Impeller diameters 2

[EngMecBra]

Hi,

What kind of problems may occur if a pump operates with a rated impeller diameter of 584mm, with minimum and maximum impeller diameters of 562 and 740mm?
The difference between minimum and rated impellers is too small, only 22mm, while the difference between maximum and rated impellers is 156mm!
Please see annex file!

Thanks!

Rodrigo Alves.

 

[SNORGY]

As far as the pump is concerned, there won't be any problems, since preumably it is designed by the OEM to accommodate the full range of impeller sizes.

You might not (and based on what I see, *likely do not*) have a big enough motor to run the bigger impellers. You also will need to evaluate your piping system dP and velocities downstream (discharge side) and upstream (suction side) of the pump to see if the pump continues to be an acceptable fit in the system. With increasing impeller size will come increasing flow and increasing pressure drop, which will tend to force the pump operation leftward along its characteristic and give rise to unbalanced radial forces, increased shaft deflection, somewhat shorter bearing and seal life, and lower efficiency; while on the suction side you might begin to see cavitation. Based on what I see, as you increase impeller size you will probably see problems in this order:

(1) Available motor power
(2) Suction side hydraulic bottleneck
(3) Increasingly poor process fit in piping system
(4) Shortened life on pump parts.

 

[Artisi]

A quick answer to your question - "What kind of problems may occur if a pump operates with a rated impeller diameter of 584mm, with minimum and maximum impeller diameters of 562 and 740mm?"

None.

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.)

 

[1gibson]

There are not going to be any hydraulic problems with the pump at min/max diameter, the only negative consequence will be possible limitation of your ability to re-rate the pump in the future, for a single stage pump without additional impeller designs that fit the same case.

How do you think minimum and maximum diameters are determined by the pump manufacturer? Do the vendors say "let's set the minimum and maximum diameters just outside the safe range for hydraulic design, just to trick people who don't put a clause in their spec to stay 5% or more away from these max/min values" ?

The intent of a spec imposed limit is to provide a margin of error for system design, in case the conditions need to be modified without having to change pump selection. If there are options other than impeller diameter (different hydraulic, additional stages, VFD overspeed) then you need to look closely before arbitrarily rejecting selections at max/min diameters.

I will say, you want to confirm that the NPSHr is adjusted based on impeller diameter for small impellers. At the same flow, a smaller impeller's NPSHr will increase a small amount. This is partially due to how NPSHr is measured (3% head drop at 100 ft is 3 ft, 3% head drop at 66 ft is only 2 ft) and partially due to the reduced BEP flow of the trimmed impeller. But the selection software should take this into account, and it is more of a system issue than a "hydraulic problem" with the pump.

The pump vendor will not sell you a pump with unacceptable impeller diameter at the conditions you specify. If you need additional flexibility, check all of these cases and ensure your selection can accommodate worst case or future scenarios.

 

[bimr]

It is difficult to say what the effect will be on the pump system without knowing the details of the system curve.

The point on the curve where the flow and head match the application's requirement is known as the duty point or rated capacity. A different impeller will cause a change in the flow with a corresponding change in the system curve.

It is possible that that a different impeller size may not allow the pump system to meet the application requirements.

In addition, if the electric motor is undersized for increased flow with a larger impeller size, there is a possibility that the higher amperage draw caused by use of the larger impeller may burn up the pump's electric motor.

Other than the possibility of burning up an undersized motor, a change in impeller size should have little effect on just the pump.

 

[BigInch]

Discharge pressure will tend to increase. Be sure that it will still be below maximum operating pressure limit of downstream pipelines.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

 

[77JQX]

The pump will work OK. The issue I see is that when your rated impeller is close to the minimum diameter you tend to get poorer efficiency. The efficiency (if Rend. translates to eff.) at your design point isn't even 65%. I don't know what you're pumping, and that may effect your pump design, so I may be wrong here. But, for a pump of this size, if I was pumping clean water, I'd want 85% efficiency. At the electric rates I pay, I'd expect the 65% efficiency pump to cost about $40/hour more to operate than an 85% efficient pump.

 

[bimr]

Sorry, I missed the pdf.

At 650 cu meters/hr and 335 meters head, your pump will be using 592 kw.

If you install the larger impeller, then your pump will be putting out approximately 576 to 600 meters of head according to your pump curve. Now you will be using at least 1016 kw.

