inst questions for interview
DESCRIPTION
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Q No. 1: What is the difference between absolute pressure and gauge pressure?These are two units of pressure measurement.Gauge Pressure: Pressure above the atmospheric pressure is called gauge pressure. A gauge pressure device will indicate zero pressure when bled down to atmospheric pressure (i.e., gauge pressure is referenced to atmospheric pressure). Gauge pressure is denoted by a (g) at the end of the pressure unit [e.g., kPa (g)].
Absolute Pressure: Absolute pressure includes the effect of atmospheric pressure with the gauge pressure. It is denoted by an (a) at the end of the pressure unit [e.g.,
kPa (a)]. An absolute pressure indicator would indicate atmospheric
pressure when completely vented down to atmosphere - it would not
indicate scale zero.
Absolute Pressure = Gauge Pressure + Atmospheric Pressure
Q No. 2: Explain the concept of elevated zero in 4 ~ 20 mA loop?
In control loops, a 4mA signal means a process zero and a 20mA signal depicts process span i.e a transmitter would generate a 4mA signal when instrument measures LRV and a 20mA signal when it measures URV. 4mA signal is used instead of a 0mA signal for LRV so that loop errors can be properly identified. For example, if 0mA is used for LRV and there is some breakage in the wire carrying loop current to control room, so ideally there would be 0mA current in the loop. It would really be difficult to find out whether it is actual process zero or some open circuit in the loop. On the contrary, a 0mA signal in 4 ~ 20mA current loop would indicate loop fault and 4mA would essentially mean process zero.Q No. 3: How does thermocouple and RTD work?
Thermocouples and RTD, both are used for temperature measurement.
RTDEvery type of metal has a unique composition and has a different resistance to the flow of electrical current. This is termed the resistively constant for that metal. For most metals the change in electrical resistance is directly proportional to its change in temperature and is linear over a range of temperatures. This constant factor called the temperature coefficient of electrical resistance (short formed TCR) is the basis of resistance temperature detectors. The RTD can actually be regarded as a high precision wire wound resistor whose resistance varies with temperature. By measuring the resistance of the metal, its temperature can be determined.
Temperature Transmitter in the form of Wheatstone Bridge
ThermocoupleA thermocouple consists of two pieces of dissimilar metals with their ends joined together (by twisting, soldering or welding). When heat is applied to the junction, a voltage, in the range of milli-volts (mV), is generated. A thermocouple is therefore said to be self-powered. Shown in Figure is a completed thermocouple circuit.
Q No.4: What is Level Compensation? Briefly differentiate between zero suppression and zero elevation?
Level Compensation
It would be idealistic to say that the DP cell can always be located at the exact the bottom of the vessel we are measuring fluid level in. Hence, the measuring system has to consider the hydrostatic pressure of the fluid in the sensing lines themselves. This leads to two compensations required.
Zero Suppression
In some cases, it is not possible to mount the level transmitter right at the
base level of the tank. Say for maintenance purposes, the level transmitter
has to be mounted X meters below the base of an open tank as shown in
Figure
The liquid in the tank exerts a varying pressure that is proportional to its level H on the high-pressure side of the transmitter. The liquid in the high-pressure impulse line also exerts a pressure on the high-pressure side. However, this pressure is a constant (P = S X . ) and is present at all times. When the liquid level is at H meters, pressure on the high-pressure side of the transmitter will be:
Phigh = S H . + S X . + Patm
Plow = Patm
P = Phigh - Plow = S H . + S X .
That is, the pressure on the high-pressure side is always higher than the actual pressure exerted by the liquid column in the tank (by a value of S X . ). This constant pressure would cause an output signal that is higher than 4 mA when the tank is empty and above 20 mA when it is full. The transmitter has to be negatively biased by a value of -S X . so that the output of the transmitter is proportional to the tank level (S H . ) only. This procedure is called Zero Suppression and it can be done during calibration of the transmitter. A zero suppression kit can be installed in the transmitter for this purpose.
