Methane Removal, Pipe - PVC vs HDPE 1

[GeneralAssociate]

GeneralAssociate (Mechanical) 22 Aug 05 21:50
Value Engineering was requested by CM/Owner. Commercial Project includes methane mitigation using air intake/exaust/blower system, slab on grade, 3/4" washed stone bedding, new construction.

One item sticks out - 8500LF 4" Perforated SDR 17 HDPE pipe. Using Perforated PVC under slab would lower project costs, seems logical. Engineer prefers HDPE?. But only on perforated? Sch 40 pvc approved on non-perf headers. Make sense?

Cost savings would be substantial enough to include in proposed VE. Any thoughts on industry/design standards.

Thanks All, Steven

 

[Backfill]

GeneralAssociate,
Hopefully I understand enough of your query to respond intelligently:

1. 4" perforated PVC may calculate out to be too brittle/fragile to withstand settlement. 8,500 lf of removal line tells me its a fairly large slab (or sets of slabs), so there will be a bit of rockin & rollin.

2. HDPE will deform while maintaining serviceability right up to pinching tight. When that happens, there still should be piping along the exterior until the gas can re-enter past the pinch. That works whether the pipe is under positive(intake) or negative(exhaust) pressure. If its all part of a grid or array, it may be designed to withstand a few pinch-offs, hairballs, or gremlins.

3. Flanging HDPE to PVC is annoying, but the adapters are off-the-shelf.

4. What is the facility intended service life? What is the methane system intended service life?

5. What are repair costs? Do direct costs entail excavating slab portions? Do indirects entail process interruptions?

6. Bear in mind, my PVC and HDPE experience has predominately been in fluids and gasses on positive pressure. There have been cases where we mixed them. The only degassing system I've been involved with was in hard rock using drill holes and a high pressure blower (exhaust) system.

Hope the discussion helps. Good luck with the matter, and please keep us posted.