Delegates at Water Expo 2023 — an international water industry conference which took place at the Miami Airport Convention Centre, US — learned that bearing selection is critical in vertical turbine pump (VTP) performance, and that application, medium and bearing material should be considered during pump manufacturing or refurbishment, according to polymer bearing and shaft line products manufacturer Thordon Bearings’ strategic account manager, Keith Brand.
In his paper, ‘Selecting the right bearings to improve vertical turbine pump reliability’, the abrasiveness of the pumped media is a variable affecting pump bearing selection and lifespan. The other variable is application. “Abrasive wear and dry starts are the most common causes of bearing failure,” Brand said.
Common oil and water-lubricated bearing materials, such as bronze, carbon graphite, thermoset laminates, Teflon, thermoplastics and elastomers, Brand said that some bearings have better dry-running capability but poor abrasion or chemical resistance, while others may be suited to high-temperature environments but fall short on mechanical strength. All tend to be more OPEX-costly, according to the manager.
“In some applications, the upper bearings are not lubricated upon startup. This could mean 30sec or more of high-speed dry bearing operation, ruling out materials such as rubber,” said Brand.
“Rubber bearings can burn up from frictional heat created with little to no lubricant and most phenolic bearings cannot handle running dry for more than 60sec. Oil-lubrication or a water flush is required to lubricate the bearings until process water can lubricate the shaft, but this also adds to the cost and can be detrimental to the environment,” he added.
Brand said that some manufacturers have attempted to make pump components more chemical and abrasion-resistant by isolating the pump’s oil-lubricated bearings and shafts from process water. “But the drawback to this approach is the additional cost,” he said.
With the different types of water-lubricated, grease-free polymer bearings available from Thordon, Brand added that the company has products for different types of operating mediums and applications.
For example, the company’s composite bearing material — an elastomeric polymer alloy — is for use in highly abrasive operating environments. The SXL material balances the operational requirement between abrasion resistance, low friction and good dry startup performance. Meanwhile, Thordon’s ThorPlas-blue, an engineered thermoplastic, has chemical resistance and can handle dry starts and operate at higher temperatures. ThorPlas-white has similar qualities but it is for food equipment, drinking and potable water applications and has NSF/ANSI 51/61 and independent UK certification body for plumbing products and materials Water Regulations Approval Scheme (WRAS) approval.
Brand said that understanding pumped mediums and dry startup requirements will narrow down options. “Understanding bearing material selection will keep pumps pumping longer, reduce downtime and maintenance costs, and prevent failure,” he said.