Zero Elevation
When a wet leg installation is used (see Figure below), the low-pressure side of the level transmitter will always experience a higher pressure than the high-pressure side. This is due to the fact that the height of the wet leg (X) is always equal to or greater than the maximum height of the liquid column (H) inside the tank.
When the liquid level is at H meters, we have:
Phigh = Pgas + S H .Plow = Pgas + S X .P = Phigh - Plow = S H . - S X . = - S (X - H)
The differential pressure .P sensed by the transmitter is always a negative number (i.e., low pressure side is at a higher pressure than high pressure side). P increases from P = -S X . to P = -S (X-H) as the tank level rises from 0% to 100%.
If the transmitter were not calibrated for this constant negative error, the transmitter output would read low at all times. To properly calibrate the transmitter, a positive bias is needed to elevate the transmitter output.
This positive biasing technique is called zero elevation.
Q No. 5: What is the relationship between differential pressure (DP) and Flow (Q)?The output of the D/P cell acts on a pressure to milli-amp transducer, which transmits a variable 4-20 ma signal.
This simple system although giving an indication of the flow rate (Q), is actually transmitting a signal proportional to the differential pressure (DP).
However, the relationship between the volume of flow Q and P is not linear. Thus such a system would not be appropriate in instrumentation or metering that requires a linear relationship or scale. In actuality the differential pressure increases in proportion to the square of the flow rate.
We can write this as: P Q2
In other words the flow rate (Q) is proportional; to the square root of the differential pressure.
Volumetric Flow Rate = Q P .
To convert the signal from the flow transmitter, (figure 9 above) to one that
is directly proportional to the flow-rate, one has to obtain or extract the
square root of the signal from the flow transmitter.
Q No. 6: What are the applications of wet leg level measurement through DP transmitters. How do they work?
In a wet leg system, the low-pressure impulse line is completely filled with liquid (usually the same liquid as the process) and hence the name wet leg. A level transmitter, with the associated three-valve manifold, is used in an identical manner to the dry leg system. In most of the closed tank level measurement applications, wet leg system is mostly used especially when the liquid whose level is being measured is condensable, for example water at its boiling point.Figure shows a typical wet leg installation.
Q No. 7: What is the difference between open loop control and closed loop control?Open Loop control is manual control. In case of open loop control there is no feed back to the controller about the status of the process at any instant. It is up to the operator to decide what parameters to change in order to keep the process at the set point. The operator would provide feedback and apply the necessary correction to the system whilst observing the effect on the controlled variable. This is termed open loop operation.
On the other hand, closed loop control is automatic control, where the feedback from the process is sent back to the controller which subsequently compares that with set point and necessary compensation is made automatically to overcome the disturbance caused. Closed loop control has 3 types of controlling strategies or schemes.
1. Feedback Control
2. Feedforward Control
3. Cascade Control
Q No. 8: Narrate the disadvantages of proportional control?
1. After a disturbance, proportional control will provide only a new mass balance situation. A change in control signal requires a change in error signal, therefore offset will occur.
2. Proportional control stabilizes an error; it does not remove it.
Reset of Integral Action can be used to remove offset generated in case of proportional control.
Q No. 9: How can pressure transmitter be used for level measurement?
The pressure at the base of a vessel containing liquid is directly proportional to the height of the liquid in the vessel. This is termed hydrostatic pressure. As the level in the vessel rises, the pressure exerted by the liquid at the base of the vessel will increase linearly. Mathematically, we have:
P = S H
where
P = Pressure (Pa)
S = Weight density of the liquid (N/m3) = g
H = Height of liquid column (m)
= Density (kg/m3)
g = acceleration due to gravity (9.81 m/s2)
The level of liquid inside a tank can be determined from the pressure reading if the weight density of the liquid is constant. Differential Pressure (DP) capsules are the most commonly used devices to measure the pressure at the base of a tank.
