In today's business environment, paper companies are looking for new avenues to lower energy and operating costs. There is a growing awareness that centrifugal pump optimization can significantly reduce electrical energy demand while improving pump and process reliability. Proper pump selection, sizing, and operation are important to mill economic performance.
Historically, pump design has focused on mechanical performance; today, the focus must shift to pump system performance in order to achieve available life-cycle cost savings. These potential savings are a significant portion of mill operating cost. Motor and valve performance improvements can have a major impact on the bottom line.
Thinking Outside Traditional Process Design
When deciding to build a new facility or modernize an existing one, initial design considerations are focused on sizing the major capital equipment items. Once mass balances are determined, the reactors, vessels, and other capital equipment items are selected. The next phase typically includes sizing the pipes and motor-driven systems -- in our case, centrifugal pumps -- to meet production targets.
In anticipation of future load growth, the end-user, supplier, and design engineers routinely add 10 to 50 percent "safety margins" to ensure the pump and motor can accommodate anticipated capacity increases. Once the piping isometrics and pump sizing are completed, near the final design phase, process control engineers select the instruments and valves needed to implement process control strategies. As each design phase progresses, the various engineering disciplines rarely collaborate on the subtleties associated with pump, pipe, and valve sizing to consider their overall impact on operating stability.
As a result, optimum process control is seldom achieved at plant startup. Furthermore, as control loop performance is known to decay over time, unless addressed, the performance gap will continue to widen over the life of the plant.
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In 1996, a Finnish Technical Research Center report entitled "Expert Systems for Diagnosis and Performance of Centrifugal Pumps" revealed that the average pumping efficiency, across the 20 plants and 1,690 pumps studied, was less than 40 percent, with 10 percent of pumps operating below 10 percent. Pump over-sizing and throttled valves were identified as the two major contributors to this sizeable efficiency loss. Besides hindering overall plant efficiency, poor pump performance results in lower product quality, lost production time, collateral damage to process equipment, and inordinate maintenance costs.
Pump and motor manufacturers have made substantial improvements in mechanical efficiency over the years. Yet, once the pump is installed, its efficiency is determined predominately by process conditions. The major factors affecting performance include efficiency of the pump and system components, overall system design, efficient pump control, and appropriate maintenance cycles. To achieve the efficiencies available from mechanical design, pump manufacturers must work closely with end-users and design engineers to consider all of these factors when specifying pumps. In the future, pump selection and sizing should be considered in the context of the overall system, not just the efficiency of the individual components.
The vast majority of pumping systems run far from their best efficiency point (BEP). For reasons ranging from shortsighted or overly conservative design, specification, and procurement to decades of incremental changes in operating conditions, most pumps, pipes, and control valves are too large or too small. As a result, pumping systems fail to convert the electric power they consume into fluid motion with anywhere near the economy, reliability, and control inherently available in the finely engineered individual components.
Design Considerations for Improve Pumping Efficiency
In the process industries, the purchase price of a centrifugal pump is often 5 - 10 percent of the total cost of ownership. Typically, considering current design practice, the life-cycle cost (LCC) of a 100-horsepower pump system, including costs to install, operate, maintain, and decommission, will be more than 20 times the initial purchase price. In a marketplace that is relentless on cost, optimizing pump efficiency is an increasingly important consideration. Centrifugal pumps consume, depending on the industry, between 25 and 60 percent of plant electrical motor energy.
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Based on current design practice, energy accounts for about 50 percent of life-cycle costs (LCC), with maintenance averaging around 20 percent. In poorly designed systems, maintenance may reach as high as 40 percent of LCC, even more.
Initial process design considerations help identify opportunities to improve pump system efficiency. The following criteria offer the highest potential for efficiency improvements:
- Reduced load on the motor through optimum process design
- Best match between component size and load requirement
- Use of speed control instead of throttling or bypass mechanisms.
Among all rotating assets in a process plant, centrifugal pumps typically have the best overall potential for electrical energy savings. In addition, the excess energy in fixed-speed systems, not used for moving fluid, is often dissipated into the infrastructure and contributes to noise, vibration, and lower equipment reliability, i.e., instruments, valves, pipes, and the pumps themselves.
