TRIMMING OF THE PUMP IMPELLER
Impeller trimming can save up to 20 % of energy
Main topics:

When the pump impeller trimming is necessary?

Why does pump user decide to trim the pump impeller? Which malfunctions in the pump operation indicate that it is necessary to cut the impeller.

How the pump characteristics change when cutting the impeller? What formulas can be used to calculate impeller diameter.

How cutting of impeller affects the power saving?

Examples of impeller trimming calculation

Defining impeller diameter using catalog.

Defining the impeller diameter for required pump parameters.
When the pump impeller trimming is necessary?
The pump has parameters more then necessary or oversized.
Evidences of the pump oversizing:

Throttle control

Electric motor overloading. Control panel shut off the electric motor.

Hight level of vibration of the pump pipes, valves.

Extra maintenance cost.

Cavitation.
In what cases impeller trimming is most effective?
 When the characteristic of the system does not change in time. Those pump operation does not change over time. For example filling a tank.
How much energy can be saved by impeller trimming?
According to statistics, trimming of impeller can save up to 20% of electricity.
In addition to direct savings, it is also necessary to take into account indirect savings. For example, if a pump is controlled by a valve, then increased wear and, accordingly, its repair and replacement. Repair of the pump itself.
Dependence of pump parameters on impeller diameter?
Affinity laws for trimming of impellers.
Q1  initial flow rate
H1  initial head
P1  initial power
D1  initial diameter of the pump impeller
Q2  required flow rate
H2  required head
P2  required power
D2  required diameter of the pump impeller
Characteristics of a pump with impellers with different diameters
Head curves for impellers with different diameters 510, 460, 410 mm.
IMPORTANT. Trimmed impeller usually has less efficiency than a full size impeller.
Te BEP moves toward the less flow.
IMPORTANT. NPSH rises. It is necessary to take into account when you decide to trim an impeller. Especially at big flow rate.
Dependence of pump shaft power on impeller diameters 510, 460, 410 mm.
What is important when you decide to trim an impeller ?
Trimming of an impeller is used to adapt a pump curve to a system curve so that to provide the efficient pump operation.

What parameters pump should have after trimming?

What is the real diameter of impeller now?

Does the system curve change over time. Are there maximum and minimum operating modes? the maximum operating mode should be provided after impeller trimming,

Take into account the difference between real pump speed and in the catalog if you use the characteristic from the catalog.

If this is the new pump take into account the pump will wear out and his head curve will degrade,
Methods of defining the impeller diameter?

Send request to the pump manufacturer to calculate the diameter of impeller.

To calculate the diameter and develop curves using affinity laws.

Using catalog with characteristics with different impeller diameters.

Using the pump selection program of the pump manufacturer. Usually all pump selection programs have function of changing of diameter of impeller.

Purchase the reduced impeller and save the impeller with the bigger diameter for the future application if bigger parameters will be required.
What parameters pump should have after impeller trim?
Step 1. Define the real pump parameters. Perform pump test at the facility.
Step 2. Measure pressure difference at the valve before and after. The pressure after the valve is the required system pressure.
Step3. The goal of the impeller trimming is excluding or minimizing of the throttle control. The safety margin 1015 to the head can be added to compensate the pump wearing.
Example
When the pump with impeller 510 mm is selected the duty point will be on the cross of HQ curve and System curve
with a flow 1540 m3/h, but required flow is 1000 m3/h.
In addition, the pump consumes extra power and overloads the electric motor.
To meet system requirements, a control valve is used. Operating point shifts to a flow 1000 m3/h.
But control valve throttle a pump head what cause the Energy loss.
The difference in shaft power is 120 kW.
The saving potential is very substantial.
The difference in NPSH between fullsize and trimmed impeller is 0.5 m.
The difference is necessary to take into account.
Diameter Shaft power
510 mm 325 kW
510 mm with the throttling 240 kW
410 mm 120 kW
It is necessary to remember that diameter of the impeller calculated with using formulas gives less diameter than required. Some times the error can reach up to 1520 %. For impeller with lower nq (higher head lower flow) formulas give better result than for impellers with high nq (low head big capacity) . It is explained by the fact that shrouds of impeller with low nq are parallel shrouds of impellers with big nq are not parallel and at the trimming not only diameter reduces but increases the width of the impeller.
Defining the impeller diameter with using the catalog characteristics.
In the catalog we can find characteristics for different impeller diameters. Look at the characteristics.
The required operating point can lay between characteristics.
Required pump operating point
Determining the impeller diameter for the required pump parameters.
A flow of 615 m3 / h is required, a static head of the system is 37 m, taking into account friction losses, the required pump head will be 55 m.
In order to provide these parameters, a pump with a wheel diameter of 375 mm was chosen.
After starting the pump, it turned out that in order to maintain the required flow rate, it is necessary to close the valve and the actual operating point will be A.
The reason is that the actual characteristic of the system turned out to be lower than the calculated one. The pump head is more than required.
Step 1. We build a parabola of similar modes based on the equations. We take a random feed (720 m3 / h) and find the appropriate pressure for it. (red line).
Step 2. We find the point of intersection of the parabola with the characteristic of the pump with an impeller of 375 mm. We get a head
Step 3. Now we need to determine a new impeller diameter corresponding to a head of 53 m using an affinity laws.
Step 4. Calculate the power reduction. Calculate ratio of diameters.
The pump flow corresponding to the maximum diameter.
615/0.92=668 m3/h.
Pump shaft power corresponding to full diameter is 130 kW.
Pump shaft power corresponding to trimmed dimeter 130x0.78=101.4 kW
Shaft power difference = 130101.4=28.6 kW