Diffuser redesign saves $100,000 per year

A chemical processing plant had been experiencing high vibration and bearing failures on its boiler feed water (BFW) circulation pumps. This application used a sealless pump, due to the high pressure and high temperature water being circulated. This vibration and wear required that the motors be overhauled approximately once per year at a cost of more than $100,000.

Investigations after initial commissioning of the units indicated that the vibration frequency being measured, 177 Hz, also matched the first bending mode frequency of the rotor. However, the phenomenon which excited this mode was not conclusively identified.

Hayward Tyler was contracted to perform a thorough review of the existing hydraulic design, specifically focused on the impeller and diffuser blade combination. This design used a mixed flow, six-blade impeller and a radial, seven-blade diffuser. The review identified that the blade pass frequency was also 177Hz and the corresponding radial load was exciting the bending mode of the shaft resulting in excessive damage.

Hayward Tyler’s engineers evaluated different hydraulic options (impeller blades and diffuser blades) to remove the radial load and settled on a new eight-blade diffuser. Computational Fluid Dynamic analysis of the new combination showed a 92% reduction in load at the problematic 177Hz frequency. Furthermore, this new diffuser was designed to fit in the existing footprint meaning only a new diffuser was needed to solve the problem.

The plant ordered new diffusers manufactured by Hayward Tyler, and confirmed upon installation that the 177Hz vibration had been eliminated from the pump. The total cost of the engineering and new diffusers was less than the cost of one motor overhaul. www.haywardtyler.com