Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
(OP)
I've got a project where we're installing 2.5 inch PVC slotted pipe in 4 inches of 90% compacted sand beneath a concrete slab as part of an air sweep system for methane mitigation. One of the reviewers of the system design asked if it would be possible to displace the compacted sand at a given air velocity flow through the slotted PVC pipe. I know the predicted velocity through the pipe but don't know what velocity would trigger displacement of the compacted sand. Any ideas on finding the answer?





RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
Is the question regarding compacted sand chunks that have entered the air piping, or the external compacted sand bed? I don't think you need to worry about the external bed, if it displaces at all, it just becomes more compacted. The bulk bed of sand is presumably being held in place by hydrostatic forces from the surrounding bed, and the weight/bearing forces of the overlying slab, no? Thus my statement that the only displacement possible is due to further compaction.
If you need to worry about sand particles being entrained moving in the air piping, use published data for drag coefficients of spheres at the sizes/diameters possible from your sand sieve analysis. Calculate the air velocity at which aero drag forces exceed the friction force between the particle and the wall. You'll find published data as well, using a search term such as "aeolian deposits".
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
If that arguement doesn't work, you are stuck with trying to find a source of data in published literature for sand (or other particle bed) compaction under fluid dynamic forces, or trying to generate one yourself. From what I know of dry particle beds subjected to fluid forces (catalyst bed packing in hydrazine rocket thrusters), the steady flow doesn't do much, it's the shocks that come from rapid start/stop flow that causes the particles to crumble and recompact to open up voids.
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
Do you know or can you guesstimate the forces imparted by the tamping/compacting tool? Then calculate the worst-case pressure at the face of the sand, and compare to the tamping forces used.
Another thing to go back at this regulator with - how large a void is acceptable? There must be some void space near the pipe, or you run the risk of having sand fall into the pipe.
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
Rather than blowing the sand away, oil flows by not allowing the sand to "fall into" the pipe. The slots are typically cut by laser, somewhere around 0.002 inch wide. I'm old fashion, because it works, and cut the slots about 1/128 inch wide then cold roll the pipe. I save a bundle on the cost per well.
You may want to look at API references on the subject. I don't have the exact specification in front of me, but anything relating to Pump Jacks or Sand Screen/Injection would pretty much be more than what you are looking for.
Good luck. Where I come from, methane is good! We would like to harvest it, particularly the new technologies developing in Coal Bed Methane Recovery.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
Presumably the void space is viewed as sites where methane may accumulate (and PZas is right, the pipe is void space too!), and the purge cycle is done both to mitigate explosion/fire hazard as well as noxious gas hazard. The void space is then treated as an explosion/flame enclosure?
Can you worst-case bound your analysis assuming all of the sand becomes compacted, and the 10% void space is clustered around the slots?
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
I'll try that approach and see if it flies with our regulator.
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand
RE: Air Exit Velocity Through Slotted PVC Pipe to Displace Sand