Sewer Pumping Downhill 2

[djcbgn]

Hi
I am designing a sewage pump station with a flow of about 700 gpm. The force main will run entirely downhill, where the discharge elevation is about 120' below the pump station. The proposed 8" force main is about 25,000' long. There really are no prominent high point in the profile, some sections are flatter than others but the pipe is sloping downhill the entire length. I have designed many pump stations, several of these with intermediate high points but I have not had a situation like this. Running gravity sewer is not an option with this situation. My initial concerns are air in the line and maybe developing "vapor lock" and solids separation.

Any thought to approaching this.
Thanks.

DJC

 

[1503-44]

Vapor lock occurs at high points. You don't seem to have any.

If the flow rate velocity is sufficient to keep the line flowing full, i.e. pressure >15 psiA at all points, especially at the inlet and outlet, and at a minimum 3ft/s velocity, solids should be swept along, so no apparent problems.

If velocity is slower than 3ft/s, you might want to try to reduce that diameter.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

That's a pretty low flowrate so you might find it difficult to get a standard sized pipe which is just the right size so that your pumping losses are not too big but not so big that gravity takes over and you don't have positive pressure at the entry point.

Your issue is probably more one of how do you stop and start the system without draining it or pumping into no back pressure for a while.

Options are:
Close the arrival valve before you turn the pump off.

Create a min back pressure of 120 feet at the arrival point so the pipe is full all the time ( but needs more pumping power), but might not be easy with sewage.

Like mr 44 says, go at > ~1m/sec and you will have no air and no drop out.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[swazimatt]

I have recently had two projects that combined would match yours. One had the downhill pumping, the other the long rising main.

As mentioned the main issue with downhill pumping is that it will drain when you are not pumping. When you turn the pump back on you will need to expel 25,000' worth of foul air (i know this because with that length pipe you will have very long retention times in the pipe - septicity will be an issue) and this may require controlled pumping as there may be some air pockets that need to be pushed out the end with compressed air (in the pumping process) behind it. you will have multiple air valves along this length, but depending on the terrain this may make the situation worse - more air pockets or whistling as the rising main drains.
To close of off the end and keep the line charged would be tricky to manage i do not think you can get an 8" automated valve, possibly get a weighted valve but this would have to withstand 120' of static pressure to stay closed.
I am not sure if a barometric loop would work in this situation, but may be worth looking into - possibly with strategically placed air valves and designed high points to break the static head

The next issue i am sure you have discovered already - the length of your rising main and associated friction losses. you need to achieve slime stripping velocity of about 0.99m/s for an 8" pipe(slightly more than 3ft/s) and it ends up being a fine balance to achieve this without having massive pump head (and associated operating costs). As mentioned if this is a WWPS pumping regular waste from a gravity network with peaks during the day you will have retention time and septicity issues, hence the velocity requirement to be able to strip slime from the rising main walls.

On our long rising main we had the added parameter of a max allowable receiving flow rate, so had to add attenuation. on the septicity issues we have just allowed provision for future options like dosing or aeration and included H2S sensors in wetwell and receiving manhole so the council can monitor this and the effect of the selected treatment once it is operational - as septicity does not have a single solution and wastewater will change as the development and tenants change (industrial development).

What we ended up doing on our falling main project is discharging to a manhole and the having the rest of the pipe operate like a gravity water main to the discharge point with sealed rodding-eye points that enable sections of the pipeline to be jetted if needed. this was only a few hundred meters though and not 25,000'

 

[LittleInch]

"Slime striping velocity"...

Uurgh

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MedicineEng]

With the utmost respect , I'm stealing the expression "slime stripping velocity" to use in one of those endless meetings where I hear BS left and right.

I can already see myself saying: "Guys, we need to reach slime stripping velocity on this one"...

 

[swazimatt]

love it!!

 

[swazimatt]

what was the outcome here?

 

[1503-44]

We can not foresee any particular problem with pumping sewerage downhill, given the minimal information provided. No vapor lock is expected to occur, nor will solid deposition typically be an issue, if velocities are kept above 1m/s.

Other questions not asked, such as maximum velocities and scour of pipe material itself, max pressure required, pump power, waterhammer and discharge velocity control, etc,etc, are left to the op to resolve on his own, or return with additional questions.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[bimr]

The pipe scouring velocity will be between 3.8 to 4.1 ft/sec. The recommended pipe size for 700 gpm is 8-Inch which has a velocity of approximately 4.5 ft/sec.

The best solution is to install a back pressure-sustaining valve on the discharge. The valve will stop siphonage. The valve needs to have a full port for wastewater applications.

You can get one from Red Valve, GA Industries or Ross Valve:

Ross

Similar discussion on another site:

blog

 

[swazimatt]

The best solution is to install a back pressure-sustaining valve on the discharge. The valve will stop siphonage. The valve needs to have a full port for wastewater applications.

I have never used one of these, but it would need to provide backpressure greater than 120'. Does this mean your pump head would be >120' + friction losses over 25,000'? or would it be friction losses - 120' static head + <120' discharge valve, so basically the pumping head would just be slightly more than friction losses for the 25,000'.

 

[LittleInch]

A simple valve would be the former as it makes life easy, but you can vary the back pressure based on flowrate.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Flow rate, or pressure at the high point.

The valve only provides the 120'+/- backpressure when the pump is stopped. When the pump is turned on, the valve is opened slowly as pressure just upstream of it starts building up and when the pipeline reaches full flow the valve is fully open. It can work automatically like a pressure control valve, but it controls and holds pressure at the pipeline high point keeping it at just slightly above the minimum pipeline operating pressure. When shutting down operation, the valve will close, keeping pressure at the high point always slightly above minimum and when flow stops, it is fully closed again.

If you control via a flow rate signal, then you have to keep one eye on the flow dial and the other on the pressure at the high point, adjusting the flow dial such that you do not fall below minimum pressure at the high point. It's far easier to do that (with only one eye) by setting it up as a pressure control valve with the high point pressure telemetry providing the signal to the valve's controller. You don't really know what the flow rate should be (especially during start up and shut down) to keep Pmin at the high point, but you do always know that the pressure there needs to be >Pmin. If the pipe could collapse from vacuum, I would definitely go with the high point pressure control method.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."