Vibration in steam turbines

The vibration in a steam turbine is not a fault in itself , but a symptom of a problem that exists in the turbine and that could lead to serious consequences. For this reason, the steam turbines are specially protected to detect high levels of vibration and cause stop it before serious damage even happen .


Figure 0 : Interior SKODA turbine of 3.5 MW . It has taken the complete rotor to check their status . They can appreciate the different rows of blades
Vibration has many different causes, so that when it occurs it is necessary to study which one is causing the phenomenon , for, of course , correct .
The vibration is especially evident during the boot process , since during this period cross one or several turbine critical speeds , speeds at which vibration, molecular resonance , is greatly amplified . It is a common mistake not to study and correct the problem that is causing the abnormally high level of vibration and just taking any action that facilitates starting point , the damage that can occur can become very high. Normally , after a serious malfunction turbine usually gross negligence of operation and / or maintenance.
Eleven most common causes that cause a high vibration level are:
- Poor condition of vibration sensors or signal conditioning cards . It is possible that what we are considering as a vibration is actually a false signal , which originates sensor malfunction detect charged . When a high vibration trip should carefully consider the graph of vibration sensor that caused the trigger before this period (perhaps 2-4 hours before ) . An indication of the poor condition of a sensor that is usually increased vibration does not occur gradually, but the graph reflecting a very high momentary increase of vibration. Mechanically it is unlikely that this phenomenon occurs ( the instantaneous increase in the level of vibration) , so if this is observed , probably due to a spurious signal caused by the poor state of the sensor or the influence of an external element that is causing a disturbance in the measurement.- Misalignment between turbine and desmultiplicadora gearbox ( gearbox) . Is the cause of at least 20 % of cases of high levels of vibration in turbine. Although the coupling is resilient and supports some misalignment in theory , almost all manufacturers of flexible couplings recommend align it like a rigid coupling. It is important to respect the tolerances specified by the manufacturer, both horizontally and vertically , with the reducer . Also keep in mind that the line hot and cold can vary. Therefore, it is necessary to perform a cold lineup , preferably with a laser aligner ( for accuracy ) , and then performing hot alignment to see the variation. If this second is necessary to correct something, it is worth noting the misalignment that is necessary to make cold ( in the horizontal and / or vertical axis) for use in the future need to disassembly and need to repeat these alignmen
 

Figure 1. Technical testing the turbine and gearbox alignment with a laser aligner SKF
- Poor condition of coupling between turbine and reduction gear . It is convenient to perform a visual inspection of the coupling periodically (at least once a year ) and monitor developments especially vibrations
- Poor condition of reduction gear -generator coupling . This is a typical case of vibration induced by external equipment to the turbine but joined to it . The vibration is not really of the turbine, but comes from an external cause. As in the previous case , it is advisable mesh periodic visual inspections and monitoring changes in the level of vibration.
- Vibration the alternator or demultiplexer , which is transmitted to the turbine. Vibration is another case detected in the turbine but from an external device to it. The vibration in the alternator or demultiplexer discussed below
- Problem in the lubrication of the bearings, which makes the lubricating oil does not arrive correctly ( in flow or pressure ) to the bearings . Must differentiate the problems associated with flow and pressure related problems oil quality . In reference to the first , the clogging of the channels through which the oil circulates , the poor condition of the filters and a breakdown in lubrication pumps (remember that a turbine usually takes several : a mechanical pump whose driving force is provided by the turbine shaft itself , a pre-lube pump , electrical, for starters , and an emergency pump that starts before a power failure) . When the amount of oil being insufficient, the position of the shaft and bearing cyclically vary , resulting in vibration. In more severe cases , the shaft and the bearing without lubricant film touch , which causes a degradation of the shaft rather quickly .
- Poor quality of the oil. Lubricating oil, over time, lose some of its properties through degradation of the additives and is contaminated with metal particles and water. The presence of water, foams , variability of viscosity with temperature, the viscosity change in the degraded oil usually causes behind a vibration caused by the poor quality of the oil. Of these, the presence of water is the most common , so that the periodic analysis of the oil, water purging and repair the cause which makes the water entering the lubricating circuit are best preventative measures.
- Poor condition of bearings. The three bearings which usually has a steam turbine to those used in cogeneration plants ( front, rear or thrust or axial ) wear out over time, even with perfect lubrication . These bearings are coated with a layer of antifriction material , which is to be lost. For this reason, it is necessary to periodically measure the clearances between shaft and bearings, and shaft displacement , to ensure that the bearings are still in a position to allow proper operation of the turbine. These tolerances are indicated anywhere in the book of operation and maintenance that the manufacturer delivery, and must respect the measurement intervals of these gaps and change if this check reveals a problem. Proper lubrication system maintenance undeniably contributes to a longer life of these bearings , and in the same way , improper maintenance of oil, its pressures and flow rates causes accelerated degradation of these.

