Struggles with industrial production economics, a limited global energy supply and the realities of environmental conservation aren’t going to abate, so as energy costs rise, pump manufacturers are enhancing equipment efficiencies. Engineers have long known that matching the pump to system requirements is the way to achieve the highest level of pumping efficiency and equipment reliability. They know that applying a total-system-optimization approach to both existing and new pumping systems offers significant savings opportunities.
Understand the operating range
Figure 1. A rotodynamic pump operates at the point where the pump and system curves intersect – the duty point.
Effective energy reduction achieved through design and modification requires a thorough understanding of pump operating ranges. A pump curve is a graphical representation of the range of flows a rotodynamic pump can achieve. Likewise, a system curve graphically represents the flow characteristics of a given piping system. If the rotodynamic pump is installed in the system, their interaction also can be represented graphically (Figure 1). The x-axis is the volumetric flow rate and the y-axis is the head (pressure). The intersection of the pump curve and system curve is the duty point.
Notice that increasing the system pressure reduces the flow rate. At some point, increased pressure pushes the flow rate to zero, a condition to be avoided. To allow for unforeseen increased pressure demands, pumping system designers often select an oversized pump. The downside consequence of oversizing the pump is that it either produces excess flow or it needs to be throttled. Both conditions increase energy consumption and maintenance requirements while shortening pump life.
Measure specific energy
Specific energy is a useful metric for evaluating pump types, models and system configurations. It’s the power consumed per unit volume of fluid pumped. You calculate it by measuring the kilowatt-hours used during a test period and the fluid volume delivered into the system during the same test period.
Specific energy reflects the factors that influence both pump and installation efficiencies. It accounts for where on its curve the pump is operating when delivering flow into that particular system. Thus, it’s possible for a lower-efficiency pump to consume less power than a higher-efficiency pump, simply because of how well the pump’s characteristics mesh with a specific system configuration.
Process optimization is defined as identifying, understanding and eliminating unnecessary losses in a cost-effective manner while reducing energy consumption and improving pump system reliability. It’s the key to improving the performance of existing pumping systems, and begins by identifying system inefficiencies such as the following:
Symptoms of an inefficient pumping system
- Highly throttled flow control valve
- Bypass line (recirculation) to regulate flow
- Batch-type process in which one or more pumps operate continuously
- Continuous process with frequent pump on/off cycling
- Cavitation noise at the pump or in the system
- Parallel-pump configuration with the same number of pumps always operating
- Changed pump system function, but without hardware modification
- Inability to measure a pump system’s flow, pressure or power
Evaluate any pumping system that exhibits one or more of these symptoms. Give priority to any large, high-maintenance system that shows a symptom. Occasionally, your analysis will reveal that one or more pumping systems can be disabled without compromising the process.
The next system optimization step is measuring flow rate, pressure and power consumption using either installed process transmitters or portable instruments. Comparing the measured flow and head to the required flow and head might reveal a discrepancy, which can be indicative of an inefficient system. Comparing the current operating conditions to the design conditions also can reveal an improperly sized pump.
If the original pump performance curve is available, compare it to a curve representing the current system operating points. This can provide a general understanding of the current pump condition. Even a comparison of a single test point to the original curve can determine whether you should investigate the system further or merely overhaul a worn pump.
Minimize life cycle cost
Every rotodynamic pump has a sweet spot, its best efficiency point (BEP). A pump operating outside its acceptable operating range (even if reasonably close to its BEP) will be inefficient and exhibit higher energy use and shorter mean time between failures (MTBF).
Incorrectly sized valves can produce excessive pressure drops, and different valves can introduce different loss coefficients. If you use throttling valves or bypass lines to control flow, conduct an analysis to determine the most efficient means of flow control. Such variable-flow systems might benefit from some form of pump speed control, such as variable-speed drives.
Figure 2. Energy dominates the life cycle cost for a typical pumping system.