Forcemain sewer pipe mystery?

[seasar]

All,

I have a problem I hope you can help me with...a forcemain sewer pipe that pumps effluent through a 25.4" ID pipe at 1,000 to 2,000 gpm, 30,000 ft away, elevation change from the pumping station to the open tank discharge point of +240 ft. There are a dozen air relief/vacuum breakers installed along the line and they are in working order. The highest point in the line occurs 5000 ft from the discharge (see attached sketch). We have had problems with the air relief at this point and had the idea of replacing it with a standpipe. The elevations are known and given the relatively low velocity in the pipe we figured there wouldn't be enough head to spit water out of the top...what actually happens is a mini-geyser that goes on continuously. Upon first opening it there was so much gas it sounded like choked flow gas discharge but after a few minutes it got very quiet then a few minutes later a continuous stream of water (eyeballed at over 100 gpm) started coming out and continued for several hours before we closed it off.
We're OK with reinstalling the air breaker but I don't understand why this didn't work. The pipe must be experiencing a pressure drop of roughly 20ft between the high point and the discharge. We have been into this pipe on occasion noticed no significant buildup of debris, there is 1 20" valve butterfly valve and it is fully open.
I have included my math for f*L/D*v^2/2g on the sketch and came up with ~ 1ft of drop...even taking into account the fittings/etc it can't get to 20ft...something else is going on.

 

[rconner]

I'm not going to claim any specific expertise in such system design, but design flow velocity sounds quite low (and very likely e.g. not high enough to scavenge air along with the flow). Wonder what effect flow somehow ramped up, I guess with pumping at shorter intervals to move same amount, might be? Also, don't know exactly where 20" bfv is, but if installed with concentric reducer I guess it also might be creating a sort of high spot where also maybe air good bit of might accumulate. Maybe air is more of a problem to what is going on than "debris"?
Like you I will await more learned responses to your specific questions.

 

[bimr]

If you search this website, you will discover numerous posts regarding forcemain problems. Most are caused by low velocities. The textbook Pump Station Design by Sanks has an excellent discussion of this issue.

A force main should have a minimum continously operating flow rate of 2 ft/sec to avoid solids deposition.

If you operate the force main intermittently, the flow rate when the pump starts should initially be 3.5 ft/sec to resuspend the solids that settle out when the flow stops.

It appears that the force main design flow is too large for the application or perhaps the forcemain is designed for the future.

Unless you have extreme elevations differences that will collapse the pipe, you should consider closing the combination air/vacuum valves. They are probably not necessary. Most wastewater textbooks as well as experience engineers recommend NOT using combination air/vacuum valves on wastewater force mains. Air release valves are a better idea but should also be avoided if possible.

See the attachment for a discussion:

The main issue that you have is inadequate forcemain velocity that will not allow the entrained air to be pushed up the hill. Because of this issue, the air is trapped in the forcemain and/or renters the forcemain from the combination valves..

Installing a standpipe is a terrible idea. It will make the problem worse as it will allow more air to enter the forcemain.

Try to eliminate as many of the combination air release valves as possible. Install air release valves instead of combination air release valves. You do not want to allow any additional air to enter the pipeline.

In addition, see if there is any way that the velocity can be increased.

 

[seasar]

bimr...thanks for the response. The attachment was very interesting.
Increasing the velocity isn't a simple option as
1) The continuous rate of generation of effluent isn't high enough
2) If the flow were changed to intermittent with starting velocities >3.5 ft/sec the pumping station would have
to be redesigned and enlarged....even then at 5000+ gpm it would only run ~ 20% of the time.

The line has been in service for nearly 30 years...I haven't been able to locate the design premise that justifies the size.

I should have clarified in my original post that the air reliefs and vacuum breakers are separate devices. At each manhole/service station there is a tee to a 3" isolation valve...at the outlet of the valve is a header with an air relief and a vacuum breaker.

Interestingly the air relief giving us trouble is a vent-o-mat RGX...the kind called out in the pumping station design book.

 

[SteveWag]

Velocity in the 24-inch main is not a part of your problem. I agree that the headloss is less that 2-3 feet at the conditions given, but it enough to submerge the 6-inch tap and that will in effect convert your air vent into an airlift pump. If the lower end of the air riser is submerged, any air traveling in the 24 will turn vertical, carrying fluid with it. Your vent pipe is too small and the velocity too high. What kind of problem are you having with the air releases? Is this subject to freezing or vandalism or random bullets? It may be possible to shorten the riser pipe to say 5-feet and install a vent on it. This should put the vent above normal water level. You could try putting a larger pipe, keeping the 6-inch tap, up 20 feet. Imagine the stack on an old west wood burning locomotive.
Merry Christmas!!! HoHoHo
Steve

 

[bimr]

Velocity not part of the problem? Really? If there was adequate velocity, the air would be pushed out and you would not have any problem.

