Pump efficiency drives cost savings
Editor’s note: The following article is based on a Plant Engineering webcast presented by Mike Pemberton, ITT Goulds Pumps, in September 2012. Readers can find the presentation slides and audio under the webcasts section of www.plantengineering.com/webcasts.
The drive to save energy in pump systems is not just about conservation or environmental concerns; it is primarily a bottom-line cost issue. Recent surveys show that energy efficiency is among the top three concerns for most industrial plant managers. Many plants continue to consume far more power than is necessary to run their pump systems, simply because there is a lack of awareness of the losses—out of sight and out of mind. In recent years, educational efforts initiated through public, private, and nonprofit sectors have begun to bring these inefficiencies to light.
According to the U.S. Dept. of Energy, electricity dedicated to electric motor-driven systems accounts for more than $33 billion annually. The American Council for an Energy Efficiency Economy recently published estimates that reductions in motor system energy costs could save plants up to 42% of their annual spend dedicated to energy and maintenance for pump systems, which in some cases can amount to millions of dollars. This article will lay out the framework for adopting an energy-efficient model that meets the needs of continuous process industrial plants and that will ensure smoother and more reliable operation going forward.
Seeing excess energy as a potential resource
Plant managers continue to invest most of their available capital in implementing traditional expansion and modernization projects, instead of earmarking funds for long-term energy savings efforts. Furthermore, short-term, expense-based solutions do not address fundamental changes that need to be made for plants to grow and profit in the future. The Finnish Technical Research Center has found that, on average, process pumps operate at less than 40% efficiency, and more than 10% of pumps run at less than 10% efficiency. This is a sizable efficiency loss, adding up to millions of dollars, which can only be addressed by a long-term commitment to reengineering 10% to 20% of a given plant’s pumping and process subsystems.
While the efficiency achieved by a pump is dependent on make and model, pumps generally are designed to operate between 65% and 85% mechanical efficiency. Every watt of power wasted is converted to heat and vibration. This reduces equipment reliability and can eventually cause infrastructure damage, raising equipment maintenance costs and degrading process control loops.
On August 30, 2012, President Obama issued an executive order, directing the Departments of Energy, Commerce, and Agriculture and the Environmental Protection Agency to help states secure energy efficiency investments. The government estimates manufacturers could save at least $100 billion in energy costs over the next decade.
Case studies: Efficiency in practice
Some industrial plants already are reaping the rewards of optimizing their pump efficiency to conserve energy and reduce costs.
A food processing plant operating a cooling tower pump system required 60,000 to 90,000 gal. of water depending on the season. An in-depth analysis determined that the parallel pumps in the system were oversized and operated against isolation valves that were 40% open, on average. By fully opening the isolation valves to increase flow, the cooling tower system was able to meet seasonal demand with five fewer pumps. This represented, based on their $0.04 kWh cost, an annual savings of $380,000. Plus, after qualifying for local utility rebates, the plant received another $100,000 in rebates.
In another example, a pulp mill digester faced similar inefficiency issues with its pump system. The digester releases chemicals when cooking wood chips and subsequently removes them through a stock washing process. The mill was running an oversized pump system designed to deliver more than twice the needed capacity. The pump was routinely damaged during start-ups and shut-downs, and the end-user control valves were normally 20% to 40% open. As a result, excess energy caused pipe and gasket failures with resulting downtime. The pulp mill achieved annual energy savings of $32,000 by installing an automated isolation valve and a low voltage (460V) 200 hp motor and VFD. The mill has not experienced any major equipment failures since the system improvements were made over five years ago. In addition, the pump and valve automation eliminated a $1 million annual downtime expense from lost production and repairs.
Energy efficiency starts with changing perspectives
To get started on the road toward energy efficiency, first take a step back to see the bigger picture—and understand that energy usage, process control, and equipment reliability are all interrelated. The amount of excess (destructive) energy used in a pump system will seriously impact plant productivity as well as long-term sustainability.
Pumps are designed to operate near their best efficiency point to ensure smooth flow through the system with minimum energy use and maximum reliability. An enormous cost that often goes unrecognized is in using more than the required energy to move fluid through multiple pump systems. Process pumps are the biggest consumers of motor energy in many continuous process plants. These pumps are often mis-sized and, as a result, cost more than most people anticipate. Maintenance costs are inordinately high, unplanned downtime lowers productivity, and employee safety can become an issue.
Poorly designed pump systems are a major cause for concern. These pumps can lead to costly problems, such as process leaks and fugitive emissions. Replacing worn-out parts of a pump in a piecemeal way can also degrade a pump’s performance; i.e., delivering less head and flow.
In evaluating process control, take a hard look at the number of control valves that are operating in manual. It’s not atypical to find that 30% to 60% of automatic control loops have been switched to manual. While throttling valves reduce pump discharge pressure and flow rate, doing so excessively can decrease the reliability of pumps, consume excess energy that is destructive, and decrease control loop utilization, thereby lowering the return on control system investment.
Practical advice can lead to big savings
Start saving on energy costs by screening pump systems to identify the top 10% to 20% that consume the most energy. First, check to see which pumps can be turned off. Then look for the following symptoms to determine which pumps systems might be using too much energy:
- Throttled valve-control systems
- Bypass (or recirculation) line that is normally open
- Constant pump operation in a batch process
- Frequent cycling on or off in a continuous process
- Presence of a cavitation noise at a pump or valve
- Multiple parallel pump systems with units that are always on.
Check your repair history to see which parts of pump systems have required the most maintenance. More likely than not, those pumps are unreliable and should be repaired or replaced.
Another important step to take is to conduct an energy efficiency audit. The U.S. Department of Energy offers a valuable publication, Guiding Principles for Successfully Implementing Industrial Energy Assessment Recommendations, to help industrial sites improve efficiency. Suggestions include assigning the right people to assess energy efficiency, holding them accountable, and clearly communicating the benefits of an energy audit.
Excess energy is a destructive force as well as a source of energy when reduced. By optimizing efficiency, plants can realize millions of dollars in savings. Adapt the simple screening methods mentioned above and regularly track the data garnered from condition-based monitoring to develop a plan that is specifically tailored for your plant. Doing so will help not only protect the environment but also protect your profit margin.