Figure 2: Radial bearing or support in poor condition, with marks of metal-metal frictionbetween the shaft and bearing
- Poor condition of the shaft in the bearing area . If a turbine has been running with the oil in poor condition, or poor lubrication , it is possible that your bearings are in poor condition, but may have ended up affecting the shaft. If either rub at some point , it is possible that the latter free of scratches or marks can cause vibration and damage the new bearing. In case of detecting damage to the shaft, it is necessary to repair , sanding with a rectified on-site or in workshop , supply of equipment , etc. . The best way to prevent this damage is periodic analysis of the quality of the oil , replacing if necessary , proper lubrication system maintenance , and replacement of the bearing when it detects that the clearance exceeds the limit specified by the manufacturer or when visual inspection shall so advise .

Figure 3. In the figure can be seen a turbine shaft supported on the radial bearing or support. The shaft bearing marks . Although replace the bearing , while the spindle is damaged and cause markings turbilencias present in the oil and cause the lubricant layer is not continuous and homogeneous. This will cause vibration. The figure shows very good or radial support bearing and at the bottom , the beginning of axial or thrust bearing

- Imbalance of rotor blades foulers . The imbalance is the most common cause of vibrations in rotating machinery , representing approximately 40 % of cases of vibration. An improper chemical treatment of boiler water and steam that drives the turbine ends up hurting not only this , but also the water-steam cycle and boiler. The chemical treatment of boiler water is as important as lubricating oil control : without these two points is impossible perfectly solved properly maintain a cogeneration plant equipped with a steam turbine . The first problem that will manifest improper chemical treatment is the presence of foreign particles deposited on the blades of the turbine. As this deposition will not be done equally on all rotating parts , rotor imbalance present that will result in high levels of vibration. Inlaid in the turbine blades may be caused by inappropriate levels of carbonate , silica , iron, sodium and other metals . To remove them , it will be necessary to clean the blades , which sometimes can mean severe blasting it . Following the cleansing will be required dynamic balancing of the turbine.
- Imbalance of a broken rotor blade. It is rare , but if a foreign particle enters the turbine and hits a blade can cause loss of, or damage that will affect the rotor balancing . To avoid this , you wear filters that retain objects of a certain size that can be in circulation by the steam pipes . If the filter is damaged or removed , large particles could damage the blades. Repair means replacement of damaged blades , interior cleaning of the turbine and balance . It is an expensive failure . To avoid this , make sure you can not let any element that can be in circulation by the steam pipes and steam filter is perfectly capable of performing its function. Is desirable visual inspections with one borescope or endoscope , to observe the state of the surface of the blades without disassembling the turbine housing . (See section on predictive maintenance )
In other cases the damage to blades can be caused by friction between these and fixed parts of the turbine. In these cases the origin of the fault could be bad bearings or thrust support that made the rotor shaft position was out of specification. The symptom that reveals he is having a problem is a high level of vibration. If there is a high vibration level and still keep the turbine running, you are leaving the door open for this serious failure occurs .

