Diffuser redesign eliminates $100,000 per year on unplanned maintenance

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Chemical plant was making frequent repairs due to high vibration issue

A chemical processing plant had been experiencing high vibration and bearing failures on their Boiler Feed Water (BFW) Circulation pumps. This application uses a sealless pump, due to the high pressure and 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 the initial commissioning of the units indicated that the vibration being measured, 177Hz, also matched the first bending mode of the frequency of the motors; however, the phenomenon which excited this mode was not conclusively identified.

Hayward Tyler engineers were 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, 6-blade impeller, and a radial, 7-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 engineers evaluated different hydraulic options (impeller and diffuser blades) to remove the radial load and settled on a new 8-blade diffuser (the original was 7). Computational Fluid Dynamic (CFD) analysis of the new combination showed a 92% reduction in load at the problematic 177Hz. Furthermore, this new diffuser was designed to fit in the existing footprint meaning only a new diffuser was needed to solve this 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.

SITE/LOCATION:

Chemical/Syngas Plant, USA

SCOPE AND WORK:

• Evaluate source of vibration issue
• Diagnose how to resolve
• Design new hydraulic option
• Evaluate new design using Computational Fluid Dynamics
• Manufacture and install new design on existing pumps
• Support on-site testing of redesigned diffusers
• Verify vibration has been eliminated

BASIC DESIGN DETAILS:

• Vertical, Motor Under, Glandless Wet Stator Circulating Pump
• Rated Flow: 4,270USgpm
• Rated Head: 143ft
• Design Pressure: 750psig
• Design Temp: 550° F
• Rated Power: 200hp
• Speed: 1780rpm
• Power Supply: 460 V/ 60 Hz / 3 ph
  
  For more information please contact Evgeny Polyakov, Sales Director at Evgeny.Polyakov@haywardtyler.com
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