12. What are two wire and three wire R.T.D. system?
‘Two wire R. T.D. system: Two wire RTD system use for short distance like a
compressor field local panel.
‘Three wire System: Three wire systems use for long distance coke a field to
control Run. The third wire is used for compensation of lead wire resistance.
13. Why is a converter used in a temp. recorder?
‘The converter is designed to convert D. C. input voltage into an A. C. input
voltage proportional in amplitude to the input.
14. Why are Thermowells used?
In numerous application it is neither desirable nor practical to expose a
temperature sensor directly to a process material. Wells are therefore used to
protect against damage corrosion, erosion, abrasion and high-pressure
processes. A thermowell is also useful in protecting a sensor from physical
damage during handling and normal operation.
15. What is the specialty of thermocouples lead wires?
‘They should be of the same material as the thermocouple
16. What is the difference the Wheatstone bridge and a potentiometer?
‘The difference between a potentiometer and a Wheatstone bridge measuring
instrument is that potentiometer is a voltage measuring instrument and
Wheatstone bridge is a current measuring instrument.
17. Explain tuning of controllers
‘Tuning basically involves adjustment of proportional. Integral and derivative
parameters to achieve good control. The gain, time constants, and dead times
around the loop will dictate the settings of various parameters of the controller.
Tuning methods = are —broadly ~— classified ~— into _two:
1. Closed Loop Method: e.g, Ultimate. Gain Method.
2. Open Loop Method: e.g., process Reaction curve
18, What is the use of single seated valve?
The single seated valve is used on smaller sizes, and in a valve of larger sizes,
where an absolute shut off is required. The use of a single seated valve is limited
by the pressure drop across the valve in the closed or almost closed position.
19, What is the use of double seated valve?
In double seated valves the upward and downward forces on the plug due to the
reduction of fluid pressure are nearly equalized. It is generally used on bigger
size valves and high-pressure systems, Actuator forces required are less i.c., a
small size actuator.
20. What types of bonnets would you use of high temp? and very low temp.?
High temperature: Bonnets are provided with radiation fins to prevent glad
packing from getting damaged. On very low temperature: Extended bonnets are
used to prevent gland packing from getting freeze.
21. How will you work on a control valve while it is line?
While the control valve is in line or in service, it has to be bypassed and secondly
the line to be depressurized and drained.
22. What is the use of a valve positioner?
Quick Action control valve.
Valve hysteresis.
Valves used on viscous liquids.
Split Range.
+ Line pressure changes on valve.
+ Valve Bench set not standard.
+ Reversing valve operation.
23, What is transmitter?
A transmitter is an electronic device that is generally mounted in the field in
close proximity to a sensor. The sensor (also known as a transducer) measures
a physical variable such as temperature or pressure and outputs a very low-level
electronic signal. The basic function of the transmitter is to provide the correct
electrical power to turn on (or excite) the sensor then to read the low-level sensor
signal, amplify it to a higher-level electrical signal and send that signal a long
distance to a control or read-out device.
24, What is a “Smart” Transmitter?
A “Smart” transmitter is a transmitter that uses a microprocessor as the heart
of the electronics. They are useful in remote area communication and calibration
process. The sensor signal transmitter into a unified standard signal: 0/4-
20mADC, 1-5VDC, 0-10VDe. The output control signal is standard such as 4-
20mA. A good smart transmitter id insensitive to noise.
25. Difference between 2 wires, 3 wire and 4 wire transmitters.
In 2 wire transmitter the power and signal are transmitted through same cable.
In 3 wire transmitter the data signal and power are with respect to common
ground.
In 4 wire transmitter two wires for power supply and two for signals.
26. Why 4-20 mA preferred over 0-20 mA signal?
In 4-20mA signal, 4 represents the minimal value and 20mA is maximum value
in the signal. With 0- 20 mA you cannot distinguish between minimum field
value and connection break, in both situations the output will be Oma.
4-20 mA, internal circuit can distinguish between connection break of minimum
value. Normally when the value is minimum the transmitter will give you 4 mA
while in case of connection breakage it will give 0 mA.
27. What is Actuator?
In a closed-loop contro! system, the part of the final control element that
translates the control signal into action by the control device.
28. What is Field bus?
Fieldbus is a general term for a digital only, high speed communications protocol.
‘The key attribute to Fieldbus communications is higher speed communications
with the possi rs all on the same field
wiring.
of addressing multiple trans
29, What are the flow measuring instruments used in Flow measurement?
Differential pressure meters
Positive displacement
Velocity meters
30. What type of pressure sensors used in pressure measurement?
Manometers
Bourdon tubes
Bellow elements
Diaphragm elements
DP transmitters
31. What is Control Valves?
A control valve is a final control element, which restricts the flow of liquid
through the channel.
32. What are the specifications of the control valve?
Flow medium and operating temperature
Flow rate kg/hr. or Nm3/hr. Max/Min/Normal
Inlet and Outlet pressure: kg/cm2 Max/Min/Normal
Max. allowable diff. Pressure: kg/cm2
A density of medium: kg/m3
Viscosity
CVS: Valve Flow Coefficient
Actuator, Body, Trim, Diaphragm, Diaphragm plate, Actuator stem
Actuator spring, Seat, Travel Indicator, Valve stem, Gaskets, Yoke, Handwheel.
33. Explain Working of RTDs.
Resistance temperature detector (RTD) uses material which change their
resistance which exposed to temperature. Temperature is determined by
measuring resistance and then using the RTD Resistance vs Temp characteristic
to detect temperature,
34. Can I split my one T/C signal to two separate instruments?
The T/C signal is a very low-level millivolt signal, and should only be connected
to one device. Splitting to two devices may result in bad readings or loss of signal.
‘The solution is to use a “dual” T/C probe, or convert one T/C output to a 4-20
mA signal by using a transmitter or signal conditioner; then the new signal can
be sent to more than one instrument
35. Explain Working of Thermocouple:
‘Thermocouple consists of two different metals, which are joint together. When
the junction of two dissimilar metals is exposed to temperature, a voltage
induced across the metals.
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