When a DP transmitter is used for the purpose of measuring a level, it will be called a level transmitter.
Q No. 10: What is three valve manifold? How does it protect DP cell from damage?A three-valve manifold is a device that is used to ensure that the capsule will not be over-ranged. It also allows isolation of the transmitter from the process loop. It consists of two block valves - high pressure and low-pressure block valve - and an equalizing valve. Figure shows a three valve manifold arrangement.
During normal operation, the equalizing valve is closed and the two block valves are open. When the transmitter is put into or removed from service, the valves must be operated in such a manner that very high pressure is never applied to only one side of the DP capsule.
Operational Sequences of Three-Valve Manifold Valving Transmitter into Service
To valve a DP transmitter into service an operator would perform the following steps:
1. Check all valves closed.
2. Open the equalizing valve . this ensures that the same pressure will be applied to both sides of the transmitter, i.e., zero differential pressure.
3. Open the High Pressure block valve slowly, check for leakage from both the high pressure and low-pressure side of the transmitter.
4. Close the equalizing valve. This locks the pressure on both sides of the transmitter.
5. Open the low-pressure block valve to apply process pressure to the low-pressure side of the transmitter and establish the working differential pressure.
6. The transmitter is now in service.
It may be necessary to bleed any trapped air from the capsule housing.
Removing Transmitter from ServiceReversal of the above steps allows the DP transmitter to be removed from service.
1. Close the low-pressure block valve.
2. Open the equalizing valve.
3. Close the high-pressure block valve.
The transmitter is now out of service.
The transmitter capsule housing still contains process pressure; this will require bleeding.
Q No. 11: How does temperature effect level measurement when it is being done by DP transmitter?
Level measurement systems that use differential pressure P as the sensing
method, are by their very nature affected by temperature and pressure.
Recall that the measured height H of a column of liquid is directly
proportional to the pressure P exerted at the base of the column and
inversely proportional to the density of the liquid.
H P/Density (mass per unit volume) of a liquid or gas is inversely proportional to
its temperature.
1/TThus, for any given amount of liquid in a container, the pressure P exerted at
the base will remain constant, but the height will vary directly with the
temperature.
H T
Q No. 12: Why 3 wire RTD are used?
As in the case of a thermocouple, a problem arises when the RTD is installed some distance away from the transmitter. Since the connecting wires are long, resistance of the wires changes as ambient temperature fluctuates. The variations in wire resistance would introduce an error in the transmitter. To eliminate this problem, a three-wire RTD is used.
The connecting wires (w1, w2, w3) are made the same length and therefore the same resistance. The power supply is connected to one end of the RTD and the top of the Wheatstone bridge. It can be seen that the resistance of the right leg of the Wheatstone bridge is R1 + R2 + RW2. The resistance of the left leg of the bridge is R3 + RW3 + RTD. Since RW1 = RW2, the result is that the resistances of the wires cancel and therefore the effect of the connecting wires is eliminated.
Q No. 13? What is the difference between transducer and transmitter?
A transducer is a device that translates a mechanical signal into an electrical signal. For example, inside a capacitance pressure device, atransducer converts changes in pressure into a proportional change in capacitance.
In process control, aconverter used to convert a 420 mA current signal into a 315 psig pneumatic signal (commonly used by valve actuators) is called acurrent-to-pressure converter.
A transmitter is a device that converts a reading from a sensor or transducer into a standard signal and transmits that signal to a monitor or controller. Transmitter types include:
1. Pressure transmitters
2. Flow transmitters
3. Temperature transmitters
Q No. 14: A thermocouple is brought to the workshop for maintenance purposes. How much mV would it show?
A thermocouple measures the difference between two temperature junctions i.e hot junction and cold junction. When T/C is brought in workshop, both the junctions are at the same temperature. So, there wouldnt be any voltage across the terminals ideally 0 mV.
