Design reviews and inspections of gear units, couplings and auxiliaries, from the latest technology systems to old-fashioned equipment, are vital to ensuring peak performance and long-term reliability.

Amin Almasi

In the current economic climate, the need to keep operating costs low but efficiencies high are greater than ever.

So the operators of power generating trains must evaluate every component in terms of lower costs, simpler maintenance and higher availability. Yet with the approach to selection, design and fabrication of power generating trains and their auxiliaries – including gear units and couplings – evolving apace, operators must keep up with each evolution.

Gas turbine drivers are the backbone of the power generation industry. Whether in simple-cycle or combined-cycle power units, gas turbines allow for immediate starting capability, ideal where considerable variations in demand are expected and a fast response is required.

Aeroderivative gas turbine emergency units can come online very fast. Micro-turbines in the 50 kW to 500 kW range are used in offshore units and remote onshore locations, while steam turbines are commonly used in combined-cycle power plants where hot exhaust gas (gas turbine exhaust) is used to generate steam and drive steam turbine-generator trains. Good examples of best steam turbine use are geothermal sites, coal fired power plants and nuclear power plants.

Small power generation units are used in reciprocating engines, such as diesel and gas engines, which while very efficient, require relatively high maintenance.

Gear units for power trains

Gear units are used in power generation trains that require a speed match between driver and generator. Gear operation is a combination of rolling and sliding motions: at first contact between two teeth, the motion is mostly sliding, but as the two pitch circles become closer, more rolling occurs.

When the pitch circles intersect, and the teeth are on the centre-line between the two shafts, the contact is all rolling. Then, as the teeth go out of mesh, there is progressively more sliding. Gear units are designed, manufactured and selected according to some important rules and some very useful codes. The gear unit bearings are based on a certain hydrodynamic or rolling element design, while the gear teeth have to withstand the operating fatigue stresses. These stresses are complex to calculate and evaluate. Extensive simulations, studies and rigorous design reviews are required for gear units.

Figure 1: An example of a very complex modern parallel-shaft gear unit.

A special purpose gear unit is like a ‘black box’ for many power engineers. In general, parallel shaft gear units are always preferred. Double helical, one- or two-stage gear units of parallel shaft design are the preference of reliability-oriented operators. There are competent manufacturers which offer high power parallel shaft gear units. High gear ratios, as high as 20, are also available in the market.

An epicyclic gear unit is generally not recommended. The epicyclic unit may be considered as a special case if the desired result is for a critical reduction in the overall size and/or weight of a power generation train. Care is needed when pinion speeds higher than 3000 rpm, pitch line velocities more than 20 metres/second (m/s), or journal velocities above 7 m/s are used. Generally, critical power generation units are ‘un-spared’ and it imposes higher expected reliability on every piece of equipment in train, particularly the gear unit, so ample rating is always encouraged.

The gear unit rated power is identified by the driver nameplate rating power multiplied by a suitable application and service factor. Gear units located next to the driver should be capable of continuously transmitting the rated driver power, multiplied by an application factor (minimum 1.2). As an indication, service factor 1.4–1.7 is usually specified for gear units. An accurate gear service factor depends primarily on loading irregularities, particularly the type of driver.

The gear unit’s internal coating is the subject of debate and internal coating is not recommended. Gear units are sized for proper resistance to tooth pitting. Considerations such as the radii of the curvature of the contacting tooth surfaces, dynamic load safety factors, maldistribution of tooth loading across the face, and the strength of the materials in terms of pitting resistance are important. Pinion and gear wheel material hardness have a considerable effect on trouble-free and smooth operation, reliability and noise.

Figure 2: An example of a modern parallel-shaft gear unit (double, one-stage).

In gears that work under heavy load and very high sliding velocities, depending on conditions, the lubricant film may not be sufficient and the result is a localised damage known as ‘scuffing’. Scuffing will exhibit itself as a rough finish. The risk of scuffing damage varies with the material of the gear, the lubricant being used, the viscosity of the lubricant, the surface roughness of the tooth flanks, the sliding velocity of the mating gear set under load and the geometry of the gear teeth. Improvements in any of these factors can reduce the risk of scuffing.

