Pressure Measurement and how it relates to TDH needed.

[sta07750]

I've got a header that is charged by two parallel pumps. I'm trying to tie a third pump in downstream. I'm trying to come up with a number for TDH for the third pump. My only available pressure measurement on the header is about 12 vertical feet below my available tie-point. When calculating the TDH needed for the third pump should I be subtracting the 12' from my TDH required to account for the vertical rise. Instead of considering it a 50 psig pipeline should I be considering it ~45 psig during my calcs? Thanks.

 

[LittleInch]

Yes, on the basis that the tie in point is the same elevation as the discharge flange, assuming that what you're trying to do is add a third pump to the header??, - a quick diagram would things a lot to understand your system.

I assume you are aware that the pressure measured is presumably in flowing condition and a higher pressure might result in no flow condition which pushes up your final discharge pressure/head from your third pump. Also more flow in the header means higher pressure in many cases unless you have some sort of fixed discharge pressure.

There are many threads here on parallel pumping - it might be a good time to read some of them.

My motto: Learn something new every day

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

 

[sta07750]

Diagram attached. I've done a bunch of reading on parallel pumping. I've run the two main pumps independently (both off, one on, other on, both on) and got some pressure readings. My question stems from finding a TDH needed for the third pump. It's entirely different in flow, head requirements, operating time, pump style, etc. The flow is an order of magnitude lower than the #1 Sump system and is very intermittent (few times a day for one or two minutes) as opposed to the relatively constant flow of the #1 Sump. If the pressure gauge is reading 25 psig before a vertical rise, can I assume the pressure at the tie-point located at the top of the rise is 25 psig - 5psig = 20 psig? The system discharge is unknown as it is a pipeline that goes offsite to a treatment plant but it does run underground before leaving our property. I just need to make sure I can empty the #2 Sump into the header regardless of what the #1 Sump is doing. The volume of #2 Sump is low enough that if the #1 Sump isn't running, we won't have enough liquid to make a difference in the larger pipe of the header so we don't have to worry about pumping this stuff all the way to the treatment plant.

I've done some modeling in EPAnet as well and those results don't closely match my Excel numbers but I don't know that I trust EPAnet as it may be a case of garbage in, garbage out. This is my first foray into adding a downstream pump to a force main.

The piping and infrastructure already exists, I'm trying to shoehorn a pump into the system that will work without any serious piping overhaul as it's all either in the air or underground.

 

[sta07750]

Please ignore the first attachment. I forgot to add the downturn at the end of the pipeline. New attached diagram shows this.

 

[LittleInch]

Yes you can assume that and a good diagram by the way - answers a lot of questions.

The issue you might face is what happens when you pump no 3 alone as the back pressure might only be your lift height - 30 feet. The friction losses would be very low at that flow. Unless you're using a PD pump you might find the pump flows too much and trips on high amps unless you've got a big motor. it's sometimes not the worst case which gives you the operating problem...

You could add some extra pipe and make it 40 feet high and then drop it back into the header to alleviate this or use a PD pump which tends not to bother too much about discharge head so long as it isn't more than the design.

My motto: Learn something new every day

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

 

[sta07750]

Even with the two main pumps off, the header reads ~25 psig due to check valves and static head from other vertical runs further down the pipe. Either way, pump 3 should never pump alone due to the design of our sump system but to alleviate that possibility I was thinking about a back pressure valve set to open just underneath the pressure seen with one of the larger pumps running. Other option in my mind is to hook up a power monitor and try to catch the scenario that way. The piping is already in place so I can't do much there. Thanks for the help.

 

[Artisi]

Without getting too involved in the discussion at this stage, a point for consideration -- you also need to review the change to the total head the existing pumps see due to adding additional flow into the system.
For a complete answer you need a full hydraulic calculation to decide on head / flow for the main pumps and flow / head required at the tap-in point for the 3rd pump.

Not straight forward unless of course you are prepared or happy to add a third pump that can achieve something like 150 gpm @ 30psi + 30ft vertical component + a bit of margin - lets say 45psi at the pump discharge -- "suck it and see"

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

Artisi, I wondered about that as well, but with the relative flows from the current pumps being quite a lot bigger than the third (1/2 @ 500-700 gpm) vs 3rd at 150 for only a few minutes at a time, there really shouldn't be much of an impact. if it was another rone of the same size then yes, a third one often doesn't add much to a system total flow.

sta, if you effectively have a static pressure of approx. 25psi, then I wouldn't bother with anything other than a check valve in your third pump line. At this sort of flow and head, the pumps and motors tend to be fairly robust so can stand a bit of over flow if it occurs, just don't set the breaker too low...

My motto: Learn something new every day

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

 

[GPRnD]

I agree it would be worth just checking where each pump might be forced to on its curve in this parallel operation.

We are assuming that the high flow sump pumps are operating in a happy place on the curve right now.

 

[sta07750]

In parallel operation the larger pumps should still be okay. If we end up cutting the flow of the large one for a few minutes we'll be fine. The pumps/motors are large enough to take the swing and those sumps are plenty large enough to accommodate a small delay. Everything falls kosher on the system curve as well when they are running together as far as I can tell.

Thanks everyone.