From sensors covering every steam turbine measurement to centralized or distributed machinery protection systems to powerful condition monitoring software to a broad portfolio of supporting services delivered by deeply experienced professionals, vibro-meter solutions are engineered to ensure you get the most from your steam turbines, whether a 150 kW single-stage unit in mechanical drive service or a 1200 MW compound unit in nuclear power generation service.
Moving-Coil Velocity Sensors
Accelerometers with integral electronics
Housing Expansion LVDTs
Valve Position Transducers
RTDs and Thermocouples
Magnetic Pickups (Variable Reluctance Sensors)
A hallmark of vibro-meter’s expertise with sensors is the ability to support either voltage-mode or current-mode signal transmission, affording you more flexibility. Current-mode transmission provides many advantages including longer field wiring distances, lower installation costs, better signal-to-noise ratios, and superior immunity to interference. Our sensing solutions are robust – specifically designed for the harsh environments that are associated with industrial use. And, our sensor solutions provide the flexibility of industry-standard outputs that allow them to be used with our own monitoring systems as well as others.
vibro-meter is the only supplier that provides an option for either current-mode or voltage-mode outputs on proximity measurements, allowing more flexibility and the ability to transmit signals over longer distances without interference. We are also one of the few suppliers to offer all of the proximity probes needed for steam turbines, covering the full spectrum of applications from conventional measurement ranges for vibration to the extended ranges needed for differential expansion.
Used on steam turbines for radial vibration, shaft axial (thrust) position, differential expansion, rotor expansion, rotor position, speed, zero speed, rotor acceleration, phase reference, and zero speed. Models with extended ranges and/or flange-mount form factors are available for applications such as differential expansion where the linear range of a conventional probe is insufficient and a right-angle cable exit is ideal. Proximity probes can also be used for overspeed measurements and because they are powered and use a bias voltage, they provide inherently superior self-check capabilities compared to passive speed sensors such as magnetic pickups.
vibro-meter is the only supplier that provides an option for either current-mode or voltage-mode outputs on many of our velocity sensors, allowing more flexibility and the ability to transmit signals over longer distances without interference. Our velocity sensors also provide a much stronger voltage output than most other commercially available sensors, ensuring better signal-to-noise ratio while remaining compatible with our own monitoring systems as well as those of most other suppliers.
Used on steam turbines for both casing absolute and shaft absolute measurements. A variety of models are available with or without integral cable for different temperature ranges and with different frequency responses.
vibro-meter is world-renowned for our accelerometer technology. For machinery vibration applications, very few can match the breadth of our portfolio or rival our quality. We are also the only supplier to offer the option for voltage-mode or current-mode transmission, providing superior immunity to noise and signal degradation over long wiring distances.
Accelerometers can be useful when high-frequency vibrations must be examined, beyond those available from the 1 kHz upper limit of our velocity sensor offerings. However, the need to monitor such frequencies is rarely the case on steam turbines unless the turbine employs rolling elements instead of fluid-film bearings. Consequently, velocity sensors are preferred for steam turbine applications2When accelerometers are used in conjunction with a shaft absolute measurement, the native acceleration signal must be double integrated (from acceleration to displacement). In addition, the acceleration amplitudes for bearing cap measurements on a steam turbine will often be considerably smaller than the corresponding velocity amplitudes, resulting in a poorer signal-to-noise ratio. A sensor with a native velocity output is thus a better choice. as they are well-suited for casing measurements on any machine with fluid-film bearings and mounting surface temperatures below 200° C.
NOTES1) vibro-meter offers accelerometers with or without integral electronics. Those with integral electronics are preferred when possible because they are easier to install and do not require a junction box for separate electronics, lowering installation costs. Those with separate electronics are generally reserved only for applications where the mounting surface temperatures would damage integral electronics and a segregated design must be used instead. Models with Integral electronics are available for applications up to 350° C. Models with non-integral electronics are also available and specifically intended for high-temperature applications up to 700° C.
vibro-meter is committed to providing a complete portfolio of transducers for steam turbine measurements. Our housing expansion LVDTs have been field-proven over decades to provide accurate, trouble-free measurement of this critical parameter.
