Gas flow rates based on pressure

[Sampler]

I work on a landfill and we recently noticed that we had very low pressures of sub-surface gas migrating out of several monitoring wells. The wells are 2 inch schedule 40 PVC, 25 feet deep, reading about 70 mm/Hg. The screen length is 10 feet. The gas make-up is 50% methane and assuming 45% CO2. The pressure was measured using a standard gauge, placed on top of the well. We would like to devise a spreadsheet to calculate gas flows from these points. Can anyone help? This is out of my area!?

 

[quark]

I would suggest you to use a vane type velometer (anemometer)rather than going with formulae. This is very cheap and can work well within 2% error. There is a likely chance that you can hire it from any HVAC contractor who does the balancing work.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[EGT01]

Of course spreadsheets are like computers and as the old saying goes "garbage in equals garbage out". Sorry, couldn't resist that one.

Actually, I think Quark's method still requires a calculation but a simple one which you can enter into a spreadsheet
Q = V * A
where Q = cuft/min
V = velocity, ft/min (measured my velometer)
A = area, sqft (cross-sectional area of pipe)

There's a chance you could use the data you presented to calculate flow but I can think of a couple of uncertainties, the effect of the screen and the permeability of material around it. If the pressure you indicated is the "deadhead" pressure when the well is closed in (no flow), then you have essentially measured the pressure in the formation around the screen. Now, if you ignore the effect of flow through the formation and screen, then it would be a straight-forward flow calculation through the piping. You could use the Darcy equation (see Crane Technical Paper 410 or other). Upstream pressure is the pressure you measured, downstream pressure would be vent to atmosphere. Pressure drop is low so you could assume gas density essentially constant, and determine the equivalent length of piping based on the straight length of pipe plus an exit loss. However, based on the assumption, this approach will likely produce higher than actually flow.

You would probably be better off trying to "model" your gas flow based on flowing conditions to account for uncertainties underground, that's an advantage of Quark's method. As another approach, you could use some type of orifice plate assembly and measure the differential pressure across it as gas flows from the wells. To give you an idea of what I'm talking about, you may have seen water well drillers use a orifice plate assembly when they test pump a well, of course those are setup to measure liquid flow.

I suppose you know your gas problem is not going to get any better. A local landfill, which had operated for over 50 years, is producing about 1 mmscfd of gas similar to your composition. They recently installed a gas recovery system to collect and flare the gas but there is talk now about a local industry trying to use the gas for fuel.

 

[Sampler]

I did some checking around this weekend, and found an inexpensive stick anemometer that I can place in the sides of the wells. Then I can get a fairly decent idea of flow based on velocity. Last I had ever checked, anemometers were fairly pricey and didn't think we could get it in the budget.

This particular site is a very small one. Based on preliminary calculations of methane generation, we won't generate enough to make it worthwhile to sell. We just don't have the "life" left. It has been closed for 10 years. I just am having a hard time seeing this much gas in a test well. I honestly think the site is older than we realize, and the wells are in waste.

Thanks for all the help....