Chronic problems are rarely solved unless the underlying issue is addressed. Often in pumping systems, diagnosing these problems is difficult and it may not be the first culprit suspected. Here, a paper mill was able to reduce operating, maintenance and capital costs while increasing system performance by fixing a chronic maintenance problem.
The mill was experiencing persistent cracking on the discharge header on a vat at a dilution pump resulting in a discharge header that required repair. To fix the discharge header, the plant had to be shut down, the pipeline drained of its valuable product and sent to the environmental treatment ponds.
In addition to the pipe cracking, there was severe movement of the discharge pipe, along with excessive pump vibration causing additional damage. After years of dealing with the issues caused by ongoing vibrations and cracking in other parts of the mill, plant managers decided it was time to correct the problem.
Time-consuming and costly hit-and-miss diagnostics were initially used to determine the root cause of the issue. The primary problem to address was the extensive cracking in the pump-discharge piping system. The mill reasoned it was due to thermal stresses caused by rapid heating of the discharge piping during plant startup. As a result, the focus on plant modifications to reduce the rate of temperature change during plant startup was the first initiative. Second, the team determined it was necessary to look for ways to minimize the pump vibration and bring the pump maintenance cost in line with similar pumps in the plant.
Their first efforts in correcting the cracking problem by minimizing the rate of temperature change in the discharge header were not effective. After a false start, management decided to look at the operation of the piping system.
Discovering the Root Cause
After careful review, the team decided to focus on the interaction between the pump and its system. The pump was initially specified for a 6,500 gallon per minute (gpm) flow rate and 110 feet of head. It was supplied with a 250-horsepower (hp) motor running at 1,750 revolutions per minute (rpm). The best efficiency point (BEP) for the installed pump occurred at 7,900 gpm (see Image 1). Looking at the operation of the vat dilution system using piping software, it was discovered the required flow rate was only 2,800 gpm. Running at this lower flow rate resulted in the pump operating below its original design point and far to the left of the BEP flow.
After performing an analysis of the entire system, it was determined the pump was oversized both in the specified flow rate and pump head. The results of the study determined that the system required only 26 feet of the 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 pump was able to meet the flow rate of 2,800 gpm with the required system head. The pump operation at 850 rpm is displayed in Image 1.
Reducing Operating Costs
Notice on the pump curve that with the pump operating at 1,750 rpm, the pump produces approximately 145 feet of head with a flow system flow rate of 2,800 gpm. The pump operating at this flow rate requires approximately 170 hp.
At the lower speed of 850 rpm, the pump is able to meet the system flow and head requirements by consuming only 70 hp. 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.
Using the plants low power cost of $0.025 kilowatt hours (kWh), installing the VFD resulted in a calculated energy savings of $16,500/year. Based on the cost of the VFD and the savings in energy consumption, the project had a return on investment (ROI) of 20 months.