Another FM / Siphon Problem
Another FM / Siphon Problem
(OP)
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
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





RE: Another FM / Siphon Problem
I don't seee how a high point on the force main would affect the gravity portion of the line.
RE: Another FM / Siphon Problem
thanks!
RE: Another FM / Siphon Problem
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.
RE: Another FM / Siphon Problem
RE: Another FM / Siphon Problem
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.
RE: Another FM / Siphon Problem
RE: Another FM / Siphon Problem
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.
RE: Another FM / Siphon Problem
RE: Another FM / Siphon Problem
Thanks again for all the comments
RE: Another FM / Siphon Problem
RE: Another FM / Siphon Problem
RE: Another FM / Siphon Problem
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
RE: Another FM / Siphon Problem
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.