Tuesday, November 6, 2012
Instrumentation Questions for Freshers - 2
1. Briefly explain
the different methods of
level measurement?
There are two ways of measuring level:
1. Direct
Direct level measurement:
(a) Bob and tape:
A bob
weight and measuring Tape provide
the most simple And direct
method of liquid leve glass:
This consists of a graduated glass tube mounted on the side of the vessel. As the level of
the liquid in the vessel
change, so does the level of the
liquid in the glass
tube.
Indirect
level measurement: (A) Pressure gauge:
This is the simplest method, for pressure gauge is located at the zero level of the
liquid in the vessel. Any rise in level causes an increase
of pressure which can be measured by a gauge.
(b) Purge
system:
In this method a pipe is installed
vertically with the open and at zero level. The other
end of the pipe is connected to a regulated air r supply and to a pressure
gauge. To make a
level measurement the air supply is adjusted
so that pressure is slightly higher than the pressure due to height of the liquid. This is accomplished by regulating the air pressure until bubbles cab be seen slowly leaving
the open end of
the pipe
The air pressure
to the bubbler pipe is minutely
in Excess of the liquid pressure in
the vessel, so that Air pressure indicated is a measure of the
level in The tank.
The method above is suitable
for open tank applications. When a liquid is in a pressure
vessel, the liquid column pressure can't be used unless the vessel pressure
is balanced out. This is done through the use
of different pressure meters.
(c) Differential pressure meter:
Connections are made at the vessel top and bottom, and to the two columns of the D.P. meter. The top connection is made to the L.P. column of the
transmitter and the bottom to H.P.
column of the transmitter. The difference
in pressure in the vessel is balanced
out, since it is fed to both the
column of the meter. The difference in pressure deducted
by the meter will be due only to the changing, level of
the liquid.
(d) Displacer
type level measurement:
The leveltrol is one of the most common instruments used measuring
level in closed tanks. This instrument works of Archimedes principle. The displacer in immersed in the
liquid due to which there is loss of weight
depending on the specified gravity of the
liquid. This displacer hangs freely on a knife transmitted
to the pneumatic or electronic
counterpart at the other end.
2. Explain how you
will
measure level with a
different pressure transmitter.
The bottom
connection of the vessel
is connected to high pressure side of
the transmitter.
Different Pressure = H X D
This difference pressure is applied
to H.P. side of the transmitted and calibrated.
3. How is D.P.
transmitter applied to a close tank?
In close tank the bottom of the tank is connected
to the high pressure side of the transmitter and top of the tank
in connected to L.P. side
of the transmitter. In this way the
vessel pressure is balanced.
4. How is D.P.
transmitter applied to an open tank?
On an open tank level measurement the L.P. side is vented to atmosphere. Whatever
pressure acts is on the
H.P. side which is a measure of
level.
SPAN = (X) (Sp. Graf)
ZERO SUPPRESSION = (Y) (Sp.Grav)
5. How is D.P
transmitter applied to a close tank & open tank with Dry leg?
Span = (X) (GL)
HW at minimum level = ( Z )
( GS ) + ( Y ) ( GL )
HW at maximum level = (
Z ) ( GS ) + (
X + Y ) ( GL )
Where:
GL = Specific
gravity of tank liquid. GS = Specific gravity of seal
liquid.
HW = Equivalent head of
water. X, Y & Z are shown
in fig (1.1)
Example:
Open tank with X = 300
inches
Y = 50
inches Z = 10 inches GL = 0.8
GS = 0.9
Span = (300)
(0.8) = 240 inches
HW at minimum level
= ( 10 ) ( 0.9 ) + (
50 ) ( 0.8 ) = 49 inches
HW at maximum level = (10 ) ( 0.9 ) + (
50 + 300 ) ( 0.8 ) = 289
inches
Calibrated range = 49
to 289 inches head of water
2.3 DEFINITION OF INSTRUMENT RANGE
Legend:
Range Defined in IEC 60902 as follows: The region of values
between the lower and upper
limits of the quantity under consideration.
It is expressed by stating the lower and upper limits
(e.g. minus 1
to 10 bars (ga)). Defined in IEC 60902
as follows: The algebraic difference between
the upper and lower limit
values of a given range.
It is expressed as a figure and unit of measurement
(e.g. 8 bars).
LRL Lower Range Limit; the lowest
quantity that a device
is designed to measure.
URL Upper
Range Limit; the highest
quantity that a device
is designed to measure.
Instrument range the region in which the instrument is designed to operate. It is a physical capability of the device. The region limits are expressed by stating the LRL
and URL.
Instrument minimum span the minimum distance
between the URV
and LRV for which the instrument is designed.
It is a physical limitation
of the device.
Instrument maximum span the maximum distance between the URV
and LRV for which the instrument is designed.
It is a physical limitation
of the device.