These transducers are used for measuring housing expansion on the high-pressure case of large steam turbines. They allow operators to ensure that the sliding feet on such cases have not become stuck, resulting in a warped or cracked case. The measurements are made via a DC LVDT in a special IP54 housing that is suitable for the temperature and humidity conditions routinely encountered at the high-pressure casing of a steam turbine.
Valve position measurements generally involve very close proximity to a steam turbine’s valves and thus elevated temperature and humidity levels compared to those incurred with housing expansion measurements. As such, AC LVDTs are often used instead of DC LVDTs as they are available for operation in temperatures up to 500° C and with sealing to IP68.
When valve measurements involve rotary instead of linear motion, a variety of sensor types are available including RVDTs, rotary potentiometers, and rotary Hall-effect sensors. All of our monitoring systems are designed to be flexibly and easily configured for a variety of third-party valve position transducers and measurement schemes.
These sensors are used for steam turbine bearing temperatures as well as other machinery-related temperatures such as generator winding temperatures, lube oil temperatures, and relevant process temperatures. vibro-meter monitoring systems are designed to be universally compatible with a very broad range of third-party RTDs and thermocouples including:
vibro-meter monitoring systems such as the SpeedSys300 are designed to be universally compatible with a very broad range of third-party magnetic pickups, both active and passive types. These sensors are typically used for overspeed measurements and are often the default sensor type provided by the OEM with steam turbines. Although magnetic pickups can be used for phase-reference measurements, proximity probes are generally advocated instead.
Passive-type magnetic pickups are usually not designed to work at rotative speeds below 200 rpm and are thus not suitable for zero speed measurements. Although active-type magnetic pickups are available for such applications, a proximity probe or Hall-Effect sensor will usually be a better choice. However, we can supply magnetic pickups upon request.
Hall-effect sensors are powered and offer advantages over passive magnetic pickups, including the ability for better diagnostic self-checks. vibro-meter monitoring systems such as the SpeedSys300 are designed to be universally compatible with a very broad range of third-party Hall-effect sensors.
However, Hall-effect sensors are not used for phase reference measurements; proximity probes are advocated instead, whether for phase reference, general speed indication, or overspeed applications. vibro-meter can supply Hall-effect sensors for speed and even rotary valve position measurements upon request.
vibro-meter is unique in the industry by offering both distributed and centralized platforms with very similar channel types between the two, allowing you to choose the platform that fits your field wiring and topology preferences rather than forcing you to choose between “full capability” and “limited capability” platforms. Each platform provides the flexibility of stand-alone condition monitoring, stand-alone machinery protection, or seamless integration of the two in a “zero footprint” fashion that requires no additional modules. And, consistent with industry best practices and standards, we offer a completely independent platform for overspeed protection of steam turbines – our SpeedSys300.
Our “one card does it all” approach revolutionized the industry more than two decades ago and our 2nd generation of this popular platform provides new levels of value, power, cyber security, and flexibility.
The VM600Mk2 is our centralized monitoring platform in a conventional 19” EIA rack-mounted form factor. It provides integrated protection and condition monitoring capabilities for all steam turbine measurements, including TSI, and builds on the enormous success of our original VM600 platform by providing numerous second-generation improvements while maintaining backward compatibility with the substantial installed base of first-generation racks (more than 240,000 protection and 88,000 condition monitoring channels).
Released in 2000, the original VM600 introduced the concept of “one module does it all” – a feature many others have since emulated but which was pioneered by vibro-meter.
You can learn more in our all-new whitepaper or explore the full capabilities, specifications, and ordering options of our VM600Mk2 offering.
Developed in conjunction with one of the world’s leading turbine OEMs, the VibroSmart architecture can reduce wiring costs without sacrificing functionality – employing our “one card does it all” philosophy pioneered in the VM600.
The VibroSmart System is our distributed monitoring platform in a 35mm DIN-rail mounted form factor. It provides integrated protection and condition monitoring capabilities for all steam turbine measurements, including TSI, and is ideal for new installations where wiring costs can be dramatically reduced by mounting the monitoring modules near the machine and using single or redundant network cables to bring necessary status and current values back to the control room.
