Impeller cutting is the process of machining the diameter of the impeller (blade) to reduce the amount of energy added to the system fluid. Cutting the impeller can make useful corrections to pump performance due to oversizing, or overly conservative design practices or changes in system loads.

When to Consider Impeller Cutting?

End users should consider cutting the impeller when any of the following conditions occur:

1. Many system bypass valves are open, indicating that system equipment can obtain excess flow

2. Excessive throttling is required to control flow through a system or process

3. High levels of noise or vibration indicate excessive flow

4. The operation of the pump deviates from the design point (operating at a small flow rate)

Benefits of Cutting Impellers

The main benefit of reducing impeller size is reduced operating and maintenance costs. Less fluid energy is wasted on bypass lines and throttles, or is dissipated in the system as noise and vibration. Energy savings are roughly proportional to the cube of reduced diameter.

Due to the inefficiencies of motors and pumps, the motor power required to generate this fluid power (power) is higher.

In addition to energy savings, cutting multistage vertical turbine pump impellers reduces wear and tear on system pipes, valves and pipe supports. Pipe vibrations caused by flow can easily fatigue pipe welds and mechanical joints. Over time, cracked welds and loose joints can occur, leading to leaks and downtime for repairs.

Excessive fluid energy is also undesirable from a design perspective. Pipe supports are typically spaced and sized to withstand static loads from the weight of the pipe and fluid, pressure loads from the system's internal pressure, and expansion caused by temperature changes in thermally dynamic applications. Vibrations from excess fluid energy place unbearable loads on the system and lead to leaks, downtime and additional maintenance.

limitation

Cutting a vertical multistage turbine pump impeller changes its operating efficiency, and nonlinearities in similar laws associated with impeller machining complicate predictions of pump performance. Therefore, the impeller diameter is rarely reduced below 70% of its original size.

In some pumps, impeller cutting increases the net positive suction head (NPSHR) required by the pump. To prevent cavitation, a centrifugal pump must operate at a certain pressure at its inlet (i.e. NPSHA ≥ NPSHR). To reduce the risk of cavitation, the impact of impeller cutting on NPSHR should be evaluated using manufacturer's data over the entire range of operating conditions.