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In addition to energy cost reduction, a top priority is to solve and eliminate recurring operating problems experienced by mill production, maintenance, and engineering departments. Typically, the asset group with the highest failure rate is centrifugal pumps, with seal leakage being the fault that causes the highest downtime and maintenance cost. Pumping system optimization helps minimize unscheduled downtime and contributes to productivity improvement.
Growing Use of Variable Speed Drives
Pump over-sizing causes the pump to operate to the far left of its best efficiency point (BEP) on the pump hear-capacity curve. Variable speed drives (VFDs), assuming a low static head system, allow the pump to operate near its best efficiency point (BEP) at any head or flow. In addition, the drive can be programmed to protect the pump from mechanical damage when away from BEP -- thereby enhancing mechanical reliability.
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Furthermore, excessive valve throttling is expensive and not only contributes to higher energy and maintenance cost, but can also significantly impair control loop performance. Employing a throttled control valve, less than 50 percent open, on the pump discharge may accelerate component wear, thereby slowing valve response. Because of increased stiction and backlash, operators often lose confidence in valve performance and switch the control loop into manual mode.
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VFDs allow pumps to run at slower speeds with for further contributions to pump reliability and significant improvement in mean-time-between-failure (MTBF). In new applications, variable speed drives are often
less expensive to purchase and install than flow-control valves and motor starters. Capital costs are also reduced by downsizing the motors, pumps, and pipes, in some cases, and eliminating the need for more expensive medium voltage power equipment. Considering the capital and operating cost savings, the total LCC of a given pumping system can be significantly reduced.
Today, there is a growing use of pump intelligence in variable frequency drives to improve pumping system performance. The added intelligence can contribute to smoother startups and production changes, tighter control during continuous operation, and faster diagnosis of potential system problems before product quality or process operation is negatively affected.
Pump Intelligence - Enhancing Mechanical Reliability beyond Variable Speed
Intelligent pump performance provides value beyond a traditional variable frequency drive (VFD). While similar energy savings can be achieved by employing a standard VFD, there is no assurance of decreasing the various failure modes of a centrifugal pump. An intelligent drive utilizes variable speed as a method for delivering enhanced pump reliability.
For academic purposes, assume variable speed is being used for flow control. A flow meter provides the process value to either an intelligent drive or standard drive to adjust speed to deliver 100 gpm. Now, assume a control valve is closing on the discharge side. What happens? The standard drive speeds up to "compensate" for increased resistance. An intelligent drive will do the same.
However, what happens when the valve closes to a point where less than 10 percent of the 100 gpm is being delivered? A standard drive will continue to ramp up to full speed (running against a closed valve), whereas, the intelligent drive can identify that a detrimental condition is occurring and notify the user. Furthermore, the intelligent drive can intervene to slow down the pump or turn it off, with periodic attempts to restart, and thereby avoid a premature failure.
In another example, as above, the pump is operating in flow control. Now, let us assume that the suction pressure (tank level) starts to decrease to a point that causes the pump to cavitate. If suction pressure drops, a standard drive will continue to speed up to meet the flow setpoint. In this case, increasing motor speed will only exacerbate the situation. The intelligent pump can address this problem by notifying the user that the net positive suction head available (NPSHa) has decreased to the point that cavitation exists. In addition, the pump intelligence can decrease motor speed to allow the NPSHa to increase to the point that cavitation is not occurring, and then resume normal operation.
In the future, plant design should consider the pumping system as an integral component of the process control architecture. Pump intelligence can enhance existing reliability-centered maintenance programs. Today, due to a lack of real-time information on process pumps and motors, many of the preventive maintenance checks on pumps, motors, and valves are unnecessary.
Mike Pemberton is manager of energy performance services at ITT Industrial Process, the parent company of Goulds Pumps, Bornemann pumps, Pure-Flo valves, and other industrial process equipment brands. The company is based in Seneca Falls, N.Y. This article was adapted from the white paper "Strategies to Improve Pump Efficiency and Life Cycle Performance" that appears on Pump Systems Matter, the education initiative of the Hydraulic Institute. Pump Systems Matter assists North American pump system users gain a more competitive business advantage through strategic, broad-based energy management and pump system performance optimization. Hydraulic Institute is a value-adding resource to member companies, engineering consulting firm,s and pump users worldwide by developing and elivering comprehensive industry standards and expanding knowledge through education and tools.