The VibroSmart platform is often an excellent solution for smaller, mechanical drive steam turbines that do not warrant a larger, rack-based solution like the VM600Mk2.
vibro-meter is committed to helping you address all classes of machinery with affordable yet robust online monitoring solutions, and this includes the smaller steam turbines that often comprise balance-of-plant mechanical drive applications.
For “Balance of Plant” (BoP) mechanical drive steam turbines that do not warrant a more full-featured approach and can instead be addressed with very basic protection, our portfolio of vibration and position transmitters and single-channel monitors represents an economical “right sized” solution. These devices can be particularly suitable for steam turbines with rolling element bearings as well as smaller units with sleeve bearings but in the sub-megawatt range.
Transmitters and single-channel monitors are available in a variety of form factors to measure casing vibration from velocity and acceleration sensors, shaft axial position from proximity probes, shaft-relative vibration from proximity probes, phase reference from proximity probes, markers, and rotative speed indication*Speed transmitters are not intended for use as part of an overspeed detection system and should be used only for speed indication – no protection. When overspeed protection is required, see our SpeedSys300 system instead.. It is important to note that these devices do not offer condition monitoring capabilities and are thus appropriate for machines where only basic protection is required. Multiple devices can be used in tandem to monitor not only the steam turbine driver, but also the driven machine such as a pump or fan.
Designed from the ground up for SIL-certified, stand-alone overspeed protection, this platform reflects innovative design features that set it apart in the industry while ensuring it can evolve with enhanced functionality while maintaining its SIL ratings.
The SpeedSys300 platform is an innovative overspeed protection solution design for stand-alone operation and independence from all other systems in accordance with API Standard 670 and industry best practices. It can be used in simplex, duplex, or triple-modular-redundant configurations for 1oo1, 1oo2, 2oo2, or 2oo3 voting and is certified for SIL 2 and SIL 3 applications.
Overspeed is a critical protection measurement on steam turbines – whether small single-stage units or the largest compound units – and the SpeedSys300’s adaptability means it can be used economically on your smallest steam turbines while scaling to fit your largest steam turbines.
Our condition monitoring software is designed to provide a seamless, unified environment for your machinery information regardless of what underlying hardware you may be using. Our configuration environments are designed for exceptional ease of use, allowing you to accomplish in minutes what formerly took hours, establishing an industry leading benchmark for power, flexibility, and intuitiveness with a highly graphical approach. And, our expert system environment is designed to automate your machinery diagnostic and anomaly detection tasks while providing a highly intuitive dashboard of machinery status suitable for operators – not just machinery specialists.
Full-featured condition monitoring and configuration software that allows you to unify your underlying protection, condition monitoring, and other data sources into a single, powerful environment for maximizing machinery availability, reliability, profitability, and safety.
VibroSight is a suite of powerful applications used for not just condition monitoring but also communications, data import/export, and configuration of our monitoring hardware platforms. When using VibroSight for condition monitoring, all of the plot types required for deep analysis of steam turbines and their driven equipment is available, under steady-state and transient operating conditions. For an extensive overview, visit the VibroSight landing page.
Configure and test your SpeedSys300 overspeed hardware in an intuitive, convenient, and powerful environment.
Our SpeedSys300 software is used for the configuration and maintenance of our SpeedSys300 overspeed systems. It is a stand-alone software package that, like our VibroSight suite of tools, is designed for highly intuitive ease-of-use. Configuration can be performed offline and then downloaded to the modules. The software can also be used in online mode to retrieve diagnostic information from modules, to generate reports on device configuration and statuses, to view detailed event logs and speed trends, and to fully exercise alarm and other functionality during proof testing.
Machinery Protection System Verification Services
Sensor Calibration Services
Condition Monitoring System Upgrade Services
Project Engineering, Management and Planning Services
Factory Acceptance Testing (FAT) and Integrated Factory Acceptance Testing (IFAT) Services
Machinery Diagnostic Services
Machinery Balancing Services
Advisory and Consultancy Services
Long Term Service Agreements (LTSAs)
vibro-meter provides comprehensive services that extend beyond just our steam turbine monitoring and protection instrumentation to encompass your broader needs such as machinery diagnostics, training, system integration, product rental, and project management. Some of our customers have a high degree of self-sufficiency and need little more than occasional technical support, while others prefer to outsource the installation, maintenance, and even operation of their systems.
