1B3 Chapter 7 Rotating Equipment Monitoring

1st Class Power Engineering

What is the primary purpose of rotating equipment monitoring?

To ensure that operating limits are not exceeded and to minimize wear and deterioration on components.

What are some types of rotating equipment monitoring?

Vibration monitoring, thermal expansion measurement, temperature monitoring, pressure monitoring, and oil condition monitoring.

Why is monitoring important for rotating equipment?

It minimizes operating and maintenance costs, achieves high reliability, and maintains performance and efficiency.

What is turbine thermal expansion?

It refers to the expansion of turbine components due to heating during startup and contraction during shutdown.

What is the significance of turbine differential expansion?

Differential expansion occurs when different components expand at different rates, potentially causing rubbing and increased vibration.

What is turbine eccentricity?

Eccentricity refers to the misalignment of the rotor within the casing, which can cause vibration and wear.

What are typical causes of vibration in rotating equipment?

Imbalance, misalignment, bearing wear, and resonance.

How is vibration measured in rotating equipment?

Using vibration sensors that detect oscillations and provide data for analysis.

What is turbine critical speed?

The speed at which the rotor's natural frequency matches the operational speed, leading to excessive vibration.

What are oil whirl and oil whip?

Oil whirl is a stable oscillation of the rotor due to oil film instability, while oil whip is an unstable oscillation that can lead to failure.

What are common oil problems in rotating equipment?

Contamination, degradation, and improper viscosity can lead to increased wear and failure.

What is a typical oil sampling and testing program?

It involves regular collection and analysis of oil samples to monitor condition and detect potential issues.

What is the role of a turbine supervisory instrumentation (TSI) system?

To monitor various indicators of equipment condition, including rotor eccentricity and differential expansion.

What parameters does a TSI system monitor?

Rotor eccentricity, differential expansion, steam/metal temperature, vibration, and rotor speed.

What is the effect of thermal stresses during startup?

Thermal stresses can lead to distortion and potential fatigue cracking in turbine components.

What is the best steam-to-metal temperature difference for turbines?

About 40°C - 80°C is considered optimal for minimizing thermal stress.

What happens to the turbine casing during startup?

The inner surface becomes hotter than the outer surface, leading to differential expansion and potential stresses.

How does the mass of the turbine casing affect thermal expansion?

The casing, having a greater mass, expands and contracts more slowly than the rotor, leading to differential expansion issues.

What is the safe rate of increase of metal temperature in turbines?

About 200°C per hour is considered a safe rate.

What is the consequence of excessive differential expansion?

It can lead to rubbing, increased vibration, and potential failure of the turbine.

How does monitoring integrate with control systems in rotating equipment?

Instrumentation is used for monitoring equipment condition, controlling operations, and providing protection from abnormal conditions.

What is the impact of startup and shutdown on turbine wear?

Starting and stopping can increase wear and induce high stresses in turbine materials.

What is the importance of monitoring steam flow in rotating equipment?

It helps ensure optimal performance and efficiency of the equipment.

What is the role of temperature monitoring in rotating equipment?

To detect overheating and ensure components operate within safe temperature limits.

What is the significance of condition monitoring of auxiliary systems?

It helps maintain overall system reliability by monitoring pressures, temperatures, and fluid levels.

What is the relationship between temperature gradients and turbine stresses?

Temperature gradients can create thermal stresses that lead to distortion and potential damage.

What is the optimal steam-to-metal temperature difference for turbines?

About 40°C - 80°C

What is the safe rate of increase in metal temperature per hour?

About 200°C per hour

What happens if the HP cylinder cover joint is not properly secured?

It can cause straining of the clamps due to excessive temperature differential.

How quickly can a 60 MW machine be run up to speed after being shut down for 8 hours?

In 15 minutes

What is the significance of thermal expansion in turbines?

It allows the supports to slide easily to accommodate axial and radial expansion.

What can cause distortion of the turbine shell?

Improper preheating or maintenance of the turbine.

What is turbine cocking?

It occurs when the turbine slider hangs up on one side of the foundation.

What is the purpose of measuring devices installed on the turbine case?

To monitor expansion and detect distortion or cocking.

What principle do coil induction detectors operate on?

Electromagnetic induction principle.

What is an LVDT?

Linear Variable Differential Transformer, an electromagnetic device used for measuring displacement.

What is the typical operating range of a standard LVDT?

25, 50, or 100 mm (1.0, 2.0, or 4.0 inches)

What happens if a turbine casing does not expand properly during heating?

The sliding surfaces may be binding, which can be corrected with lubrication.

What is flange warming?

A process where steam is admitted to longitudinal channels to heat the casing metal.

What does differential expansion monitor?

The change in axial clearances between the rotor and casing due to thermal changes.

Why is monitoring differential expansion important?

To guard against axial rub between rotating and stationary parts.

What can excessive differential expansion cause?

Catastrophic failures of rotary machinery.

How does differential expansion differ from rotor position measurements?

Differential expansion measures thermal changes, while rotor position measures axial position with minimal thermal effect.

What is the consequence of not monitoring differential expansion?

It can lead to very expensive repairs or machine replacement.

What is the role of instrumentation in turbine operation?

To monitor eccentricity and vibration for safe operation.

What is the impact of startup on gas turbine blade life?

It reduces the life of the blades equivalent to several hours of standard operation.

What is the maximum temperature differential allowed between flange and clamp?

It must be kept to a safe maximum to prevent straining.

What is the effect of uneven heating on turbine casing?