LRV Lower Range Value; the lowest quantity that a device
is adjusted to measure.
URV Upper
Range Value; the highest quantity that a device
is adjusted to measure.
Adjusted range The measurement region.
It is expressed by stating
the LRV and URV.
Adjusted span The distance between
the URV and LRV.
Example: A differential pressure transmitter is used to measure the level in a vessel,
using a wet reference leg.
0% level corresponds with a
differential pressure of -800 mbar and 100% level with - 100 mbar. The catalogue of the selected
transmitter lists -1800/+1800 mbar for LRL/URL respectively
and span limits of 300 to 1800 mbar, so:
Instrument range
|
=
|
-1800 to +1800 mbar
|
Instrument minimum/maximum span
LRV / URV Adjusted range
|
=
=
=
|
300 / 1800
mbar respectively;
-800 / -100 mbar respectively;
-800 to -100 mbar.
|
Adjusted span
|
=
|
700 mbar.
|
NOTE: ‘Adjusted’ range and ‘adjusted’ span are frequently referred to as ‘calibrated’ range and
‘calibrated’ span. This term is however only correct, if a calibration facility is used to set the LRV
and
URV. For ‘intelligent’ measuring devices,
the supplier is usually
calibrating the device
at the
LRL/URL and the user is setting the required LRV and URV by remote communication.
2.4
SELECTION OF RANGES
The accuracy (2.8) and adjusted range of
an instrument should be selected to cover the
operating window (2.1), which includes
applicable abnormal operation and alternative
operating modes. Unless otherwise stated, the normal design
value should lie between
50% and 75% of
the adjusted range
NOTE: For certain applications it might not be possible to combine all normal and abnormal operating conditions in one measurement at the required
accuracy. In such cases, a case-by-case analysis
should disclose whether additional instruments are required or the accuracy requirements
and/or operating window may be relaxed. It might be acceptable
to present measured values during
some of the abnormal operating cases at a lower accuracy or it might be justifiable for the
measurement not to produce a sensible
signal under some of the abnormal process conditions during
start-up, commissioning, regeneration, emergency conditions and the like.
IPF transmitters
should have the same instrument range, adjusted range and accuracy
as corresponding process transmitters in order to facilitate measurement comparison. For details and exceptions, see DEP
32.80.10.10-Gen. Trip settings should lie between 10%
and 90% of the adjusted
range.
The LRV should be selected so that
the displayed result
represents the zero or sub-zero value of the process
variable (e.g. 0-150 tons/day, 0-100% level, 0-10 bar (ga), -1/+3 bar
(ga), 0-500 °C, -50/+50
°C etc.). Elevated
zero’s (100-300 tons/day, 100-200
°C) should be avoided.
6. What
is purge level system?
This method is also known as bubbler method of level measurement. A pipe is installed
vertically with its open end at the zero level. The other end of the pipe is connected to a regulated air supply and to a pressure gauge or to ^P transmitter. To
make a level measurement the air supply is adjusted
so that pressure is slightly higher than the pressure due to the height of the liquid. This is accomplished by regulating the air pressure
until
bubbles can be seen slowly
leaving the open end of the pipe. The gage then measures the
air pressure needed to over come the pressure
of the liquid.
/\ P = H X D
USE: On for corrosive liquids where the transmitter cannot be directly connected to process eg... Acids,
Some organic liquids.
7. Explain the working of a leveltrol.
The leveltrol
is used for measuring
level of liquids in a closed
vessel.
1.
PRINCIPLE. : It works on Archimedes principle "The loss in weight of a body
immersed in a liquid
is equal to amount of liquid displaced by
the body". The leveltrol basically consists of
the following :
2. DISPLACER: It is consists of a cylindrical shape pipe sealed and filled inside with sand
or some weight.
The purpose of this is to convert change in level to primary
motion. The variation in buoyancy
resulting from a change in liquid level varies the
net weight of the displacer
increasing or decreasing the load on the torque arm. This change is directly proportional to change in level and specific gravity
of the liquid.
3. RELAY: Amplifies pressure variations at the nozzles.
4. REVERSING ARC: It is used for the following purposes.
• Motions take of from
Torque tube.
• Means of reverse control action.
• Adjustment for specific gravity.
5.
PROPERTIONAL UNIT. : Converts primary motion to a proportional
output air pressure.
6. CONTROL SETTING UNIT: Provides
motions of varying the set point.
8. Explain the working an electronic level trol.
The variation in buoyancy resulting from a change
in liquid level,
varies the net weight of
the displacer increasing
or decreasing the load on the torque arm. This change is directly
proportional to the change in level and specific gravity
of the liquid. The resulting torque tube movement varies the angular
motion of the rotor
in the RVDT (Rotary Variable Differential. Transformer) providing a voltage change proportional to the rotor displacement, which is converted and amplified
to a D.C. current.
