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Pumps & Systems, September 2008
Editor's Note: This is the fourth part of a series based on Optimizing Pumping Systems: A Guide to Improved Energy Efficiency, Reliability, and Profitability, written by pump systems experts. This new guidebook continues the mission of Pump Systems MatterTM and the Hydraulic Institute to advance knowledge on pumping systems.
Click here for Part 1, Part 2, Part 3, Part 5 and Part 6
Pumping systems are used worldwide to transport fluids and operate in most industrial processes. Commercial and residential buildings also rely on pumps for essential services. Pumping systems account for nearly 25 percent of the energy consumed by electric motors, and for 20 to 60 percent of the total electrical energy usage in many industrial, water and wastewater treatment facilities. Figure 1 shows the potential saving in gigawatt hours per year for optimization opportunities in key energy intensive industries.
The process of identifying, understanding and effectively eliminating unnecessary losses while reducing energy consumption, improving reliability and minimizing the cost of ownership over the economic life of the pumping systems is commonly referred to as systems optimization. Besides reducing energy costs, improving the performance of an existing pumping system yields other benefits (see Table 1).
Assessing the Current System
The first step in pumping system optimization is to assess the current system for existing deficiencies in need of correction. Many pumping system problems result from improper pump selection and operation, and these pumps can require considerable maintenance. The symptoms in Table 2 appear when improper sizing, selection, operation or other issues result in suboptimal performance.
The more symptoms present, the greater the likelihood of potential energy savings.
Next, the pump systems selected for assessment should be thoroughly evaluated to determine the true system requirements. In some systems, the operating pressures or rates of flow may be excessively high. Occasionally, this analysis will reveal that one or more pumping systems can be turned off without compromising the process.
Diagnostics and Data Collection
The next step in system optimization is to collect performance data such as flow rate, discharge pressure or power consumption, using accurate and repeatable instrumentation. If the testing is not detrimental to the process, then consider a permanent installation. Continuously or intermittently measuring important parameters directly affecting process energy consumption can pay off in a short time.
Compare System Curve with Pump Performance Curve
Once the data have been collected, it is possible to compare the existing operating conditions to the design conditions and determine whether the pump is appropriately sized. The original pump performance curve is useful to construct a curve for the operating points of the existing system. Comparing acquired test data to the original performance curve provides an immediate sense of the current pump condition. Even comparing a single test point to the original curve can help the user determine whether the first step is to overhaul the pump or investigate the system further.
Depending on the system operation and fluid properties, a curve may be corrected for speed, temperature, specific gravity or viscosity. In most cases, measurements that balance quantitative and qualitative aspects will be sufficient for major decisions regarding system optimization. Figure 2 shows a typical pump performance curve with two data points. Point A shows a case where the pump is undersized and point B where a pump is oversized, both in capacity performance.
How to address each case can be determined without spending excessive time calculating error margins. Assuming the piping cannot be changed, simply moving the pump curve closer to the desired operating condition will improve system efficiency.
Pump Duty Point
Figure 3 illustrates that the pump delivers the head and flow determined by the intersection of the pump curve and the system curve, commonly called the duty point. Maximum efficiency and lower power consumption will be achieved by ensuring the flow and head at the BEP closely match the duty point.
Pump industry best practice is to ensure that the system curve intersection should be ±10 percent of BEP flow, which will reduce energy costs and provide benefits (see Table 3).
Energy Costs From Passive Throttling Devices
Other components of the existing system must also be assessed. When throttling valves or bypass lines are used to control flow, conduct an analysis to determine the most efficient means of flow control. These variable flow systems may benefit from pump speed control, such as variable speed drives.
Evaluate the system piping configuration for optimization opportunities. A proper configuration includes a straight run of pipe leading into the pump inlet to ensure a uniform flow distribution into the pump. Use turning vanes or other means of "straightening" the flow when this is not possible. Finally, size the suction piping to minimize friction losses.
Evaluate Opportunities
Data analysis should indicate the system components that need to be changed for better efficiency. Some typical ways to optimize pumping systems include:
Motors
Every pump needs a prime mover. Electric motors are frequently used drivers for pumps. The electric power a motor consumes is another important piece of data to evaluate when looking for opportunities to save. Measurement of low voltage motor power is usually easy and an important point in data collection. The power consumption can uncover any potential problems (mechanical or electrical) with the motor and provide a quick method to check for wasted energy.