Q No. 15: Differentiate between direct acting and reverse acting actuators.With an increase in the error signal, if the output of the controller also increases then the controller would be direct acting. If controller o/p decreases with an increase in error signal, it would be regarded as a reverse acting controller.
Q No. 16: What is Set Point (SP), Manipulated Variable (MV) and Process Variable (PV)?
Set PointThe set point is a value for a process variable that is desired to be maintained. For example, if a process temperature needs to kept within 5 C of 100 C, then the set point is 100 C. A temperature sensor can be used to help maintain the temperature at set point. The sensor is inserted into the process, and a controller compares the temperature reading from the sensor to the set point. If the temperature reading is 110 C, then the controller determines that the process is above set point and signals the fuel valve of the burner to close slightly until the process cools to 100 C. Set points can also be maximum or minimum values.
Process variableThe measured variable is the condition of the process fluid that must be kept at the designated set point. Measured variable and process variable terms are used inter-changeably sometimes.
Manipulated variableThe factor that is changed by the controller to keep the measured variable at set point is called the manipulated variable (MV). Normally, its the output of controller.
Q No. 17: Is it possible to maintain set-point in an On-Off control loop?
In On-Off control, control signal is either 0% or 100%. So, it isnt possible to maintain the PV at set point in this case. The graph of PV would be pulsating around a set point.
Q No. 18: What are the typical parts of a control loop?
Following are the three major components comprising control loops
1. Primary Element (Transducer and Transmitter collectively)
2. Controller3. Final Control Element (Control Valve, dampeners etc.)
Q No. 19: What is an I/P converter and where is it used?Output from the controller is a 4 ~ 20 mA signal called the manipulated variable. But the final control element, control valve, is a pneumatic device that understands pneumatic signal only. In order to convert that standard 4 ~ 20mA signal into a standard pneumatic (3 ~ 15 psi) signal, an I/P converter is used.
Q No. 20: Is driving a car (in a normal manner) an example of feedback control or feedforward control?
Whenever a driver anticipated a disturbance, say a car coming from the opposite side in this case, he applies brakes before any collision is made. So, its an example of feedforward control.Q No. 21: How can offset inherent in proportional control be overcome?Offset is an inherent characteristic of proportional control loops. To overcome that, reset of integral action is used. If we wish to restore the process to the setpoint after a disturbance then proportional action alone will be insufficient. Consider again the diagram (Figure) showing the response of a system under proportional control.
If we wish to restore the process to the setpoint we must increase the inflow over and above that required to restore a mass balance. The additional inflow must replace the lost volume and then revert to a mass balance situation to maintain the level at the setpoint. This is shown in figure. This additional control signal must be present until the error signal is once again zero.
Q No. 22: What do we mean by span?
The algebraic difference between the upper and lower range values (for example: Range = 0 to150oF; Span = 150oF; Range = 3 to 15 psig, Span = 12 psig).Q No. 23: Explain Split Range Control In Control Valve Briefly.
split range control is which the output of a controller is
split to two or more control valves. For eg,
For 0% controller output,valve A fully open & B fully
closed.
For 25%, Valve A 75% open,B 25% open
For 50%, Valve A& B- 50% open,
For 75%, Valve A 25%open, B-75%open..
For 100%, Valve A fully closed & valve B Fully Open..
Like this, d controller output action can be set depending
upon the application...i have one tank of 500mm height, which is in under vacuum of 735 mmHg, I want measure level of the same using DPT , SO WHAT WILL BE THE URV & LRV, DENSITY OF THE TANK FLUID IS 0.95. is there necessary to fill the LP side with fluid.
First of all there is no difference between a vessel with a
process pressure of 10 or 50Bar or a vessel with a vacuum.
These pressures cancel each other across the LP and HP legs
on our tx's, and are therefore not taken into account in our
calibration.
To explain in more detail:
If there is 100Bar on the LP side there is also 100Bar on
the HP side so the DP across the tx is still zero.
If there is 750mmHg vacuum on the LP side there is also a
750mmHg vacuum on the HP side so the DP across the tx is
still zero.
