Reisizing a Sump Pump Replacement
Reisizing a Sump Pump Replacement
(OP)
Hi Everyone,
This is my first time sizing a pump in the real world and feel the need to refresh my mind on a few things.
I have been assigned the task of replacing one of our old sump pumps with a new pump. The original pump was a submersible pump that pumped the fluids out of the sump. The data books for this old pump indicated a suction head of zero due to it being submerged and a discharge head of 44 ft, so a total dynamic head of 44 ft. This new replacement pump is to be above ground with piping going into the sump to pump the water out of the sump. I have been told we want to keep the same discharge head of 144 ft.
I've calculated the suction head and the NPSHA to be around 23 ft and 21 ft respectively. I'm confident in my methods to calculate these values as I've seen them done before and did them in school numerous times.
What I can't seem to wrap my head around for some reason is what the new head I should size my pump for. My gut tells me that I need to size it for around 167 ft (23 ft suction + 144 ft discharge) since the pump needs to work harder to pump that water out since its not submerged anymore. Is this correct, or am I missing something?
Thanks, in advance, for the help.
This is my first time sizing a pump in the real world and feel the need to refresh my mind on a few things.
I have been assigned the task of replacing one of our old sump pumps with a new pump. The original pump was a submersible pump that pumped the fluids out of the sump. The data books for this old pump indicated a suction head of zero due to it being submerged and a discharge head of 44 ft, so a total dynamic head of 44 ft. This new replacement pump is to be above ground with piping going into the sump to pump the water out of the sump. I have been told we want to keep the same discharge head of 144 ft.
I've calculated the suction head and the NPSHA to be around 23 ft and 21 ft respectively. I'm confident in my methods to calculate these values as I've seen them done before and did them in school numerous times.
What I can't seem to wrap my head around for some reason is what the new head I should size my pump for. My gut tells me that I need to size it for around 167 ft (23 ft suction + 144 ft discharge) since the pump needs to work harder to pump that water out since its not submerged anymore. Is this correct, or am I missing something?
Thanks, in advance, for the help.





RE: Reisizing a Sump Pump Replacement
Think of reference points when you are sizing the discharge head. If the pump is at surface, it will only see the 144' above it as discharge (plus friction losses).
RE: Reisizing a Sump Pump Replacement
Maybe I worded that a bit wrong. Let me elaborate.
The elevation to the pump center line is about 8 feet.
The sump is open to atmosphere, so surface pressure is 14.7 psia, or about 32 feet.
The velocity head is around 0.5 feet.
I calculated the friction head loss to be around 1.5 feet.
So 32 ft + 0.5 ft - 1.5 ft - 8 ft = 23 ft at the pump suction.
At this point, I assumed that the discharge head of 144' included everything (elevation, friction, etc.). This pump was sized ages ago (before I was even born!), so I can't find any additional info such as the height the fluid is being pumped to, frictional losses, etc. This is all I have to go on for now.
The NPSHR at the desired flow (60 USGPM) is around 3 feet if I remember correctly.
With that in mind, is my analysis okay, or am I way out to lunch?
RE: Reisizing a Sump Pump Replacement
It seems like you figured the NPSHa correctly for the new pump. But, you should never add NPSHa to differential head unless you just want to know the discharge head in absolute units. And, I don’t know what value there would be in that number.
Johnny Pellin
RE: Reisizing a Sump Pump Replacement
RE: Reisizing a Sump Pump Replacement
http://www.mcnallyinstitute.com/07-html/7-01.html
RE: Reisizing a Sump Pump Replacement
My job is selecting pumps for different applications and I have done a quick analysis; NPSHr=3.2ft. Small pump with 5HP motor can do it. When you change from submersible to sump pump, imppeller of the sump pump is is still at the bottom of the sump (with ground clearance and strainer, usually) even though the motor is above ground.
See this; http://gouldspumps.com/Products/3171/
RE: Reisizing a Sump Pump Replacement
RE: Reisizing a Sump Pump Replacement
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Reisizing a Sump Pump Replacement
RE: Reisizing a Sump Pump Replacement
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Reisizing a Sump Pump Replacement
Ya, I should have mentioned earlier; we are using a self priming centrifugal pump for this application. The system I am installing is present at other parts at our site, so I'm not designing this directly from scratch. This pump is preferred as we have many more around the plant in the same applications, so we have spares available as necessary. I've become aware of suction lift limitations for this kind of application during this and have been keeping this in mind.
The main issue I had is that I couldn't find what these other pumps were sized at when they were installed. I just wasn't sure if it was okay or not to size the new pump based on the previously used pump (60 USGPM @ 144 ft) since the piping has changed (not by much, just some minor suction piping and additional discharge piping). The general consensus seems to be that I should be fine if I size this replacement pump at the same flow and head as the previous pump; perhaps slightly higher just to be safe.
Thanks again