Figure 3. Scuff marks from both fixed and moving parts of the turbine can apreviarse in this figure. Scratches can be seen in the ' shroud ' or ring that protects the outer part of the blades . Those scratches and scuffs these were responsible for the vibrations that are appreciated in this tubina of 3.5 MW , SKODA brand , installed in a production of electricity from biomass
- Imbalance in the wrong rotor dynamic balancing , or for loss or damage to any member rotating (screws , washers, nuts ) . The imbalance may be a fault of origin (the initial balance of the turbine was poor ) or may be a failure occurred . In the second case , it is important to carry out repairs on the turbine rotor leaving no element unassembled or assembled improperly. It is even advisable to number the screws and washers for mounting exactly dismantled . If the shaft is damaged , you have to make amends by providing materials , grinding , cleaning , sanding , etc. . It is convenient to have a spectrum of vibrations from the commissioning of the equipment . This first spectrum will be useful , and always will be a reference to whether initial or no problem occurred .
- Curvature of the rotor due to a hot soak with toner system stopped. Steam turbines are equipped with a system that facilitates jacking the shaft does not bend when hot. The mission of this system is to redistribute the weights uniformly over the axis of rotation, and prevent the rotor unbalance curvatures . If the turbine is to warm and toner does not enter the system up shaft may be bent upward . The problem is always detected when trying to boot , and check that the vibration level is higher than allowed . If so , the best solution is to keep the turbine spinning with no load and a lower speed than the nominal for several hours. After this time, if this is the cause of the problem , the vibration will be gone and back to normal.
- Permanently curved shaft . The shaft can be bent permanently, ie non-recoverable deformation following the procedure outlined in the previous section . Not easy to happen after the initial startup of the turbine, and is usually due to a failure existing and coming from the manufacturing process. Typically, the dynamic balance has masked the problem , although in the initial spectrum of vibration, it is advisable to make the start of the operation of the equipment , is sure to be present.
- Crack in the shaft. Sometimes a shaft surface defect advances and ends up becoming a fissure or crack , causing an imbalance in the shaft. It can happen by a manufacturing defect in the shaft (most common) or may be related to the rotor corrosion may be suffering . When this occurs , it is detected through vibration analysis, and in most cases are visible to the naked eye or with the aid of a magnifying element . The solution is often to change the rotor axis, although in some cases it is possible to repair companies specializing in this type of work in specialty metals through sanitation , supply of materials, grinding and stress relief treatment . It will be necessary to conduct a balancing shaft. As a preventive measure to prevent corrosion that convert a surface defect in a crack or fissure , is the chemical control of steam turbine.
- Corrosion or fouling of the shaft, blades , etc. . If steam conditioning was not adequate, can cause corrosion on the blades or loose foreign material from the turbine in these. Such fouling and corrosion by modifying the turbine unbalanced weight distribution along the rotational axis. When this occurs the solution is cleaning the rotor assembly by blasting or mechanical cleaning. It is normally necessary to remove the rotor and cleaning these out of the turbine. In case of embedding , it is desirable to sample and analyze the deposited materials , to know the origin of the foreign particles and take the appropriate corrective measures . After cleaning the shaft , it is necessary to balance it again. The best preventive measure is to perform a careful chemical control in the water contribution in the degasser , in the condensed water in the boiler and steam.
- Presence of particles in the water or steam. If the input steam turbine has liquid water particles , the collision of the drops against the turbine can cause vibrations and imbalances. Vapor may contain liquid water overheating failure by a tempering excessive because tempering valve is in poor condition , or because on the road between tempering valve and turbine entry to suffer abnormal cooling . If this occurs it is necessary to detect and correct as possible, as will cause erosion of turbine blades , and will be damaged. Vibration analysis and inspections Boroscope help in the task of early detection of the problem. The solution is inevitably correct the problem that is causing the presence of water vapor.
- Failure on the bench. A bench poorly designed or poorly executed can cause vibration . When vibration is detected , you should first check the state of the bed , trying to find cracks , lack of equipment, etc. . If vibration is present from the start and have ruled out other causes , it is very likely that the problem is related to the design or implementation of the bench . The solution , in this case , will review the design of the bench , and if this is correct , run it again.
- Defect in subjection to the bench. Although the bed can be properly executed , the turbine can not be conveniently subjected to this . This may occur because the screws do not have the proper torque or because the screws are not properly anchored to the bench . This failure is much more common than it may seem. Some authors call this failure ' pedestal lame ' , and vibration analysis reveals this decision relatively easy . When this problem occurs, it is observed that by loosening one of the screws ( the one causing the problem) strangely vibration level decreases.
- Voltage steam pipes . If the pipeline alignment is not perfect or are not properly considered the thermal effects of the expansion , pipe stress can be induced to make that strange force is exerted on the turbine housing . These forces may cause vibrations , inter alia . The steam inlet pipe in small turbines usually flexible , and the output is usually equipped with a compensator that connects the turbine housing to the outlet pipe . To check for any problem in this regard , it should drop the inlet and outlet pipes and check which is your natural position without being connected to the turbine.