Any rotating machine exhibits the effects of wear and tear over time. The function of a rotating machine is the result of the fine-tuned cooperation of many different components. The emphasis of degradation study should be on the entire rotating machine as a system rather than on isolated components. Treating the rotating machine as a system reveals the effects of degradation on the match of the components. Several mechanisms cause the degradation of rotating machines. Fouling is caused by the adherence of particles to flow path surfaces. Corrosion is the loss or deterioration of machine component material exposed to fluids caused by chemical reactions. Erosion is the abrasive removal of material from the flow path by hard or incompressible particles impinging on flow surfaces. Abrasion is caused when a rotating surface rubs on a stationary surface. Foreign objects can also cause damage. While some of these effects can be reversed by cleaning or washing, others require the adjustment, repair or replacement of components.
Fig. 1 Damage of blades and material removal in steam turbine.
Fig. 2 An example of solid particle erosion in steam turbine.
Fig. 3 Deposit on blades.
This study will cover all main rotating machines in process plants, as well as power generation industry equipment including steam turbines, gas turbines and electrical generators. Presented degradation analysis is useful in rotating machine design, selection, purchase, vendor drawing review, long term operation and maintenance technical management. It can also help in analysis of the unit health (diagnostics) and predict future failures (prognostics).
Study of degradation
Three major effects determine the performance deterioration of rotating machines: increased clearances, changes in rotating part geometry (Figure 1, Figure 2) and changes in flow path surface quality (Figure 3). The first two effects typically lead to non-recoverable degradation; the latter effect can be at least partially reversed by some corrective processes such as online washing.
Stage degradation has a cumulative effect. A degraded stage will create different exit conditions than a new stage, and each subsequent stage will operate further away from its design point. While in the new machine all stages were working at their optimum efficiency point, the degradation will force all stages to work at off-optimum efficiencies. Degradation also limits the operating range. Typically, degraded rotating machines will have reduced margins such as surge margins or stall margins.
Pressure and flow are not independent, and the efficiency is determined by the resulting operating point. Deterioration will shift the pressure-flow relationship to lower efficiency. In this study, degradation effects that normally occur together are separated. Degradation will impact pressure, efficiency and capacity, albeit to various degrees depending on the type of degradation. First it is necessary to study the individual impact of reduced efficiency, predominantly due to fouling and erosion, and reduced machine capacity due to corrosion, erosion, opening of clearances and fouling as isolated events. Since in many instances, the operating point on the map is different between the new and the degraded machines, the actual efficiency is the result of degradation and the move of the operating point.
Increase of clearances
Maintaining clearances is, in particular, a problem in rotating machines, due to various dynamic loadings and changes in temperature between a cold machine and a machine accelerating to full load or in transient condition. Some new rotating machine designs use special seals, such as abradable seals, to minimize these clearances. However, the most severe case will determine the minimum clearance for the rotating machine. Optimization is necessary for rotating machine clearance design.
Studies show increase in clearances due to degradation affects machine performance. In a study on an axial compressor of a gas turbine, the clearance was increased from 2.9% (design value) to 4.3%, and this lead to a 20% increase in surge flow coefficient, 12% reduction in design pressure coefficient, and 2.5% efficiency loss. Similarly for another air compressor of a gas turbine, an increase in the clearance from 1% to 3.5% reduced the pressure coefficient by 9%. A study on a gas turbine with blades cropped to simulate increased clearances shows the same results. A 3% crop on the gas turbine stages reduced flow by 4.6%, pressure ratio by 3% and machine efficiency by 2.5%.
Corrosion, erosion and deposits
Corrosion tends to alter the flow path in two ways. It increases the surface roughness, but it may also remove material at leading edges, trailing edge or nozzles. Increased surface roughness causes thicker boundary layers on both moving components, such as blades or impellers, and stationary components, and thus reduces the flow capacity.
Furthermore, changes in the flow capacity of the rotating machine will subsequently alter the operating point. Then corrosion and erosion will affect the total performance of the machine.
Turbines reported efficiency losses of 2.5% for a 10.2 µm surface roughness when compared with smooth-surface rotating machine components. It was also reported that the most pronounced differences appear at the optimum operating point, whereas the far off-optimum efficiency is less affected. Profile losses are in the same order of magnitude as losses because of clearances. When the degradation of the rotating machine leads to material removal, specifically in the nozzle area, complex effects and interactions may be seen. Because the flow capacity is limited by the effective throat area, erosion can cause flow angles to deviate. It will lead to reduced efficiency. Any change in the flow capacity will impact the operating point and consequently will reduce efficiency.[pullquote]
Deposits on the fluid path both increase the flow path surface roughness and may, if thick enough, decrease the flow area of the machine. Solid particles that travel through the flow path are subjected to dynamic forces. Fluid carries these particles though the rotating machines where they may impact on the machine parts and cause solid particle erosion. As these particles lose momentum they move outward toward the tips of the rotating parts and can cause erosion or lodge underneath some part. These deposits may cause a flow disturbance or affect the rotating part performance. The impact of condensate droplet erosion, such as a water droplet in a steam turbine, on rotating machine efficiency may be often negligible, but it is an important factor to consider when it threatens the machine component. The main change in flow path efficiency attributable to erosion is an increase in surface roughness of the leading edge of the rotating parts.
Mechanical damage’s negative effects are not limited to damaged parts. Any damage will cause power losses and disturbances several stages before and after the affected stage. Foreign object damage and other mechanical damage to the fluid path can cause change in flow area or a flow blockage.
