Slopes for forced sewage pipe

[strelok]

Hello,

I need a help and advise, please!
5 independent source of saniraty sewage will pump the sewage through pressurized piping to collecting tank.
Can we lay the pressurized piping with no slope?
Also what other procedures can we apply to prevent clogging (is flushing with water practical thing to do?), because at the worst case scanario, when we have one pump operated, the speed of the flow will reach only 0.3m/sec.
regards,
strelok

 

[dicksewerrat]

Will you have asmall holding tank in each 'bldg? Will they all be pumping through the same pipe?

 

[strelok]

Dicksewerrat,

yes, each of the source will have a pit, from which they will pump to the system. The main pressurized sewage collector is 12". each source of sewage will be connected to the main pressurized collector through 6" line.
Hydraulics calculation shows that when we have all sources under operation we can reach 0.6m/s flow velocity. But when we have only one pump under operation, the velocity is around 0.16m/sec which I believe is very low. Because the system is temporary and will exist only couple of years, we would like to introduce some procedures to prevent clogging, but I don't know which? Flushing with water seems a way to go? I need an advise!!!!

 

[KRSServices]

Why are the lines so large in diameter? I suspect that either the flow rate compared to the pipe diamter may be a little too conservative. I would recommend that you have an experienced engineer look at the flows and provide recommendations for this system, particularly if it is a retrofit into an existing system.

KRS Services

http://www.krs-services.com

 

[semo]

Your lines need to be smaller. The main doesn't have to be sized to provide scour (2 fps) only when all the pumps are running. You should do that with a lot fewer pumps. So what happens if all the pumps come on at the same time (not a standard occurance with only 5 pumps). You may have a pump deadhead (or reduce flow) for a few minutes until one pump shuts off. You've got storage in your wetwells. As soon as the pressure relieves a little, the smaller pump will do it's thing.

I agree with KRS that you need someone that routinely handles pressure systems to look at it. Especially if you have concerns.

 

[strelok]

Thank you for advise. the lines are so large in diameter, because they bought them originaly for gravity system, but it became not possible to slope them, and dicission was made to make them pressurized. The system will serve only this 5 sources (there will be no more) and end up at the treatment unit. The pumps at each source will be identically.
I will try to look at other options. thank all of you for help.

 

[KRSServices]

I appreciate the circumstances surrounding your qustion, however, you cannot force a gravity system to perform functions it was never intended to perform. If you want a pumping system, then a pumping system must be designed.

You cannot "purchase" some gravity pipe, add a few pumps and expect to live happily ever after, so to speak. I have had the pleasure of putting in several pressure systems, similar to the needs you wrote about, and each had unique considerations for design. You stated that the pipe was purchased with the intent to use as a gravity system, but then discovered that sloping the pipe was not possible. Your boss could have saved his money and hired guys like me who do this for a living. We could have then recommended a few options, but now, your options are very limited without significant expense.

Gravity pipe is really not meant for presures, and if you are not careful, someone is going to get hurt or killed (if for example the pipe is overhead and not properly supported and it fails due to pressure) because the original problem has not been properly thought out.

KRS Services

http://www.krs-services.com

 

[taylor62]

This doesn't apply specifically to the question above, but for some reason it won't post. I'm running a FM generally downhill and am having problems with gas generation. I haven't found any experts yet, so my question is below. Thanks.

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.

 

[semo]

Sounds like you have air-lock in the system. I've had similar systems that pump downhill and used air/vacuum valves so that the line can drain quickly.

When you start pumping, the water probably completely fills the pipe at some point down the hill, trapping an air bubble above it. That bubble will (if the water velocity is not high enough to expell it) already be at or work its way to the high point in the pipe. There it will restrict the available flow area of the pipe. The smaller flow area will cause the point of operation to move left along the pump curve and create higher system pressures with lower flows.

In order to eliminate it, you will probably need to install air-release valves, especially at the high points.