When gear pitch line velocities exceed 120 m/s, special design considerations are given, such as deep sump requirement (of perhaps 500 mm depth). For gears at pitch line velocities above 150 m/s, additional requirements are recommended to ensure smooth and high-quality gear operation such as material cleanliness, superior material mechanical properties (such as hardness, strength and fracture toughness), and extra ultrasonic and wet magnetic capabilities.

For critical high-power gear units, gears are usually integral forged with their shafts. Single-helical gear units are rarely used in critical power services. Tooth accuracy is an important manufacturing factor. Large changes in diameter from the gear’s outside diameter to shaft or bearing diameters can create difficulties in manufacturing and heat treating that may lead to unexpected stress concentrations and material flaws. These deficiencies are usually difficult to detect. When operating at high pitch line velocities or high powers, these defects can lead to sudden failure. Shafts with diameters of 200 mm and larger are forged while shafts below this size may be from hot-rolled bar-stock, with same quality and heat treatment criteria as forgings.

Coupling for power trains

Correct coupling selection is a difficult task since detailed installation and operation procedures, torsional analysis conditions, train arrangement considerations and thermal growth situations should be matched with available coupling models, as well as commercial conditions.

The preferred coupling type is almost always a metallic flexible-element coupling known as high torsional stiffness coupling. The coupling is selected mainly based on the train loading and required misalignment. Correctly selected coupling is capable of transmitting the maximum steady-state torques and cyclic torques as well as the maximum transient torques under all conditions of angular misalignment, axial displacement, speed and temperature variation, simultaneously, to which the train will be subjected in service.

The maximum angular misalignment is very important. It is specified with respect to expected misalignment during start-up, normal operation and shutdown of the power generation train, based on known effects from thermal, pressure and dynamic forces.

Another important factor for coupling selection is maximum axial displacement. It depends on the amount and direction of the relative movement of the shaft ends towards or away from each other in various operating conditions. As a rough indication, the minimum steady-state axial deflection capability is determined by the largest shaft diameter divided by 125. For example, if the largest shaft diameter in a train is around 500 mm, steady-state axial deflection around 4 mm is expected.

The coupling service factor is very important and allows for various modes of off-design operation that can result from factors such as a change in power demand, unequal load sharing between generator units, fouling, torsional oscillations, driver output at maximum conditions or possible future up-rating.

A good example of future uprating is augmentation of the power output of a gas turbine by water injection. For metallic flexible-element couplings, the minimum service factor is recommended as 1.5. Coupling capable of withstanding the cyclic torque associated with start-up or transient conditions is required particularly for drivers that introduce transient load variations such as reciprocating engines.

The worst transient case for power generation trains is usually generator short circuit. Limited life fatigue stress analysis with respect to transient cases is carried out to verify coupling selection suitability. For initial coupling selection, a large cyclic torque requirement is typically assumed until all conditions are known so that the torsional response analysis can be completed. Sufficient spacer length (known as shaft separation) is required for removal of coupling. It allows for the maintenance of adjacent bearings and seals without the removal of the shaft or disturbance of the train alignment.

The distance between shaft-ends of about 450 mm or more is preferred for trouble-free installation and maintenance. As a rule of thumb, for low speed (around 1500–3600 rpm, generator coupling) and high speed couplings (above 6000 rpm, coupling between high speed driver and gear unit), an engineer may consider potential mass-centre displacement of 50 μm and 20 μm respectively.

Metallic, deformed-thread, self-locking fasteners are only acceptable for special purpose couplings. Keyless connection methods (coupling hub type) are always preferred. Attention is also needed for coupling guard details.

Coupling guards need a rigid, liquid-tight and spark-resistance design. Coupling guards should withstand a 100 kg static load with a deflection of less than 0.0005 times the unsupported length. Proper radial clearance, of perhaps a minimum of 25 mm, is required. The coupling guard thermal analysis also needs attention, with the preferred guard operating temperature below 70 °C.

