Interview questions on Vibration Measurement

1.What is ‘vibration’?

Vibration is defined as the motion of the equipment or its part to and from its rest (static) condition.

 

Interview questions on Vibration Measurement

2.Explain following with their sensitivity (output) and the measuring units:

Radial: A vibration measurement across the radius of a rotating shaft. It is measured in terms of Micron. The sensitivity of a radial vibration pickup (eddy probe) is 200mv DC/mill (refer to the drawings for the exact parameters).

Velocity: It is defined as the rate of change of distance traveled by the equipment. Velocity measurement is generally used for measuring the equipment body vibration. The sensitivity of a velocity pickup is 500mv DC/inch/sec (refer to the manufacturer drawings for the exact parameters).

Acceleration: It is defined as the rate of change of velocity. Acceleration measurement is generally used for measuring the equipment body vibration. The sensitivity of a velocity pickup is 100mv DC/inch/sec2 (refer to the manufacturer drawings for the exact parameters).

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3.What is the resistance of an ‘Eddy probe’?

An eddy probe resistance should be between 5-8 ohms.

4.Draw a sketch of an eddy probe calibration graph?

                        Eddy probe calibration graph

 5.What is gap voltage? Why and how is it set?

Gap voltage is feed back voltage derived by setting the standard gap between the eddy probe tip and the rotating shaft.

The gap voltage is the base DC voltage set to get the AC pulses peak per the radial vibration measured on the shaft. The gap voltage is set at –8.00V DC i.e. equal to a gap of 40 mill (between the probe tip and the measuring surface). or at -10.00V DC as per the system standard.

6.Why an eddy probe and its probe driver is a matched pair?

Eddy probe output is always measured after the probe driver. The characteristic of the eddy probe slightly differs from one to another. Always an eddy probe is installed on the shaft, after measuring and plotting an eddy probe calibration graph with its driver. The calibration is done by adjusting the sensitivity potentiometer on the eddy probe driver assigned for that particular eddy probe.

7.What do the AC and DC signal represent in radial vibration, where ‘AC is super imposed on DC’?

DC signal is the gap voltage set for –8.00V DC. DC signal is always present whether the unit is running or stopped.

8.How much is the radial vibration, if the signal measured on a DVM is 135.00mv AC?

The DVM measures the AC voltage in RMS (root mean square value).

AC peak to peak =135.00*2 root 2

                                     =181.837mv

Eddy probe sensitivity=200mv/mill or 200mv/25.4 Microns

Hence, the vibration in Micron =(381.837*25.4)/200

                                                    =48.49 Micron

9.Why is vibration measurement very important on gas turbine and compressor?

Gas turbines and compressors are high speed rotating equipment. On equipment when vibration exceeds the manufacturer limits can cause sever damage to both itself and to its associated components/parts. Hence it is important to measure and monitor vibration on the running equipment. Generally a high vibration pre-alarm and shutdown limits are set as per the manufactures recommendations.

  

10.What type of vibration measuring instrument is used on a gas turbine? Where are the vibrations measuring points?

Since a gas turbine is hot engine, its bearing vibration measurement cannot be done through an eddy probe system. Generally a velocity or an acceleration pick-up is used for measuring and monitoring the vibration on the body of the gas turbine. The vibration pick-ups are generally installed on the turbine’s CT/GP and PT points.

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11.What is the type of vibration pick-up (contact or non-contact type) commonly installed on a gas turbine?

On gas turbine vibration measurement, the ‘contact type’ peck-up such as a velocity or acceleration pick-up is used.

12.What type of vibration measuring instrument is used on the compressors? Where is the vibration measuring points?

On a gas compressor shaft/bearing, measurement is carried out using a eddy probe displacement system.

On each gas compressor, on both forward and aft radial bearings (journal bearing) two eddy probes are installed in the ‘x’ and ‘y’ positions.

13.What is the type of vibration pick-up (contact type or non-contact type) commonly installed on a gas compressor?

A ‘non contact type’- eddy probe is installed on gas compressors.

14.What is ‘x or y function’ and, ‘x and y function’ selection on a radial vibration monitor?

X or Y function: in this mode, the unit shutsdown when either ‘X’ or  ‘Y’ probe detects a high vibration exceeding the setpoint.

X and Y function: in this mode, the unit shutsdown only when ‘X’ and ‘Y’ probes detects a high vibration exceeding the setpoint.

15.What is a ‘Probe driver’? What does is it do? What is the other name for a probe driver?

Probe driver is an amplifier installed in the field closer to the sensor. It transmits the field vibration signal to the remote control panel. The other name for the probe driver is ‘proximeter’. Radial, velocity and accelerometer probe drivers are of different types.

16.What are the three wires used on a probe driver?

The three wires terminated on the probe driver are: -24V DC (power supply), common and output signal.

17.What is a ‘charge amplifier’? Where is it used?

Charge Amplifier is a probe driver installed for the accelerometer pick-up.

18.What is an ‘axial displacement’? How is it measured?

Axial displacement is the movement of the rotary shaft in the axial direction to and fro. Every centrifugal shaft is permitted to have a fixed axial movement designed by the manufacturer, this is to allow the compressor to take the load on the thrust collar.

Axial displacement is measure using an eddy probe system. It is measured using the proportional change in the DC voltage (gap voltage) caused by the movement of the shaft from its center position to either side (+or-).

19.What is the unit measurement of axial displacement?

Axial displacement is measured in terms of “Microns”

20.What is the term ‘float/end float’ in a compressor?

It is the free axial movement of the compressor shaft. End float is the maximum possible movement of the shaft. It is a small amount approximately 250 Microns (refer to the manufacturer drawings for drawings for the exact parameters).

21.What are the following assessments/recommendations of a vibration analysis? How are they corrected?

Misalignment: misalignment may cause two time RPM vibration. This is corrected by re-aligning the compressor shaft one to another.

Unbalance: unbalance in the compressor shaft may cause one time RPM vibration. This is corrected by balancing the compressor shaft on a balancing wheel.

Overhauling: multiple RPM vibration is symptom of damaged bearings. This is corrected by a total overhaul of the compressor or replacement of parts.

(The above are only a simple clue and not a standard confirmed judgment)

 22.What precautions are to be taken when installing a vibration measuring instrument?

Vibration measuring instrument such as a eddy probe, velocity and acceleration pick-ups, eddy probe drivers, charge amplifiers are sensitive instruments. The following care has to be taken while installing in the field.

-Do not drop the instrument, this may result in the loss of their characteristic.

-Fixing and tightening to be done as per the manufacturers recommended torque.

-Proper care of the signal cable insulation.

-Prevent any loose connections.

-…Etc.

23.What are the reasons for a vibration ‘spike signal’?

The following may cause a ‘spike signal’ in the vibration measurement.

-Loss of signal cable insulation.

-Signal cable passing next to high voltage lines.

-Improper earthing facility.

-Grounding of the wires.

-Loose mounting of the instrument field components

-…Etc.

24.List the manufacturers names of vibration measuring instruments?

The common vibration measuring instrument manufacturers are:
Bently nevada
Forbes Marshall Shinkawa
Dymac
Bell and Howell

25.Which of the above manufacturer’s product is more reliable and why?

With past experience, it is observed that the ‘Bently nevada’ vibration measuring system is more reliable due to its component layout on the control panel, accuracy and high gauge signal cable system in the field.

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