How do you calculate velocity

[productiondude]

I have a 4 inch cup pid behind it I have 7500 kpa.In front of it I have 2500 kpa. when i start the well up it takes some time before the pressure on the 2500 kpa side reaches 6500 kpa so you still have a diffrental of 1000 which pushes the pig but until that happens I would think the pig is moving quite rapidly? 2 phase flow of gas and fluids.

 

[zdas04]

Pig velocity is a very iffy thing to calculate. First, you have to assume a value for bypass flow (there is always some). Then you have to assume a value for total mass (the pig's mass is easy, but how much liquid and solid stuff is it pushing?). Then if can figure the real dP across the pig, you can use the line diameter to determine force and back into an acceleration number. Unfortunately the acceleration is ever changing. If you make the (incorrect) assumption that it is constant then you can integrate the acceleration value to get to a velocity equation, which you can integrate to get to a distance equation that you can solve for time to get the expected duration of the pig run and feed that back into the velocity equation to find the pig velocity at the end of the run (or any other point in time).

With your rapidly increasing downstream pressure, the constant acceleration assumption is even worse than normal. Another twist is that the mass of the stuff in front of the pig is also ever changing, but you can make assumptions about the rate of change that aren't horrible.

I've been doing this silly arithmetic for years and have never been close to predicting an actual duration (an therefore a velocity). I keep trying because my clients expect it, but I always start with a serious disclaimer.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com