Instrumentation Interview Preparation Guide

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Pre-commissioning and commissioning activities before a plant or facilities handed over for normal operation it should be inspected checked clean flushed verified and tested, in short pre-commissioning is the testing of non energized system and commissioning is the testing of energized system

By checking the terminal block, thermoucouples should have a polarity sig or indication

Gas Turbine Having varies instrumentation measurement system to monitor and control the turbine. Temperature monitoring protection for bearing, Cooling air, TIT,TAT. Speed Monitoring protection and control, varies flow and pressure measurement, Pulsation monitoring, vibration monitoring and protection. VIGV Hydraulic valves control, turbine gas valves, blow off valves, varies type of on/off and solenoid valves. As a maintenance technician, He is capable to install and commission above instrumentation system in turbine. To explain more details, More specific questions to be added. Let me prepare and add more questions.

Input : 24v DC
Output : 5-18v pulse which frequency is proportional to turbine rotor speed (number of teeth in 360 deg varies with different turbine). Speed sensor having sensor, amplifier and controller. Sensor works basses on eddy current principle.
F = (#teeth* RPM)/ 60
Example(120 teeth and 3000 RPM)
F= 6000 HZ

Mass flow meter works on the principle of "coriols effect"

Instrumentation is the variety of measuring instruments to monitoring & control a process.

For pneumatic Control valve : air Lock Relay holds the Air in locked condition when the supply Air pressure falls below the set value.

MODBUS is a commonly used industrial communications protocol. It allows the exchange of data between PLCs and computers. It was originally designed for Modicon (Schneider Electric) PLCs but has become widely used by many PLC manufacturers and industrial networks.

principle - karmans
working: biuff body is placed in pipeline. There is no flow pressure sensor getting zero. When flow in the pipe line bluff body beyond generate votices if floe is increase vortices are increased these vortices are sensed by pressure sensors and fed to transmitter generate output.

NEMA VE2 is followed in installation of Cable tray

RTD you can measured resisitance Thermocouple you can measure millivolts.

Rpm means rotating per minute... We can only checking the rotating by usen of talkometer and reflecting sticke(stripes)

normally in electronics and instrumentation cct the pnp transistr is used thats y the olp is 24vlt bt in vibartion he npn transistr is used thats y v use -24vlt

You are probably referring to a proximity probe, and specifically a probe manufacturered by Bently Nevada (now GE Power). Actually the negative DC voltage is bias power that the sensor requires in order to return a voltage (also negative) corresponding to the actual gap between the sensor tip and the target. The gap between the sensor and target must be set prior to using the probe at a distance relative to the probe sensitivity, this gap varies depending on what type of probe is being used. The output DC voltage from the signal conditioner used by the probe can be checked to set the gap. At the far end of the sensitivity range, the output voltage may be close to -24 VDC. As this gap decreases the voltage becomes more positive and at a nearly closed gap it is almost 0 VDC.
There is no good reason for this supply voltage or output voltage to be negative, as other types of proximity probes use positive bias voltage. I suspect that the Bently company used a minus bias voltage for product protection. They have been selling their hardware for 50 years, so it\\'s impossible to switch now.

Pt100 is a type of Resistence temprature detector(RTD).It is made by platanium.It means 100 ohms,resistance of 0 degree c.

An electric potential is developed when a conductor is moved across the magnetic field. In most electrical machinery the conductor is a wire. The principle is equally applicable to a moving, electrically conductive liquid. The primary device of commercial magnetic meters consist of straight cylindrical electrically insulated tube with a pair of electrodes nearly flush with the tube walls and located at opposite end of a tube diameter. This device is limited to electrically conducting liquids. The magnetic meter is particularly suited to measurement of slurries and dirty fluids.

Turbine meters consist of straight flow tube within which a turbine or fan is free to rotate about it s axis which is fixed along g the centre line of the tube. Mostly, a magnetic pick up system senses the rotation of the rotor through the tube walls. The turbine meter is a flow rate device
, since the rotor speed is directly proportional to the flow rate. The output is usually in the form of electric pulses from the magnetic pick up with a frequency proportional to the flow rate.

The most common range for differential range for liquid measurement is 0-100. This range is high enough to minimize the errors caused by unequal heads in the seal chambers. It is also dependent on the differences in the temperature of the load lines. The 100 range permits an increased in capacity up to 400. While decrease down up to 20 by merely changing the range tubes or range adjustments.

Variable area meters are special form of head meters. Where in the area of flow restrictor is varied. So as to hold the differential pressure constant. The rota meter consists of a vertical tapered tube through which the metered fluid flows in upward direction. As the flow varies the ‘float’ rises or falls to vary the area of the passages that the differential across it balances the gravitational force on the ‘float’. The differential pressure is maintained constant. The position of the ‘float’ is the measure of the rate of flow.

When, in some processes, e.g. batch processes, long transient responses are expected during which a sustained deviation is present the controller integral action continuously drives the output to a minimum or maximum value. This phenomenon is called ‘integral saturation of the control unit’. When this condition is met, then this unit is de-saturated.

In automatic reference junction compensation, variable nickel resistor is used. As the temperature changes, so does its resistance. This reference junction compensator is located, so that it will be at the temperature of the reference junction. The reference junction is at the poset where the dissimilar wire of the thermocouple is rejoined. This joint is invariably at the terminal strip of the instrument.

A solenoid is electrically operated valve. It consists of solenoid coil in which magnetic plunger moves. This plunger is connected to the plug and tends to open or close the valve. There are two types of solenoid valves:

1. Normally Open
2. Normally closed

Use: It is used for safety purpose in different electric work

A ratio control system is characterized by the fact that variations in the secondary variable don’t reflect back on the primary variable. A ratio control system is the system where secondary flow is hold in some proportion to a primary uncontrollable flow.

If we assume that the output of a primary transmitter is A. and the output of the secondary transmitter is B, and that the multiplication factor of the ratio relay is K, then for equilibrium conditions which means set valve is equal to measured valve, we find the following relation:

KA-B=0 or B/A = K where ‘K’ is the ratio setting off the relay.

1. Emitter +ve of meter and base -ve output =Low resistance
2. Emitter -ve of meter and base +ve output =High resistance
3. Collector +ve and base -ve output =Low
4. Collector -ve and base +ve output =Low

Emitter: Collector = High resistance
PNP: Opposite Results

A controller which generates an output signal by motion of its parts. The increase in the baffle is to move towards the nozzle. The nozzle back pressure will increase. This increase in the back pressure acting on the balancing bellows, will expand the bellows. The nozzle is moved upward due to this. The nozzle will move until motion almost equals the input baffle motion.