Limits on allowable maximum and minimum impeller diameters
Limits on allowable maximum and minimum impeller diameters
(OP)
I have seen specifications for pumps which state limits on the actual impeller diameter: no more than X% of the maximum diameter and no less than Y% of the minimum diameter. Has anyone seen specific values on these limits? What are they typically?





RE: Limits on allowable maximum and minimum impeller diameters
RE: Limits on allowable maximum and minimum impeller diameters
The main reason for the requirement is to cover future head rise, if a pump isn't sized correctly this can prevent the need for a completely new selection. Suppose this also applies if a pump is grossly oversized.
In some pump types (single volute OH2) there could be effects on radial loads with max diameter, and on any pump type there could be discharge recirculation with min diameter. But there is a reason the pump vendor has a max and min diameter and they should be designed to avoid these issues.
Example: a multistage vertical pump (VS1 or VS6) can meet conditions with 19 full diameter impellers, or 20 trimmed impellers. Provision for future head rise can be had by designing for the addition of a future stage.
If customer doesn't understand this, the reject the 19 stage pump, demand the 20 stage. This increases their shutoff head (generally not desireable) and then in the future when they need to increase head, they have to purchase multiple full diameter impellers, instead of 1 impeller and 1 case.
So, if the pump vendor can meet future head rise requirements in a reasonable manner, let them provide full diameter impellers.
Minimum diameter, there might be some merit in avoiding but only if there is not an alternate impeller.
RE: Limits on allowable maximum and minimum impeller diameters
RE: Limits on allowable maximum and minimum impeller diameters
Between 5-10% is typical for specs.; more than that and you've selected the wrong pump. You may also find concurrent requirements that the motor and baseplate be ORIGINALLY sized to accomodate the max diameter impeller; no matter what size is required by original C.O.S. If conditions change or were miscalced originally, it can become VERY expensive to upsize existing baseplate, foundation, motor, starters, etc......
Just get a copy of Shell, Exxon, Conoco, etc. pump specs and use as a draft if you are trying to write your own specs. These specs are all based upon decades of experience, and though they have become a bit overbearing these days, some of the basic intentions remian; one of them being the head reserve clause. JUST DO IT.
Verticals, instead of having a head reserve value for each stage, should make sure there is room in the original hole/can to add a couple stages for future use. Multistage verticals should be designed with all full trim impellers; just trimming the intial stage to fine tune the head. Trimming one stage preferable to trimming all of them.
RE: Limits on allowable maximum and minimum impeller diameters
The most reasonable way to specify the pump is to simply list what you want the pump to be capable of in the future and let the manufacturer determine how to size for present and future conditions whether it's increasing the speed the motor runs at or the diameter of the impeller.
RE: Limits on allowable maximum and minimum impeller diameters
"But the spec says....."
API 610 does not include any relevant requirement for % of impeller diameter, only for future head rise.
RE: Limits on allowable maximum and minimum impeller diameters
RE: Limits on allowable maximum and minimum impeller diameters
My experience has been for a well engineered design a max reduction would be somewhere between 90 -95% of full diameter, provided it doesn't sacrifice pump efficieny, this of course also depends where the pump duty falls on the H/Q curve
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.)
RE: Limits on allowable maximum and minimum impeller diameters
On diffuser pumps for example is is customary not to trim them much beyond 90% hence you can see the futility in a spec that calls for a 90% maximum diameter....
My recommendations are:
For volute pumps
Maximum diameter 95% to 97.5% of full diameter (97.5% is API 610)
Minimum diameter (at <= 1000 Ns) 80% of full diameter
Minimum diameter (at >= 2000 Ns) 85% of full diameter
If you are using a VFD, forget the above. You only need a trimed impeller if you want to guard against ever having going above a motor limiting speed.
For diffuser pumps
Maximum diameter 97.5% of full diameter
Minimum diameter 90% of full diameter