sec. Fortunately, the pressure will dissipate quickly once the pig is out of the pipeline, but imagine something like that hitting the #1 pin at the bowling alley.

All three factors, velocity, mass and shape are important risk parameters. Consider, an M16 bullet, small mass, high velocity, shape causes tumbling on impact = massive dammage. Shape also spreads out the energy on impact. Shapes that hit and conform to a target profile deliver minimal energy, those that retain their shape deliver more effective energy density to the target.

"Non-lethal" riot control projectiles generating 120-160 ft/lb. of energy have the greatest potential for incapacitation with minimal potential for causing serious injury and death, although some deaths have been recorded from their use. In comparison, a thrown fastball is estimated to deliver 97 ft/lbs. of energy. A typical 38 caliber bullet is around 200 ft/lbs, but of course has a very high relative impact energy density on the target.

Out in the oil patch, we had 1460 psig sand traps with a valve on the bottom open to atmosphere to blow out the sand. Of course the sand had a tendency to plug the valve. NEVER look into a valve to see what's clogging it. Cost one guy his life. Sandblasted the top of his head off, so even sand grains can do it if there's enough of them and the velocity is high.

If you're going to let "pigs fly" dig a big pit and make a good size embankment behind.

Going the Big Inch!

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