TRIMMING OF IMPELLER OF CENTRIFUGAL PUMPS

(5) FOR A FLOW OF Q_d WITH TRIMMED IMPELLER, THE FLOW EQUIVALENT WITH THE ORIGINAL IMPELLER HAVING DIAMETER OF D_{original ,}Q_eq-original= ( in M³ / Hr. )

Q_eq-original= Q_d * ( D_original / D_trimmed)

(6) NOW THE HEAD WE GET WITH ORIGINAL IMPELLER FOR A FLOW OF Q_eq-original

( READ FROM THE H Vs. Q OF THE ORIGINAL IMPELLER ) = H_eq-original

(7) HEAD THAT WILL BE DEVELOPED BY TRIMMED IMPELLER FOR THE DESIRED FLOW OF Q_d , H_trimmed( MLC ) =

H_trimmed = H_eq-original * ( D_trimmed / D_original)²

(8) NOW COMPARE H_trimmed AND DESIRED HEAD H_d AND IF THEY DON'T MATCH CHOOSE ANOTHER DIAMETER FOR THE TRIMMED IMPELLER ( D_trimmed) AND REPEAT THE ABOVE STEPS UNTIL THEY MATCH.

IMPORTANT THINGS TO REMEMBER WITH REGARD TO TRIMMING :

(1) The affinity laws are not accurate if the extent of trimming is more than 10% of the impeller diameter because of (a) there is increased turbulence at the vane tips as the impeller is trimmed because the shroud to casing clearance is increasing which is called "slip" (b) head - capacity characteristic curve becomes steeper as the liquid exit angle changes because of trimming.

(2) Mixed flow pumps are more affected than low specific speed radial vane impellers ( high head & low capacity )

(3) The effect of impeller trimming is more pronounced in high specific speed pumps. The greater the impeller reduction and the higher the specific speed of the impeller, the more the pump efficiency will decrease with impeller trimming.

(4) It is important to note that impeller diameter reductions greater than 10% will increase the NPSHR ( Net Positive Suction Head Required ) and it may lead to cavitation if there is a close margin between NPSHA & NPSHR. Check with your pump manufacturers.

(5) Excessive shroud to casing clearnace 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. This can be a real concern in large pumps of over 250 horsepower (195KW ) or pumps having heads in excess of 650 feet (198 M ).

(6) The pump efficiency normally decreases with trimming. It can be improved with corrocoating of the impeller. This was reported to be done in some chemical industries and efficiency improvement was reported.