Centrifugal Pumps- Part V- Inducers

During my course of career, I face multiple questions regarding pump inducers. This is the reason I decide to write a (mini) article on #inducer.

NPSH margin

The main reason that manufacturers employ inducers is low NPSHA. If you uncertainties regarding NPSHA please visit this article (Centrifugal Pumps- Part II- NPSH and Cavitation). Low NPSH margin will cause cavitation during operation. There are two methods to increase NPSH margin

1. Decreasing NPSHR: This method shall be done solely by pump manufacturer since NPSHR is defined only based on pump parameters. Of course there is a limit on the amount that NPSHR can be reduced, so this option is limited.
2. Increasing NPSHA: There are multiple methods to increase NPSHA.

• First method is to increase the suction drum height. By using this method NPSHA value can be increased to great content however, it is costly, and sometimes impossible.
• Second method is to decrease pump suction elevation. This method means lower pump skid height or even digging the ground. This can affect the pump vibration or digging is not possible due to passage of underground piping.

Third method is by employing the inducers. Inducers act as a preliminary pump and increase the pressure at pump suction nozzle. (Side note on this method: As stated in KSB website, as well as other sources: "The inducer's task is to reduce the NPSH required by the pump, which is achieved by increasing the static pressure upstream of the impeller". To calculate NPSHA you shall calculate static head upstream of the pump. The Inducers will increase static head upstream of the pump. Best way is to rephrase this sentence into this new statement: inducers increase the margin between NPSHA and NPSHR by increasing the static head upstream of the pump. Thanks to @Sander Langenberg for his valuable comment!)

In order to calculate NPSHA we consider pump battery limit. In the pump and inducer case, battery limit is suction of pump and before inducer, thus inducer does not have relation with NPSHA.

Thanks Aref Vaezpour for his valuable comment

Working principal of Inducers

Inducers are typically an axial flow pump impellers that are mounted on shaft extensions. Pump shaft shall be extended through the casing. These impellers, as shown on figure 1, will mount on the pump shaft. As the shaft starts to rotate, this impeller will rotate and pump the fluid, increasing its pressure a little. This little pressure increment will increase NPSHA at pump suction and cavitation will not happen.

Figure 1- Different sizes of inducers

Pros and Cons of inducers

Pros:

1. Increasing NPSHA: Primary reason of using inducers. They increase the NPSHA and cavitation will not happen

Cons:

1. Pump modification: In order to install inducers, the body of the pumps shall be modified. Manufacturers have less experienced with modified body of the pump.
2. Smaller operating range: The inducers are designed for a specific point of operation. If flow deviates from that point, inducers will cause additional NPSHA drop. So inducers cannot be used in the pumps with wide range of operation.
3.  Higher initial cost: Since pump needs modification and additional parts are installed, the initial cost of the pump will rise. However, it shall be mentioned that other methods of increasing NPSHA such as lifting suction drum or reducing skid height may cost more!
4. Higher maintenance cost: The inducers themselves are prone to cavitation. This means that inducers shall be replace regularly.
5. Shorter maintenance intervolves: As a rule, adding a mechanism to a device will increase the risk of break down. Adding inducers may cause break down. However this can be avoided by inspecting and changing the inducers during overhauls.
6. Lower efficiency: The efficiency of the pump will drop since the driver needs to provide extra power for inducers to rotate. Even thou the efficiency drop is small, typically 1-2%, in long run it can cost a lot.

Conclusion

In our experience, we avoid inducers as much as we can. In order to increase NPSH margin we usually employ combination of methods aka asking manufacturers to reduce NPSHR as much as they can, checking the isometrics drawings for individual pump with real dimensions. Increasing the suction drum elevations and other methods but avoid inducers at all cost. If then the NPSH margin is still insufficient (below 0.6m) then we accept inducers.

Related Article

• Centrifugal Pumps- Part II- NPSH and Cavitation

References

1. Hydraulic Institute "Hydraulic Institute Standards for Centrifugal, Rotary & Reciprocating Pumps", 14th Ed. (1983)
2. Cover figure (Industrial Electronics)

#API #Inducer #tips