9. How will you
reverse an action of the leveltrol?
The reversing are serves as motion take off arm from the torque tube. It is provided
with a slot on each side of the center so that link can be connected
either for reverse or direct action.
10.
What is interface level?
How do you calculate it?
When a vessel is filled with two liquids of two different specific gravities the level
measurement refers to as interface level.
DP = H (D - d)
On a level
set the difference of two specific gravities.
11. How will you
calibrate a leveltrol in the
field?
Calculation # 1 If the calibrating liquid is water: Process Liquid Density / 1* Displacer height = mm of
H2O.
Calculation # 2 If the calibrating liquid is other liquid: Process Liquid Density / Calibrating liquid density * Displacer
height = mm of H2O. (Calibrating liquid height in mm).
1. First close both the primary isolation valves and drain
the liquid inside the chamber.
2. Adjust the zero to get 0% output.
3. Connect a transparent PVC tube
to the drain point as shown in hook up.
4. Fill it to the center of the top flange.
5. Adjust the specific gravity
or span adjustment (Electronic Level).
6. Fill it up
to 50 %, check linearity.
12. How will you
calibrate on interface level
control. ?
On an interface leveltrol there are two liquid of two different specific gravities.
1. The level will be zero when
it is full of lighter liquid.
Zero % level
= H X d. H = Displacer length
d = Specific
gravity of lighter
liquid.
2 The level will be 100
% when it is full of heavier liquid.
100 % level = H
X D.
D = Specific
gravity of heavier liquid. Calibration with water:
1. Fill H X d level with water adjust zero.
2. Fill H X D level with water adjust Sp. gravity or span.
3. Check linearity.
Displacer length:
L in mm X
Density Low : dl in Kg / 1
Density High : DH in Kg
/ 1
Alarm / Trip point %: X HH Trip Point: Y in mm
Y= L* (DH X + (1- )
* dl)
100 100
13. How will you
apply wt. lest calibration to a
leveltrol.
Wt. test calibration method:
1. Remove the displacer from the torque
arm.
2. Apply equivalent weight on the torque
arm that is equal to the wt.
of the displacer.
Adjust zero % output.
3. For Span :
V = πr2h
Loss in weight
= Wt. of float
- wt. of the float immersed in liquid
Loss in weight
= [ wt. of float - Vol. x d ] Span wt. = (wt. of float - Loss in wt.)
r = radius
of the displacer. h = ht. of
displacer.
4. Apply equivalent wt. equal to the (Wt. of float - Loss in weight).
Adjust Span to get 100 % out put.
5. To check linearity applies
average of the
two weights.
14.
What will happen if the displacer has fallen down while
in line?
The output will be maximum.
15.
What will happen if the displacer has a hole in it while in line?
The output will be minimum.
16.
What is the used of Suppression and elevation?
Suppression and elevation are used on Level applications where (1) transmitters are not
mounted on some level (2) Wet leg. I.e. condensable vapors are present.
17.
What are the limitations of leveltrol?
The limitations
of a level control
that it cannot be used for
lengths more than 72
inches.
18. How will you
commission D.P. transmitter in field in pressurized vessel.
1. Close both the isolation valves, Vent the H.P.
side.
2. Fill it with the sealing
liquid.
3. Open the
L.P. side vent valve.
4. Adjust zero with suppression
spring.
5. Close the L.P. side vent valve.
6. Open both
the isolation valves.
19. How will you
check zero of a level D.P. transmitter while is line?
1. Close both the isolation valves.
2. Open the
vent valve on L.P.
leg and H.P. leg
drain.
3. Check and adjust
zero if necessary.
20.
Explain the working of an Enraf
level gauge?
The Enraf precise
level gauge are based on servo powered
null-balance technique. A
displacer serves as a continuous level
sensing element. Principle:
A displacer with a relative density higher than that of the product to be measured is suspended from a stainless steel wire B that is attached to a measuring drum. A two phase
servo meter controlled by a capacitive balance system winds or unwinds the measuring wire until the tension
in the weighing springs is in balance with the weight of the displacer partly immersed in the liquid. The sensing system in principle
measures the two capacitance formed by the moving center sensing rod E provided
with two capacitor plates and the side plates. In
balance position
the capacitances are of equal value.
A level variation wills
a difference in buoyancy of the displacer. The center sensing
rod will move in the
direction of one of the side
capacitor plates. This causes a difference in value of
these capacitances. By an electronic circuit this change is detected and integrated.
During the rotation of the servo motor the cam driven
transmitter continuously change the voltage pattern to a remote indicator of which the receiver
motor drives a counter
indicating level variation.
Instrumentation Questions for Freshers
Quick Facts About Control Valves
Labels: common interview instrumentation questions, dp type transmitters, epc-school, instrument questions, level questions, Rahul Kapoor
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