Wherever you fall within this spectrum of needs, we have both standard and tailored service offerings to fit. In addition to the short descriptions below, you can read more in our Services Brochure.
These services verify the operation of your already installed protection system, whether VM600, VM600Mk2, or VibroSmart. Although our monitoring systems do not require calibration, periodic functional testing to verify that the system working within published specifications is recommended at 2-year intervals. These services cover the monitoring system, its connected sensors, and its communications with associated automation platforms such as the plant’s distributed control system.
These services verify that your sensors are performing within published specifications and calibrate them where required. Sensors can be returned to the factory for calibration or can be calibrated in the field. Factory as well as field calibration services are performed using equipment calibrated to NIST-traceable references.
These services are similar to System Verification Services, but are performed at time of initial system deployment and include the installation activities – not just the verification activities. These services also include training so that operators and others will be proficient in using the newly installed systems.
These services address the migration of hardware and software from older vibro-meter condition monitoring environments to our latest VibroSight software. Where the VM600 platform is used with CMC16 modules, the hardware is upgraded to our latest XMx16 modules. Servers are upgraded, mimic diagrams are updated, and integration with other automation platforms such as plant distributed control systems is updated so that functionality is maintained or enhanced under the new environment.
These services provide a comprehensive turnkey solution for upgrades, new installations, migrations, retrofits, and system expansions by delivering all design and project management activities in addition to execution activities. They are integrated and harmonized with your overall planning to ensure that critical tasks are not impaired and schedules do not conflict.
These services allow robust functional testing of new systems before they leave the factory, and typically occur once the systems are mounted in cabinets and pre-wired to terminations, ready to accept field wiring at site. When condition monitoring is included with the machinery protection, this functionality is tested as well. This testing can also be carried out at locations other than vibro-meter premises when full integration with other systems, such as the plant DCS, must be exercised and verified.
Field engineers with machinery expertise are available to collect data from your machinery using portable data acquisition equipment or your installed condition monitoring systems. The collected data is reviewed and diagnostic reports are produced regarding machinery health such as the likely malfunction, its severity, and recommended corrected actions along with any corresponding urgency.
Our machinery diagnostic services can also be used to generate customized algorithms within your installed VibroSight Rulebox software, allowing it to perform automated diagnostics by embedding the same analytical processes and domain expertise utilized by our own engineers.
We provide expert capabilities in machinery balancing, ranging from simple single-plane balancing of Balance of Plant machinery to complex multi-plane balancing of larger, more critical machinery trains that may operate under varying speed and load conditions with multiple measurement and correction planes.
We strongly recommend that our Balancing Services be combined with our Machinery Diagnostic Services (see above) so that imbalance can first be positively confirmed as the source of high vibrations and other causes (misalignment, thermal bows, cracked shaft, etc.) can be ruled out.
For machinery with no (or insufficient) condition monitoring instrumentation, we provide rentals that can be used on a short-term or extended-term basis. For customers without embedded vibration and machinery diagnostic expertise, the data collected by rental instruments can also be used in conjunction with our Machinery Diagnostic Services. This data can often be transmitted electronically to avoid the cost of site travel by our field diagnostic engineers.
These services are designed to address a wide range of a la carte or bundled activities such as generating project specifications, site surveys to assess machinery and corresponding recommended monitoring, ongoing predictive maintenance services, recurring audits of machinery condition at specified intervals and after major events, and many others that can be custom-tailored to your unique needs.
An LTSA is a bundle of tailored services provided on an ongoing basis rather than a la carte, on-demand basis. LTSAs can cover the entire system, or only selected parts thereof, such as the condition monitoring portion of the system but not the sensors or protection hardware. The composition and duration of the LTSA is formulated based on your needs as we understand that one-size-fits-all solutions are rarely able to address the specific needs and economics of each installation and each customer.
Training can be delivered virtually via video conferencing or in-person at our facilities, your facilities, or other facilities such as hotels or conference centers. Training is delivered by certified professionals and is assembled based on your specific needs to cover sensors, monitors, software, vibration diagnostics, measurement fundamentals, system maintenance, system operation, and other specialized topics pertaining to machinery monitoring and protection such as cybersecurity and gas turbine combustion monitoring. We maintain dozens of standard training modules that can be mixed, matched, and customized to meet your needs, and modules to address new topics can be developed as required.
