1. Explain what is the use of double seated valve?
In double seated valves the upward and downward forces on the plug due to reduction of fluid pressure are nearly equalized. It is generally used on bigger size valves and high pressure systems. Actuator forces required are less.
2. Tell me what happens if transmitter wires get shorted?
The barrier if installed limits electrical energy flowing into hazardous area. If there is no barrier, typically a fuse in the power distribution system will blow.
3. Explain how pressure limit switch occurs?
A pressure switch typically consists of a spring-loaded plunger actuated by process pressure to operate a micros witch. The pressure at which the micro switch operates is dependent upon spring force, which may be varied by an adjusting nut.
4. Tell me what is the use of single seated valve?
The single seated valve is used on smaller sizes where an absolute shut off is required. The use of single seated valve is limited by pressure drop across the valve in the closed or almost closed position.
5. Tell me how the sizes of proximity switches are selected?
The sizes of proximity switches are selected based on sensing distance and mounting requirements.
6. Explain what will happen if thermocouple wires get shorted?
A cold junction compensated instrument will typically indicate temperature of the location where the T/C wires are shorted.
7. Explain what is solenoid valve? Where it is used?
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
8. Tell me how D.P. transmitter can be applied to close tank?
In closed tank, bottom of the tank is connected to the high pressure side of the transmitter. Top of tank is connected to the lower pressure side of the transmitter. In this way vessel pressure can be measured.
9. Tell me what is SPDT relay?
The term SPDT refers to single-pole-double-throw relay contact.
10. Do you know types of instrument cables?
IS cables & NIS cables
IS – Intrinsic safety & NIS – Non Intrinsic safety
Which cable to use, depends upon hazards condition.
11. Explain difference between PSV & PRV?
A pressure safety valve is used to relieve overpressure by releasing relatively small volumes of material and installed on smaller vessels. A pressure relef valve is used to relieve overpressure by releasing relatively large volumes of material and installed on large vessels/tanks.
12. Tell me how do you identify an orifice in the pipeline?
An orifice tab is welded on the orifice plate which extends out of the line giving an indication of the orifice plate.
13. Explain construction of micro switch?
A microswitch is a small switch consisting of a tiny springloaded plunger which operates an electrical contact when actuated. This arrangement is encaged in a plastic housing. It is very commonly found in pressure switches.
14. Explain what is adapter flange?
It is a transmitter part for allowing process connection to pipe/tube.
15. Tell me what is the full form of ASTM standard?
ASTM stands for American Society for Testing of Materials.
16. Explain application of PCV?
A pcv is a self-actuated pressure control valve used to maintain upstream or downstream pressure at a set pressure using force from the process fluid if necessary.
17. Do you know what is Zero speed?
In some equipments a speed below a threshold value but close to zero is referred to as zero speed.
18. Explain what is the sensor used in coriollis mass flow meter to measure density?
Density is measured here by measuring the resonant frequency of a vibrating U-tube.
19. Tell me when do we use SOV of rating 110vac and 110vdc?
Primarily depends upon the availability of reliable power supply source.
20. Tell me what is differences between DCS & PLC?
Distributed control system(DCS) and Programmable logic controller. These are the control systems which handles fields I/Os.
21. Explain what does DN stand for in valve size?
DN stands for Diameter Nominal. It is a flange size of the valve.
22. Do you know what is the working of Rota meter?
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.
23. Tell me what is control valve sizing?
Control valve sizing refers to calculation of flow capacity of a valve for a given fluid, operating conditions and piping arrangement.
24. Tell me in which applications do we use 4 wire RTD?
Three wire is a better alternative. Primary objective of 3-wire and 4-wire ararrangements to eliminate effects of lead resistance on temp. measurement.
25. Do you know instrument JB's?
Instrument JB's depend upon hazards area classification.
JB's also can be IS or NIS
For IS signal IS JB's used for NIS signal NIS JB's used
For analog I/p & o/p signal we can use same JB.
But for Digital I/p & o/p we have to use separate JB's. Because digital outputs are powered signal, by wrong connection there may be chances to damage the card.
For DCS I/p & o/p and PLC I/p & o/p we used separate JB's.
26. Tell me how Control loop should be tuned in process loop?
You may use Ziegler-Nichol's method ( open loop / closed loop ) or special tuning software tools.
27. Tell me how is cold junction compensation in thermocouple carried?
This is typically performed in modern programmable instruments by means of measuring actual reference junction temperature using a temperature sensor mounted close to the ref. junction and compensating for the same using appropriate look-up table stored within the instrument's memory.
28. Tell me what does BENCH RANGE refer to in an actuator?
The bench range/set of an actuator/control valve combination refers to the air pressure required to fully open/close the valve out of line when it is not subjected to process fluid forces.
29. Explain how spectacle blind occurs?
A spectacle blind is a blind plate and a plate with line size bore connected together and installed between pipe flanges. This blind may be reversed to blind/deblind a line. It also provides a visual clue as to whether a line is blinded at that location or not.
30. Explain how would you choose differential range?
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.
31. Tell me what is the Relationship between AWG & mm2?
A) AWG was used prior SI standards to specify the diameter of the core of the electrical wires.
B) Both awg and mm2 are used to specify size of cross section of a wire/conductor. The relatioship is found in a standard table – westermann's tables.
32. Tell me what are examples of temperature elements?
Thermocouples & RTD's.
33. Explain what is Ground Loop? Preventive steps to avoid ground loop?
When ground wiring is not done properly, grounding of various points is not effective and potential differences exist between them resulting in currents flowing between them. This leads to measurement errors and is not desirable. It can be eliminated by proper ground wiring.
34. Explain what is force balance principle? State some of its' advantages?
Force balance principle: A controller which generates an output signal by opposing torque. The input force is applied on the input bellows which moves the beam. This crackles nozzle back pressure. The nozzle back pressure is sensed by the balancing bellows which brings the beam to balance. The baffle movement is very less about 0.002 for full scale output.
Advantages:
a. Moving parts are fewer.
b. Baffle movement is negligible
c. Frictional losses are less
35. Explain why control mode specification is given in the data sheet of the transmitter?
Not mandatory. This is controller specification. Direct OR reverse.
