Pumping into Pressurized Line

[sta07750]

I'm spec'ing out a new pump for our waste water lift station. The lift station will pump out via trash pumps through some discharge piping and into a common sewage header that runs to the treatment plant. The issue I'm having is that the common header is filled mostly by a another, larger lift station. That means the common header can be anywhere from 5psig to 25 psig. How can I pipe up these new pumps so that I don't see pump run out when the forced main isn't running at the higher pressure? We need to run these at about 140gpm and 80' TDH (25 feet up and into a 25psig header).

Our previous setup was driven by sewage ejectors and those seemed to work just fine until we ran into compressor reliability problems and check valve problems.

Thanks for any ideas.

 

[MikeHalloran]

It sounds like you'll still need check valves, and an easy way to service them, because there will be problems with them.

OR, you could install a 150+ foot standpipe into the top of your common header, i.e. tall enough so nothing would come out, and rig your new lift station to squirt through an air gap into the top of that standpipe.



Mike Halloran
Pembroke Pines, FL, USA

 

[bimr]

You can use a pressure sustaining valve on the discharge line. The valve will automatically hold the back pressure at the set point, so the pump will be operating at the same point.

http://rossvalve.thomasnet.com/prod...back-pressure-sustaining-valve-for-wastewater

Another alternative is that you may install an oversize motor on the pump so that the pump will not overload the electric motor at pump runout condition.

 

[racookpe1978]

Can that pressure reg valve (pressure sustaining valve) work on the garbage and trash coming out of his sewage lift pump?

 

[LittleInch]

You have a fixed flow you want to maintain, but a variable system pressure for the same flow. Either you

1) increase the system pressure losses to compensate
2) Vary your pump output by varying speed (VFD)
3) change pump to a PD type unit

for 1, you could in theory just build a big high loop which more or less equals your 80 foot and then fall back into the header. At low pressure the down leg would start to slug and create a vacuum but would maintain your pump output. Alternatively install a control valve suitable for the fluid you have - there are many available, but tend not to be cheap

2) As you have a fixed minimum head this could be a worthwhile option to investigate as it would reduce the head whilst maintaining flow

3) PD pumps are essentially fixed flow devices which are relatively unaffected by pressure other than varying the power input - max at max pressure. Probably the least attractive option, but remains a possibility.

Even if you don't go for 2 now, it costs very little extra to make sure the motor is suitable for VFD drive in the future and is normally a good plan B.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[jgailla]

For sewage, I've usually been able to find a pump that performs acceptably in both scenarios.
It runs to the left under full pressure and to the right under no pressure, usually an oversize, inefficient motor as bimr suggests.
It never, or almost never, runs near the BEP, but it stays within a narrow enough range to work without excessive wear.

 

[sta07750]

Bimr: That's an interesting piece of equipment there, I'll have to look into that.

LittleInch: I'm trying to avoid a VFD just to avoid the complexity. This is an industrial site with a relatively small lift station and is a little behind the times in terms of controls. I don't want to be the guy that brings a VFD in to pump poop. I'll definitely be getting VFD compatible motors though just to hedge my bet. The control valve is an option and it's only a 4" line so it wouldn't be over the top expensive. The only issue there comes back to controls. I don't want to control hydraulically because there will be some solids in the line and there isn't any kind of remote indication on the lines anywhere near there. We would have to tap the forced main (which means shutting the plant down as it is a common header with some other important sumps) for a local pressure reading to brought to the control valve. I'm trying to do this mechanically to avoid major system changes.

Jgailla: I've been trying to find a pump that is good in both scenarios but I'm struggling. The potential head difference is about 50' so it's not small. All of the pumps that I've looked at so far have me operating pretty far left already at full pressure and then at double or triple the flow when the pressure drops down. The pumps I'm looking at seem to have a very flat curve, I need something more vertical.

My current gameplan is to use self-priming trash pumps for space reasons and solids reasons. We need to be able to handle hard solids (bolts, screws, small rags, fasteners, etc) that I think would tear up a grinder pump. Our wet well is only 3' across at the bottom and then steps incrementally larger up to 5' at the top for 3 distinct sections. I don't think we can reasonably fit a duplex submersible setup in that hole with the way the wet well inlet/outlet piping is set up. We would get a new wet well but like I said, we are an industrial site and we don't like to dig if we don't have to.

 

[bimr]

racookpe1978

There are a few firms that make these valves for sewage applications. I believe that GA Industries also makes one. They are full port valves.