Secondary effects come from repair of damages. The rotating machine components are designed to satisfy aerodynamic and mechanical conditions. Their design is limited to minimize flow separation and any kind of disturbances and losses. Repair methods such as partition weld repairs to correct solid particle erosion and mechanical damage often leave components deviated, such as thicker than the original design, when compared to new machine. These deviations introduce a flow disturbance. This flow disturbance is reflected in the velocity coefficient of the fluid and causes more loss and disturbances.
Seal and packing degradation
Seal and packing degradation and problems cause large portions of shutdowns and losses. Some typical causes for uneven top-to-bottom or side-to-side wear patterns in packing and seals are:
- Rotor to casing misalignment
- Response of the rotor to unbalance, especially when passing through critical speeds
- Differences in temperature between the top and bottom, or side to side, of the machine casing causing distortion during hot or still-warm starts and during stops
- Differences between the thermal expansions of the casing supports and rotating parts of the machine, especially in the vertical direction.
Temperature and pressure profile distortion
Deterioration could potentially lead to a variation in the machine, or each stage, exit temperature or pressure profile. The problems with a distorted temperature or pressure distribution are threefold. Local temperature or pressure peaks can affect and damage the machine sections. The altered temperature or pressure profile will increase secondary flow activities, thus reducing the efficiency. Correlation between the measured temperature, or pressure, and the true parameter is established and confirmed in site or shop performance tests. If the temperature or pressure field is altered, this correlation is no longer valid and the rotating machine, if controlled or protected by temperature or pressure, could therefore be overloaded with more power loss, but shortening its life, or under-load, thus additionally losing power.
Electrical generator and electrical systems degradation
Generator degradation may be identified as rotor problems, misalignment, unbalance, eccentricity, mechanical faults and electrical faults. Bearings are the source of trouble in generators. Electrical hardware such as coils and windings, if properly sized, will last for a long time. Insulations may degrade in a relatively shorter time. The latest techniques, such as ultra-wide band partial discharge, can be employed to identify insulation degradation. Generally electrical system degradation can be evaluated by measuring voltage imbalance, high harmonic content of currents and voltages, and other electrical parameters.
Degrading a component in a rotating machine will always lead to observable changes in machine parameters due to the impact of altered operating points, not only at the degraded component, but also for all other machine components. In a reverse sense, this finding can also be used for diagnostic purposes, because different types of degradation on different components will alter the machine in a different way. The data indicate that the rotating machine flow tends to react more distinctly to some degraded components. It can be recommended that monitoring rotating machine pressure against a reference is the best and optimum way to monitor degradation. This is also for the practical reason that flow is usually not as easily monitored, when compared to discharge pressure.
In many cases deterioration and degradation may result in increase of vibration and noise. Vibration, acoustic and noise condition monitoring methods can be effectively used to identify degradation and preventing catastrophic consequences. Typically these methods utilize instrumentation and software to acquire and analyze data to monitor unbalance, misalignment, bearing and seal deterioration, rotor bows, resonance as result of component deterioration, wear, rubs, interaction, foundation degradation, rotating component cracks, loose rotating parts, gear system degradation and similar effects.
Thermography is also a useful condition monitoring method for mechanical and electrical machines and systems. The overheating is usually because of degradation or malfunction. Temperatures in excess of expected ones can show degradation or a problem.
The focus of the study is on three areas: rotating machine degradation’s impact on total plant efficiency, on production and emission characteristics, and on measurable parameters.
The power generation company has power generation facilities in four locations. Performance degradation of rotating machines, including gas turbines, steam turbines and generators, after three years of operation were reported at 2% to 6%, depending on the machine, service and location. It caused around 4% loss of total power generation capacity for the company. This degradation caused around 60,000 tons of extra emissions per year, a loss of 250 GWh per year and more than $12 million in revenue losses per year.
For rotating machines working without a permanent inlet filtration system, erosion was one of the key contributors to degradation. Rotating machines with appropriate inlet filtration system were more subject to fouling caused by smaller particles and corrosion.
Various upgrading and renovation were performed on these power generation units including dedicated rotating machine renovation plans, extensive component replacement of degraded components, dedicated cleaning processes, installation of advanced condition monitoring systems, installation of an advanced control system with the capability to mitigate and reduce degradation effects, using the latest design and technology in critical components such as renovation and replacement using new technology, installation and provision of online washing systems, and installation of dedicated permanent inlet filters for all rotating machines.
After this renovation and revamp program, performance degradations of rotating machines were reported around 1% to 2.5%. It corresponds to less than a 1% loss of total power generation capacity efficiency, which is around 3% of total efficiency recovered by the above-mentioned correction activities.
Rate of degradation
It’s difficult to identify the rate of degradation for rotating machines, because it’s subject to a variety of operational and design factors that typically can’t be controlled entirely. Quantifying performance degradation is also difficult because consistent, valid field data is hard to obtain. Correlation between various sites is impacted by variables such as mode of operation and contaminants. Another problem is that test instruments and procedures vary widely, often with large tolerances.
Typically, performance degradation during the first 24,000 hours, or three years, of operation — the recommended interval for a fluid path inspection — is around 2% to 6% from the performance test measurements (corrected to guaranteed conditions) assumes degraded parts aren’t replaced. If replaced, the expected performance degradation can be assumed around 1.5% to 3%. An extensive cleaning, revamp and renovation program can limit degradation in the range of 1% to 2%. Maintenance and overhaul decisions are ultimately based on economic and safety considerations. Understanding performance degradation, as well as factors that influence degradation, can help in these decisions.