Unlock a pump’s full potential while saving energy and maintenance dollars.
by Jack Creamer, Schneider Electric
July 16, 2014
Figure 3. Comparison of two efficiency scenarios at different flow rates: 8 to 9 percent more efficient with variable speed drives at 60 percent flow

Asset Management

Physical assets, such as pumps, need to be maintained on a regular basis. Maintenance costs represent 25 percent of TCO (see Figure 1), so examining maintenance practices can make a positive contribution to energy-influenced savings.

The challenge behind reducing maintenance costs is ensuring system integrity remains stable, but with the right steps, operators and maintenance specialists can strike an affordable balance.

All pumps should be operated within the parameters of a given pump’s specifications. As discussed, pump efficiency varies according to operational parameters. The pump is designed for optimal operation at the BEP, but 75 percent of pumping systems are oversized by about 30 percent to meet anticipated lifetime peak production or to rationalize spare parts inventory.

Figure 4 illustrates how pumps begin to waste significant energy when appropriate maintenance practices are neglected. For example, discharge recirculation can occur if the pump operates at 65 percent of the BEP flow rate, damaging the impeller.

Figure 4. Maintenance-related issues that impact pump performance (Courtesy of Barringer & Associates, “Pump practices & life”)

Variable speed drives keep the operating point close to the BEP and protect the pump against inefficiencies. Monitoring the operating point of the pump and its efficiency can help predict when potential system problems—especially extreme ones—will occur. Dry running, low flow operation or cavitation (due to low net positive suction head), which can cause instant damage, is avoided.

Figure 5 illustrates how operating away from the BEP not only decreases the efficiency but speeds the wear and tear on the pump, reducing reliability.

Figure 5. Effect of the distance from the BEP on reliability (Courtesy of Barringer & Associates, “Pump practices & life”)

Cost-Effective Maintenance Strategies

There are three main approaches to cost-effective maintenance. Preventive maintenance is the systematic inspection and detection of potential failures before they occur. Condition-based maintenance is a type of preventive maintenance that estimates and projects equipment condition over time, using probability formulas to assess downtime risks. Corrective maintenance is a response to an unanticipated problem or emergency.

Figure 6 illustrates the cost curves of these three types of maintenance. Condition-based maintenance is the most cost-effective of the three approaches. Condition-based maintenance monitors system data on an ongoing basis and provides an accurate assessment of the system’s status. of its With pumps, variables such as suction pressure, discharge pressure, pump speed, power, flow and temperatures are monitored to detect a loss of efficiency. Combining the efficiency and process variables identifies potential problems.

Figure 6. Cost curves of the different maintenance approaches (Courtesy of Penn State University Applied Research Laboratory, “Open systems architecture for condition-based maintenance”)

Variable speed drives measure process variables, temperature and power with high accuracy and assess pump efficiency. If connected to the automation system or a server, they can continuously monitor the health of the system and can indicate precisely when proper maintenance is needed.