Unless your pump was installed with a significantly oversized electric motor, the larger impeller will not work. You will overload the existing electric motor and wiring. The higher amperage that will be used will burn up the pump's electric motor.

The power required varies as the cube of the diameter ratio.

 

[stanier]

Suggest you look at these websites and learn about the issues you have raised. Then if you have more questions post them on the forum.

http://www.pumpsystemsmatter.org/

http://www.mcnallyinsitute.com

http://www.pump-zone.com

http://www.pumpfundamentals.com/pump_glossary.htm#gl441

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[Artisi]

For a simple question I would think that the OP must now be completely confused with all the tangential comments re, power, efficiency, changing impellers, motor sizes, NPSH etc etc. when all he was asking was there any problem operating at the reduced impeller diameter.



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.)

 

[SNORGY]

I think Artisi just eloquently slapped me upside the head for pointing out the pre-conceived bias from which my reply was spawned. Star for that...

 

[Artisi]

After so many years in the pump industry, I like to keep things simple - (KISS) - Keep It Simple and Specific otherwise the issue becomes clouded and out of control, likened to "shotgun therapy", fire a load of shot when only 1 well-placed pellet would have been sufficient.

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.)

 

[BigInch]

Me2. For some reason (Snorgy mentioned "impeller increasing" ), but then didn't say the pressure might go high, so I stumbled along further down the wrong path mentioning that. My trigger finger was quicker than my aim yesterday. Snorgy your answer was 98%.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

 

[bimr]

Are cost and efficiency somehow excluded from the definition of a problem?

One problem is reduced efficiency which will be observed by a higher electrical operating cost.

A pump fitted with a smaller impeller will be less efficient than a smaller pump fitted with a full size impeller when matched to the same duty.

A 5% efficiency difference for a pump of this capacity results in at least $10-15K more annual operating power cost (when operating 12 hours per day).

Maximum efficiency in a pump is achieved when using the largest impeller size.

Another problem is that the pump will have a higher initial installed cost than a smaller pump.

This is not a good pump selection for this application if that is what you are asking.

 

[TenPenny]

"Maximum efficiency in a pump is achieved when using the largest impeller size."

Well, not always. Seeing as how we're getting picky and pedantic and all.

 

[1gibson]

Selecting a pump in the largest impeller size also limits you on options in the future. It may or may not be safe to assume that he's picked the best hydraulic selection available to him.

Life would just be too easy if the right answer was always "calculate your system perfectly, and select a pump near max impeller diameter that runs at exactly the best efficiency point."

But as pointed out already, we are digressing. No problem was ever stated in this thread (not by the original poster, at least!) We're one step away from discussing the magic of plastic/composite close clearance wear rings, and cost savings of VFD's...

The only thing I would suggest is change the impeller diameter in the selection software, keep flowrate the same, and confirm the NPSHr also changes (probably a very small change.) If it doesn't change with diameter, then the software is not set up properly and NPSHr should be clarified by vendor.

 

[Pumpsonly]

Agree with Artisi, a simple question has attracted many unrelated comments. The OP did not mention about fitting larger impeller and a barrage of comments about motor sizing, efficiency , BEP etc..

This is obviously a 2 or 3 stage pump with very large wheel impeller..and a relatively narrow passage. It is not easy to cast this type of impeller with good finishing, therefore the efficiency is not too bad.
The next smaller impeller pump may not give the required head, therefore this pump is selected.

It is easy to say select a pump to operate at or close to BEP. But how offer you can do that in real life? Unless you are want a custom make pump for your application.

 

[xabichika]

when you have rated diameter impeller very close the the minimum impeller diameter, the chance pump will suffer recirculation is higher. This is due to the gap or clearance between impeller vane tips and cutwater is wider. This recirculation will cause the unstable flow on impeller outlet location. The unstable flow manifest in vibration, noise and cavitation. The minimum rated impeller diameter should not lower than 75% of max diameter impeller, this what OEM has informed me.

 

[1gibson]

Xabichika, centrifugal pumps come in various specific speeds. What is true for a low specific speed pump will not necessarily be true for high specific speed pump. I would not make any decisions based on your 75% rule of thumb, and would not second guess an OEM's available range of impeller diameters based on this concept either.

As far as relevance to the original post, 740mm x 0.75 = 555mm. The impeller is 584mm, so you are saying there should not be any issues. More accurately, you are saying that some OEM said there should be no issues.