Impeller Trimming is done upto 70% of full impeller size? Is this correct? if no what is limit?
My Question:
In a performance curve, Impeller size given for particular model is 10" to 7.5" inch. My impeller is 7.5" inch at duty point on these performance curve. Is it Technically correct, to trim the impeller for this model with above limit percentage?

Trimming should be limited to about 75% of a pump’s maximum impeller diameter. If impeller diameter is trimmed down excessively, there will be excessive clearance between the impeller and the pump casing resulting in increased internal flow recirculation, head loss, reduction in pumping efficiency, more power consumption. Hence impeller trimming should always be as per pump supplier's guidelines which is a design parameter. Pump supplier has to give you details about suitable minimum diameter of that model.

You have specifed the designed impeller diameters as 10" to 7.5". i.e. Minimum allowable impeller diameter for that model as specified on perfromance curve is 7.5". If you want to go below 7.5", then you need to take confirmation from pump supplier. To go beyond design specs may cause troublesome pump performance, may be in terms of reduced efficiency, excessive power consumption & at extreme end, may lead to cavitation, noise & vibrations in the pump.

In short its not possible to go below, 7.5" for that particular model.

Many a time people take the above argument incorrectly. They feel whatever impeller offer to them by manufacturer is full size impeller and they want to trim the impeller upto 20% as per manufacturer instruction.

Have you come across anything like this in WILO.

Whenever you are scrutinizing the technical details of the pump provided by pump manufacturer, you should look for the details of Maximum Impeller Diamater, Minimum Impeller Diameter & Impeller diameter offered by pump manufacturer for the desired duty parameters. If these details are not provided by pump manufacturer, you should ask him to provide the same. This evaluation will give you clear idea about minimum & maximum impeller diameters for that perticular model. We always indicate these details in pump data sheet. Hope this clears your doubt.

You have not specified impeller inlet dia. It is not uncommon to fine umpeller inlet dia at 75% of OD. Trimming of mixed flow impellers can get tricky as trimming is on an inclined conical envelope. It will be unwise to lay down any arbitrary minimum dia as ratio of its OD.

The allowable impeller trim depends on several factors:

The impeller specific speed. Low specific speed impellers (<20 metric, 1000 US), can typically be trimmed more. This is because the blade loading is lower on these impellers and the vane overlap is higher.

Hydraulic limitations. As I noted above at some point the blade loading becomes excessive and the performance will suddenly drop. Large trims reduce the intensity of discharge side recirculation and can make the HQ curve unstable.

Mechanical limitations. At some point you can't trim further without hitting a feature like the impeller wear ring. Diffuser pumps require that the shroud remain untrimmed to maintain the A gap. This limits the amount of trim that can be applied without the unsupported shroud becoming too weak.

Efficiency limitations. Efficiency will drop with increasing trim. The efficiency drop with diffuser pumps will be significantly more than with volute pumps. At some point the life cycle cost due to excessive trimming will outweigh the benefit of purchasing a pump with a smaller (correctly sized) impeller.

The 70% limit you mention might be ok for small water pumps (< 100 KW) with volute casings. For bigger pumps I'd seldom recommend trimming below 80 to 85% for reasons of life cycle cost. For large (>1000 KW) diffuser style pumps we'd generally not go below 90% trim.

Does that help ?

From the vane developmental point of view ( according the “Grid “ shape) straight portion ( constant blade angle distribution @ trailing edge) only we have freedom to trim the impeller. As we know, for low sp. speed ; that straight length is more and it will be diminishing towards high sp. speed. So high sp. speed it is not advisable trim more because exit velocity vectors will change drastically even a small change.

Dear Simon, pls correct me if I am wrong.

Sorry i forget to mentioned that this is radial closed impeller as specific speed is 687.

My question is very simple, in performance curve, impeller diameter is mentioned is 10" to 7.5". Even if my duty point is lying on 7.5" , Will you advice to trim the impeller further or select a new model with full impeller size.

It is technically and financially better to select a new model having full impeller size of 7.5".
Technically because it will have less losses and financially because the Volute Casing and other parts of the pump will be small and hence will have lesser cost.