Interview Questions Answers in Electrical Engineering, Instrumentation

Answers / { sam }

What is the working principal of Mass flow transmitter?

Answer

It is a combination of a flow meter and a density meter
combined either as one unit or two separate instruments and
the final mass flow calculation gets done at a remote
location. Good example of a all in one unit is the Coriolis
meter. You can read the flow, the density or the mass flow
directly from it and configure its output to give you
whatever you need to be displayed on your supervisory system.
The principle it works on is vibration tubes. The variance
in vibration is a direct and proportional result due to the
variance in flow. So the frequency of the vibration is
monitored and converted into the output signal obviously
with some calculations inside to get the density and mass
flow as well.
Good luck

Is This Answer Correct ?

23 Yes

3 No

Question { ABB, 14096 }

I have one DP transmitter of range 0-500mmWC i have to
calculate its range in M3/hr for flow measurement so whats
the formula.

Answer

You first need to have a flow element like a orifice plate
installed in order to get a differential pressure across the
orifice. You then need to measure this differential pressure
with the differential pressure transmitter or a u-tube or
two pressure gauges at various flow rates. If this is not
possible the orifice plate manufacturer will give you the
minimum and max diff pressure across the orifice plate for
various flow rates and liquids. You should already have the
pipe size, the orifice plate details and the liquid you will
use in this application.
Once you have this minimum and maximum diff pressures for
the minimum and max flow rates you can see on the
transmitter range if it will be big enough to measure these
diff pressures. The signal can be send from the transmitter
directly to the DCS and the squire roof extraction can be
done there or you can do the SQ extraction on the
transmitter itself by modifying it's configuration to
measure flow.

Is This Answer Correct ?

12 Yes

2 No

Question { 6871 }

Please anyone answer my question, what is the difference
between single acting and double acting control valve, in
what condition we better choose single acting rather than
double acting

Answer

Normally a control valve is refer to by it's fail position.
This means "what position will the valve move to should the
supply air or control signal to the valve falls away". This
is important to safe guard the process at various places so
some valves will be fail open and some fail close. In order
to have valve as a fail open or close the valve the actuator
have to be spring loaded. So by having the spring on top or
bottom of the actuator piston, will determine if it will be
a FO or FC valve. This kind of valve is also called single
action since it will only have one output from its
positioner to either the top or bottom of the actuator. The
positioner on the valve is also setup as a single acting
positioner since it will only give a single action to the
actuator, the reverse action will be done by the spring. The
problem with this setup is that it is possible that the
process might be so strong or the pressure so high (during a
blow down or ESD shutdown in the plant) that the spring
might in certain instances be to week to push the valve into
the fail position quick enough, due to the back pressure
from the process and can cause damage to the plant or even a
explosion. To make sure that the valve will go to the fail
position we install a double action positioner with two
outputs. One goes to the top of the actuator and one to the
bottom. This is also very helpful to do very accurate and
stable control on a high flow line since the pressure from
the position do the actual control and not spring control
one way and positioner control the other way as in single
acting control valves. It is also solving the problem that
the valve will now be forced into the fail position by the
spring as well as the positioner supply pressure during a
emergency.
In shutdown valves (open/close ESDV's) the same is true and
sometime at critical and high pressure points we use
hydraulics instead of pneumatics as the double acting agent
to make sure the valve will close during a emergency.
So to summarize the double acting action in ESD and control
valve is just there to make sure the valve will do what it
was designed for. Call it a extra fail safe if you want. In
theory not needed since a single acting valve should do the
trick just as well,but in practice you are at time very glad
you did it especially if you look at the kind of pressures
the valves are working on. With those kind of flows and
pressures you don't want to leave anything to chance.

Is This Answer Correct ?

5 Yes

0 No

Question { ABB, 11249 }

Why three wire is using with a RTD?

Answer

You are right, and the temperature indication does not
increase so significantly that it is worth the effort to use
a 3 or 4 wire RTD.
It might increase with something like 0,001Deg C so that is
so small you might as well say there is no difference in the
accuracy of a 2,3 and 4 wire RTD, using a local or smart
transmitter.
In the old days we use to use a 2 wire RTD in the field and
then run a cable say 200m to the temperature indicator. By
the time it gets to the indicator the temperature is
completely different from what it was in the field due to
the cable resistance that add itself the the RTD resistance.
We compensated for that by installing a 3de wire for the
sole propose to measure the resistance of the cable itself
and deduct that from the total resistance measured at the
temperature indicator.
So the actual resistance of the RTD as measured at the
temperature indicator is RTD - RLine1.(or [(Rline1/2)x2]if
you want)
With the 4 wire it makes it more accurate in that you can
now measure line one and line 2. The theory is that the one
line might have a small difference compare to the other line.
So the actual RTD resistance at the temperature indicator is
measured RTD - [(RLine1/2)+(RLine2/2)]
But since we all use the small compact local and smart
temperature transmitters these days, 3 and 4 wires are no
longer needed since the distance from the RTD to the
transmitter is only from about 50 to 500mm and but it seems
it have stayed due to some design engineer always saying,
why buy a 2 wire if you can get the 3 and 4 wire for just
about the same price. It will make the indication just more
accurate, but they never say by how much (0,001Deg C)
Good luck

Is This Answer Correct ?

