Rotary Screw Pump, Submersible Motors

[noel0589]

Hi all,

Thanks for the help. I am dealing with a submersible system of an electric motor driving a rotary screw pump which in turn drives a piston(plunger) which raises a hydraulic elevator system.

Pump Data says that for pump to operate at 3400 RPM, the minimum flow rate must be 185.2GPM at 350PSI, a BHP of 46.2

As a small aside, I am assuming, but please correct me if I'm wrong, that the pump rpm is equal to the rotor rpm in a typical application. If so, why would the manufacturer say 3400rpm and not 3500rpm? 3500rpm is more closer to the full load rpm of a motor. Operating motors at 3400rpm could burn out the windings. So does this mean that if the motor is running at 3500rpm and psi is 350, minimum gpm would be more than 185.2?

I also wonder about the following. the pump data says that to run the pump at 3400rpm against a discharge 350psi we need a minimum flow of 185.2 and bhp of 46.2. At 400psi we need a minimum flow of 183.9 and bhp of 52.4. At 450psi we need gpm 182.8 and bhp 58.3.

why does gpm decrease as psi increases to have a constant rpm of pump? I would think that as discharge psi increases, slip would increase meaning gpm would have to increase to compensate.

Although I have more questions, I am tired and want to go home now.
Any comments would be appreciated!!

 

[MikeHalloran]

I'm guessing that 3400 rpm is intended to represent the speed attainable by a very hot motor with bad bearings and a marginal power supply.

If the motor can find enough torque to drive the pump at more than 3400 rpm, then yes, the pump will deliver more. As a first approximation, the pump flow will track shaft speed linearly.

As a second approximation, increased discharge pressure increases internal leakage, so discharge flow falls off a little with rising pressure.

Most motors in this size do not run at exactly constant speed. The pump curve is normalized that way to simplify your calculations a little.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[Artisi]

Your enquiry is a litle confused in terms of your understanding regarding pump hydraulics.

"Pump Data says that for pump to operate at 3400 RPM, the minimum flow rate must be 185.2GPM at 350PSI, a BHP of 46.2"

Pump speed governs pump output, not output governing speed.

I would say that you are running at above 3400 rpm - more like 3550 / 3560 rpm.

If the pump was operating at 3400 rpm the output would be 185.2 GPM at a pressure of 350psi and will require 46.2 HP. but in your case you will be slightly above this flow and pressure and absorbed power.

"I also wonder about the following. the pump data says that to run the pump at 3400rpm against a discharge 350psi we need a minimum flow of 185.2 and bhp of 46.2. At 400psi we need a minimum flow of 183.9 and bhp of 52.4. At 450psi we need gpm 182.8 and bhp 58.3."

If you need increased output in terms of flow and pressure the pump speed must be increased - BUT this can not be achieved with a synchronous speed electric motor.

A 2pole 60Hz motor speed is nominally 36000 rpm - slippage etc results in a speed usually of around 3560 rpm for a motor of this size.

Hope this helps you somewhat in your understanding.



Naresuan University
Phitsanulok
Thailand

 

[EVdave13]

Mike and Artisi are right regarding the curve. I have designed progressing cavity pumps, and am familiar with the confusion that can result from the curves.

The problem is that this is a positive displacement type pump, and the curves are different from centrifugal pump curves. With a positive displacement type, a given RPM generates a given flow rate. As back pressure increases, some slippage results, slightly reducing the flow. Also, as you increase back pressure, the pump takes more power to drive.

I don't know the exact type you are discussing, but if it happens to be the progressing cavity type (with a metal rotor in a stator constructed of molded elastomer inside a pipe), then 3400 RPM is a bit fast, though it may be OK with clean oil.

Most people read the RPM they need to turn their pump at to obtain a given pressure and flow rate. It is usually fairly low. There is usually some reduction by belts, gearbox, or other means involved, sometimes in combination with a variable speed drive, in order to match it to a motor. The required HP is also read off of the curve.

 

[MikeHalloran]

Dave, a rotary screw pump has a driven rotor that looks similar to that of a progressing cavity pump, but instead of an elastomer sleeve, it has one or two meshing rotors on parallel axes with complementary threads in their surface. The intermeshing screws form cavities that progress along the the rotor axis, but no elastic deformation, and little or no physical contact is involved. The quoted speed is not excessive for the pump type.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[checman]

Noel0589: The 3400 rpm maybe the nominal rpm of the submersible motor normally used by the manufacturer. Have you looked at the motor data plate for this information? As far as the minimum flows they are just stating the capacity of the pump at a given rpm and pressure. A rotary positive displacement pump must discharge a certain amount per revolution minus any slippage. That is why you always have a safety relief valve in the system. They will try to put out their rated flow minus any slippage no matter what. They will continue to build pressure in an effort to pump their rated flow until something gives. If something limits them from pumping their rated flow something will happen either the motor overloads or things break or burst but hopefully the system RV opens and bypasses the flow back to the supply.

Regards checman