In most pulp and paper applications, energy and unscheduled maintenance are two of the largest components of ownership cost. For this reason, improving pump efficiency is key to pump system optimization.
Although certain mechanical and control modifications can improve reliability, selecting the best hydraulic fit is an often overlooked approach to system optimization. Energy and reliability are two sides of the same coin: A good rule of thumb is that where there is excess energy beyond what is required to move fluid through the pipes, there will also be unreliability.
Three primary factors affect pump system reliability:
- operating speed
- operating point (percent of best efficiency point [BEP])
- impeller diameter
The No. 1 predictor of pump reliability is speed, and the second is distance from the BEP. For example, in pumps with a nominal speed of less than 900 revolutions per minute (rpm), the pump can operate near shutoff or at the other end of the curve—right of the BEP—and still be reliable. Once speed increases to 1,200 rpm and above, the allowable operating range narrows.
At existing mills, about 20 percent of installed centrifugal pumps offer 80 percent of the potential life-cycle cost savings. In general, the best candidates for energy reduction exceed 50 horsepower (hp). However, lower-horsepower pumps that incur high maintenance costs are also good candidates for optimization. Most pumps in paper mills are low static head systems with relatively short pipe runs between tanks, making these systems both low static and friction head systems, which are ideal for variable speed control.
Systems with static heads that are less than half of the pump’s total dynamic head are considered low static head systems. While high static head systems are typically not considered good candidates for variable speed control, there are always exceptions. An example of high static head pumps that will benefit from variable speed control is the makeup liquor pumps that feed continuous digesters.
In high static head systems, there are minimal potential energy savings to justify variable speed drive (VSD) application. In makeup liquor pumping systems, because of high backpressure from a steam pressurized vessel, these pumps often experience cavitation and have mean time between repairs (MTBR) of less than one year.
Pump users can achieve tight control of suction-side tank levels more effectively with variable speed control than fixed-speed valve control. The rapid and linear response of variable speed proportional-integral-derivative (PID) control reduces tank level upsets and air entrainment and, as a result, extends pump life and improves overall system performance. These pumps incur high maintenance costs in most mills, but VSD control provides improved control performance and extends pump MTBR by three times or more over fixed-speed valve operation.
Pump system performance is affected by several factors:
- efficiency of the pump as well as other system components
- overall system design (sizing and balancing measures)
- effectiveness of piping systems (e.g. reduced frictional pressure losses, matching of pump and system characteristic)
- efficient pump control (e.g. variable hydraulics, on-off control, etc.)
- efficiency of drives (motor and VSD)
- appropriate maintenance cycles
These symptoms indicate opportunity for optimization, including potential benefits from VSD application:
- Throttled valve-controlled systems. Often the valve is sized so that it operates less than 50 percent open, resulting in significantly higher energy costs. Also, the pump is forced to run back on the curve, away from the BEP.
- Bypass (recirculation) line normally open. Use discrete control logic to open and close bypass based on flow or pressure demand. Minimizing bypass flow rates lowers pump system speed and, subsequently, energy costs.
- Multiple parallel pump system with same number of pumps always operating. Implement multi-pump sequencing (two to four pumps) to meet flow and pressure demand. Turning off one or more pumps in low-flow demand periods provides energy as well as maintenance cost savings. Tighter pressure and flow control improves overall process reliability by reducing pipe vibration and collateral damage to instruments, valves and pipes.
- Constant pump operation in a batch environment or frequent cycle batch operation in a continuous process. Variable speed control logic can help prevent deadheading the pump against a closed discharge valve.
- Presence of cavitation noise (at pump, valve or piping). Intelligent variable frequency drive control offers dry-running and cavitation protection. Intelligent pump control offers vibration monitoring with alarming.
Across integrated mills, there are many good applications for pump system optimization. Significant opportunities for improvement exist because of oversizing, valve throttling, bypassing (recirculation), frequent start/stop (batch) and multi-pump systems where all pumps run continuously.
The paper industry is a high-value industry for pump system assessments prior to project implementation. This is because integrated mills generally have a large pump population in the range of 400 to 1,200 pump systems, a general-purpose work environment, and a recent history of using more variable speed control and mechanical modifications, such as impeller trims, to improve system efficiency.