Reisizing a Sump Pump Replacement

[BanditKeith]

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.

 

[DubMac]

Are you saying that the pump volute/impeller will be 23' above the low water line?? That will be a heck of a suction lift, what is your NPSHR at required flow?....better have a decent margin or you will be sucking air. Remember, pumps do not suck.....the atmosphere is "pushing" the water up the suction line; when atmospheric pressure can't overcome the static and friction heads, you will lose prime.

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).

 

[BanditKeith]

Hi, thanks for the quick reply.

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?

 

[JJPellin]

The total pump head required is unchanged. The old pump was in the pit with zero suction pressure (it still had positive NPSHa). This old pump had to put up 144 feet from that location to meet the process need. The new pump is located on the surface, 8 feet higher than the old pump. It has a lower suction head by virtue of the higher elevation. But, it also has a lower discharge head required by virtue of the same fact.

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

 

[TenPenny]

One way to think about it is that the pump head required is to move the water from where it is, to where it needs to go, since the sump hasn't changed, and the discharge location hasn't changed, the total tdh required by the pump doesn't change.

 

[bimr]

Here is an example for you to follow:

http://www.mcnallyinstitute.com/07-html/7-01.html

 

[ciise]

As TenPenny said, fluid is still moving from/to the same points.

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/

 

[BanditKeith]

That makes 100%. Simple enough. Thanks for the replies everyone.

 

[LittleInch]

The thing missing here is what type of pump you're planning to use. A centrifugal is normally reluctant to self prime. Have you thought about how to start the pump and get the water into the pump inlet to start with? Don't rely on an nrv to hold the column of water in the suction line. Submerged sump pumps don't have this issue. I assume three is some good reason why you can't replace like with like, but if unless your above water pump is a pd pump of some sort then you could run into problems starting it.

My motto: Learn something new every day

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

 

[bimr]

LittleInch, a self priming centrifugal will have no problem operating in this application.

 

[LittleInch]

My point exactly, i.e. you need to specify a self priming pump which has a higher inlet than a normal pump and the OP starts off saying this was his first time sizing a pump. If you don't know something then its difficult to specify it. Lots of people just think about how everything works in steady state without thinking through how to make a system start and stop properly. Even self priming pumps have limits of suction lift and volume of pipework to fill and then also introduce air into the system which it may not have had it before causing potential air locks and problems downstream. Solving one problem sometimes leads to two more ;-)

My motto: Learn something new every day

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

 

[BanditKeith]

Hi, thanks for your comments

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