Pumps with heads greater than 200 m (650 ft) per stage and with more than 225 kW (300 hp) per stage

Pumps with heads greater than 200 m (650 ft) per stage and with more than 225 kW (300 hp) per stage
may require special provisions to reduce vane passing frequency vibration and low -frequency vibration at reduced flowrates. 

For these pumps, the radial clearance between the diffuser vane or volute tongue (cutwater) and the periphery of the impeller blade shall be at least 3 % of the maximum impeller blade tip radius for diffuser designs and at least 6% of the maximum blade tip radius for volute designs. The maximum impeller blade tip radius is the radius of the largest impeller that can be used within the pump casing . 

Percent clearance is calculated as follows:

  P = 100 (R 3- R2)/ R2

where
P is the percent clearance
R3 is the radius of volute or diffuser inlet tip
R2 is the maximum impeller blade tip radius.

The impellers of pumps covered by this clause shall not be modified after test to correct hydraulic performance byunderfiling, overfiling, or “V” cutting without notifying the purchaser prior to shipment.