Another FM / Siphon Problem 1

[jahodges01]

I have a pump station that discharges to a high point and from that point forward runs gravity downhill and the gravity portion is backing up into the my high point (fortunately I have emergency overflow that sewage goes to). Basically similar concept as pumping uphill and then downhill (65’ drop) except I truly created gravity downhill (the high point the forcemain dumps into is an open atmospheric manhole so the pipe down the hill is no longer pressurized). The problem I have is that gravity portion down the hill and outlets has a high point the contractor installed.

I have ran hydraulic model and obviously demonstrates surcharging due to the high spot (see attached)…however my question is shouldn’t it act as a siphon still and be able to overcome the high point on the gradual slope due to the excessive grade / static head I have initially? The surcharging doesn’t stop. I suspect I have another high spot on the forcemain somewhere that currently doesn’t have an air release….or am I wrong?

Thanks so much

 

[Maury]

Your hydraulic model is correct, but the line may be getting plugged in the sag prior to the high point in the gravity line. If the contractor was sloppy about maintaining grade, he may have made other mistakes, such as a broken pipe or bad joints. I suggest having the gravity cleaned and inspected (TV inspection.

I don't seee how a high point on the force main would affect the gravity portion of the line.

 

[jahodges01]

Let me be a little more specific....the model I attached is forcemain but treated as gravity the entire run down the hill and to the existing manhole it outlets into. My question is initially at the top of the hill at the open atmosphere air release manhole there is alot of surcharging occurring, air expulsion and flow restriction in pipe. I believe at the top of the hill from this open atm air relief positive grade was not maintained to the hill and an unknown high point exists for the air to trap and cannot be vented and reduces pipe capacity. I am wondering if the additional high point at the bottom of the hill as the pipe continues to run gravity to outlet would cause this problem. I do have an air relief installed and should act as a siphon if anything. I did televise and found that this portion on top of the hill does not maintain positive grade so if we fix it should solve problem...correct??

thanks!

 

[bimr]

You seem to be trapped in the no mans area between a gravity sewer and a force main where weird phenomena occur.

I assume that this is a large diameter pipe because you are able to televise.

The minimum velocity to force air down the pipeline is around 4 ft/sec. If you can not maintain that velocity, your problem is going to reoccur.

What was the idea regarding opening to atomosphere at the top of the hill? Would it not be better to keep the pipe sealed. If this is sewage, you would not want the air vent at the top.

On a typical sewage force main, you would release the air on startup and then close the valves to keep air out of the pipeline.

Gravity sewers are oversized and this allows the air to travel on top of the pipe.

Is there a large difference in elevation?

You probably should revisit your design concept.

 

[micalbrch]

bmir: You must have the possibility to release air at the high points because you cannot avoid that air enters the pipe. No chance. And if the air pocket accumulates at the high points your whole system will not work.

 

[bimr]

I am familiar with air release valves and the use of them. Have you ever seen an air release on a petroleum pipeline?

One should minimize the use of air release on sewage applications because the sewage will cause the air release valves to jam and leak. Leakage of sewage is considered to be a spill.

 

[micalbrch]

bimr: Don't be upset. But you recommended "Would it not be better to keep the pipe sealed." And this was related to the origin post and not related to petroleum pipes. Therefore I pointed out that you must not have a completely sealed pipe without air release valves for sewage.

 

[bimr]

My point is that the use of automatic air release valves on sewage force mains is not a good practice.

As I stated: "On a typical sewage force main, you would release the air on startup and then close the valves to keep air out of the pipeline." That is what I meant by sealing the air out. Introducing additonal air with a air release/vacuum valve should be avoided.

Refer to Metcal & Eddy for a more detailed description:

“Automatic air release valves should not be installed if their use can be avoided. From past experience is has been found that automatic air release valves require frequent maintenance in order for them to function as intended. In adequate maintenance causes these valves to clog and malfunction, often soon after they are installed. In most cases, manual air valves could be used instead of air valves. For example, if after the force main has been put into service, the need develops for frequent use of a manually operated valve to relieve entrapped air or gas, the valve may be left at a part open setting for continuous bleeding of air or sewage. As a last resort, an automatic air release valve may be installed.”

“In general, automatic air and vacuum valves should not be used on sewage force mains.”

Wastewater Engineering: Collection and Pumping of Wastewater, Metcalf & Eddy.

 

[micalbrch]

bimr: I fully agree now. Thanks for clarification.

 

[jahodges01]

It was more intended as a unique design to advantage of our grade and implement both pressurized and gravity flow. The flow was intended to be forced to a high point by the pump station. From this high point (which is an open manhole / open atmosphere with an emergency overflow integrated into the manhole) and then run gravity down the hill and discharge into an existing manhole. The drop down the hill was 65' and would provide ~9 fps of velocity. The other portions flatter portions that ran gravity just before the hill and after the hill ran at minimum grade and provided ~4 fps velocity. My question is I have surcharging back into my high point open atmosphere manhole and believe the contractor installed the pipe with some "unknown" high points (very minimal) just before going down the hill....I believe this is causing my problem. But also a 7' high was installed on the gravity line that is after the hill. This one we were aware of and allowed an air release to be installed on (no vaccuum). I believe this would not cause the problem of the surcharging at the top of the hill but wanted others opinions and should just siphon.

Thanks again for all the comments

 

[cvg]

9 fps should be plenty of velocity to move the air through the line. 4 fps will not and could allow air to accumulate in high points and also to move back up the pipe. this air is "burping" out the the manhole and could cause a flow restriction. Proper aeration of the pipe could eliminate this. Possibly some flow control to maintain a full pipe could also help. See the attached document, especially pages 10 and 11.

 

[cvg]

another paper that gets into the details more is attached. Also, an anti-vortex cover / deflector might also be useful

 

[jahodges01]

That is exactly what is happening. We are going to have the contractor open cut the min slope gravity line just before the hill that was televised and showed isolated high spots and he will reinstall at min 1% grade (was originally directionally drilled) to ensure gravity flow to the hill and down

Just want to make sure with the repair, I shouldn't have problems with the other high spot that I know about which is past the hill (this area had 3-4 fps but an air relief on top high spot and open manhole at the other) and should act as a siphon. Thoughts??

Thanks so much cvg

 

[bimr]

One normal method of minimizing turbulence is to align the crown of the force main outlet with the crown of the sewer at the receiving manhole. The invert of the manhole would then be sloped smoothly between the force main and sewer inverts.

If the velocity in the force main is higher than about 6.5 ft/sec, it is probably necessary to design a special entrance to the sewer.

If the force main is considerably higher than the sewer, then the force main should stop in a manhole at ordinary depth, and the connection to the lower level sewer should be made with a short gravity sewer and a standard drop inlet.

Wastewater Engineering: Collection and Pumping of Wastewater, Metcalf & Eddy.