Thursday, November 1, 2012
Instrumentation Questions for Freshers
1. What
are the process Variables?
2.
Define all the process Variable and
state their unit of measurement. ?
Flow: Any fluids or liquids flowing from one place to another place is called flow and it
is defined as volume per unit of time at specified temperature and pressure Conditions, is generally measured by positive-displacement or rate
meters.
Units: kg / hr, litter / min, gallon
/ min, m3 / hr, Nm3 / hr. (Gases)
Pressure: It is defined as Force per unit Area. P
= F/A
Units
: bar, Pascal,
kg / cm2, lb / in2.
Level: The height
of
the
water
column, liquid and powder
etc.,
at
the
desired
measurement of height between minimum level points to maximum level point is called level. The measurement principle is, head pressure method.
Units: Meters, mm, cm, percentage.
Temperature: It is the degree
of hotness or coldness of a body
is called temperature.
Units : Degree Centigrade, Degree Fahrenheit, Degree
Kelvin, Degree Rankin.
Quality: It deals
with analysis.( pH, % CO2, % 02, Conductivity, Viscosity )
3. What
are the primary elements
used for flow measurement?
The primary elements used
for flow measurement are:
• Orifice Plate.
• Venturi tube.
• Pitot tube.
• Annubars.
• Flow Nozzle.
• Weir & Flumes.
4. What
are the different types of orifice plates and
state their uses?
The different types of
orifice plates are:
• Concentric.
• Segmental.
• Eccentric.
• Quadrant Edge.
Concentric:
The concentric orifice
plate is used for ideal liquid as well as gases and steam service. This orifice plate beta ratio fall between
of 0.15 to 0.75 for liquids and 0.20 to 0.70 for gases,
and steam. Best results
occur between value of 0.4 and 0.6. Beta ratio means ratio of
the orifice bore to the internal pipe diameters.
(45º beveled edges
are often used to
minimize
Friction resistance to flowing fluid)
Eccentric :
The eccentric orifice plate has a hole eccentric. Use full for measuring
containing solids, oil containing water and wet steam. Eccentric
plates can be used either flange
or vena contracta taps, but the tap must be at 180º or 90º to the eccentric opening.
Eccentric orifices
have the bore offset from center to Minimize problems in services
of solids-containing materials.
Segmental :
The segmental orifice place has the hole in the form segment of a circle. This is used for colloidal and slurry flow measurement. For best accuracy, the tap location should
be 180º from the center
of tangency.
Segmental orifices provide
another version of plates useful for solids containing materials.
Quadrant Edge : It common use in Europe and are particularly useful
for pipe sizes less than 2 inchs.
Quadrant edge orifices produce a relatively constant Coefficient of discharge for services with low Reynolds numbers in the range from 100,000
down to 5,000.
5. How do you identify an
orifice in the pipe line?
An orifice
tab is welded on the orifice plate which extends outer
of the line giving an indication of the orifice
plate.
6. Why is the orifice
tab provided?
The orifice
tab is provided due to the following reasons.
• Indication of an orifice plate in a line.
• The orifice diameter is marked on it.
• The material
of the orifice plate.
• The tag no. of the orifice plate.
• The mark the inlet of an orifice.
Advantages and
Disadvantages of Orifice Plates
Advantages of orifice plates include:
• High differential pressure
generated
• Exhaustive data available
• Low purchase price and installation cost
• Easy replacement
7. What is Bernoulli's theorem and where it is applicable?
Bernoulli's theorem states the "total energy of a liquid flowing
from one point
to other remains constant." It is applicable for non compressible liquids.
8. How do you identify the H. P. side or inlet of
an orifice plate in line?
The marking is always done H. P. side of the orifice tab which gives an indication of the
H. P. side.
9. How do you calibrate a D. P. transmitter?
The following steps are to be taken which calibrating:
1. Adjust zero of
the Tx’r.
2. Static pressure test : Give equal pressure on both sides of the transmitter.
Zero should
not shift. If it is shifting carry out static alignment.
3. Vacuum test : Apply equal vacuum to both the sides. The zero should not shift.
4. Calibration Procedure:
• Give 20 psi air or
24Vdc supply to the transmitter.
• Vent the L.P. side to atmosphere.
• Connect output of the Instrument to a standard test gauge or Multimeter and adjust zero.
5. Apply required
pressure to high pressure side of the transmitter and adjust the span.
6. Adjust zero again if necessary.
10. What is the seal
liquid used for filling
impulse lines on crude
and viscous liquid?
Glycol.
11. How do you carry out piping for
a
Different pressure flow transmitter on liquids, Gas and steam services Why
?
Liquid lines: On liquid lines the transmitter is mounted below the orifice plate because liquids have a property of self
draining.
Gas Service: On gas service
the transmitter is mounted above the orifice plate because
Gases have a property of self
venting and secondly condensate formation.
Steam Service: On steam service the transmitter is mounted below the orifice plate with
condensate pots. The pots should be at the same level.