So work with the transmitter as if there is no pressure or
vacuum in the vessel.
The best transmitter to use in a vacuum application like
this is a capillary type, but according to your question it
seems you already have a piped transmitter in place. This
makes it a bit more troublesome to do, but by working
carefully and accurately you can achieve accurate and
reliable results.
Take your time, this is one of the MOST DIFFICULT DP level
setup's you will ever came across in any industry.
It is always better to use a wet-leg since condensation will
cause your DP to chance in time. If I look at the sg of the
product you most probably are working with hydrocarbon
condensate, so you would want to install a wet leg in a
application like this.
Process zero with LP leg filled:
1st open both legs to atm and do zero trim. Even better if
you know how to do a factory reset and then do a zero trim.
Connect a 1/2" T-piece to top of LP leg just on the bend
before it goes to the top tap-off point with a needle valve
pointing upwards.
With main process isolation valves still close, fill LP leg
to max and close needle valve.
Try to use glycol since it's density is higher than water's,
and will prevent contamination of the wet leg.
You can also use glycerin or diesel.
Close 5-way manifold equalization valve and open main
process isolation valves and then only open both isolation
valves on manifold.
Open needle valve and fill again LP leg as much as possible,
start closing needle valve slowly and keep filling
to make sure lp leg is filled properly. Playing with the
main LP isolation valve and the needle valve will give best
results to get the LP leg filled to max.
This should put you in the situation that your tx has been
zeroed at atmospheric pressure, so the vacuum pulled
on both sides of diaphragm now and LP leg filled to max,
should now give a accurate process zero to work from.
Write down this displayed value.
What ever this value is is not important you will use this
reading as your process zero reference point to work from
so it can be anything as long as you are sure this is a
accurate and reliable process zero.
To double check if the process zero is good, isolate the tx
again and open it up to atm again not draining the LP leg.
It should still give a zero indication with both sides open
to atm.
Put it back on line and make sure the LP leg is still filled
to max by making use of the needle valve and LP main
isolation valve again.
You should be back at the previously displayed value. Do
this a couple of times to make sure you get to the same
values every time. Only then can you be sure that your
process zero value is reliable and accurate.
To calculate the LRV and URV:
Like I said I am assuming now that the tx had been installed
300mm below the bottom tap-off point.
I am also assuming that the bottom tap-off point is zero
position and the top tap-off point is 100%
You should now have something like (+/-) -750mmH2o on the tx
display.
Let's say the value is exactly -750mmH2o.
Measure from the middle of the tx's diaphragm to the bottom
tap-off point. We make this say 300mm
Measure from the bottom to the top tap-off points. We make
this say 500mm.
Calculation:
LRV is -750 + 300 = -450mm x .95 = -427,5 mmH2o
URV is -750 + 300 + 500 = +50mmH2o x .95 = +47,5 mmH2o
Modify your L/URV's to these new values. There is no need to
use a hand pump when working with smart tx's, just modify
the values with the HART, make sure it's on line and give it
back to production.
Since it is such a small span the level might be to
sensitive so you might want to increase the damping as well
on the Tx.
which type of rtd is good for any system,2wire,3wire or 4 wire
If the distance between transmitter and RTD is not long,Any
rtd (2,3 or 4 wire)is suitable.
But Generally 3 wire RTD only preparred for Industrial
purpose. Most of the Industires & Design document suggest 3
wire RTD is suitable for any application.
depend on distance if distance too short its should be two
wire
3 and 4 wire used for long distance
in instrumentation why we are using 4-20mA , why not
voltage ? if somebody design a instrument which works on
voltage then can we use that instrument?no, because voltage makes fluctuation & current remains
constant.
because there is voltage drop as per the length of conducting wire. but current has no drop
-20 mamps is best choice because interference level is
minimum in current application where as in 0-10 V it is
comparatively high.Thus in using 4-20 mamps signal is
distortionless
How to ensure vibration sensor functioning accordingly.With out using an other sensor
By using Vibration Transmitter calibration equipment,fix
the sensor on the calibration equipment.Gap voltage shhould
be adjusted as per requierd.Power on the calibration
equipment and vibration transmitter and increase the
vibration as per range.check the output of the transmitter
in milli amps across millivolts generated as per
vibration.
