REDUCING VIBRATION

AN INTEGRAL SQUEEZE FILM DAMPER REDUCED DOWNTIME BY ADDRESSING

IMPELLER VIBRATION

By BILLY TALBOT, JONG KIM & ALLAN MATHIS

Figure 1: Bearing with integral squeeze film damper installed on the rotor of an overhung blower[/caption]

A Dow Chemical Company specialty chemical operation in Deer Park, Texas covers 700 acres along the Houston Ship Channel and relies on a fleet of rotating equipment to meet production demands. However, an overhung blower that was part of the sulfuric acid unit required shutdown every three to four months for cleaning and rebalancing. The resulting downtime and constant demands for maintenance had a severe impact on plant productivity. The trouble was traced to an impeller in the blower.

Mass imbalance

Product buildup on the machine’s impeller led to mass imbalance and high synchronous vibrations that, if not addressed, could cause catastrophic bearing machine failure. Every few months, therefore, the machine had to be shut down for four days for maintenance. To eliminate frequent cleaning and rebalancing, plant personnel looked for the best way to dampen critical speeds, reduce dynamic bearing loads and improve vibration response. The solution was the installation of an integral squeeze film damper (ISFD) on the impeller end of the shaft to reduce vibration amplitudes, make the machine less sensitive to imbalance and shift critical speeds.

ISFD technology integrates the damper and bearing and provides engineered stiffness and damping in order to change rotordynamic behavior. For the blower at Deer Park, this technology was paired with a low-profile Flexure Pivot tilt pad bearing (Figure 1). Prior to installation, a lateral rotordynamic analysis was carried out. Baseline rotor models were created without the integral squeeze film damper to provide a comparison with field vibration data. Mapping of the first forward and backward rotor modes, stability analysis, the synchronous response to imbalance, and dynamic bearing loads on the models all matched field data.

Figure 2: Rotor modeling of overhung blower with new bearing[/caption]

With confidence established in the rotordynamic modeling through this baseline analysis, modeling was conducted on rotor performance with the proposed ISFD technology/Flexure Pivot bearing on the impeller end only (Figure 2). A support stiffness of 250,000 lb/in was selected so that the radial deflection at the impeller end bearing (for 2,038 lbs of bearing reaction force) was approximately 8 mil. At this stiffness, the optimum damping value for the first mode was approximately 2,300 lb-s/in. Based upon this combination of stiffness and damping coefficients at the impeller end bearing support, analysis showed elimination of the critical speed peak in the machine’s vibration response at both the impeller and coupling ends of the shaft (Figure 3).

Figure 3: Comparison of synchronous response to rotor imbalance[/caption]

Analysis indicated that the new system was less sensitive to imbalance loading from product buildup on the impeller, which would increase the mean time between maintenance. Field results The ISFD technology was then installed in the field. Dow tested the bearing with 31 gm imbalance (at 17" radius) to obtain a forced response.

Field results confirmed that the technology reduced synchronous vibration response on both the impeller and the coupling ends even though ISFD technology was applied to the impeller end only. In regular operation, the first critical speed was completely dampened. In contrast, the original bearings experienced vibrations of up to 5-6 mils when process carryover occurred and product built up on the impeller. The optimized stiffness and damping with the ISFD technology absorbs the energy of the vibrations to allow continued operation without damage to the bearing (Figure 4).

Figure 4: Vibration levels before and after the upgrade[/caption]

With more than three years in operation on the original blower and two more bearings installed on sister blowers at Dow’s Deer Park operation without a shutdown, it is estimated that ISFD technology has saved Dow around 1,200 hours of production and eliminated the associated maintenance cost of opening and closing the equipment, hydro blasting and field balancing.

Authors: Billy Talbot is Sales Manager at Bearings Plus (BPI). With more than 3,200 bearings supplied with ISFD technology, BPI provides bearing repairs and replacements, custom technology upgrades and rotordynamic analysis. For more information, visit www.bearingsplus.com.

Jong Kim, PhD, is Senior Consulting Engineer for Bearings Plus and Senior Principal Engineer at its parent company, Waukesha Bearings.

Allan Mathis is Rotating Equipment and Vibration Engineer for Dow Chemical Leverage Services for the Houston Hub. Mathis holds a BS in Mechanical Engineering, an ISO Certified Category III from the Vibration Institute, and has more than 25 years of experience in solving rotating equipment and piping problems. For more information about Dow Chemical, visit www.dow.com.