According to the Department of Energy, pumps are critical to our industrial plants and the planet. They account for 50% of a typical plant’s rotating equipment energy usage, followed by air compressors at 27%, fans at 10%, and others at 12.5%. Further, it is estimated that an integrated Pulp & Paper mill will have approximately 2,000 pumps in service. Most have had to specify a pump or two over the years. Pumps are mainly categorized as Positive Displacement Pumps and Centrifugal Pumps. Today’s focus will be on Centrifugal Pumps.
This is often a replacement-in-kind of a pump at the end of its useful life. On the other hand, it may be a pre-mature pump failure that you are trying to troubleshoot. Typically, one knows the GPM (gallons per minute) requirements and the fluid’s characteristics (viscosity, specific gravity, pH, etc.) Unmistakably, understanding the fluid characteristics to spec out a pump is essential, as illustrated below. When selecting a centrifugal pump, you want to choose based on roughly 75% of the pump’s flow rate and discharge pressure. When specifying a pump, review the published pump system curves from the manufacturer to the sweet spot for runnability.
Further, it is recommended to spec out a variable frequency drive to be able to adjust the process flow as needed. As a sidebar, pulp refiners are specialized pumps moving pulp slurries through their rotor and stator. Of course, it uses a lot of energy to alter the fibers to provide needed breakage of fiber bundles, exposing the edges to be frayed and creating a crill to provide greater fiber-to-fiber bonding.
Most topics covered during the seminar focused on the physics of why and how pumps work. How to calculate the velocity of the fluid (Fluid Velocity = .4085*GPM/pipe ID2), calculate proper impeller size, total discharge (dynamic) head (Head = PSI * 2.31), how read pump charts, and how to troubleshoot pump issues.
In an example provided during the presentation, the fluid’s velocity became the key cause of the cavitation. As expected, a municipality’s wastewater treatment plant had four identical centrifugal pumps moving sewage, which has methane gas entrapped in it. All four pumps had severe cavitation. However, the acceptance test showed no issues and performed well, albeit water was the medium tested. So, a lesson learned is to be aware of problems that can occur when it isn’t practical to test the same medium to be pumped and try to compensate for that.
Back to the waste treatment plant: the telltale signs of cavitation were readily apparent: high vibration when two or more pumps were engaged, and it sounded like rocks were being pumped. Why? After a quick calculation, it was determined that the velocity at the inlet was 17.5 ft/sec. As the Rule of Thumb in the blue box above noted, this was outside a good practice standard. This high velocity forced the liquid with methane gas to implode against the impeller, pitting the impeller and creating rock-sounding cavitation. (See Classic Vaporization Cavitation picture.) Long story short, reducing the motor RPM (using a variable frequency drive) from 1800 rpm to 1200 rpm dropped the fluid velocity so low that the methane gas wasn’t imploding. As a result, cavitation stopped, and while the process flow was reduced, there was more than enough capacity with the other pumps to keep up with demand. Further, from Pump Affinity Laws, recall that lowering the RPMs will dramatically lower power consumption. Affinity laws follow this equation: Power1/Power2 = (RPM1/RPM2)3.
As a takeaway, here are some considerations to review when troubleshooting pumps. These checklists should quickly determine if your pump is healthy or sick.
Two good reference sites on pumps from TAPPI are:
1. TAPPI TIP 0420-10:2022 Horizontal End Suction Centrifugal Stock Pumps.
This requires a TAPPI Membership and access to TAPPI Standards and TIPs.
This Technical Information Paper provides guidelines developed for the horizontal end suction stock pump to supply the user and the pump and pump component manufacturer with vital information to consider when designing the system and sizing, acquiring, or providing a stock pump. Reliability, operating efficiency, and maintenance costs are critical to developing these guidelines. The end suction centrifugal stock pump was designed to handle pulp and paper stock. This pump ranks between the ANSI horizontal end suction centrifugal pump for chemical processing and the demanding standards of an API pump. Along with the demand for higher flow rates, more flexibility in system design, and a more comprehensive pH range of fluids and stock consistencies, there is also a need for a high-quality yet cost-effective stock pump. To further aid the reader, a list of beneficial pump and component construction, mechanical and hydraulic features is provided to augment the selection process and specification of end suction centrifugal stock pumps. Considerations for pump installation and ongoing maintenance and operation are also briefly addressed for the life cycle of the equipment.
2. Excellent presentation on pumps from PaperCon 2013. Unthrottle Your Profits: Tips for Boosting Pump Efficiency, Reliability, and Process Control, PaperCon 2013. Here’s a snippet from the presentation:
Potential for Optimization: Across virtually all industrial market segments, whether a facility operates thousands of pumps or just a few, centrifugal pumps represent the most significant opportunity for electrical energy savings compared to other processes and equipment. In an evaluation of nearly 1,700 pumps at 20 process plants, the Finnish Technical Research Center found that the average pumping efficiency fell below 40 percent. Furthermore, over 10 percent of the pumps ran at only 10 percent efficiency or less. While a broad range of best efficiency points depends on make and model, pumps are designed to operate between 65 and 85 percent mechanical efficiency. Every watt Unfortunately, every waste is converted to heat and vibration that reduce equipment reliability and can eventually cause damage, raising equipment maintenance costs and degrading process control.
This presentation is from Mike Pemberton of ITT Goulds Pumps. You can download a copy free from TAPPI – included in your TAPPI membership.
