Pump Motor RPM

Vader?

Your consultant had 1500 rpm in mind and probably his mindset doesn't conceive less. Is this an existing replacement? Besides starting currents and couples both items you propose, will do the work. The lower Rpm (985) will have a little more slip, but will run smoother, considered the housings and impellers have the same quality. The 985 rpm solution will probably be more expensive - bigger pump house, more expensive motor, wiring and protection accordingly. His argument doesn't convince me that much. Ask him for a meaningful explanation, because what you 'quote' is poor. These are no small pumps anyway.

This is a new equipments to be procured and install in a Pump Station, we have also an argument that if the motor is design in a 1500 rpm and we used 985 rpm, the motor will give more hotter temperature than the 1500 rpm (thats is one of the reason why they prefer the higher rpm in the motor), speaking of the big pump house, and motor we are aware in the expenses part which fits the budget, but if they ask now to used the 1480 rpm (or option 2) I am getting a higher pump capacity also which will require a big capacity in the VFD side this will give me also a more big cost, so either of these two option I used it will lead to cost implication.

As of now they're asking for the 1480 rpm or Option 2 so far.

Correct me if i'm wrong, In my point of view if I will used the 985 rpm I could end up with longer life of the pump compared to the 1480 rpm. In which both will satisfy the duty points.

To add to the thoughts expressed, and maybe your dilemma jho wel,

If a VFD is slowing down the 1500 motor to 985, yes it will run hotter. If your application do not require the control of volume at any stage, why even think of VFD? The cost alone here will be prohibitive and unwarranted. You do get different pump designs and you can buy a pump off the shelve running at the slower speed. However remember the rule of thumb, the thicker the product the slower you need to pump. Centrifugal water pumps are typically, depending on design, run at 1490 or 2900 rpm.

are you suggesting now the selection of 1480 rpm is more correct than the 985 rpm?

I will be thankful if you can give me the list of standard or code where I can find that the Centrifugal pumps should be running at 1490-2900 rpm.

In the 60-Hz world, many pumps are 1800 or 3600 rpm, but 1200 or 900 rpm are also quite common.

In the 50-Hz world, 1500 or 3000 rpm would be common, with 1000 or 750 rpm not too far behind.

(These are all synchronous speeds; owing to slip, inductive motors would be a bit less rpm in each case.)

You can mix-and-match this in many ways; the peripheral velocity of the impeller governs (specific speed). If you examine the curves for belt-driven pumps (any speed you want), you will get a more complete picture.

The horsepower (or kw) should be the same in either case, so the motor starter and wiring would also be the same.

I agree with your reasoning that the larger, slower pump would tend to last longer. The motor will also be a frame size or so larger. So the initial cost would be higher.

I had not heard before of the slower motor running hotter, but it is plausible that the larger size would not dissipate heat as well.

Have you received any budgetary quotes on these options!

It may be that there is concern for lower motor cooling fan speed, but as this is a centrifugal pump, the load (and heat) would decrease with speed. These factors may or may not offset each other. The motor manufacturers can usually provide answers.

I agree,

There are tens of different pump designs. Chose a pump that fits the capacity and loads, depending on the system, you can play with frequencies, but don't forget the slip between the pump and the medium. Some pumps don't perform at low speed. You should tell a lot more. I see new components join the show? Without full process specifications, this is not just asking how we like our eggs.

In advance many thanks for all the ideas for this kind of discussions.

To give you all a clear view.

Given is the ff:

Specification states that the pump should be selected from min. - max. duty as mention below,

Duty = 550 lps/40M head; min=220lps/54M head; Max=815 lps

Speed (max) = 1500 rpm & Min. Efficiency is 85%

NPSHa = 8 M at the Bottom Water Level of Reservoir

The pumps manufacturer are having hard time in selecting the pump up to max. flow (of 815 lps & including all parameters) that will fit in a 8 M head suction pressure, most of the selections using the three points it ends up always with a higher NPSH required (w/c range from 9.7 - 13.2 M Head on different manufacturer),

Now in view of the above: Since the Pumps are running on VFD and to run at duty point (as specify in the instrumentation specs.), we did try in selecting the pumps by Option 1: ( meets the required 3 points but low RPM)

Duty = 550 lps/40M head; min=220lps/54M head; Max=815 lps/20M head

Speed (max) = 985 rpm & Min. Efficiency is 86%

NPSHr = 3.76 M; Motor = 280 Kw

Option 2: ( meets the required 2 points but the max flow is 720 lps only)

Duty = 550 lps/40M head; min=220lps/54M head; Max=720 lps/11M head

Speed (max) = 1480 rpm & Min. Efficiency is 85%

NPSHr = 5.7 M; Motor = 355 Kw

These are the two best selection they've done so far.

Note: Option 1 requires NPSHr=5.8M Head (max) while the Option 2 requires 9.8M head (max),

I would appreciate it which of the two has the best option or give me an advice for these selections.

With this additional information, the choice becomes clear: Option 1.

Option 2 fails to meet the NPSH_a condition near the upper range of its speed. This would risk cavitation and possible damage to the pump.

I hope you can deal with the politics of anyone still favoring the higher speed.