Every measurement customarily made on steam turbines for both protection and condition monitoring is available in our monitoring system platforms. In addition, we provide most of the sensors you will need either directly or via partnerships. In instances where we do not provide sensors (such as temperature), we can provide guidance to assist you in sourcing them yourself or we can source and install them for you as part of turnkey installation and project management capabilities. For details on TSI measurements beyond those available here, please download our publication A Practical Guide for Understanding Turbine Supervisory Instrumentation.
* Although Meggitt vibro-meter® does not provide temperature, pressure or valve position sensors, our protection and condition monitoring systems can integrate these readings.
† These sensors can also be used in conjunction with shaft relative vibration sensors to obtain absolute measurements if orientated to coincide with shaft relative measurement planes.
This measurement is made by means of a proximity probe, usually affixed to the bearing housing and observing the vibratory motion of the shaft within its bearing clearance. The probe can return both AC and DC signal components, corresponding to dynamic motion (vibration) toward and away from the probe, as well as the average position (DC component of signal). The average position shows the location within the bearing clearance where the shaft rides on its film of lubricating oil. It is about this point that dynamic motion occurs.
Although this can be a single-channel measurement with a probe mounted in only a single vibration plane, it is more common – particularly on critical machinery – to mount a second probe in an orthogonal axis so that position and vibration in both an X- and Y-plane can be observed, ensuring that any motion within the bearing clearance is detected.
These measurements are routinely used for both protection and condition monitoring. The amplitude of the signal corresponds to the amount of vibration and can be related to bearing clearances. It is made in units of displacement, either micrometers or mils.
This is the axial movement of the shaft at the thrust bearing, relative to the thrust bearing housing. It may be made at the end of the shaft or at the thrust collar. A similar measurement is called rotor position and is when the axial position is made relative to the machine casing rather than the thrust bearing.
Like shaft-relative vibration measurements, shaft axial position is made via a proximity probe. Unlike shaft-relative vibration, it is generally the DC component of the signal (position) that is of interest rather than axial vibration (AC component of signal). Axial thrust movement in excess 2mm (80 mils) is rare. In such cases, larger diameter probes are available with longer measurement ranges. However, must thrust bearings are designed to have a total range of 1 – 0 – 1 mm where the thrust collar can move in the normal or counter direction against the normally active or inactive thrust bearing pads.
Excessive movement corresponds to damage of the thrust bearing and an axial rub between rotating and stationary parts such as between blades and diaphragms on a steam turbine. As such, this measurement is extremely important for machinery protection, and also has diagnostic value for condition monitoring purposes. While not as useful on steam turbines, the AC component of the signal (axial vibration) has diagnostic value as well, and can be indicative of problems with the driven machine, such as surge or incipient surge on a centrifugal or axial compressor or blower.
Because the shaft axial (thrust) position measurement is so important, it is usually made by means of two redundant probes that compare their readings and use logical AND voting. The measurement can also be done by means of a single probe but will have not voting. The thrust bearings on steam turbines are generally large enough to easily accommodate two probes – either to observe the thrust collar directly or to observe the end of the shaft near the thrust bearing.
To ensure radial and axial (thrust) bearings are not too heavily loaded, or starved of necessary lubrication, it is customary to embed an RTD or thermocouple into the bearing pad(s) carrying the shaft load. When excessive temperatures are observed, the machine must be shut down because the bearing babbitt material can melt, resulting in damage far beyond a replaceable bearing pad – such as a scored shaft or an axial rub.
Often, sudden catastrophic changes in a machine will result in instantaneous changes in shaft position or vibration, and the thermal inertia of the bearing materials will cause temperature to rise more slowly than vibration levels, but still relatively fast.
For this reason, it is not always advisable to vote temperature and vibration using AND logic. By the time both measurements indicate a problem, it may be too late to trip the machine and prevent damage.
