Unthrottle your profits
Editor’s note: This article is based on a Plant Engineering webcast presented by Mr. Pemberton in late 2011. Readers can find the presentation slides and audio under the media library section of www.plantengineering.com.
Regardless of company or product, all industrial facilities have the same goals. From top executives to the plant floor, everyone wants to trim production costs while improving reliability and control of the process. A recent poll confirmed that energy efficiency, maintenance, and aging infrastructure are the top three concerns of industrial pump users for 2012. But what are we really doing to address these concerns?
Just imagine, if you remove the specialized production equipment from your facility, you’d be left with a maze of pumps, pipes, tanks, and valves—subsystems that are critical to overall operations. Improving the reliability and efficiency of these subsystems is the key to achieving three objectives: reducing energy consumption, lowering maintenance costs, and improving process control. It’s all about optimizing the entire fluid system.
This article will address the importance of a shift in thinking—taking a systems approach that will improve production efficiency and help industrial facilities to realize higher product quality and, thus, higher company profits.
Pumps: Potential for optimization
Across virtually all industrial market segments, whether a facility operates thousands of pumps or just a few, centrifugal pumps represent the greatest opportunity for electrical energy savings compared to other processes and equipment. In an evaluation of nearly 1,700 pumps at 20 process plants, the Finnish Technical Research Center found that the average pumping efficiency fell below 40%. Furthermore, over 10% of pumps were running at only 10% efficiency or less.
While there is a broad range of best efficiency points depending on make and model, pumps are designed to operate between 65% and 85% mechanical efficiency. Every watt of power wasted is converted to heat and vibration that reduce equipment reliability and can eventually cause damage, raising equipment maintenance costs and degrading process control.
Factors driving pump inefficiency
Two of the main causes of plant inefficiency are the installation of oversized pumps and excessive throttling.
1. Improper sizing
Conservative engineering practices can often result in the specification and installation of pumps that exceed application requirements. Engineers add safety margins to original sizing calculations, thinking ahead to anticipated expansions or hoping to compensate for process uncertainties. For example:
- An engineer assesses system demands and determines a replacement pump needs to move liquid at 2,500 gallons per minute (gpm) to a 100-ft head.
- Operations then decides that 3,600 gpm may be needed in the future.
- A junior engineer considers potential piping losses and adds 15% to head.
- A senior engineer adds 10% more to head, just to be safe.
The result? The pump arrives “super-sized,” adding to system operating costs in terms of both energy and maintenance requirements. In addition, concurrent sizing is rare—pumps are chosen at different stages in the process design. So even if a pump is sized properly at installation, it may not be right down the road due to changes in production.
2. Throttled control valves and seal leakage
In systems with oversized pumps, valves tend to remain in restrictive positions. Many pumps are continuously running throttled-back by valves in the range of 20% to 40% open, forcing the pump to operate against a high resistance, which makes it susceptible to inordinate bearing damage, seal wear, and leakage. Under these conditions, it is common for the pump shaft to also crack or break.
Excessive valve throttling is very expensive, not only in terms of energy but also due to degraded process control. Sub-optimal process control increases variability and, as a result, many control loops are switched into manual mode to stabilize the process.
Improving control loop performance
According to a study by the U.S. Dept. of Energy, motor systems equipped with variable frequency drives (VFDs) account for only 4% of motor energy usage. This represents a significant opportunity, as VFDs could be applied to 18% to 25% of total energy used, or up to 60% at new or refurbished plants.
VFDs have brought what is essentially a mechanical system into the digital age. They embed firmware programs with pump protection technology and other functions. One reason pump intelligence has been underutilized is that, mechanically, the basic design and appearance of industrial pumps hasn’t changed significantly in decades. People may not associate pumps with information technologies and controls or aren’t aware that pumps are becoming an integral part of the automation architecture.
In fixed-speed pumping systems, where flow is adjusted by manual throttling, change in process conditions affects the efficiency of the pump. Variable speed controls integrate the pump into the process control systems as part of the continuous loop. As a result, the hydraulics can be controlled in such a way that the pump can continuously operate near peak efficiency. Today’s VFD controls are highly reliable and promise relatively quick payback—from 6 to 24 months in retrofitted plants and, also, as little as 2 months or less in new or retrofitted pumping systems. The energy savings justify the project, and less excess energy improves reliability as well.
Tips for optimizing pump performance
To make systems more efficient, the modifications don’t have to revolve around process. You can perform mechanical modifications like trimming the impeller to reduce the head and allow the pump to run closer to best efficiency point. Rather than changing the size of the motor or the impeller, you can also rerate pumps, modify casings, or change hydraulics, essentially generating a new pump curve.
It’s not difficult to find the information needed to optimize pump systems. Maintenance supervisors know and will often have a list of bad actors and high vibration systems that they are monitoring. When identifying candidates for optimization, the presence of cavitation noise is a good indication that change is needed. However, you should also be sure to ask, can it be turned off?
The best way to save energy is to turn systems off entirely when they are not needed. In parallel systems where there is a series of pumps in a row, not all of them may need to be on. For other systems, look at the gap between supply and demand. The capacity of the system may be much greater than what is required.
Another remedy to inefficient systems is obvious: replace the pump or motor with a downsized version. While expensive, this action may be warranted.
A systems approach
Plants invest heavily in automation to improve performance and productivity, spending millions on IT systems. But overall, they have not always been satisfied with the results. This is because while their automation systems are used to control specialized process equipment, they are often disconnected from the underlying manufacturing process—the motor systems and valves. About 40% of manufacturing revenues are devoted to maintenance of pumps and valves, yet there is little or no real-time data for them. There’s good news in the new equation: up to 60% of scheduled maintenance checks on valves and motor systems are avoidable.
It’s imperative to consider pumps as part of the process control architecture, to monitor them as components of an overall process system rather than as individual assets. Using a “systems approach” to achieve process optimization shifts the focus from the separate components to the total system performance. And seeing the forest for the trees yields energy and cost savings that far outweigh the sum of all the savings through component optimization.
Mike Pemberton is manager of energy performance services for ITT Goulds Pumps.