Optimizing Pumping Systems

Interest in energy efficiency is not a fad. The economics of industrial production, the limitations of global energy supply and the realities of environmental conservation will be enduring themes for decades, if not the millennia. As energy costs increase, pump manufacturers respond with an understanding of the importance of making equipment more efficient at saving energy.

Traditional methods of specifying and purchasing piping, valves, fittings, pumps and drivers often result in lowest first cost, but also often produce subsequent unnecessary, expensive energy consumption and higher maintenance costs. An organization that incorporates the energy, reliability and economic benefits of optimum pumping systems can enhance profits, gain production efficiency and move ahead with essential capital upgrades necessary for long-term business survival.
System fundamentals

Pumping systems are typically designed to support the needs of other systems, such as process fluids transfer, heat transfer and the distribution of water and wastewater. Systems are generally classified as closed-loop or open-loop. Closed-loop systems recirculate fluid around set paths, whereas open-loop systems have specified inputs and outputs, transferring fluids from one point to another. For closed-loop systems, the frictional losses of system piping and equipment are the dominant loads. Open-loop systems often have significant static head requirements due to elevation and tank pressurization needs.

Pumps, piping, valves and end-use equipment typically compose these systems. Other common components include filters, strainers, and heat exchangers. Any evaluation of a pumping system should consider the interaction between these components, not just the pump itself. This is referred to as a systems approach to pumping system evaluation. The pumps and the system must be designed and treated as one entity, not only to ensure correct operation, but also to reap the benefits of energy efficient pumping.

The Hydraulic Institute (Parsippany, N.J.; www.pumps.org) recognizes about 40 different types of pumps, broadly classified into two categories that relate to the manner in which the pumps add energy to the working fluid: positive displacement and rotodynamic also known as centrifugal.

Rotodynamic pumps are much more common and have a variable flow-pressure relationship, which is described by a performance curve that plots the rate of flow as various pressures. Positive displacement pumps have a fixed displacement volume. Their flowrates are directly proportional to speed.

The other major components of typical pumping systems have a large effect on the system efficiencies. The selection of efficient and properly sized electric motors is vital, along with the use of variable speed drives when appropriate. Proper piping inlet and outlet configurations are also important for efficient system operation. Additionally, the appropriate selection and operation of valves is critical, especially any throttling or bypass valves.

Along with pump-speed control and multiple-pump arrangements, bypass valves and throttling valves are the primary methods for controlling rates of flow in pumping systems. The most appropriate type of speed control depends on the system size and layout, fluid properties, and sometimes other factors. Bypass arrangements allow fluid to flow around a system component but at the expense of system efficiency since the power used to bypass any fluid is wasted. Throttling valves restrict fluid flow at the expense of pressure drops across the valves.
Proper systems design

Pump engineers have long been trained that the highest level of pumping efficiency and equipment reliability is achieved by matching the pump to the system. Applying a total-systems-optimization approach, for instance, Pump Systems Matter (see box on additional information resources) advances significant savings opportunities with both existing and new pumping systems.

The Figure shows that increasing the system pressure will reduce the rate of flow. If the pressure reaches a certain point, the flowrate may approach zero, a condition to be avoided. To allow for unforeseen pressure increases, pumping system designers often select an oversized pump. The consequence of this oversizing is that the system will operate with excessive flow or will need to be throttled, thereby increasing energy use, increasing maintenance requirements and decreasing the life of the pump.

Specific energy is a useful measure to consider when evaluating combinations of pump type, model and system. Specific energy is the power consumed per unit volume of fluid pumped. It is determined by measuring the flow delivered into the system over a period of time and calculating the power consumed during the same period of time. This measure takes into account all of the factors that will influence the efficiency of an installation, not just pump efficiency.

Specific energy also takes into account where the pump is operating on its curve when delivering flow into that particular system. Thus, a pump with a lower efficiency may consume less power than a higher efficiency pump, simply because of how its characteristics fit with the system in question.

Another benefit of using specific energy as a measure is that it allows some approximate comparisons between similar pumping installations.
Steps to improving efficiency

Existing systems. Process optimization is the process of identifying, understanding and cost effectively eliminating unnecessary losses while reducing energy consumption and improving reliability in pumping systems. Pumping systems possessing one or more symptoms that are typical of an inefficient system (see box) should be considered for further investigation, with priority given to large, high-maintenance systems that are mission critical to the process or facility operation.

Next, the pump systems selected for assessment should be thoroughly evaluated to determine the system requirements. In some situations, it may be determined that the system is operating with excessively high pressure or rates of flow. Occasionally, this analysis will find one or more pumping systems that can actually be turned off without compromising the process. An awareness of system-demand variability will help to better match flow and pressure requirements more closely to the system need.

The next step in the system optimization process involves data collection. Data may be acquired with installed process transmitters or portable instruments to determine discharge flowrate, discharge pressure and power consumption. The instruments used should be both accurate and repeatable. The data acquisition equipment should be matched to the application, and the length of data collection should provide statistically valid averages. Systems with varying or seasonal loads may require long-term data logging equipment.

The collected data can be used to compare the measured rates of flow and head to the required rates of flow and head. This may reveal an imbalance between measured and required conditions, which is evidence of an inefficient system. Comparing the existing operating conditions to the design conditions can also reveal an improperly sized pump.

If the original pump performance curve is available, it will be useful to construct a curve for the operating points of the existing system. Comparing the two 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 the first step is to overhaul a worn pump or to investigate the system further. Every rotodynamic pump has a best efficiency point (BEP). A pump operating outside of an acceptable operating range (within a reasonable range of BEP) will be inefficient and have higher energy use and shorter mean time between failures (MTBF).

Other components of the existing system must also be assessed. Incorrectly sized valves can create excessive pressure drops across the valves, and the different types of valves have different loss coefficients. When throttling valves or bypass lines are used to control flow, an analysis should be conducted to determine the most efficient means of flow control. These variable flow systems may benefit from pump speed control, such as variable speed drives.

The system piping configuration should be evaluated for optimization opportunities. A proper configuration will include a straight run of pipe leading into the pump inlet to ensure a uniform velocity of fluid entering the pump. Turning vanes or some other means of "straightening" the flow should be used when this is not possible. Also, the suction piping should be of sufficient size to minimize friction losses.

New systems. The design and selection of new systems provide the opportunity to optimize for minimum lifecycle costs, including energy, maintenance and other costs. Significant lifecycle opportunities exist through optimal pipe sizing (larger pipes can deliver fluid at lower pressures), variable-speed pump control, and pump and valve selection.

The selection of pump type and size, the impeller size and pump operating speed all impact the pump operating point and determine the pump’s BEP. Getting the BEP matched to the actual system operating point is an important part of designing an efficient system. The piping size, material, and associated fittings and other components influence the system resistance and hence the system curve and operating point. These materials should be selected through the consideration of lifecycle costs, especially since they are the most difficult parts of the pumping system to change in the future.

It is also important to note that all pumping systems change over time, affecting their operating points. As the systems age, corrosion, abrasion or solids buildup are likely to occur in the piping, altering the effective piping diameters. Cyclic mechanical and thermal loadings may cause piping fatigue damage over time. Valves, gaskets and other components are subject to wear and corrosion as well. Worn or damaged impellers and other parts in the pump itself will impact system performance. This also has a degrading impact on the process control loop associated with the pumping system.

Additionally, operational changes over the life of the system will influence system efficiency, as industrial processes are often evolving or changing to changing demands. Thus, the pump operating parameters can change as well as the duty cycles.