Maintenance costs and energy costs are often interrelated. Hydraulic experts have published numerous papers demonstrating the relationship between pump selection and reliability. They show that pump flow rate, speed, impeller diameter and suction energy (SE ) all affect reliability (Figures 4-7 and the SE sidebar). These four factors can be quantified using reliability factors whose values range between 0 (very poor reliability) and 1 (excellent reliability).
Figure 3 shows how Fo, the reliability factor, is related to the best efficiency point (BEP ). At BEP the reliability factor is 1 (excellent), whereas at low flow rates the reliability factor decreases, as expected. Relatively speaking, smaller pumps are more robust and can tolerate low flow rates better than larger pumps.
Figure 3. Smaller pumps are more reliable if demand roams below the design flow.
Figure 4 shows a straight-line relationship between the speed and reliability. Although pumps are designed to operate up to their maximum design speed, their reliability is greater at reduced speeds. The speed reliability factor (FN) of Figure 4 shows that a pump operating at half of its maximum design speed (e.g., 1,750 rpm operating speed for a 3,500 rpm maximum design speed pump) is 3 times as reliable (FN of 0.6 versus 0.2).
Figure 4. Not surprisingly higher speed pumps sacrifice head for degraded reliability.
The impeller diameter, compared to the maximum trim, significantly affects reliability. Observations of pressure gauge fluctuations for pumps with maximum diameter impellers show much higher pressure spikes than for pumps with trimmed impellers. This is especially true for higher speed pumps. These pressure spikes are felt by the pump, its bearings, and via shaft deflections, by the mechanical seals. The impeller diameter reliability factor FD of Figure 5 reflects this observation. To account for different pump designs the diameter of the impeller (the trim) has been expressed as a % of the amount that the impeller can be cut.
Figure 5. The best reliability is achieved by running the largest trim; reducing trim to achieve energy efficiency might have a hidden maintenance cost.
The net positive suction head (NPSH) margin and the SE ratio also affect pump performance and reliability. Figure 6 illustrates the relationship between reliability (FNPSH), NPSH margin ratio, and SE ratio. Suction Energy is a measure of the potential for damaging suction recirculation whereas NPSH Margin Ratio (NPSH available/NPSH required) is a measure of the ability to suppress suction recirculation damage (see sidebar). Most process pumps are designed with relatively low values of Suction Energy and have values for FNPSH of 1.
Figure 6. A powerhouse application shows pumps operating at a wide range of conditions.
These reliability factors can be used in evaluating the benefits of making changes to a pumping system. In comparing options for a particular application the “relative reliability index” or RI , should be evaluated. Use the following equation based on the factors from Figures 3-6:
RI = Fd × Fn × Fq × FNPSH
Because of the nature of the individual factors, the RI will be between 0 and 1 and will be less than these factors. In most cases reducing energy consumption also lowers maintenance expense; however this is not always the case. For example, reducing impeller diameter can increase recirculation in the suction; this leads to pump damage (Figure 5).
Although reducing energy consumption in pumping systems may seem straightforward, it can be a complicated task. Energy savings are often exaggerated by using simplified analysis found in the literature. A thorough understanding of the hydraulics of the system and the plant needs are necessary. Maintenance departments and project engineering ultimately serve production.
A telling example
Inspection of one of the pumps in a complex system (Figure 6) revealed extensive suction and discharge recirculation which damaged the impeller. This led to a suspicion of problems with the NPSH. Additional study revealed that the pump had a very high SE ratio and a relatively low NPSH Margin Reliability Ratio (Figure 7).
Figure 7. The closer the NPSHA is to the NPSHR, the lower the reliability.
Testing of the pumps identified significant pump deterioration and a wide variation in performance capabilities. Because these pumps operated in parallel, one of more of the pumps probably operated very near shutoff. Table 1 summarizes the financial and reliability aspects for each option evaluated for the pumphouse:
Option 1: Identical replacement pumps with improved characteristics and materials (to stainless steel from cast iron). This was the option ultimately selected.
Option 2: Replacement pumps with larger impeller pumps with upgraded materials. This option was rejected as a result of an even higher SE ratio.