Is This Pump Motor Too Small?

[jeffvdp]

The pump is a submersible type with a design point of 139 gpm at 589 ft of head. The purpose is pumping out a wastewater tank with worst case fluid properties of SG = 1.25 and viscosity = 3.0 cP. It is designed to operate from 1' - 45' of submergence as measured from the bottom of the suction strainer (the lowest point of the pump). Pump is custom-made for the application (nothing "off-the-shelf" available that met all design requirements) with a gearbox connecting the 1750 rpm motor to the 3600 rpm pump.

I have included the following:
-Motor manufacturer's data
-Pump manufacturer's curves
-Pump performance curve of assembled motor/pump (performance test conducted with water, SG = 1)
-Data from pump performance curve of assembled motor/pump & my calculation determining pump input power for a SG = 1.25 (I wasn't sure how to incorporate a viscosity of 3.0 cP)

 

[BigInch]

It's a lot easier to check your calculations if you'd write the equations you used somewhere. I don't really want to try to rebuild your spreadsheet.

Power with 1.25 SG will be 25% higher. Viscosity of 3 doesn't matter for the pump, but could affect the head loss in attached piping. You don't tell us anything about that.

Without the attached piping, or details on elevations, it is not clear why you need 589 ft of head, but assuming you are pumping to that head, the power calculations appear correct at about 100 HP required. If you don't really pump to that head, power required will vary linearly with head and with whatever flowrate you do get at the output head.

The pump's efficiency leaves a lot to be desired.

Normally you'd expect to see motor power capacity of 110% to 120% of what's needed, so that's marginal, but it should work at design flow and head. Just don't try to change flowrates too fast, or plan on doing higher flow x head combination for more than 15 minutes.

From "BigInch's Extremely simple theory of everything."

 

[jeffvdp]

Thanks for answering my post, BigInch!

Top part of spreadsheet is from test with water with SG=1. Bottom part of spreadsheet is me trying to calculate what motor requirement would be at test points if pump encountered fluid with SG=1.25. The motor that was provided is a 100 HP 3-phase AC motor with a service factor of 1.00. Spreadsheet breaks down as follows:

Pressure, flow, current and volts are all measurements from performance test.

Fluid power calculated by - flow*pressure*SG*2.31/3960 flow in units of gpm and pressure in units of psi

Input power calculated by - ((phase a current*phase a voltage)+(phase b current*phase b voltage)+(phase c current*phase c voltage))/746

Pump efficiency calculated by - fluid power/input power

 

[BigInch]

So that's a wire to water efficiency. Alright. I usually get to see the pump hydraulic efficiency curve too. No problem. Motor HP is marginal. Normally a bit more HP is provided as a matter of course.

What about the piping. Surely you have the 600 ft head for some reason. It's not apparent why you need so much, if the pump is only 1 ft under a -45 low level., but I'll leave that reason to you. You may however have to provide some backpressure to keep flow from running away, if for some reason the 600 some odd feet of head resistance is not provided.
















From "BigInch's Extremely simple theory of everything."

 

[DubMac]

Didn't you say there was a gearbox?? Don't forget to factor in the losses to the box. Get gearbox efficiency from manufacturer.

 

[BigInch]

Wire to water, he's got gearbox inefficiency included already.

From "BigInch's Extremely simple theory of everything."

 

[DubMac]

aye,aye matey.

 

[chicopee]

Your point of intersection between pump performance and system demand curves should somewhat higher in your linear graph since the system performance curve will be near or a quadratic form.

 

[chicopee]

should read "....be near or at a quadratic form."

 

[PumpSmart]

Pump is a submersible type.
I'm familiar with grindex, flygt, toyo, dragflow and tsurimi submersible pumps curves and this curve is different to those.
This motor spreadsheet looks like reliance submersible motor.
If so, it will be very difficult for reliance submersible motor to be conected to gearbox.
Reliance submersible motor uses to work with some USA pump mfg.
In other hand , gearbox needs to be submersible too?
I see pump curve, and it seems to be a horizontal pump.
GE 1.25 is like 32% solids by weight assuming fine sand solids,
it is a high solids content.

Please confirm all data you sent.

 

[jeffvdp]

Chicopee, the pump performance curve was generated from the company that the custom pump/motor combination is being purchased from. The "system curve" on the pump performance curve is actually the power. I think it is motor horsepower and not brake horsepower though the graph doesn't really say.

 

[jeffvdp]

PumpSmart,

Indeed, the motor is a Reliance submersible motor. The pump is a Goulds. They are connected via a gearbox. All data included with original post is correct.

 

[BigInch]

It is Brake power. Motor HP is brake power. Hydraulic, or sometimes, "water power" is what is used inside the pump.

From "BigInch's Extremely simple theory of everything."

 

[PumpSmart]

jeffvd,
Reliace motors needs to have a minimun height of water for cooling.
Pump is LF3196 Horizontal Low Flow Pump - it is "NO submersible pump" and "NO handlings solids".
32% in solids by weight is a high number.
You need a slurry pump, low rpm, high chromium metallurgy.
So it is slurry fluid, if your pumpen 32% by weight of sand d50, 100 mesh, fluid will be settling slurry type that means solids could settle on piping and plug.
So you have to calculate settling pipe velocity and derate pump performance for handling settling slurry.
You will use a Dragflow or Toyo pumps and in Goulds, you may use JC or HSU submersible heavy duty pumps.

I hope this helps