Because pump systems can be complex, determining the root cause of problems in an operating system can often lead to a hit-and-miss approach that is time consuming and costly. For six years, that was the case for a vat dilution pump in a bleach plant within an integrated paper mill. This case study will explore how the plant reduced operating, maintenance and capital costs while increasing system uptime by fixing this chronic maintenance problem.
The mill experienced persistent cracking on the discharge header of the vat dilution pump. To repair the discharge header, the plant had to shut down, and the pipeline was drained of its valuable product, which had to be sent to environmental treatment ponds.
In addition to the pipe cracking, severe movement of the discharge pipe and excessive pump vibration occurred. After years of living with these problems while “fighting fires” in other parts of the mill, the plant decided that a solution was needed.
The primary problem was the extensive cracking in the pump discharge piping. The plant managers reasoned that the problem resulted from thermal stresses caused by rapid heating of the discharge piping during plant startup. As a result, they focused their efforts on plant modifications to reduce the rate of temperature change during startup. They also sought ways to minimize pump vibration and bring the pump maintenance cost into line with similar pumps in the plant.
Their first efforts to correct the cracking problem—minimizing the rate of temperature change in the discharge header—were not effective. After this false start, the plant decided to look at the operation of the piping system.
Discovering the Problem
In 2004, the managers decided to focus on the interaction between the pump and its system. The pump was initially specified for a flow rate of a 6,500 gallons per minute (gpm) and 110 feet of head. It was supplied with a 250-horsepower motor running at 1,750 rpm. The best efficiency point (BEP) for the installed pump occurred at 7,900 gpm (see Figure 1). After looking at the operation of the vat dilution system, the required flow rate was only 2,800 gpm. This lower flow rate resulted in the pump running below its original design point and far to the left of the BEP.
After performing an analysis of the entire system, the managers determined that the pump was oversized in the specified flow rate and in the pump head. The results of the study determined that the system requires only 26 feet of pump head to meet the 2,800-gpm flow requirement of the vat dilution system. To better match the pump to the system, a variable frequency drive (VFD) was installed. With the pump operating at 850 rpm, the installed pump could meet the 2,800-gpm flow rate with the required system head. The pump operation at 850 rpm is also displayed in Figure 1.
Reduced Operating Costs
Notice from the pump curve in Figure 1 that, with the pump operating at 1,750 rpm, it produces approximately 145 feet of head with a system flow rate of 2,800 gpm. The pump operating at this flow rate requires approximately 170 horsepower.
At the lower speed of 850 rpm, the pump met the system flow and head requirements and only consumed 70 horsepower. This reduction in speed resulted in a 60 percent savings on the cost of power to operate the pump. The lower power requirement at the slower speed also allowed for the use of a low-voltage motor and VFD, resulting in significant savings in capital costs.
Using the 2004 power cost of $0.025 kilowatt hour (kWh), installing the VFD resulted in an energy savings of $16,500 per year. Based on the cost of the VFD and the savings in energy consumption, the project had a simple return on investment (ROI) of 20 months.
Reduced Maintenance Costs
Projecting future maintenance savings is difficult when evaluating a pumping system. However, in this example, with the pump operating at a slower speed after adding the VFD, 100 less horsepower entered the fluid in the system. That extra horsepower had to be removed from the fluid, and this was accomplished with a highly throttled control valve. The dissipation of this power across the control valve generated heat, noise and vibration. Another way of looking at the system before the VFD is that the excess power was damaging the equipment, which increased maintenance costs.
In addition, the pump curve in Figure 1 shows that when the pump operates at 850 rpm with a flow rate of 2,800 gpm, it is much closer to its BEP. Operating a pump closer to its BEP has been proven to increase reliability and reduce maintenance costs.