Submersible Pump Motor Application

[noel0589]

Hi all,

I am new to this forum and basically new to my job trying to learn about hydraulic elevator systems. We use submersible pumps with submersible motors to drive a piston which does the actual lifting of the elevator.
My question is how motors are sized for pumps. From gross weight and desired speed of lift in FPM, we can get required GPM. and from a pump table I can see that HP for a certain pump at 350 PSI and 185 GPM is 46.5.
Does this mean that the pump will supply 46.5 HP at 350psi and 185gpm? and that I would have to use a motor of at least 46.5 HP capacity? Would I also have to match torque requirements?
If I used a 40HP motor what would happen?
If I used a 60HP motor what would happen?
Anything anyone would like to share would be great. As you can see I'm a newbie.

Thanks!!

 

[buzzp]

Just a little FYI: A submersible pump is submersed in fluid such as water. Is your pump motor submersed in hydraulic fluid?

 

[noel0589]

yes, it is submersed in hydraulic oil

 

[Scipio]

Actually the power delivered to the fluid (hydraulic horsepower) can be calculated as hp = psi * gpm / 1714. In your case, your operating conditions have a hydraulic power requirement of about 38 hp. Assuming the 46.5 came from a pump selection chart, that will be the brake horsepower. The pump must be about 81% efficient, so it takes 46.5 hp being supplied by a motor to produce the 38 hp of actual work done on the hydraulic oil. The rest of the power goes into parasitic losses like seal drag, as well as energy lost due to heat & noise generation, which varies depending on the type of pump you're looking at. Beyond calculating power requirements at the rated conditions, depending on the style of pump you may want to specify the motor be large enough to cover the entire pump curve. Some pumps actually require more power at shutoff than the rated point. I once saw one unit (a small regenerative turbine type pump) that required only about 4 hp at rated capacity, but over 10 hp at shutoff. Result was the motor over-amped and tripped the starter when we tried to turn it over.

As for torque requirements, yes, you have to meet them, but on a pump like this it's usually not much of a problem, the startup torque is frequently the highest torque requirement due to machine inertia, however it's not that high in these pumps. It's easily enough checked by requesting torque vs. speed curves for both the pump and motor from 0 to 100% of full load speed.

If you put a 40 hp motor in there, it would likely run for a little while if it had a high enough service factor, but within a few months you'd likely be replacing the motor - they tend to overheat and burn out prematurely if you run them that high.

If you put a 60 hp motor in there you're fine - you might be wasting a few bucks as a 50 hp motor would likely work fine though. Easiest way is just to ask the vendor what the rated and maximum horsepower requirements are for the pump, and make them responsible for putting the right motor on to cover the entire operating range.

 

[noel0589]

I know that we can get torque-speed curves for motors. But we can also get this for pumps?!?! But pumps do not deliver any torque do they? If I have to meet starting requirements to lift an elevator cab what would I have to calculate between motor, pump, and cab?

 

[CESSNA1]

NOEL0589: There are no torque speed curves for pumps, there are pump curves. Pump curves typically show pressure verses flow with lines of constant horsepower. You may have an accumulator in the system to smooth out pulsations and ease the starting load by applying initial pressure while the pump comes up to pressure.

Regards
Dave

 

[Scipio]

Depends on the pump and pump manufacturer, but yes, in some (not all) cases you can get torque curves. The pump doesn't deliver torque, but it does require torque to do its job. Basically the motor converts energy (electrical power) into torque, which is transmitted through the shaft into the pump, which then converts the torque back into energy (added head to the fluid, the hydraulic power) in different ways, depending on the pump construction.

The type of curve CESSNA1 describes is usually catalog info covering the hydraulic envelope a particular pump model & size can handle, or in the case of a variable speed unit a family of speed curves. If, however, you have a fixed speed pump and you have a specific model & size selected, a more specific curve can be generated. The specific curve will usually show flow vs. head, along with power requirements across the capacity range, at rated speed. If you know the horsepower requirements and the speed at a given point, you can calculate the torque.

I'm assuming you're probably looking at a gear, vane or lobe pump here, in which case I wouldn't worry about the torque. Startup torque is usually the only place you run into problems. One example is a progressing cavity pump - these operate with an interference fit between an elastomer stator and a steel helical rotor. Since the static coefficient of friction is greater than the dynamic coefficient, more torque can be required to overcome the static friction between the two and get the rotor turning than is required to keep it turning. Another example could be a large multistage centrifugal pump where the rotor is massive enough to have a significant moment of inertia resisting accelerating up to speed. The type of pump it sounds like you're looking at shouldn't have any of these concerns associated with them, but the vendor should be able to tell you for sure.

As for what's required to get the elevator cab moving, sorry, that's something I have no experience whatsoever with, I don't know what your startup scenarios would be.

 

[Artisi]

do a search on the net for pump breakaway torque, I just typed in "pump breakaway torque" at

http://ask.com/

seems to be a few sites to look at.

Naresuan University
Phitsanulok
Thailand

 

[d23]

noel0589

FYI:

Pump curves are typically based on fresh water. If you are using hydraulic fluid you need to talk with the pump manufacture. Hydraulic fluid is usually a low viscosity, but head degradation does occur due to viscosity.

HP curves are also based on fresh water. If the pump curve predicts 46 HP, that number is most likely base on fresh water. HP is a liner calculation based on fluid specific gravity.

About Pump Torque Curves:

I have personally never seen a pump torque curve; however a pump manufacture can provide you with pump inertia information for a given impeller. For your needs you could look at the motor speed torque curve and predict the RPM developed under given load conditions for that motor. You can then compare the motor slip to the pump curve and make a more accurate load calculation. Manual iteration between the two curves could take hours or days to complete, but HP required will change approximately at the cube of the speed change. I sell oil filled motors that are a little different than most motors you will encounter. For our motors I wrote a Y equivalent spreadsheet that makes these calculations for me.

This is a little late, but hope it helps some!