The decision of whether to use a gear unit can significantly affect the operation, performance, reliability, and maintenance of machinery. For nearly all machinery applications, operators in industry prefer to utilize direct-driven rotating machinery trains and avoid gear units. In other words, there is a bias against using a gear unit in any rotating machinery.
This article explains the scientific/engineering reasons and operational knowledge/experience for this preference, and also explains the risks and issues with a gear unit. In addition, special cases where gear units should be used are briefly discussed.
The key takeaway of this article is to only use a gear unit (or gearbox) where really needed; and, if a reasonable direct-drive train is achievable, do not use a gear unit.
Gear units in machinery trains
Gear units have been around for many years, and there are tens of thousands of gear units that operate over long periods of time in a wide range of sizes and power-ratings. There are also many gear units for high-gear ratios (say above “10”), and high-speed applications. It is estimated that more than 10,000 gear units are in operation for medium/large size machines worldwide. Risks, operational issues, and high capital/operational costs associated with gear units have been accepted as a fact in many plants.
Many operators know that there are operational issues, painful installation/commissioning procedures, delays in commission/start-up, numerous unscheduled shutdowns, huge costs, and other risks associated with gear units. Because of all these issues and risks, there is a feeling or a preference in operations teams worldwide to avoid gear units. However, there is a perception that there is no other solution among many other engineering and operations teams.
On the other hand, vendors and consultants who select machineries and offer gear units as the best solution will not have to live with their decisions and will not face any gear unit problems first-hand. It is the operators and maintenance teams who will face all of these issues. The usual story is that a vendor or a machinery consultant, after some initial evaluations (particularly a driver speed match analysis), informs an operator or purchaser that there cannot be an obvious machinery train solution that avoids the use of a gear unit.
For a high-speed application, many vendors and consultants still offer a conventional electric motor with a gear unit. This is particularly the case for a high-speed compressor train such as a high-speed centrifugal compressor. In other words, there is an unjustified bias against high-speed, direct-drive electric motors. The use of high-speed, direct-drive electric motors should more positively be evaluated. The risk and possible operational issues of modern high-speed, direct-drive electric motors (those with successful operating references) are usually less than the problems and issues associated with gear units.
For low-speed machinery trains, the gear unit application can be easily avoided. Proper driver types could be selected for machinery options to avoid high-speed drivers for low-speed driven machineries. Steam turbine drivers should usually be limited to direct-drive applications. For example, a high-speed steam turbine driver for a reciprocating compressor or a screw compressor using a gear unit for speed-match is usually a poor choice which can usually be avoided. Using a gear unit with a gear ratio of “15” to “25” could sometimes be offered by some manufacturers (or consultants) to match the speed of a high-speed steam turbine driver with a low-speed reciprocating compressor. This is a complicated train which potentially can cause many torsional, dynamic, and operational issues.
Gas turbine models should properly be selected to facilitate the direct drive machinery configuration. Usually proper models of aero-derivative gas turbines can offer direct-drive trains (usually with variable speed capabilities) compared to old-fashioned frame-type gas turbines which could be used only in very limited speed ranges.
The challenges of gear units
Many gear-unit manufacturers and gear-unit supporters (engineers or advisors that support gear units) have accepted the issues and failures associated with gear units. Engineers who support gear unit applications usually confirm the existence of operational problems and challenges with gear units, but then may claim that the underlying cause for most train failures in gear driven machinery is not the gear unit itself, but instead poor “application engineering” or “inadequate maintenance” of the gear units.
Short investigations into these two areas soon reveal that:
- A gear unit is a very complex piece of equipment that needs a very careful application. Any small mismatch or carelessness could lead to an issue. Obviously, there are many risks.
- A gear unit is a sensitive item which requires special commissioning procedures and intensive and costly maintenance.
Gear unit power losses should also be considered. The reality is that gear unit power losses are not so high that the application of a gear unit would be discouraged. However, major problems include operational issues, difficulties in commissioning, delays, and more importantly unscheduled shutdowns. Generally, all risks and operational costs associated with gear units cannot be properly incorporated into simplified commercial evaluations.
The operation of gear units, even normal operation, is always associated with wear because the gears are sliding and rubbing most of the time. The low life and high wear rates of gear units should also be noted. The following damaging modes need to be considered:
- Low or high cycle fatigues can affect gear teeth because of torsional vibration, misalignment, or overload.