Since it is not possible to have adequate velocity, your best path forward it to close off all the vacuum relief valves so that no additional air will enter the pipe. (Note that you should verify that the pipe will not collapse if a vacuum situation occurs.)

Vacuum reliefs are only installed on sewage forcemains when the installed pipe has a very thin thickness and may collapse.

You should also attempt to remove as many of the air release valves as possible. Air should only enter the pipe on startup. Air release valves are problems on forcemains because they will leak after fouling with sewage.

Ensure that you have a quick closing check valve on the pump station to avoid water hammer issues.

Note also that the entrapped air is effectively lowering the pipe diameter and causing increased pressure drop.

 

[seasar]

Since this is a forcemain sewer line isn't it likely that sewer gas is being generated by the effluent
as it rots? I know the pH changes substantially from the pumping station to the point of discharge...the constiuents
of the effluent would certainly harbor some biological growth.

That being the case it wouldn't just be air at startup but also sewer gas that would need to be removed (again as
the velocity isn't adequate to scour the gas away).

 

[SteveWag]

It would be nice if we could perhaps pull a 12-inch or 8-inch line through the existing main or rebuild the pump facility with enormous brand new pumps or perhaps slide a carbide die along the outside of the force main, permanently decreasing its diameter or some other such nonsense. The fact is the line and pumps have been in use for 30 years, apparently without solids deposit and without air problems. Air has successfully removed, and perhaps allowed to enter, by the air valves. The question is “why did the vent pipe not work as thought?”.

Gas may be generated by bioactivity, probably not that much. Assuming that the pumps are not adding air, most of the air might come from the air valves depending on the profile and placement of the vents a suggested by BIMR. Actually, having air in the line reduces the effective cross section and increases fluid velocity, helping scour solids.
Steve

 

[bimr]

You will not generate much gas in the forcemain from anaerobic activity. Anaerobic activity is generally undesirable because it is associated with increased corrosion and odors.

Air is undesirable in pipeline because the affects of air in the pipeline are unpredictable. The air will consolidate at high points in the pipeline and may cause flow blockages, burping, and siphoning. The air may also cause water hammers as the air is compressed and decompressed when the pump cycles on and off. Air is also associated with an increase in the rate of corrosion.

 

[BenJohnson]

First of all, 240' lift is pushing the limit for most pumps. I would work with a pump representative to make sure your system falls on the operating area of the pump curve. A centrifugal pump would have real problems - may need a progressing cavity pump.

The velocity for force mains are suppose to be 4' second. You could put in a larger pump and larger (aerated) wet well to make sure each pump cycle has sufficient sewage to meet to requirements of that large pipe.

I suspect that vacuum valve is compounding your problem - and totally unnecessary.

One idea is to change to stop the force main at the peak elevation, and allow the last portion of pipe to be gravity flow - I think that is what you are already doing with the standpipe

The original design was terrible - or was that the result of the maintenance crew trying to fix things?

 

[SREisme]

If you are blowing water out of the top of your high point at ~100gpm, then you have a pressure drop in the line from your high point to your discharge equal to 100gpm of frictional losses plus the extra head in your standpipe.
You say you have inspected the line in the past and not found significant fouling, so that is unlikely to be the cause unless your sewage conditions have changed.

You say the 20" butterfly valve is wide open. What are you using to make that statement? Are you certain that the disk is still attached to the stem? What kind of connection do you have between disk ans stem? Is it a keyed fit that could have slopped out and be allowing the disk to be less tha 100% open? Can you install pressure gauges on either side of the valve to check for excessive DP?

Your 240ft of static head is only about 100psi of discharge pressure, which isn't much in terms of centrifugal pump capacity, and if you've run for 30 years with no problem I wouldn't start blaming the pumps now. Verifying the operating conditions at the pump against the pump curve to see where you are actually operating is a valid check. You excessive line losses should show up as increased discharge pressure.

One more thing, your downhill section of pipe should be under gravity drain conditions or slight pressure depending on frictional losses. Can you block in the air vent and vacuum valves between your high point and your tank and repeat the experiment? You should not be able to pull any additional air into the forcemain on the uphill section of pipe as it should be pressurized to get over the hill. You may be pulling air in on the downhill side and vapor locking the line, but it still sounds like you have an excessive restriction causing your extra water to go out the vent stack.

Merry Christmas!