14 Yes

0 No

Question { 8895 }

Why do we need to install hook-up line differential pressure
type level transmitter when the differential pressure type
level transmitter with capillary are available?

Answer

Capillary level transmitters are the best for just about any
application but they are very expensive compare to a normal
DP Cell with tubing.
The problem with these capillary transmitter installations
are that a lot of installations and designs are done without
flushing rings and equalization tubing and valves. This then
have to be installed afterwords since they are needed during
calibration once you start working on vessels with a process
pressure of 10bar and up. Without these the calibration will
become more inaccurate the higher your vessel process
pressure is. Be aware we still have static alignment
problems even in this day and age of smart technology.
Unless you do a process zero before your calibration you are
going to pick up problems and the only way to do a process
zero is if these flushing rings and tubing is installed from
the HP to the LP tap off points.
Good Luck

Is This Answer Correct ?

1 Yes

0 No

Question { 4563 }

How can we say that accuracy of measurement increases by using 3 or 4 wire RTD?
because for example:
If resistance of RTD element is say r ohms and
Resistance of the cable from transmitter to RTD is R ohms
And if 4 wire RTD is used,

____._____________________ R ohms
|
|_____._____________________ R ohms
/
\
/ r ohms
\
\
|
|______.__________________ R Ohms
|
|_______._________________ R ohms

Then the actual resistance measured will be:
Total = r + R/2 + R/2 = r +R Ohms

Note: r+R instead of r ohms. hence error

Answer

Your maths is not right but actually you are right, and the
temperature indication does not increase so significantly
that it is worth the effort to use a 3 or 4 wire RTD. It
might increase with something like 0,001Deg C so that is so
small you might as well say there is no difference in the
accuracy of a 2,3 and 4 wire RTD, using a local or smart
transmitter.
In the old days we use to use a 2 wire RTD in the field and
then run a cable say 200m to the temperature indicator. By
the time it gets to the indicator the temperature is
completely different from what it was in the field due to
the cable resistance that add itself the the RTD resistance.
We compensated for that by installing a 3de wire for the
sole propose to measure the resistance of the cable itself
and deduct that from the total resistance measured at the
temperature indicator.
So the actual resistance of the RTD as measured at the
temperature indicator is RTD - RLine1.(or [(Rline1/2)x2]if
you want)
With the 4 wire it makes it more accurate in that you can
now measure line one and line 2. The theory is that the one
line might have a small difference compare to the other line.
So the actual RTD resistance at the temperature indicator is
measured RTD - [(RLine1/2)+(RLine2/2)]
But since we all use the small compact local and smart
temperature transmitters these days, 3 and 4 wires are no
longer needed since the distance from the RTD to the
transmitter is only from about 50 to 500mm and but it seems
it have stayed due to some design engineer always saying,
why buy a 2 wire if you can get the 3 and 4 wire for just
about the same price. It will make the indication just more
accurate, but they never say by how much (0,001Deg C)
Good luck

Is This Answer Correct ?

7 Yes

0 No

Question { 6836 }

what are the standards used for earthing instruments in the
field?

Answer

The internal screen is always floating at the instrument.
This means we just put some heat shrink on it to seal it and
then tie it off inside the instrument and just let it lie
there. The overall screen (braiding surrounding the cable
just below the pvc outside)will be in contact with the gland
once you have installed the gland. On the gland thread you
install a copper gland ring and a red IP washer, and the
gland then are attached to the instrument. From the gland
ring you use a six mm bolt and nut with copper washers to
attached a small diameter earth wire (about 4 to 6mm is
fine). This earth wire is then attached to the outside of
the instrument at the earth connection point on it's
housing. Every instrument will have this earth connector on
the outside of it's housing. From there you attached another
earth wire to the earth boss nearby. This boss is normally
just a piece of round bar about 40mm in diameter and about
30mm long and tapped in the middle that is welded to the
structure close by, specially for this earthing of the
instruments. Again it is better to use copper washers. If no
boss is available you can use some other point on the
structure as well like a stainless steel cable tray for
instance but the earth boss should really be part of the design.
On the other side of the instrument cable at the RTU the
internal screen in attached the instrumentation clean earth.
The dirty earth is used only for electrical equipment.
Good luck

Is This Answer Correct ?

