Lawmakers in Washington are talking about it. Operations teams are thinking about it, and CFOs are losing sleep because of it. A plant’s energy costs are on multiple stakeholders’ minds.
Plant managers especially want to increase uptime and throughput without sacrificing goals for reduced production costs. These two competing issues can be dealt with simultaneously, but not through piecemeal efforts. A systems approach to pump system efficiency is required to accomplish both.
Imagine if all the specialized production equipment in a plant was stripped away. A labyrinth of pumps, pipes, tanks and valves would be left—key components of subsystems that are critical to a successful operation. These subsystems represent a significant portion of a plant’s current operating costs and, directly or indirectly, have a serious impact on plant performance.
Right now, approximately 40 percent of manufacturing revenues at facilities are devoted to the maintenance of pumps and valves. The good news is that up to 60 percent of scheduled maintenance checks on valves and motor systems in these systems could be avoided.1
The key to solving many facilities’ most pressing issues is a systems approach to efficiency—not just focusing on the primary systems but also the subsystems.
Finding Optimization Opportunities
The standard in plant systems management tends to be more about “fire fighting,” but engineers need to take a more systematic approach to improving long-term performance. This does not mean ignoring pressing issues. It means finding the right solutions from a technology perspective, rather than deploying the fastest and easiest fixes. It also means tracking the sources of chronic issues, so they can be treated with permanent solutions. Instead of treating symptoms, operators must attempt to cure the disease in the systems.
In pump systems, disease runs rampant. Pumps are often oversized and inefficient. The average pump efficiency is less than 40 percent. More than 10 percent of pumps are running at 10 percent efficiency or less, according to a recent study by the Finnish Tech Research Center. centrifugal pumps represent the single greatest opportunity for electrical energy savings compared to other processes and equipment.
The biggest process disease that plagues plants today is not necessarily intuitive. It is often hiding in plain sight. Oversized pumps that result in excessive throttling are the most common culprits, with seal failure as the number one maintenance cost item. Pumps with relatively high levels of excess energy carry higher electrical costs, but increasing the pump size is considered a way to play it safe. It is exactly the opposite. The excess energy is blowing the pumps apart. Oversized pumps are less reliable and more costly.
When thinking about the amount of excess head generated by oversized pumps, the excess is often considered as reserve pressure that is available, if needed, to do useful work. Many end users think that this is why adjustable control valves are used, but valves kept in restrictive positions wear out faster. They also wear out pumps faster. Valves that are frequently less than 40 percent open can force pumps to operate against massive resistance, exposing the pumps to inordinate bearing damage, seal wear and leakage. Under these conditions, pump shafts often crack or break.
Optimizing Pump Performance
A systems approach to efficiency includes more than just understanding energy loss in throttled pumps and the increased costs associated with resulting breakdowns. Additional technologies are available that help subsystems work in better unison, which should also be considered.
In addition to pump and valve breakdowns, oversized pumps and throttling can result in degraded process control. Sub-optimal process control increases variability, and as a result, control loops are often switched into manual mode to stabilize the process. This common fix results in additional costs that are the direct result of compounding system inefficiency.
In many cases, a better solution may be installing variable frequency drives (VFDs) to adjust speed rather than control valve position.
A recent study by the U.S. Department of Energy suggests that motor systems equipped with VFDs account for only 4 percent of motor energy usage, marking a significant opportunity for efficiency improvement. VFDs are widely applicable and can be applied to 18 to 25 percent of total energy used for retrofits or up to 60 percent for new or refurbished plants.
In antiquated systems, manual throttling is the only way to adjust the flow in pump systems. Today, VFDs integrate pumps into process control systems. They make archaic, mechanically controlled pump systems high-tech, often including embedded firmware programs with pump protection technology. By incorporating VFDs into the continuous loop in lieu of a control valve, low static head pump systems can consistently operate near peak efficiency. Adopting a systems approach means building systems that have components that work in sync with one another.
Case Study: A Bleach Plant
Regular pump breakdowns and undue wear resulting from throttling can cost companies millions each year. In one case study dating back to 2001, a paper mill bleach plant suffered from major problems and financial losses as the result of an oversized pump before a plant performance team stepped in to help.