12. An operator
tells
you
that
flow
indication is more, How would you start checking?
• First flushing the transmitter.
Flush both the impulse lines.
Adjust the zero by equalizing if necessary. If still the indication is more then.
• Check L.P. side for
choke. If that is clean then.
• Check the leaks
on L.P. side. If not.
• Calibrate the transmitter.
13. How do you do a zero checks on a D.P. transmitter?
Close one of the valve either
H.P. or L.P. open the equalizing valve. The O/P should read
zero.
14. How would you do Glycol
filling or fill seal liquids
in seal pots 7 Draw and
explain.
• The procedure for glycol filling is :
• Close the primary isolation
valves.
• Open the vent on the
seal pots.
• Drain the used glycol if present.
• Connect a hand pump on
L.P. side while filling the H.P.
side with glycol.
• Keep the equalizer valve open.
• Keep the L.P. side valve closed.
• Start pumping and fill glycol.
• Same repeat for L.P. side by connecting pump to H.P. side, keeping equalizer
open and H.P. side isolation
valve closed.
• Close the seal pot vent valves.
• Close equalizer valve.
• Open both the primary isolation
valves.
15. How do you calculate new factor from new range using
old factor and old range?
Q1 √ ∆P1 Q1 = Old flow
= Q2 = New flow
Q2 √ ∆P2 √ ∆P1 = Old DP
√ ∆P2 = New DP
Q2 × √ ∆P1 = Q1 × √ ∆P2
F = √ ∆P x
10 (or) O/P % = √ I/P%
X 10
O/P % = Flow,
I/P % = ∆P
16. How will you
vent air in the D.P. cell? What
if seal pots are used?
• Air is vented by opening the vent plugs on a liquid service transmitter.
• On services where seal pots are used
isolate the primary
isolation
valves
and
open the vent valves. Fill the line from the transmitter drain plug with a pump.
17. Why
flow is measured in square
root?
Flow varies directly
as the square root of different pressure F = K square root of D/P. Since
this flow varies as the square root of differential
pressure the pen does not directly indicate flow. The flow can be determined by
taking the square root of the pen. Say the pen
reads 50% of chart.
18.
What is absolute pressure?
Absolute pressure is the total pressure present in the system
Abs.
pressure = Gauge pressure + Atm. pressure.
19.
What is absolute zero pressure?
Absolute zero = 760 mm
Hg Vacuum.
20.
What is the maximum Vacuum?
The maximum Vacuum
= 760 mm Hg.
21.
What is Vacuum?
Any pressure
below atmospheric pressure is vacuum.
Atm = 760 mm
22. What are the primary elements for measuring pressure?
The primary elements used for measuring pressure are:
• Bourdon tube.
• Diaphragm.
• Capsule.
• Bellows.
• Pressure Springs.
The above are known as elastic deformation pressure elements. Type of Bourdon
tubes.
• 'C ' type.
• Spiral.
• Helix.
Diaphragm: The diaphragm
is best suited for low pressure
measurement.
Capsules : Two circular diaphragms are welded together to form a pressure capsule.
Material Used: phosphor bronze, Ni-spanc stainless steel.
Bellows :
Bellows is a one -
piece, collapsible, seamless metallic unit with deep folds
Formed from very tin walled
tubing.
Material used: Brass,
phosphor bronze, stainless steel.
Used for high
Pressure.
Pre.spring: Pressure springs of helical or spiral
shape used for measuring
high pressures.
23. How
will you calibrate an absolute pressure
transmitter
using
vacuum manometer. Range 0-400mm abs?
The procedure for calibration is as follows:
• Connect air supply
to the transmitter.
• Connect a test gauge of 0-1.4 Kg/cm2 to the output.
• Connect Vacuum pump with tee off
to the manometer.
• Apply 760 mm Vacuum (or nearest) and adjust zero.
• Apply 360 mm Vacuum adjust span. (760 - 360 = 400 mm abs.)
24. You are given a mercury manometer range 0 -760 mm? A vacuum gauge reads
60 mm vacuum.
The test manometer reads 50 vacuums? Which of the two in correct?
The transmitter is correct because
760 - 50 = 710 mm abs.
25. Why is an inclined manometer used?
It is used to extend the scale of the instrument. Because the manometer is at an angle to the vertical.
26. What is the principle of a pressure
gauge?
Pressure
works on Hooks law.
Principle: "Measuring the stress in an elastic medium"
27. Draw and explain a pressure gauge? What is the
used of a Hair spring?
The parts of
a pressure gauge are:
1. 'C' type Bourdon tube.
2. Connecting
link.
3. Sector gear.
4. Pinion gear.
5. Hair Spring.
6. Pointer.
7. Dial.
Uses of Hair Spring: Hair spring serves two purposes namely
• To avoid backlash
error (eliminate any play into linkages).
• It serves as a controlling torque.
Labels: common interview instrumentation questions, epc-school, flow measurement, instrument questions, pressure
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