ARE YOU AFRAID OF GEARBOXES?

I(Brun) recently worked with a large oil & gas company on the installation of a multi-train 10,000 hp booster compression application. For this application, the operators preferred to utilize direct-driven centrifugal compressors. However, after some initial driver speed match analysis, it was realized that there was no obvious compressor train solution that could avoid the use of a gearbox.

Consequently, the company decided to re-evaluate the use of high-speed, directdrive electricmotors.After a lengthy evaluation, it became apparent that the disadvantages and risks of this solution exceeded those of using a conventional electric motor with a gearbox. There are also many instances in which the user insists on avoiding a gearbox, and instead accepts lower aerodynamic performance, bigger size, and higher cost of the driven compressor.

Clearly, there is bias, if not fear, in the industry against using gearboxes in compression applications. Although it is inherently obvious that one should always minimize the number of rotating machinery pieces mounted in a compression train, avoiding gearboxes at the cost of performance, or at the risk of utilizing much more complex and often unproven technologies, may not be the wisest approach.

Gearboxes have been around since the advent of the industrial revolution and there are tens of thousands of gearboxes that successfully operate over long periods of time in high horsepower, high-gear ratio, and highdriver speed applications. For example, just in the population of gas turbine driven compression applications above 2000 hp, there are an estimated 6,000 gearboxes in operation worldwide.

Most small and mid-size generator sets also use gearboxes and, although they are not variable speed, they still have to cross the full speed range during startup and shutdown. Therefore, it is surprising to find such a strong negative bias among turbomachinery experts against a technology that is mature and performs well overall.

When digging deeper, one often finds that this bias originates from a few “horror stories” that compressor operators have experienced with gearboxes. What is often overlooked is that the underlying cause for most of these horror storieswas not the gearbox design, but poor application engineering or inadequate maintenance of the gearbox. Also, over the last few years, we have seen many high-speed electric motor problems, either with the motor itself or with the variable frequency drive (VFD) and far fewer failures with gearboxes.

Gearbox horsepower losses are often cited as a reason for avoiding them, but these losses tend to be similar in magnitude as windage and cooling losses in high-speed electricmotors. It also should be emphasized that using a gearbox often allows the customer to optimize the driven compressor for the application, thus getting a smaller, aerodynamically better matched compressor. There are many instances where the better compressor performance more than makes up for the gearbox losses.

Let’s discuss some fundamentals of gearboxes in turbomachinery applications. There are many types of gearboxes, but the most commonly used are in compressor drives that are parallel shaft or epicyclic (planetary) gears. Beyond that, there are several design sub-variations which are primarily related to the shape of the individual gear teeth, such as spur, helical, or herringbone (double helical); or the general arrangement of the gear wheels, such as bevel, worm, or hypoid.

Parallel shaft gears are used for lower gear ratio and higher power applications, while epicyclic gears can handle high gear ratios in a single stage, although most applications are limited to below 50MW. Critical design criteria for proper gear selection are power, gear ratio, shaft speeds, and sometimes less critical issues, such as weight, space availability, noise, lubrication type, and design life requirements.

Gearbox life

Gearbox life limitations are primarily mechanical in nature. Gear or tooth highcycle fatigue due to torsional vibration or misalignment, overload due to excessive power, wear caused by rubbing and elastic deformation, and overheating due to inadequate lubrication are typical causes. Sometimes corrosion may also affect the life of a gearbox, but this almost always relates to poor lube-oil quality.

Most gearbox operational problems occur during startup of a new installation and are oftenrelatedtotorsionalvibrationissues.When designing and approving a gearbox design for a particular compression application, the most critical engineering step is to perform a proper torsional rotordynamic analysis.

This type of analysis can be performed by all major manufacturers and many thirdparty providers, and is generally accurate. The biggest challengewith a torsional analysis is that some of the input parameters to perform the analysis, such as operating range, coupling stiffness and damping, shaft geometries, and rotating inertias, may not always be available or the sub-supplier may not be willing to share them.

But if the torsional analysis is performed properly, one can usually design the train to avoid running on any of the torsional critical speeds, avoiding most mechanical fatigue and earlier failure problems.

The other issue that often leads to failures of gearboxes is inadequate or improper lubrication. Improper lubrication can be over or under lubrication, usage of the incorrect type of lube oil, or lube oil degradation. Utilizing the manufacturers recommended lube oil, lube oil flow rate, and maintaining lube oil quality through regular testing is critical.

However, in most gas turbine driven compression applications, the gearbox lubrication is integrated with the gas turbine lubrication so lube oil quality control should not present any additional maintenance burden to the operator. Furthermore, having proper temperature, pressure, and vibration instrumentation installed in the gearbox allows for the early identification of operating problems and troubleshooting.

Finally, gearbox failures have been caused by simple issues such as using an undersized gearbox for the application or gross shaft misalignment. These are engineering and installation problems and, as with other machinery issues, can be avoided with proper quality control. Based on millions of reliable hours of field operation, it is clear that it is possible to design and integrate a gearbox into a high-horsepower compressor train that will function well and operate for the design life of the train without significantly impacting performance or maintenance activities. But it is imperative that the design and application engineering, including a torsional rotordynamic analysis are properly done for the compressor train prior to fabrication and assembly of the equipment.

Authors

Klaus Brun is the Machinery Program Director at Southwest Research Institute in San Antonio, Texas. He is also the past Chair of the Board of Directors of the ASME International Gas Turbine Institute and the IGTI Oil & Gas applications committee.

Rainer Kurz is the manager of systems analysis for Solar Turbines Incorporated in San Diego, CA. He is an ASME Fellow since 2003 and past chair of the IGTI Oil & Gas applications committee.