Variable Speed Operation
Using control methods that reduce the power to drive the pump during the periods of reduced demand can save energy costs. Where interruption of flow can be tolerated, on-off control may be the most energy-efficient option. Varying pump performance by changing speed is most often the best energy-efficient control method. Figure 4 shows the energy consumption of other popular control methods when compared to variable speed control.
The energy that a pump consumes varies as the third power of the speed, so a 50 percent reduction in speed will reduce the power consumed by as much as 80 percent, depending on the system head curve characteristics. It is then possible to match pump operating speed to the exact conditions of service without throttling.
Stop/Start Control
In this method of control, switching the pump on or off varies the flow. It is necessary to have a storage capacity in the system such as a wet well, elevated tank, or accumulator-type pressure vessel. The storage can provide a steady flow to the system with an intermittently operating pump. This method effectively minimizes energy consumption if intermittent flow, stop/start operation and the storage facility are acceptable. By arranging the run times during the low tariff periods, this method can be used to benefit from off-peak energy tariffs.
To reduce energy consumption with stop/start control, pump at the lowest flow rate the process permits while maintaining an acceptable pump operating range. This reduced flow minimizes friction losses in the pipe and an appropriately small pump can be installed. Ensure that the maximum system flow requirements are satisfied.
Multiple, Staged Pump Operation
Another energy-efficient method of flow control, particularly for systems where static head is a high proportion of the total, is the installation of two or more pumps in parallel. Variation of flow rate is achieved by switching additional pumps on and off to meet demand. It is often necessary to install two or more pumps in parallel to achieve the head and flow required and/or to vary the flow rate into the system. By plotting the pump curves onto the same graph as the system curve, it is possible to determine the flow and head obtained when running one, two or three pumps by the intersection points shown in Figure 5.
Pump Replacement and Upgrade
Proper pumping system design is the most important single element in minimizing energy and life cycle costs. Other meaningful savings can be realized by looking at the entire pump system, including the piping, fittings and valves upstream and downstream from a pump, as well as the motor and motor driver.
Model the System and Implement Scenarios
The use of analytical methods and/or hydraulic system modeling offers benefits at various stages in the system improvement process. If a system model already exists, then use it to evaluate potential solutions.
A system model has a major advantage over other potential approaches: It accounts for all of the system interactions. Changes in pump speed or impeller, control schemes, tank levels, etc., can all be evaluated on a common platform (see Table 4).
Before committing to any construction or purchase decisions, thoroughly evaluate the impact of the modifications on all processes.
Summary
To evaluate an existing pump system successfully, be sure to:
• Understand the operational deficiencies of the system and the process guarantees
• Identify and understand any and all system problems
• Carefully assess the system under full range of operation
• Use qualified technicians to capture field data
• Utilize available historical data from the plant operating system
• Understand the true cost of energy and production downtime
• Model the system and implement scenarios in the computer to optimize energy and economic benefits
• Ensure buy-in from stakeholders in purchasing, operations, maintenance and management by presenting results that show how each group will benefit from the system upgrade
"Improving the Performance of Existing Pumps" is the fourth article in a series based on the new guidebook Optimizing Pumping Systems, A Guide to Improved Energy Efficiency, Reliability, and Profitability currently available from Hydraulic Institute (HI) and Pump Systems MatterTM (PSM). Next month's article will focus on optimizing new pumping system designs.
As you look to improve the performance of your existing pumping systems, Pump System Matter and the Hydraulic Institute websites contain useful information to assist your efforts:
- Pump System Improvement Modeling ToolTM (PSIM) is a free, educational and downloadable software program focused on helping users better understand the hydraulic behavior of pumping systems
- Case studies highlight the successful cost savings efforts of manufacturers, utilities and private industry
- Pump System Basic Assessment Guide outlines processes for identifying cost-effective system improvements
- Pump Systems Assessment Tool Matrix lists both free and commercially available downloadable tools that can help pump users and engineering consulting firms improve the performance of pumping systems and design more efficient systems
To download these resources, obtain information on the guidebook or purchase the new book, visit the Pump Systems Matter website (http://www.pumpsystemsmatter.org/) or the Hydraulic Institute website (http://www.pumps.org/).
Tags: Pumps , September 2008 Issue
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