Each independent shaft in a machine is usually fitted with a once-per-turn mark, such as a key or keyway at a coupling, allowing a proximity probe to observe the passing of this mark with each shaft revolution. This provides a precise reference in time against which all other measurements along the shaft can be synchronized. For example, a Y probe on a bearing may observe the shaft’s maximum positive excursion at 23 degrees of rotation after the phase reference mark, while the corresponding X probe might give a maximum positive excursion at 128 degrees of rotation.
This information is extremely valuable for diagnostics and allows not only simultaneous sampling of all sensors along a shaft, but also to sample at precise increments of shaft rotation (so-called “synchronous” sampling), where the vibration is sampled at some multiple of shaft rotative speed – such as 64 times per revolution or 128 times per shaft revolution.
Fundamentally, a phase reference measurement is a timing measurement, but it is usually expressed in degrees of shaft rotation after the timing mark occurs. This mark can also be used for basic speed indication, but cannot update fast enough for overspeed measurements. For this reason, a toothed wheel is usually used instead. For example, a shaft rotating at 3000 rpm cannot update its speed reading faster than one full shaft rotation (20 ms) when only a once-per-turn mark is provided.
In contrast, the same machine with a 50-tooth wheel will generate a pulse 50 times per shaft revolution, or every 4 ms. Because an overspeed system on a steam turbine must often respond in 30 ms or less, a surface that generates a pulse every 4 ms (allowing three or more successive speed measurements to be acquired before a shutdown decision is made) is far superior to one generating a pulse every 20 ms (allowing only one speed measurement before a shutdown decision is made).
As noted above, a phase reference measurement is made using a proximity probe observing a once-per-turn shaft discontinuity such as a key or keyway. However, this will rarely update at a rate suitable for accurate speed indication, much less overspeed protection.
As such, a multi-tooth surface will be used – often a gearwheel made specifically for speed measurement purposes. While such a surface is recommended for greater accuracy in speed indication purposes, it is absolutely essential for overspeed protection measurements for the reasons discussed in the phase reference section.
Also, overspeed is almost never measured by a single (non-redundant) sensor. Instead, there are usually three redundant sensors observing the same multi-tooth surface and then monitored in a 2-out-of-3 voting arrangement that represents an optimal balance of missed trips versus false (spurious) trips.
1-out-of-2 arrangements are also used (sometimes on gas turbines), but are less common on steam turbines where 2-out-of-3 is routinely used instead as part of a SIL 3 protection loop. Speed and overspeed sensors can include magnetic pickups (i.e., variable reluctance sensors), Hall-effect sensors, and conventional eddy-current proximity probes as used for vibration and position measurements.
The differences in materials and also mass between a turbine case and its rotor means that they expand at different rates. As they grown or shrink relative to one another, the differential between the two must be monitored very carefully, to prevent axial rubs from occurring between stationary and rotating blading.
These measurements are most often made with extended-range proximity probes observing a collar or other surface. To allow smaller-diameter probes with shorter linear ranges, the probes can be arranged back-to-back in a complementary arrangement that effectively doubles the linear observable measurement range.
Another common technique by some turbine manufacturers is to create a ramped surface on the rotor such that a trigonometric relationship exists between movement observed by the probe and actual relative motion between the probe’s mounting (turbine case) and the rotor. This arrangement allows a small change in the radial direction observed by the probe to correspond to a large change in the axial direction through differential expansion. Still another arrangement is by means of a magnetic collar and a probe observing a swinging pendulum that also exhibits a trigonometric relationship, similar in principle to that of the ramp approach.
The exact method used typically depends on that adopted by the turbine OEM. vibro-meter monitoring systems are designed to be adaptable to all of these.
The steam admitted into the high-pressure case of a large turbine is not only at high pressures, it is at high temperatures. This causes the HP turbine case to expand during start up and to accommodate this expansion, the case is generally designed with fixed feet on one end and sliding feet on the other end.
When one or both of these sliding feet stick, it can warp the case and the expansion must thus be monitored. This is normally done with LVDTs and can be accomplished by either a single- or dual-channel measurement. Dual-channel is normally recommended as it can display not only the absolute expansion, but also the differential between each side to ensure both feet are sliding properly – not just one. When only one foot slides and the other is stuck, it can result in a so-called “cocked case”.
When both feet are stuck, this can likewise create problems.
