Steam Turbine Monitoring Solutions

The sensors in this section are supplied by third parties, but are generally compatible with our monitoring platforms. Consult vibro-meter for additional information on these sensors for new or retrofit applications, or if you have existing sensors of the types mentioned here and want to explore compatibility. In some cases, vibro-meter may be able to source these sensors, size them to the particulars of the application, install and/or provide installation guidance depending on the project requirements, and assume full system responsibility.

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.

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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.

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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.

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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.

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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.

Click here to find out more about our advisory and consultancy services.

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• • Sensor Calibration Services – Verification that your sensors are performing within published specifications and calibration as required; can be performed at site or in the factory.
  • Condition Monitoring System Upgrade Services – Site services to upgrade your installed condition monitoring systems, whether vibro-meter or other, to our state-of-the-art VibroSight platform.
  • Project Engineering / Management / Planning Services – Turnkey site services to manage the entirety of your protection and/or condition monitoring project.
  • Instrument Rentals – Portable data acquisition and condition monitoring systems.
  • Long Term Service Agreements (LTSAs) – Tailored commercial agreements to maintain your vibro-meter instrumentation.
  • Training – Tailored to cover the topics of your choosing at our facilities, your facilities, or other facilities – or even via video conferencing.

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^* 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.

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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.

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 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.

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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.

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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.

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.

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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.

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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.

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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.

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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.

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Animation courtesy of Gerler Engineering & Design. Used by permission.

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.

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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.

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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.

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