Have you ever experienced a sprained ankle that tweaked your hip joint and then resulted in a stiff back? This is an illustration of how chain reactions can occur in complex systems. The body, like a highly engineered industrial process, can experience immobilization and downtime, i.e., each joint, bone and associated connective tissue inside works as part of a highly integrated system. When one subsystem fails, other issues may be the root cause.
If a control valve is presenting problems, it may be a sign of a greater systemic issue, with other “bad actor” system components taking the blame.
Collectively and individually, one of the biggest sources of plantwide maintenance and electrical energy savings in industrial processes today1, can be found at the heart of the facility—the centrifugal pumps.
FUZZY LOGIC ABOUNDS
Putting pure engineering analysis aside, logic states that a bigger pump can produce more available head and thus give reserve pressure capabilities that can be put to work, if and when needed. Plant managers dream of the day output will significantly increase because a new processing line comes online, leveraging as much existing infrastructure as possible.
Historically, however, the combined notion that larger pumps are insurance, as well as an investment in the future productivity of a facility, has led to the installation of many oversized pumps. In fact, according to recent research by the Finnish Tech Research Center2, the average pump efficiency is less than 40%, with roughly 10% of all processing pumps operating at 10% efficiency or less. This is largely because of valve throttling and oversizing.
EXTENDING VALVE LIFE AND MORE
The plain truth about pumps operating at or near the best efficiency point (BEP) is: They run smoother, last longer and operate on much less energy. A throttled pump can consume 75% or more excess power3, transmuted in the form of vibration, heat and noise. And here’s the kicker—pumps operating at or near BEP help the rest of the system components operate more efficiently and last longer.
Valves, seals, gaskets, pipes and other subsystem components can wear out faster when pumps are throttling flow. In fact, an estimated 60% of scheduled maintenance checks on valves and motor systems can be avoided when the pump system is operating at BEP and monitored by real-time asset systems4. This is especially pertinent given the reality that, across continuous process industries, about 40% of manufacturing revenues are devoted to maintenance. (Source: DOE Industrial Technology Program, ITP “Motor Challenge”)
Pumps are at risk from this reality as well. Valves that are less than 40% open expose pumps to massive resistance, resulting in component stress that accelerates bearing damage and seal wear5. Pump shafts often break under these constrained conditions.
But perhaps even more important is the fact that pumps operating at BEP offer plants greater control over the process. Throttling pumps can result in increased process variability from a combination of control valve, pump and pipe mis-sizing. This leaves engineers no choice but to operate control loops in manual mode. In some industries, up to 80% of process control loops contribute to increased process variability6, and valves are without question the biggest contributor to process variability primarily because of stiction and backlash.
In pump systems, two separate curves represent the hydraulics, each plotted on the same artesian plane correlating force and flow (Figure 1—TDH is total dynamic head). The total head, typically represented on the Y-axis and measured in feet (or meters), represents pressure (kg/cm2). The pump flow capacity, typically represented on the X-axis and measured in gallons (or liters) per minute, represents volume transfer per time increment.
The two key curves are the pump curve, which is convex and decreasing; and the system curve, which is concave and increasing. If these two curves cross near BEP, then the pump will operate efficiently and reliably.
When fluid flow is restricted enough to move a pump left of BEP, it is being throttled, thereby increasing pressure inside the pump casing. Potentially, this can result in undue wear and tear from increased radial loading and the resulting low-flow cavitation7 plus suction recirculation. When the flow is too great, the pump may experience a lower-pressure area inside the pump in a condition called “runout,” which can also cause vibration, recirculation and other losses in efficiency8. In some cases, seals may begin to leak because there isn’t enough pressure inside the pump to keep them seated properly.
In most systems, it’s unlikely that the pump will operate at its exact BEP all of the time. Shifting process variables and swings in end-user flow demand affect pump efficiency. However, a pump that is properly sized will maintain a flow near peak efficiency most of the time. As a general rule, maintaining a flow between 70% and 100% of BEP is acceptable performance9.
“TUNING” A SYSTEM TO BEP
When a pump is creating too much or too little force in the system, inefficiency and the problems described above emerge. There are a few ways to “tune” the system so that the pump is operating at BEP:
- Variable Frequency Drives (VFDs)—In processing scenarios where variability is important, VFDs are a great solution—they adapt to changing process conditions, enable soft starting and shutdown to help protect components from start/stop forces, and ensure that the right amount of energy flows into the pump to keep it at BEP under virtually all normal operating conditions.
- Trimming Impellers—In some cases it’s possible to trim the pump impellers on a throttling pump, reducing the head it produces. This takes careful thought. It’s not possible to “un-trim” an impeller without replacing it.
- Turning off One or More Parallel Pumps—An often misunderstood factor is that turning on two or more parallel pumps doesn’t result in doubling or tripling the flow rate. Due to backpressure in the piping, each pump turned on only adds to the flow incrementally; e.g., 1+1+1 ≠ 3, it equals more like 1.5 times the flow rate. Turning off pumps that don’t need to run saves the maximum amount of energy.
- Replacing the Pump—In some scenarios, an overrated or underrated pump cannot be brought into BEP in a given system. This occurs when the pump is dramatically outsized. The only solution becomes replacing it.
VALVE WEAR AND OTHER COSTS ADD UP
When pumps operate outside of BEP, and especially when they are throttled, inefficiency and repairs occur at higher rates. These costs can be significant, even exceeding the original cost of the equipment (Figure 2).
In typical American Petroleum Institute (API)-rated pump systems, electrical energy costs over the lifetime of a pump can be more than 10 times the cost of the initial investment in the pump. Maintenance costs can be higher or lower than the electrical energy cost of the pump, depending on the sizing. These facts and others have taught many process engineers to look beyond the cost of the initial investment and consider the cost of the investment from cradle to grave—the Total Cost of Ownership metrics.
Sometimes, the source of a faulty valve isn’t the way it’s installed, operated or constructed. Poor pump performance can be the product of how the valve is sized in proportion to the pump upstream. If a plant is suffering from a “bum hip” that’s been lingering for some time, a solution may be to optimize the rest of the system. With the right, holistic approach, a plant can be up and running better than ever, pain free and ready to tackle the expansion its managers have been hoping to implement all along.
© ITT Corporation, 2014
1 Pemberton, Mike. “A Big Picture Evaluation Can Produce Big Savings.” Pumps & Systems. September, 2013.
2 Previous Citation
3 Pemberton, Mike. “Don’t Pay at the Pump.” Webinar
4 ARC Advisory Group. Strategies to drive manufacturing efficiency on the production floor. (Cited previously in the “Don’t Pay at the Pump” webinar.)
5 Pemberton, Mike. “A Big Picture Evaluation Can Produce Big Savings.” Pumps & Systems. September, 2013.
6 Emerson Entech newsletter. (Cited previously in the “Don’t Pay at the Pump” webinar.)
7 Kernan, Daniel. “Pumps 101: Operation, Maintenance and Monitoring basics.” ITT Goulds Pumps whitepaper available here