It can cause the turbine case to cool unevenly and require reheating.

What is the output voltage range of a standard LVDT?

(-7.5)-0-(+7.5) volts direct current (VDC)

What is a potential consequence of using steam for flange warming?

It can force the flange surfaces apart and stretch the studs.

What is the typical accuracy of an LVDT?

±0.5 % full scale

What type of signal does a DC LVDT require?

Only DC power to operate.

What is the purpose of the spring-loaded roller tipped plunger in an LVDT?

To press against a bracket attached to the turbine case for measurement.

What are the two basic machine conditions related to differential expansion?

The rotor long condition and the rotor short condition.

What occurs during the rotor long condition?

The rotor thermally expands at a faster rate than the machine casing due to its smaller mass and different metallurgy.

What can happen if the rate of rotor growth is not controlled?

The moving and stationary parts of the machine may come into contact.

What is the purpose of turbine soak periods?

To allow the machine's casing to thermally expand and catch up to the rotor assembly.

What describes the rotor short condition?

The rotor shrinks with respect to the machine case during steady state operation.

What typically causes the rotor short condition?

A drastic reduction in steam temperature or flow, known as quenching the rotor.

How do internal clearances differ between rotor long and rotor short conditions?

Rotor long conditions have larger internal clearances, while rotor short conditions have smaller clearances.

What is the significance of measuring differential expansion?

To prevent internal damage to the turbine by allowing unrestricted growth of the rotor and casing.

What types of sensors are used to measure differential expansion?

Eddy current probes are commonly used for accurate measurement.

How do eddy current probes operate?

They generate a high frequency oscillating RF signal that creates eddy currents on the rotor surface, allowing measurement of the gap.

What must be ensured for proper installation of an eddy current probe?

The probe must be mounted perpendicular to the target area within a few degrees.

What effect does excessive seal water have on turbine operation?

It can cause quenching of the rotor, leading to a rotor short condition.

What is the role of differential expansion transducers?

To monitor the differential expansion by comparing signals from the rotor and casing positions.

What is the impact of temperature increase on turbine materials?

Metal alloys and castings expand as temperature increases, which must be accounted for in turbine design.

What happens to the turbine cylinders after shutdown?

The top and bottom halves of the cylinders cool at different rates, leading to potential eccentricity.

What is the purpose of maintaining steam temperatures and flows during startup?

To prevent excessive differential expansion values and maintain internal clearances.

What is a common characteristic of the rotor in a rotor short condition?

The rotor contracts with respect to the machine case.

What is the consequence of quenching the rotor beyond acceptable limits?

It can lead to a rapid rotor short condition and diminish internal clearances.

What is the significance of the location of differential expansion transducers?

Their location affects the accuracy of measurements based on turbine configuration.

What is the typical configuration for mounting dual transducers?

They are often placed on either side of a collar to extend the range of measurement.

What is the effect of excessive mounting error on eddy current probes?

It can change the calibration factor of the installation.

What is the function of ramps on turbine shafts?

They increase the operating range of measurement transducers by allowing larger measurements.

What is the relationship between rotor growth and turbine efficiency?

Rotor short conditions are designed to have smaller clearances to increase unit efficiency.

How does the configuration of a turbine affect differential expansion measurement?

Each installation is slightly different, requiring an understanding of the machine's thermal characteristics.

What is the primary concern when restricting thermal growth in turbines?

It may cause internal damage to the turbine.

What is the typical response of the rotor to sudden temperature changes?

The rotor reacts first due to its smaller mass and differing metallurgical properties.

What is the purpose of monitoring differential expansion in turbines?

To ensure safe operation and prevent mechanical failures.

What is the typical speed of barring gear for turbine spindles?

About 20 r/min

What does hogging or sagging refer to in turbine spindles?

Deformation of the spindle due to uneven temperature distribution.

What is the effect of rotating spindles during warming up?

It promotes circulation and helps even out cylinder heating or cooling.

What is the purpose of barring gear in turbines?

To keep turbine spindles straight while the machine is off load.

What is the mass of a typical high-pressure turbine spindle mentioned?

About 4.5 tons.

What is the significance of a temperature difference of 1.6°C along a spindle?

It can cause a bend of 0.025 mm in the spindle.

What measurement is crucial before running a turbine up to speed?

Correcting any stationary bend in the spindle.

What device is used to measure shaft eccentricity at low speeds?

A dial gauge.

What modern technology is used to measure eccentricity in turbines?

Eddy current probes.

What is the typical speed range for measuring eccentricity with modern systems?

1 to 240 r/min.

What happens to eccentricity measurement after a turbine is brought to speed?

The measurement is locked to zero under load provisions.

What is the formula to estimate mid-span eccentricity?

E_ms = (S_b / S_t) * E_t

What does vibration in rotating equipment result from?

The energy produced by the equipment and imperfections in balance.

What is the simplest form of vibration described as?

A regular oscillation of an object, often illustrated by a sine wave.

What force primarily causes vibration at the frequency of rotational speed?

Unbalance due to uneven mass distribution.

How does misalignment affect vibration frequency?

It causes vibration at twice the rotating speed.

What vibration frequency occurs in turbines due to rotor blades?

Equal to the number of blades in a rotor stage times the rotating speed.

What type of vibration frequency occurs in gear systems?

Based on the number of gear teeth and the rotating speed.

What is the relationship between vibration and the rotation of a shaft?

One period of the sine wave is equal to one rotation of the shaft.

What is the consequence of a stationary spindle becoming too hot?

It may lead to hogging due to uneven heating.