As rmw has pointed out, many pump curves already have this information shown, therefore if you have a curve it is only a matter of reading the information - if you don't you will need to contact the supplier.
you can trim your impeller, but you have also to modify the casing, suction and discharge pipe also. otherwise, you better go back to your pump curves as suggested by artisi and rmw
Tq
Actually, where I work we trim our trash pump and jet pump impellers without modifying the volutes or openings. One modification we make frequently is to trim a 14" jet pump impeller that is about 7-1/2" wide to 12.875" for about an inch of the width and then at a 27 degree angle. This is done to improve solids handling.
(original links from the "modifying impellers topic" are not active now).
There is increased turbulence at the vane tips as the impeller is trimmed because the shroud to casing clearance is increasing. This is sometimes referred to as "slip"
The liquid exit angle is changed as the impeller is cut back, so the head/capacity curve becomes steeper.
The greater the impeller reduction and the higher the specific speed of the impeller, the more the pump efficiency will decrease with impeller trimming.
Impeller diameter reductions greater than 5% to 10% of the maximum will increase the NPSHR (net positive suction head required).
Excessive shroud to casing clearance and the resultant recirculation to the low pressure side of the pump will produce "eddy flows" around the impeller, causing low frequency axial vibrations that can translate to mechanical seal problems.
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