- High wear rates can be caused by rubbing and excessive deformations.
- Overheating issues, typically because of inadequate lubrication, have caused many problems in gear units.
- The complex lubrication oil problems associated with gear units.
Complex Torsional Problems. Gear unit driven machinery trains are usually subjected to complex torsional issues. When designing and applying a gear unit for a particular rotating machine, one of the critical engineering steps is to perform a proper torsional analysis. One of challenges with a torsional analysis is that some of the input parameters to perform the analysis, such as coupling stiffness/damping, shaft geometries, and rotating inertias of each piece of equipment involved may not always be available, or the sub-supplier may not be willing to share them.
Inaccurate data have caused poor torsional evaluation and consequently have resulted in many operational issues or even gear unit failures. A resonance for a torsional critical speed (particularly the first torsional critical speed) may result in a high vibration, and this high vibration may cause a high vibration trip (unscheduled shutdown). If the trip level is increased to let the machinery operate, then mechanical fatigue or premature failure may occur.
- Read "Bearings are not boring"
Complicated Issues with Lubrication. Another major problem that usually leads to failures in gear units is inadequate or improper lubrication. Improper lubrication can be related to the lubrication oil flowrate, an incorrect type of lubrication oil, or lubrication degradation. It is critical to use the correct lubrication oil type, proper oil flowrate, and suitable oil quality for any gear unit.
Additionally, the lubrication oil of all machineries in a gear unit driven train is usually supplied by a common lubrication oil system. A major issue is that a compromised lubrication oil type or a compromised oil skid could be selected to serve all equipment in a train, which may severely impact the gear unit reliability.
Sizing, Selection and Commissioning. Many gear units have been integrated into large machinery trains and are currently in operation. What is not usually mentioned are the huge efforts, lengthy delays, and high costs associated with these gear-unit driven applications. The painful and lengthy commissioning, operation, or maintenance activities for gear units have been usually forgotten.
Gear unit sizing can be a major issue contributing to gear unit failures. Undersized gear units have caused many operational problems. Also, the high vibration, high noises, and significant dynamic forces for many gear units should be highlighted. Sometimes, the vibration at no-load or part-load could be more than the vibration at the full-load. This may seem a bit strange, but this is the case for some gear unit driven machineries.
Gear units can be eliminated in many machinery trains and should usually be avoided. Gear units should only be used where other direct-drive solutions cannot be employed. An optimum configuration should always minimize the number of rotating machinery pieces mounted in a machinery train, thereby avoiding gear units. This optimum solution could reduce significantly the capital cost, operational cost, and maintenance cost for the asset.
This case study is about serious commissioning problems and lengthy start-up delays associated with a steam-turbine driven reciprocating compressor train. The reciprocating compressor was a four-cylinder four-crank high-pressure compressor with a speed around 300 rpm. The train power was around 4.2 MW, which was a large machine for a critical and high-risk service that increased the gas pressure from around 8 Barg to above 130 Barg in four stages (four cylinders). The steam turbine driver was provided to operate at around 5,300 rpm and a complex gear unit was employed to match the steam turbine speed (around 5,300 rpm) to the reciprocating compressor speed (approximately 300 rpm). The gear unit was a complicated one with a gear ratio around “17”.
During the no-load test, the compressor train was tripped because of high vibration. The alignment, the levelling, and the gear mesh patterns were checked and everything was in order. The vibration was gradually increased when the turbine started. The measured vibration was around 68 microns at around 2,500 rpm. At around 4,100 rpm, a high vibration level of 173 microns was recorded, which exceeded the trip level. This high vibration was around 2.5 times the vibration of 68 microns at 2,500 rpm. The compressor was shut-down by the automatic control system.
By analyzing the recorded vibration, the root-cause of the high vibration was found. There was a resonance with the first torsional critical speed due to the complexity and unpredictability of the torsional behavior of the train using the gear unit. The solution was to change the coupling set in order to avoid the torsional resonance.
The reported problem in this case study can be a common issue for steam turbine driven reciprocating compressors. This issue could be strange to some operators because many electric motor driven reciprocating compressors (i.e., direct drive trains) have been employed in different plants worldwide without this kind of issue.