3 Yes

0 No

Question { Schenck, 35081 }

What is the difference between Analytical instruments and
other instruments?

Answer

Yes, I see what you mean it can be a bit confusing.
Instrumentation is a very wide field so the easiest to
explain would be to classify all the various areas in
different categories, but they are all part of the
instrumentation field.
Process measuring instrumentation, like pressure,
temperature, flow, density, level, viscosity, conductivity,
PH, redox, distance, angle ext.
Process control instrumentation, like control valve, on/off
valve, solenoids valves, valve positions, feedback positioners.
Monitoring and control instrumentation, like the DCS, SCADA,
PLC's and relay control systems.
Pneumatic instrumentation, like pneumatic valve positioners,
electromagnetic positioners, DP transmitters, level trolls,
switches and pneumatic relays and pistons.
Hydraulic instrumentation, like hydraulic control systems,
pneumatic/hydraulic solenoid valves, hydraulic control
valves, feedback positioners and pistons.
Fire and gas instrumentation, like flame, gas, smoke, heat
detectors and the addressable monitoring and control
systems. CO2, foam, 440VAC foam control valves and deluge
systems and their pneumatic and solenoid control systems
will also fall in this category.
Analytical instrumentation, like oil in water, gas
chromatographic, oxygen content, chlorine content,
florescence, dew point analyzers.
Good luck

Is This Answer Correct ?

21 Yes

25 No

Question { 10495 }

What standards are used for earthing of field instruments?

Answer

Is This Answer Correct ?

5 Yes

4 No

Question { HTA Instrumentation, 14910 }

what is the difference between 3,4 wire RTD sensor?

Answer

Is This Answer Correct ?

10 Yes

2 No

Question { Yokogawa, 42949 }

WHAT IS DIFFERENCE BETWEEN TWO WIRE RTD AND THREE WIRE RTD?

Answer

The temperature indication does not increase so
significantly that it is worth the effort to use a 3 or 4
wire RTD. It might increase with something like 0,001Deg C
so that is so small you might as well say there is no
difference in the accuracy of a 2,3 and 4 wire RTD, using a
local or smart transmitter.
In the old days we use to use a 2 wire RTD in the field and
then run a cable say 200m to the temperature indicator. By
the time it gets to the indicator the temperature is
completely different from what it was in the field due to
the cable resistance that add itself the the RTD resistance.
We compensated for that by installing a 3de wire for the
sole propose to measure the resistance of the cable itself
and deduct that from the total resistance measured at the
temperature indicator.
So the actual resistance of the RTD as measured at the
temperature indicator is RTD - RLine1.(or [(Rline1/2)x2]if
you want)
With the 4 wire it makes it more accurate in that you can
now measure line one and line 2. The theory is that the one
line might have a small difference compare to the other line.
So the actual RTD resistance at the temperature indicator is
measured RTD - [(RLine1/2)+(RLine2/2)]
But since we all use the small compact local and smart
temperature transmitters these days, 3 and 4 wires are no
longer needed since the distance from the RTD to the
transmitter is only from about 50 to 500mm and but it seems
it have stayed due to some design engineer always saying,
why buy a 2 wire if you can get the 3 and 4 wire for just
about the same price. It will make the indication just more
accurate, but they never say by how much (0,001Deg C)
Good luck

Is This Answer Correct ?

5 Yes

7 No

Question { 15144 }

when the instrument tapping on pipe line is located below
and transmitter is located above the tapping point for
liquid service transmitter, what should we do, is there any
additional drain valve is required??

Answer

Cannot answer your question if you cannot clearly ask the
question. What are you referring to level, press?

Is This Answer Correct ?

6 Yes

0 No

Question { 17906 }

can we connect instrument earth pit to electrical earth
pit, if not why?

Answer

Is This Answer Correct ?

2 Yes

12 No

Question { 16469 }

If we interchange HP and LP tapping in drum level
measurement and reverse LRV & URV correspondingly.What is
the difference between the normal tapping(HP,LP) and
reversed tapping(LP,HP)?