The rotor on a large turbine generator can have extremely long unsupported spans between its bearings and can thus sag if not kept constantly turning. For this reason, when a turbine is brought offline, its rotor will generally continue to slowly rotate by means of a turning gear mechanism. As the machine is again brought on line and back to speed, the amount of residual bow must be carefully monitored by means of a probe to observe the shaft “wobble” (called eccentricity).
Excessive eccentricity corresponds to excessive bow and if the machine is further accelerated in speed with excessive bow, the bow can exceed the elastic limits of the shaft and become a permanent bend. Eccentricity is measured by means of a proximity probe mounted some distance away from a bearing. This allows the bow to be more pronounced and thus more easily observable than if attempted at a bearing location by a radial vibration probe.
Valves are used to control steam admission into the turbine and thus to control its speed. For larger machines, there are often a series of valves that work together to admit the steam rather than a single large valve. These valves are frequently arranged in a so-called “valve rack” that mechanically lifts the valve stems by means of a cam and levers.
The linear travel is usually measured by means of LVDTs attached to the rack mechanisms. In some cases, the valve adjustments are measured using rotary position rather than linear, in which case a sensor measuring degrees of rotation is used such as a rotary potentiometers, RVDT, or rotary Hall-effect sensor.
At one time, these measurements were always made in the same system as the vibration and other TSI measurements. However, today the valve position measurements are often moved into the turbine control system because they are fundamentally not just for indication, but for actual control purposes.
Thus, during an instrumentation upgrade, the measurement may be migrated from the TSI environment to the control environment. In other instances, the user may wish to leave the measurement within the TSI system. vibro-meter monitoring systems are designed to accommodate any of these sensors, allowing inclusion of valve position measurements when required.
As noted in the description of eccentricity measurements, to prevent excessive shaft bow or even a bent shaft, the rotor of a large steam turbine must be kept constantly turning when the turbine in use. This is done my means of a special turning gear that keeps the rotor slowly turning – often at speeds below 5 rpm. When the rotor decelerates toward a standstill, this turning gear must be engaged at the right speed to keep the rotor turning. This is done by means of a zero-speed tachometer.
To ensure very low speeds can be measured with acceptable update rates, a multi-tooth gear is used and it is observed by a speed probe (usually proximity, but Hall-effect of active magnetic pickups can also be used).
If a simple once-per-turn discontinuity was used instead of a multi-tooth wheel, the rotational speeds would result in unacceptably slow update rates. For example, a shaft turning at 3 rpm generates only 3 pulses per minute from a once-per-turn discontinuity (such as from a phase reference probe). This would require 20 seconds between updated speed readings and is too slow. However, a 60-tooth gear – which is common – would generate 180 pulses per minute and allow speed updates every 333 ms.
When large steam turbines are mounted on compliant foundations that can move, the entire turbine can vibrate in space – not just its shaft relative to its bearings. Examples of compliant foundations would be in plants where the turbine is mounted on the top floor of a building, many stories above ground with steam piping and auxiliary machinery beneath the turbine.
In such instances, the amount of bearing vibration relative to free space (bearing absolute) is a valuable measurement along with a vector summation of the shaft-relative vibration and the bearing housing vibration. This is known as a shaft-absolute measurement and essentially measures the motion of the shaft relative to free space. The vector summation of the two signals (shaft-relative and bearing absolute) is made within the monitoring system and is thus a two-channel measurement.
Shaft-relative vibration is made via a conventional proximity probe, as was discussed above in the section of those measurements. Bearing absolute vibration is made by means of a seismic sensor such as a moving-coil velocity pickup or a piezoelectric accelerometer. Velocity sensors are generally preferred, however, because they require only a single stage of integration (conversion from velocity to displacement) to the permit vectorial combination with the displacement measurement from a shaft-relative proximity probe.
The shaft absolute measurement requires both the bearing absolute and shaft-relative measurement to be made in the same measurement plane. For example, if a shaft-relative probe is mounted at 45° right of vertical, the corresponding bearing cap sensor must be mounted in the same orientation 45° right of vertical.
|Advanced machinery protection and condition monitoring for turbomachinery.|
|Speedometer – a vibro-meter publication on overspeed|