Vertical turbine Pumps - power consumption

[techky]

Hi Guys,

I need your advise.

I am doing an apple to apple comparison of Mixed flow Vertical line-shaft pumps.
Duty conditions are 4,400m3/hr @ 15m, 2,500m3/hr @ 30.3m
Based on the motor specs compliance of 1.15, the motor rating turns out to be 500Kw / 8 POLE for the Manufacturer A.

However, I understand that the other pump manufacturers (lets say Manufacturer B) meets the same duty condition at 350Kw / 8 POLE.

& manufacturers C & D can meet the same duty conditions at 400Kw / 8 POLE ; 450Kw / 6 POLE.

This puts me under a big doubt.
Keeping aside the curve comparisons, my query is that:
- How is that all the manufacturer are coming out with very different motor rating which is sometimes 2 or 3 ratings higher? Least to say the significant imapct on cost & KwHr.

Note that the pumps are designed based on manufacturing standard.

Could you share your thoughts and advise please.
Thank you!

 

[Artisi]

First up, have you taken the time to calculate the power required for the 2 duties based on reasonable pump and motor efficiencies - if not why not?
Before you can make any decisions you need to have some idea of what's what.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[ashtree]

The curves for the pump will tell the story but a 350kw might do the job. It will depend on the pump efficiency, and the margin the manufacturer puts on the motor selection.

Consider the calculation for water horsepower.
QxH/102= Kw.

Q= 4400m3/hr =1222 l/sec

1222x15/102=179.7 kw.

If we use a pump efficiency of 70% means that you will need 256kw input to the pump. Its normal to use a motor that at full pumping rate is no more than 90% loaded which means that you would select a motor of at least 285 kw.

At 2500m3/hr and assuming the same pump efficiency (which is unlikely) and the 90% margin means that you will require at least a motor of 327kw. The next size obviously is a 350kw.

So the first step is to compare the pump efficiency at the operating point on the curves that you have selected.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Artisi]

Pumps of this size should be into the high 80's for efficiency.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[ashtree]

I agree but i was trying to stay on the conservative side.
The relative efficiencies of the pumps would be the first thing i would be looking at because an inefficent pump at this size is going to burn money fast.
The larger pumps may also be "specified" by someone who has no idea, has no product smaller or thinks that the client won't know/care and will just go with the largest pump.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Artisi]

The 500 kW is either non-sense or a mis-read of the data.
The pump data offered including curves would be nice to see, but as usual - crystal ball gazing.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[TenPenny]

If you have the pump data, look at the efficiency and the power required at the rated point for each pump. Then you can get a sense of what the various manufacturers have sized the motors for. Sometimes, they will size the motor based on the actual operating point; sometimes they will size the motor for end of curve / max hp.

Your first step is to compare the different efficiencies for the different pump selections, and that should be a prominent item listed on the quotations.

 

[LittleInch]

You need to compare the shaft power requirements of the different pumps at the two duty points to make sense of this data. Just because the motor is rated at 500kW, doesn't mean that is what it uses in operation.

Hence if you're looking at power costs as an issue, then you need to look at absorbed power of the pump, not the motor rating.

An issue worth looking at between a smaller motor and bigger one is the starting current, which can be significant for larger motors unless some sort of VFD or soft start technology is used.

8 pole / 6 pole affects pump speed which might make one more efficient than the other.

It also depend son which electric motor vendor is used for frame sizes.

The pump data sheet is what you need to post / look at properly.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JS_]

Have you looked at the thrust bearing requirements for your pumps?

Sometimes it is necessary, to go one size larger for an electric motor or right angle drive for a vertical turbine pump due to the thrust of the unit. The motor or drive often meets the design criteria for power, but lacks in the bearings capacity.

 

[techky]

Hi @Artisi,
please find attached curves. Very sorry, I should have provided this in the first place.

Hi @Tenpenny,
you are right. the motor is sized based on End or curve / shut-off (which is Max. Hp).
This goes the same rule with all the manufacturers. So how could either the manufacturers have a design advantage over this?

Hi @Ashtree,
The hydraulic Kw per manufacturer's datasheet is 286Kw. I am not sure how your calc is 179kw.
The pump efficiency is 83% (indicated in curve). Rated and Max. BP is 344 & 378Kw.
Very sorry, I should have provided this first place.
Do you think every other manufacturer's curve would deviate more than 5 or 10% than the curve provided to you. So how could possibly some other manufacturer have a lower kw rating than this.
Thanks

Hi @littleinch,
Thank you for nicely pointing it out. The core of the issue is the power cost (KwHr), whom-sover has the least gets the advantage.
Do you think every other manufacturer's absorbed power (water power I suppose you are referring to) will differ more than 5% from one manufacturer to the other, Let's say if the other offered is a [highlight #FCE94F]radial flow[/highlight].

The starting current is not a concern since it's VFD driven.
[highlight #FCE94F]May I know what do you mean by "which electric motor vendor is used for frame sizes".[/highlight]
For confidential reason, I can't post the datasheet. please excuse me on this.

Hi @JS,
I'll check on this. But this is a simple V1 motor application.
Do you mean every other manufacturer have significant advantages on transmission losses & thrust bearings etc? which could lower the absorbed power?

 

[LittleInch]

I'm referring to shaft power which as per your graph is higher for the dashed line compared to the solid line.

This seems to be related to the additional head being supplied by the alternate pump so if they trim their blades a bit then given the efficiency curves they should give you the same absorbed shaft power.

HOWEVER, your duty point is apparently 4400m @ 15m. So WHY have you got pumps giving you 24 and 27m?? Doesn't make sense and if you then need to throttle these pumps you're just burning power / money for no benefit. Ashtree used the data available to him at the time ( 15m @ 4400) compared to your pumps which are giving you a lot more head at the same flow, hence need a lot more power.

I know you've said these pumps are VFD, but why buy something much bigger than you need if you're never going to use it / need it? This is like buying a truck capable of transporting 25 tonnes of material and only ever putting 15 in it.

Pumps are only one part of a system so you need to look at this as a system to get the best efficiency out of it to match your requirements and not focus on one aspect.

What I meant before I saw your figures, was that depending on who the pump vendor goes to, they might only have a set number of motor frames and hence give the pump vendor the next one up.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

Nonsense curves, suggest you come back with some real data - that's if you really want assistance.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[techky]

Hi @Littleinch;
Noted on your reply. Thank you. yes it makes sense to try a smaller head pump. We will explore this option too with the manufacturer.

Hi @Artisi,
Noted. As mentioned above, we will try to revise our pump model for a better curve.