Answer

We use these configurations during the calibration of a
Differential Pressure Transmitter (in short DP Cell) in a
level calibration of a closed pressurized vessel. This can
only be used when you are making use of a Diff Press
Transmitter that is piped to the high and low tap off points
on the vessel with stainless steel piping. You cannot use it
on any other type of level measurement device, even if it is
also a Diff Press Transmitter with capillary tubes and pad
cells installed on the H/L tap off points and not stainless
steel piping. When you use capillaries you need to do the
calibration completely differently from normal, so be
careful when using capillaries in level applications.
Ok back to wet and dry leg calibrations.
The dry leg is the most common and the easiest to do. This
is much the same as the basic open tank level calibration.
The transmitter is mounted anywhere below the HP (bottom)
tap off point and it's HP leg is connected via S/S tubing to
the HP (Bottom) tap off point on the vessel. The LP side of
the transmitter is connected to the LP (Top) tap off point
on the vessel. The HP side will always be in contact with
the liquid in the vessel and the LP side will always be in
contact with gas since it's is tapped of from the top of the
vessel. You obviously can only achieve this if you have a
5-way manifold (isolation, vent and equalization valve
piece)installed on the transmitter.
You will start your calibration by opening up the
transmitter to atmosphere and make sure that when equal
press is applied to HP and LP side the transmitter shows
zero and 4 mA. After this zero check it is a simple matter
of measuring where your Zero and 100% positions are on the
vessel in relation to the transmitter and multiply these
with the density of the liquid you are measuring and and
install these Z AND 100% values in the transmitter.
Ok this is very easy so far but what happens when the liquid
is hotter than the ambient temperature and it's vapor in the
top half of the vessel starts to condense and run into the
dry LP leg?
In a very short time this dry leg is going to start filling
up with condensate and there goes your calibration because
the calibrated diff press (your calculated Zero and 100%
values) begins to chance.
To resolve this problem we fill the LP leg with a buffer
solution like diesel,glycerin, glycol or even the same
liquid you have in your vessel can work as well, in non
critical applications. I prefer glycol since it's density is
higher than water so if the gas starts to condensate it will
just lie on top of the glycol buffer solution and run back
into the vessel from the LP leg and not mix with it. The
mixing of the wet leg liquid with the gas condensate could
also cause problems and inaccuracies, since this could
chance the buffer density over a period of time.
To calibrate the transmitter will depend on the type and era
of transmitter you are using. The following calibration is
for smart transmitters only.
The smart transmitters that we use today can measure in the
negative (-1Bar) and you can do your calibration as normal.
The final result will be something like this, LRV =
-1230mmH2o (4mA), URV = +125mmH2o (20mA). I know it looks a
bit strange when you see it for the first time but here is
how it works.

Before you can do this calibration you need to know the ATM
value for the installation. The atmospheric value (ATM) can
be read directly from the transmitter by disconnecting the
HP side(Bottom) and open it up to atmosphere, so the only
pressure on the transmitter is on the LP side and this will
obviously push the transmitter into the negative.
Maximum negative differential pressure for a instalation =
ATM pressure.
Make sure the LP line is filled to the position where it
will start to run back into the vessel, then read off the
displayed value on the transmitter. This is your ATM value.
In this example it might be something like -1350mmH2o. This
value is determined by, where you have installed the
transmitter and what you use for a buffer solution.
To calculate the actual zero and 100% positions on the
vessel you do the same as before and just measure from the
transmitter to you zero and 100% positions on the vessel,
multiply them with the density of the liquid you are
measuring and add them to the ATM value. You can then input
these values to this transmitter's LRV and URV and the
calibration is done.
So assuming you have installed the transmitter slightly
below the lower tap off point the above LRV and URV is about
right in relation to the ATM value in this example. Be sure
to understand the difference between the ATM value and the
LRV it will in most cases not be the same. The more
accurately you can determine your ATM value the more
accurate the calibration will be.
Now the calibration of the 4to20mA and the pneumatic DP
transmitters. These transmitters cannot measure in the
negative so you need to change the HP and LP sides around so
that the HP side goes to the top of the vessel and the LP
side goes to the bottom tap off point on the vessel.
You now need to do you calibration in the reverse as well.
Again find the ATM value first, in other words max positive
differential (HP wet leg filled and LP open to atmosphere)
on the transmitter will now be your ATM value. Will be say
+1350mmH2o.
Actual zero will now be 20mA and not 4mA and will be
determined by makind use of the ATM value minus the actual
zero measured value, multiplied by the liquid density.
The actual 100% value will be determined by making use of
the ATM value minus the actual 100% measured value,
multiplied by the density. You should end up with something
like this, zero = +1250mmH3o = 20mA and 100% = +150mmH20 = 4mA.
Finally the display on you remote level indicator needs to
be changed as well otherwise it will read in the reverse. If
you use a pneumatic DP Transmitter just substitute 4 and 20
mA with 20 to 100Kps or 3 to 15 Psi the principle stays the
same.
There you have it, wet and dry leg calibrations used ONLY in
PIPED DP Cell level calibrations.
Good luck

Is This Answer Correct ?

6 Yes

1 No

Question { 25917 }

how we can calibrate level transmiiter in field?

Answer

Is This Answer Correct ?

18 Yes

6 No

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