Otherwise personnel protection from contact with the coupling guard may be considered. Oil spray to control temperature is not recommended. A useful hint for operation inspection is to ask the coupling manufacturer to study and indicate which component(s) should be inspected or replaced following the occurrence of torque greater than the peak torque rating. Some metallic-flexible couplings, such as single element convoluted diaphragm styles, can exhibit an un-damped response to forced axial vibration. Multi-disc, multi-diaphragm and non-convoluted single element flexible couplings typically do not exhibit such axial vibration behaviour.

Axial movement and thrust load study

The axial movements of the generator rotor have to be limited by locating the proper axial (thrust) bearing in the power generation train, whether in the driver unit or in the gear unit via the coupling.

Suitable coupling should be used and proper limits set. The generator has to be aligned magnetically. If the generator-rotor is pulled out of the magnetic centre during operation, considerable axial forces occur, which can cause problems.

Figure 3: An example of gear coupling. Gear couplings were used extensively many years ago. They need lubrication oil and also they require extensive maintenance due to a high rate of degradation and sometimes short life. Now they are replaced by high torsional stiffness couplings or other dry type couplings and today gear couplings tend only to be used in very special applications.

There are several reports of trouble in power generation trains regarding ‘limited-end-float’, particularly for trains using multiple-disc-pack type couplings. Considering available-in-market coupling and generator technologies, train axial movement may exceed generator allowable axial deviation from the magnetic centre. Extensive studies and reviews are required to ensure coupling selection meets the specified generator and train requirements.

There are usually two main concerns: the train rotor can make excessive force on the axial end-stop(s) of the thrust bearing(s); axial movement is usually limited by the axial bearing via coupling. Usually couplings don’t have a fixed axial clearance, but the flexible elements have axial spring stiffness. An axial vibration study may be required.

Complex power generation trains contain several machine casings and gear units. A good example is trains using low-pressure and high-power steam turbines and gear units to drive a large generator. Axial alignment is critical in complex power generation trains. It ensures that during operation, when operating temperatures have been reached and all final thermal expansions have taken place, the magnetic centre of the generator rotor is in proper position.

This is done by proper coupling selection and giving couplings defined pre-stretches. Train design and installation using coupling pre-stretches and correct alignment procedure should ensure that the generator-rotor stays in magnetic centre position during cold operation as well as hot.

During cold assembly, with the generator stopped, the generator-rotor will stay in the magnetic centre position, regardless of temperature changes, because of the large axial friction forces in the bearings during standstill compared with the coupling spring forces. At standstill after shutdown, the rotor will shrink during cool-down. Each rotor is supported by the two radial bearings with the fixed point at one side. Generally, rotor geometry is asymmetric and the bearing which offers the higher load (larger friction force) can be considered as fixed point.

Condition monitoring

Vibration monitoring systems are required for the low-speed side (generator side) as well as the high-speed side (driver side). Axial displacement is measured with proximity transducers. The common configuration is two probes mounted on the shaft end to monitor the axial displacement; two probes on the low-speed shaft of the gear unit; and two probes on the driver shaft. For diagnostic purposes, key-phasors are generally mounted, usually one on the low-speed shaft and one on the high-speed shaft.

Gear unit casing vibration measurement is critical. It is necessary to note that in direct-drive power generation trains, even a serious torsional vibration could be present without any signal in the casing or shaft vibration monitoring system. In other words, torsional failure may occur without any indication of abnormal casing vibration, shaft lateral or axial vibration.

But this is not the case in power generation trains with gear units. Torsional vibration can be identified by correct vibration monitoring of gear units. The best recommendation is to use the complete set of 3D accelerometers and velocity-meters for gear units.

Bringing together all these elements highlights the challenges of optimising power generation train design and arrangement. A complex combination of gear units, couplings, auxiliaries and cooling systems is required for power generation trains and all components, from the latest technology systems to old-fashioned equipment, should be subjected to comprehensive design review and inspection programmes to ensure the train’s performance and long-term reliability.

Amin Almasi is Lead Rotating Equipment Engineer at WorleyParsons in Brisbane, Australia

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