Air Exit Velocity Through Slotted PVC Pipe to Displace Sand

[cathyfitz]

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?

 

[btrueblood]

Cathy,

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".

 

[cathyfitz]

Thanks for the response. The concern was with the external sand bed and the displacement of sand away from the pipe with the velocity of air coming out of the slots, thus creating pockets where the methane gas could accumulate. I agree with you that the system is fluid and if any sand was displaced, it would resettle and recompact around the pipe. However, the regulator is insisting that we calculate the maximum air velocity out of the slots that could disturb the 90% compacted sand bed.

 

[btrueblood]

Supposing there were pockets formed, isn't the airflow still going to push the methane to the discharge piping? Or is the airflow only intermittently applied?

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.

 

[cathyfitz]

You're absolutely right! The way the system works is that if the methane concentrations in the subslab space exceed a threshold concentration, as monitored by sensors in the compacted sand, the air injection system will be triggered which flushes the methane toward the passive vent piping, where it is transported via risers to the top of the building. I don't think determining the air velocities through the slotted pipe are critical, because even if voids are created, it's just a more direct pathway to the passive vent piping, but we've got to satisfy the regulator. However, you've given some good ideas on where to look in the literature, thanks!

 

[btrueblood]

Cathy,

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.

 

[Cockroach]

This is a standard practice in the oil industry. Rather than using slotted PVC tube, slotted steel pipe is used. This is referred to as a sand screen.

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

 

[btrueblood]

Mmm, yeah, you may want to look for information regarding CO2 or steam injection into oil wells. They want to limit any compaction of the sand too.

 

[cathyfitz]

Thanks guys for the answers. I also looked into the literature on trenchless technology where they use compressed air to stabilize the face and prevent groundwater intrusion into the pipe. However, most of the research involves two phase flow because most of the pipes are placed below the water table.

 

[PZas]

I have to ask. If the inspector is so worried about small voids around the output area of the pipe why are they not worried about the volume inside the pipe itself?

 

[cathyfitz]

You got me - I have no idea!

 

[btrueblood]

Never too unwilling to blather, but cut me off at the knees, Fitz, if I'm wrong in the following:

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?

 

[cathyfitz]

Thanks, btrueblood,
I'll try that approach and see if it flies with our regulator.

 

[moltenmetal]

None of my business, but why attempt to "flush" instead of merely pulling on the vent? If you blow air in, the methane/air mixture may go toward your passive vent, or it may leak out in other places. If you drive the vent instead, you may have to deal with a potentially flammable air/methane mixture in your blower (no biggie- a Class 1 Div 2 motor and non-incendive internal construction takes care of that), but at least the methane will go predictably toward the vent. Any leakage will be inward.

 

[cathyfitz]

An active air injection system is required by the city's Building and Safety Code for areas with reported high methane concentrations as well as passive venting and enhanced ventilation. The system consists of 2.5 inch air injection piping that is between the 4 inch passive vent pipes, and there are also 0.5 inch 50-foot long slotted monitoring probes in the subslab space that are sucking air from the sand layer to sensors on a control panel that alarm when methane concentrations exceed 10% of the LEL.