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Editor’s Note: This case study was originally presented at the First Asia Turbomachinery & Pump Symposium, Feb. 22-25, 2016, Singapore. Adapted with permission of the authors and the Turbomachinery Laboratory, Texas A&M University, College Station, Texas.

By Jong Kim, PhD; Marcio Felipe Dos Santos and Barry Blair

Over a 13-year span, a major South American oil company’s maintenance department fought recurring high vibrations in three gas reinjection compressor trains. To reduce the likelihood of machine trips, technicians field balanced each compressor once or twice a year and replaced the worn tilt pad journal (TPJ) bearings and O-ring dampers yearly. The downtime from implementing these preventative measures and from trips in the compressor trains resulted in 1% capacity loss each year and additional flaring of the gas.

A thorough analysis of the compressors and inspection of damaged components pointed to a two-part solution: Flexure Pivot journal bearings and an integral squeeze film damper (ISFD technology).

In 2013, the compressors were placed back in service with the optimized bearings (Figure 1).

Since then, the compressors have exhibited low vibration levels that do not grow over time. They have had no trips and have not required field balancing for continuous operation. Overall efficiency has increased by approximately 1% and site profits have improved.

Finding the root cause

Put into operation in 2000, each compressor train has two casings: a low-pressure casing and a high-pressure casing. The vibration issues occurred in the first low-pressure casing only.

The original equipment manufacturer (OEM) bearings were five-pad, load-on-pad, point-contact-pivot TPJ bearings, with a bearing outer diameter smaller than the machine casing bore to provide a squeeze film damper function. O-rings at both ends of the bearings were installed to provide stiffness.

Each direct OEM bearing replacement would reduce rotor vibrations for a time. As months wore on, however, vibrations would increase again, requiring repeat maintenance and bearing replacement.

Inspection of the removed OEM bearings showed severe pivot wear on the tilt pad and bearing shell bore (Figure 2). This wear increased the bearing clearance by 63 microns or more in a five-month period, which resulted in increasing vibrations. The inability of the O-rings to provide centering capability under static deflection, together with the increased

vibration, resulted in bearings bottoming out (Figure 3) and loss of squeeze film damper performance. These factors converged to increase vibrations over time in the compressors.

Solving the challenge

With two root causes for the recurring vibrations — pivot wear and unreliable squeeze film damper performance — a combination of two solutions was implemented.

To address the pivot wear, a Flexure Pivot tilt pad journal bearing was used. The Flexure Pivot design has a pad and pivot that is integral to the bearing shell, eliminating metal-to-metal contact between the pad and shell when the pad tilts (Figure 4). The method of manufacturing the bearing, via electrical discharge machining (EDM), provides tight control of the clearance and preload for bearing optimization. With the elimination of pivot wear, the Flexure Pivot bearing maintains the original bearing clearance and original preload (changes in which can contribute to vibrations).

Figure 2. Pivot wear on the point-contact-pivot tilt pad (left) and bearing shell bore (right).Pivot wear on the point-contact-pivot tilt pad (left) and bearing shell bore (right).The second part of the solution was the use of ISFD technology in place of the O-ring squeeze film damper. Like the Flexure Pivot tilt pad, the ISFD is manufactured by EDM as part of the bearing itself, not a separate piece. The design offers many benefits over an O-ring damper, including accurate stiffness control, optimized damping and the ability to center the rotor under static load. Moreover, ISFD technology eliminates changes in stiffness and damping over time. The integral squeeze film damper will not experience degradation or cause the bearing to bottom out, as O-rings can.

The optimum stiffness and damping of the ISFD technology for the gas reinjection compressors was evaluated through a full lateral rotordynamic analysis of the compressor. Due to the low-profile nature of the Flexure Pivot and ISFD designs, the new bearings were installed as drop-in replacements for the OEM bearings.

Results

The three upgraded compressors all showed significant improvement with the optimized bearings. Compressor A’s

vibrations dropped by more than half, Compressor B’s vibrations dropped from 90 μm to less than 50 μm (Figure 5), and Compressor C’s vibrations dropped to less than 30 μm with the upgrade. The compressors have maintained the low vibration level over time. In addition, the ISFD design has eliminated the small subsynchronous vibrations experienced with the O-ring damper.

Two-and-a-half years after the bearing upgrade, the compressors have required no field balancing, experienced no trips, and had no bearing replacements. Not only were the vibration problems fixed, but also overall efficiency of the compressors has increased by 1%, bringing improvement in site profits.

A long-term solution

A rotordynamic analysis followed by implementation of optimized bearings can reduce or eliminate vibration problems in rotating equipment, while replacing worn bearings with another set of bearings of the same design is only a stopgap that incurs additional costs for maintenance, replacements and lost production. Addressing the root cause of vibrations is the most efficient and cost-effective long-term solution.