Performance curve min/max impeller diameter for a centrifugal pump

[mehrdadkntu]

As you know, the pump manufacturers supply their pump performance curves with nominal minimum and maximum impeller diameters and some diameters within this range for a particular model.

I received one proposal from a manufacturer in which a pump model had been selected for our process condition with the Min, Max and rated impeller diameter in its performance curve.

Due to change in our process condition, we requested the vendor to revise his proposal. In his second proposal, he had selected the same model for the new process condition, but he had changed the max & min impeller diameters to select the rated impeller for meeting our new process condition.
What is the criterion for determining the max & min impeller size for a particular model of a centrifugal pump?

In a particular model of a centrifugal pump, is the max/min diameter of the impeller fixed or they can be changed by vendor according to the conditions?

my another question is that how the rated impeller is fabricated.is it fabricated by trimming/cutting the max impeller down to the rated diameter or the vendor redesign and fabricate it separately?

 

[Artisi]

Answer to the first part of your 1st questions is yes, for the second part, no.
Answer to the first part of question 2 is yes,for the second part, no.
Max / min diameters are design, for your particular duty the diameter is selected to suit the duty.

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.)

 

[HPost]

In general, it is the diameter of the impeller that dictates the head that can be achieved, whilst the height of the vanes will influence the mass flow rate.

Both can be trimmed to adjust the curve, but only within certain limits. Trimming too much off will reduce the efficiency.

If you give the vendor the operating requirements, they will select an impeller that can achieve what you need. The limits around that particular impeller are what can be achieved with min and max clearances between the impeller and the housing.

 

[Artisi]

HPost: not wishing to sound critical, but to clarify what you said, changing the height of the impeller blades is opening a can of worms for discussion in this particular post,
for 1, it could only be done on some open impeller designs
2. a complete pattern design change would be required for a closed impeller.
3. Very unlikely something a manufacturer would undertake or even consider for a one off application.

Also, the max/ min shown on any pump curve or data is related to impeller diameter and not impeller / case clearance.

However you are spot on regarding giving the duty to to pump manufacturer / supplier- which was done and a different selection made, what is not clear in the OP post was "he changed the max/ min diameter".would need expanding for full understanding.

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.)

 

[DubMac]

In addition to the above, several pump vendors will have more than one impeller that will fit into a particular pump casing. Possibly your vendor just showed you a curve of the same model/size pump as the original, but with an impeller "B" or "C" installed.

Can you post a pic of the curves you've received?

 

[HPost]

Artisi...

The words "in general" are there for a reason.

The OP made no reference to open or double shroud impeller, so the comments I made remains valid.

Your comments were not taken as being critical, just as a different opinion.

Without doubt, if the impeller is already tooled up, as double shrouded, then there are limited options to trim the impeller. However, we didn't know the facts, so all options are open.

All said, no vendor will guarantee a curve can be achieved, so they are bound to give a tolerance on performance. Especially as many people are often unable to give a cast iron guarantee about the system restriction.

At least, that was my experience when designing impellers from scratch for diesel engines....

 

[mehrdadkntu]

Dear friends

Thank you all for your help and reply
Here I mention the process condition and the specification of the selected pumps by the vendor:
First proposal:
Head: 45 m Capacity: 12 m3/h
Pump model: CP-25-225
Impeller sizes: Max= 250, Rated= 187, Min=170
Second Proposal:
Head: 30.5 m Capacity: 12 m3/h
Pump model: CP-25-225
Impeller sizes: Max= 250, Rated= 159, Min=155

For another pump he proposed:
First proposal:
Head: 45 m
Pump model: CP-25-200
Impeller sizes: Max= 200, Rated= 196, Min=150
Second Proposal:
Head: 62 m
Pump model: CP-25-200
Impeller sizes: Max= 230, Rated= 226, Min=150

Dear DubMac I will provide the curves ASAP.

 

[bimr]

The pump manufacturers use various sizes of impellers to meet a range of flow and head conditions with the same pump casting. The use of different size impellers enables the manufacturer to efficiently meet the market needs. One pump with a range of impellers is more practical than having a specific pump for each specific pump application. Increasing the diameter of the impeller while keeping the revolution speed constant gives the pumped liquid more energy.

 

[gempump]

mehrdadkntu,
The greater the diameter of the impeller, the higher the Head.Different diameter of impellers to meet best operating conditions that will be close to the best efficiency point.
Reading and understanding centrifugal pump curves is key to proper pump selection.

Heavy-Duty Centrifugal Slurry Pump Manufacturer
www.gempump.com

 

[LittleInch]

mehrdadkntu,

The pump vendor has a range or impellors which all fit the same casing. Each impellor with a different max / min range will have a different set of curves, especially the efficiency curve which will vary for each impellor which you show above. The vendor, or his software, will choose whichever is best for your duty - usually lowest power so he can give you the smallest motor he can and also comply with whatever specification you ask for (usually you want duty point to be within 80-85% of the BEP).

You need to stop looking at this and concentrate on comparing the data from each pump offered with other pumps to find the one which matches your duty points at the highest efficiency.

How the vendor does it is up to him - if you order from him he needs to provide you with what you asked for and then prove on a performance test (if you ask for one) that he has supplied what you ordered.

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

 

[TenPenny]

It would help to see the pump curves and know the manufacturer. I can understand your question, and I can think of a couple of different explanations.