Air/Gas Bubbles in Sewer Force Main

[taylor62]

I recently completed start-up of a lift station only to find that the force main to which we connected is lacking a number of air/vac combo valves. The FM runs generally downhill, so I expected sections of the line to be empty (drained by gravity) between pump cycles. Instead what I've found is static head in the line that is as much as 85 feet above what would be expected based on the high point in the line. Here's my questions: Does anyone know that composition of the gasses generated in a sewer line? I know the general constituents, but not the real percentages of each. At what pressure is "normal" sewer gas forced back into solution? If one end of the force main is open to atmosphere, how can the static pressure in the line be 85 feet higher than the highest point of any element in the system? Why do the gas bubbles not expand to atmospheric equilibrium when all pumps are off? Any help or insight would be appreciated.

 

[RWF7437]

First, force mains should be laid "uphill" in the direction of flow to avoid such problems.

Second, when that is not possible or economical, air/vacuum valves must be installed at all high or low points.

I don't know any way to tell you the percentages of air, H2S, methane or other gases which form in sanitary sewage but that answer is probably irrelevant to solving your problems anyway.

good luck

 

[taylor62]

I didn't design it, but the FM runs generally downhill because of the local terrain. To run it as a gravity line would have required depths in excess of 50', so that wasn't viable. To use a combination of gravity and FM would have required a dozen or more lift stations, so that wasn't viable either. The intent was to provide a FM that was full and under pressure for much of it's length when pumps are running, but would allow air into the line when pumps stopped and local high points drained by gravity. The problem is, the air/vac valves used are not intended to release air under pressure so gaas is accumulating. Swapping the appropriate valves should solve the problem, but I'm still curious about the behavior of the line in it's current state. No one, even Apco or Crispin techs, could not explain how the pressure in the line can significantly exceed the pressure associated with the highest water elevation in the system. For instance, the highest point is 701', but the pressure at several local high points equates to an elevation of almost 800'. If the line is open to atmosphere, I can't figure how it holds this pressure. Also, the elevated static pressures increase as you move back upstream to the pump station. Perhaps this extra info will help a bit. Thanks.

 

[RWF7437]

Don't see how the pressure in the line can be higher if the pumps aren't running. If they are, then the pressure in the line at elevation 701' could easily be 800 feet of water ( about 43 psi ). Don't know how the pressure was measured or whther the gages used are reliable or whether this is based on only one or several measurements.

As for air/vacuum valves, you may have to install two or three at each high or low point to make sure you "capture" the gas bubble which forms near the high point but may move up or downstream over time.

good luck

 

[hydrae]

I have an idea on how your static pressure is greater than expected.
you mentioned you have several hills and valleys along the path.
Assume you have good check valves at the pump station.
first the line is empty, on the first 'on' cycle the line fills to the first hill and stops. You read head equal the height of pumping, next 'on' cycle some of the first cycle makes it over the first hill. During the off time any liquid on the back side of the hill falls into the following valley, gurgling the existing gas/air back to the top end of the backside of the first hill. That gas will be under pressure equal to the height of liquid in the second hill. In these conditions it will stay on the back side of the first hill forever... moving forward during 'on' cycles and gurgling back during 'off' cycles. Since this gas does not have the density of the liquid, your pressure at the pump will be equal to the height of liquid in the front side of the first hill plus height of liquid on the front side of the second hill, minus the height of liquid on the backside of the first hill (which is not the full height).

one solution: If you can augment the supply (using a fire hose or increase the active volume of the wet well?) during an on cycle and the pumps provide enough flow to push the air/gas out you may be able to push the air/ gas out to the end to replace the liquid through your largest valley in one cycle, (but the pumps must push faster than the liquid will flow downhill on the backsides of the hills.)
You may wnat to compare the active volume of the wet well to the volume of the FM to see how many 'on' cycles it takes to get from wet well to gravity main.

Hydrae

 

[RWF7437]

Just to add to hydrae's comments:

It would be useful to check the volumes of the wet well and force main and the pumps setting. Domestic wastewater goes septic in three to four hours producing lots of noxius gases. At low flows, the wet well and pump settings need to move wastewater out of the wetwell in less than 3-4 hours to prevent these septic conditions.

The trade off is that at high flows you want to prevent the pumps from cycling on and off frequently; probably not more often than every 15 or 20 minutes. This sacves energy and wear and tear on the pumps.

Finally, I hope that before the station was put into service that the entire force main was slowly filled and tested for leakage and then left filled before the first "real" duty cycle. Doing so would have ensured that there was little or no air in the line when the pumps first came on.

good luck

good luck