Improving Efficiency and Reliability with Custom-Built Impellers
It’s well understood that a centrifugal pump will perform best when the duty point (the combination of flow rate and developed head required by the system where the pump is installed) is close to the pump’s best efficiency point (BEP). This isn’t just a matter of efficient energy use; most centrifugal pumps will experience much lower vibration levels when they are operated near their BEP. Since excessive vibration causes accelerated wear in bearings and seals, it follows that pumps that are well matched to their required duty point will have longer service lives (as measured by mean time between failures – MTBF) and lower maintenance costs. (See Figure 1.)
If a pump is running roughly and requiring too much maintenance, looking at the relationship between the systems-defined duty point and the pump’s BEP is a good place to start.
Getting a Good Performance Match
Operating a pump at a flow rate that is significantly different from the unit’s BEP may be the result of inappropriate equipment selection or a change to plant operating conditions. If a pump is performing poorly because of a mismatch between its design BEP and required duty point, then an obvious remedy is to replace it with another unit that is more accurately sized for the job. However, this might be expensive, especially if it requires major changes to piping systems, mounting arrangements and drives. Another alternative is to equip the pump with a new, custom designed impeller with a BEP that more closely matches required operating conditions.
Building a Custom Impeller
Designing a custom impeller with flow and head characteristics matched to a specific set of system requirements requires the development of a suitable hydraulic design. Computational fluid dynamic (CFD) analysis is an extremely important tool for this task since it allows the designer to predict the performance of vane contours and other design features without having to build and test prototypes.
Using CFD tools to design an impeller requires a significant amount of skill and experience. The proposed design must not only meet the hydraulic objectives, but also the constraints required in order to be manufactured.
In some cases, a CFD analysis may demonstrate that a solution that meets the new performance requirements is beyond the realm of manufacturing feasibility. In this case, the strategy of building and installing a replacement custom impeller won’t work out and the complete pump will likely have to be replaced.
Case Study 1: Saving Energy and Reducing Maintenance with a Custom-Built Impeller
A reliability engineer working in a large petrochemical refinery noticed that one of the pumps in the facility was requiring major repairs at roughly 18 month intervals. He also found that the vibration level measured for this pump immediately after a full overhaul was 0.25 inches per second. Looking at the operating conditions and pump specifications, he discovered that the measured flow rate was 730 gpm, while the pumps’ rated flow rate at its BEP was 1200 gpm. This operating mismatch clearly contributed to the excessive vibration levels. It also meant that the pump was operating at roughly 64% efficiency, as compared to 75% rated efficiency at the BEP.
The pump’s owners attempted to change the pumps operating characteristics by installing a low-flow impeller supplied by the pumps manufacturer. Unfortunately, this impeller was not capable of providing adequate head.
KSB offered to develop and build a replacement impeller that would reduce the pump’s flow rate while maintaining the required head – in effect shifting the pump’s BEP to match the required duty point. Standard Alloy’s design specialists used CFD tools to test several possible impeller configurations that would accomplish these goals. The final design was predicted to both meet the flow rate target with a hydraulic efficiency of 71%.
The new impeller was ordered and fabricated in time to be available the next time the pump failed. When it was installed, the required flow rate was achieved and the measured efficiency came in on target and 7% higher than what had been achievable with the old impeller. This efficiency improvement meant an annual energy cost savings of $15,000 per year, which means that the payback period for the new custom impeller was only 8 months! Of equal importance, the measured vibration level was only 0.05 inches per second, or one fifth of what had been measured with the original impeller. The smoother running pump is expected to have a median time between repairs (MTBR) of six years.
Case Study 2: Improving Impeller Design with CFD
A municipal water utility encountered a serious problem with cavitation near the inlet of several large water pumps. KSB was asked to help solve this problem and in the course of the investigation developed a CFD model of flows through the impeller and volute. The analysis revealed that excessive pressure variations across the inlet were leading to recirculation and local cavitation. The solution in this case was to develop a new impeller design with an additional vane and improved vane contour. Once the new design had been finalized and evaluated, a new impeller was cast in Standard Alloy’s foundry and finished in the machine shop. When the new component was installed in one of the ‘problem’ pumps, it was found that the cavitation problem had been eliminated, flow volumes had been increased and efficiency improved. Moreover, since the new impeller had been cast from highly durable duplex stainless steel, impeller lifetime has been significantly increased. The utility ultimately replaced six impellers with the improved versions.
Configuring pumps so that they operate near their design BEP is important both for energy efficiency and for trouble-free service lives. A number of strategies are available for altering a pump’s BEP to better match service requirements, including replacement with a different size pump, impeller trimming and the use of variable speed drives to modify operating speed. A fourth important option – the construction of custom-designed and fabricated impellers that are precisely tailored to application requirements – has become available, thanks largely to the availability of specialized CFD tools that enable analysts to develop suitable impeller designs in a cost-effective manner. KSB has the expertise to design, fabricate and install custom impellers that can substantially improve the performance of existing pumps.
Article is based on work performed by Standard Alloys, a member of the KSB Group.