The effects of pump component wear

Pumps can also be repaired to like-new condition. In doing this, there still remains the risk that the pump’s performance will degrade before the anticipated rebuild point and cause production issues.

To further complicate matters, pump component wear invalidates most pump-performance curves. In highly variable conditions, wear cannot be accurately modeled or predicted. For processes that require tight and predictable performance over time, the solution is pumps that have tight performance ratios to begin with and are either immune to wear or can compensate for wear.

Pumps can also be repaired to like-new condition. In doing this, there still remains the risk that the pump’s performance will degrade before the anticipated rebuild point and cause production issues.
Repair or replacement to regain proper pump performance can result in high costs for rotary PD pumps. In other words, the pump works mechanically just fine, but needs to be repaired to regain performance, which can be costly.

Loose pump performance also has associated side effects. These include an increased amount of shear imparted on the fluid, greater power requirements (and reduced efficiencies) of the pump and heat generation.

Narrow Versus Wide Performance Band
This is not to be confused with tight and loose. In fact, in many cases a pump with a tight performance band gives it the ability to handle a wide flow performance range. The width of the pump’s performance band describes the range of speeds in which the pump can produce acceptable flow for the application. This is also sometimes referred to as the effective turn-down ratio of the pump, borrowed from terminology used in conjunction with motors or variable speed drives.

The performance band width of a pump is also affected by the ability to drive the pump at low to high speeds. Torque requirement, gear reduction, motor cooling and variable speed drive capabilities all play a part and are not in the scope of this article. Motor and variable speed drive capabilities, for example, set lower and upper limits. For an actual illustration of performance band width, t a pump, in this case a typical lobe pump with a 0.153 gallon/revolution theoretical displacement, effectively has a narrow performance envelope. That is because under an arbitrary worst condition-in this case pumping 1 cps (water-like viscosity) fluid against 75 psig-the pump only begins to produce flow at 185 rpm. This means that speeds between 50 rpm to 185 rpm, which are considered good speeds for ensuring the long life of rotary PD pumps, are not available to the pumping process. The performance band is therefore narrow as it ranges from 185 rpm (instead of 0 rpm) to the maximum mechanical speed capability of the pump, or some other process limitation like NPSHR versus NPSHA, or the abrasiveness of product.

In most cases, pump wear further increases slip, as is the case with lobe pumps when new, but not with wear. If wear occurs in this pump, the manufacturer-supplied performance curve no longer applies and actual performance is unknown, unless verified in the field. The point at which the pump begins to produce flow under wear conditions could be even greater than 185 rpm and prompt repairs.

In sharp contrast, the pump compensates for wear by maintaining as-new clearances. Therefore, slip does not change, and the pump performance remains tight with a wide range of flow capabilities. The example application that exploits these needs-the continuous in-line blending process-benefits from pumps that have a high turn-down ratio. This is because the recipe to produce the final product can be highly variable as far as the content percentage of each ingredient. In other words, the wider the flow rate range achieved by the pump, the wider the variation of recipes that can be produced with the system.

Conclusion
Good flow control from rotary PD pumps offers options for more advanced processes, like in-line blending, that can have far-reaching influence on a production facility’s overall capital and operating costs. Respected pump manufacturers offer performance curves that can be evaluated to determine if the performance band is comfortably suitable for the application. If not, alternative pumping technologies should be studied and considered.