Centrifugal Pump Pumping System Knowledge: What is a Cutting Impeller?

Impeller cutting is the process of machining the impeller (blades) diameter to reduce the amount of energy added to the system fluid. Impeller cutting can be a useful correction to pump performance when a pump has been oversized due to overly conservative pump design practices or changes in system loads.

• What is a cutter impeller?

The performance of a centrifugal pump can be changed by changing the diameter of the impeller, that is, by cutting the impeller.

API610 standard stipulates that when replacing an impeller with a larger diameter or a different hydraulic design, with speed regulation capability or using an empty stage, the pump head should be able to increase by at least 5%. In other words, for centrifugal pumps with constant speed and constant number of stages, the maximum diameter impeller is not allowed to be used, that is, the required performance needs to be met by cutting the impeller.

Each standard centrifugal pump has a maximum diameter impeller installed in the casing/guide vane, which is usually a standard design impeller (meeting specific performance requirements). Of course, each standard centrifugal pump also has a relative minimum diameter impeller. It is called a "relative" minimum diameter impeller because (after cutting the impeller) the performance it produces is acceptable to both the buyer and the seller.

By increasing or decreasing the diameter of the impeller, the pump output can be increased or decreased. Impellers with larger diameters produce greater head and flow than impellers with smaller diameters. Changes in impeller diameter will also affect the efficiency of the pump. When the cutting amount is small, the efficiency effect is not obvious; however, when the cutting amount exceeds a certain value, the efficiency of the pump will drop significantly. Therefore, pump manufacturers will give a recommended minimum impeller diameter for each centrifugal pump.

• 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 excess flow is available to system equipment.
2. Excessive throttling is required to control flow through the system or process.
3. High levels of noise or vibration indicate excessive flow.
4. The pump is operating away from the design point (running at low flow)

• Method of impeller cutting

There are three ways to cut the impeller:
1.Equal diameter cutting. This is the most commonly used cutting method.
2.Bevel cutting. Usually used on low specific speed single suction impellers.
3.V-shaped cutting. Usually used on double suction impellers.
For centrifugal pumps, no matter which cutting method is used, usually only the impeller blades are cut and the cover plate is retained.

• Benefits of cutting impellers

The primary benefit of reducing impeller size is reduced operating and maintenance costs. In addition to energy savings, cutting impellers reduces wear on system piping, valves, and pipe supports. Flow-induced piping vibrations tend to fatigue pipe welds and mechanical joints. Over time, weld cracking 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 handle static loads from the weight of the piping and fluid, pressure loads from the system's internal pressure, and expansion caused by temperature changes in thermodynamic applications. Vibrations from excessive fluid energy place unacceptable loads on the system and lead to leaks, downtime, and additional maintenance.