Pressure Measurement and how it relates to TDH needed.
Pressure Measurement and how it relates to TDH needed.
(OP)
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.





RE: Pressure Measurement and how it relates to TDH needed.
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
RE: Pressure Measurement and how it relates to TDH needed.
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.
RE: Pressure Measurement and how it relates to TDH needed.
RE: Pressure Measurement and how it relates to TDH needed.
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
RE: Pressure Measurement and how it relates to TDH needed.
RE: Pressure Measurement and how it relates to TDH needed.
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" [smile]
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.)
RE: Pressure Measurement and how it relates to TDH needed.
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
RE: Pressure Measurement and how it relates to TDH needed.
We are assuming that the high flow sump pumps are operating in a happy place on the curve right now.
RE: Pressure Measurement and how it relates to TDH needed.
Thanks everyone.