What is meant by potential free contacts in interconnection schemes? what is their importance?Potential free" or "dry contacts" are used when the device being controlled has its own power supply. For example, most VFDs have a 24 VDC power source in the drive that is used to control starting the drive. The external contact used to start the drive is a dry contact, and it switches the voltage supplied by the VFD to let it know it should begin running. Closing the dry contact connects the drive input to the VFD's 24 VDC and powers the drive input. The internal devices sense the voltage, start the drive, and the current and voltage is returned to the VFDs own power supply "-" through internal circuitry.
The term "potential free" is only in reference to the control contact when it is not connected to an external device. Obvioulsy, during operation, there must be potential or voltage passing through the contact or nothing can happen.
This term is meant to differentiate between a dry contact, powered extenally, and a contact that is connected to a voltage source inside the control system that is sent to an external device. An example of this would be a PLC output that uses the PLC's own 24 VDC power, and when the contact closes this voltage is connected to an external "unpowered" load such as a relay coil.
Potential free contacts (also called "dry contacts") are simply contacts which are physically operated with the main device, but not electrically connected to it. For example a motor contactor often has auxiliary contacts that are operated by the main coil and open and close at the same time as the main contacts but are not used for control of the motor starter. If they are connected to an outside circuit to indicate the status of the starter without being powered by the motor supply they would be considered potential free contacts.
I made an example to understand better what the other guys have already told you.
This example shows an "external alarm circuitry". A relay is used for the desirable voltage isolation between the system (Vc and gnd) and the external circuit (e.g. a lamp connected to an external -a.c. or d.c.- voltage source). In this way, the system alarm (i.e. Vc=5V) will close the relay's contact and make the external lamp to be turned on, showing us the alarm condition of the system.
(Of course, instead of a relay we could use e.g. an optocoupler, taking advantage of the voltage isolation between its 'led' and its 'output transistor'.)
WHAT IS A CALLIBRATION? WHAT IS MEEAN BY LOOP CHECKING INCLUDES PRESURE, THERMOCOUPLE,TT,DT?Calibration is nothing but comparisonof an unknown standard
with an known standard or in other words it is known as
checking the output is proportional to the input or not.
Loop checking is nothing but the instrument installed in
the field must give the signal to the control room without
any break. First in this the continuity of the cable will
be checked from inst to JB in field side and from the JB to
control room.
can any one pl explain me in brief that how we select the leakage classification in valve,means for liquid or gas there is any standard for selecting leakage classification or its depend upon valve size or any any other parameterI cannot remember to much about this but I remember that the
leakage rate is dependent on the valve classification. a
Class 6 valve is a tight shutoff valve with not leakage
tolerance at all. a Class 5 is classified as a modulating
valve and have a tolerances of a small amount of droplets
per minute and so on. The lower the class valve the higher
the acceptable leakage rate. This leakage rate is tested in
a valve shop either by the manufacturer or on your own site
if you have the right equipment to do it with.
We normally had it done by our valve shop after we send a
valve to them for a complete overhaul. Once the control
valve have repaired and overhauled the valve, we had to do
the final testing and acceptance with them. They would
supply us with the specs on the valve so that we can see
what class valve it was and what the acceptable leakage
tolerance was. After this we would witness the testing of
the valve and sign a acceptance certificate.
I suggest you get hold of your valve shop or the original
manufacturer to supply you with a list of the
classifications of the valves you are currently using or
want to use as well as the various acceptable leakage rate
for each class.
Good luck
Class VI for Gas service as gas requires tight shutoff &
class I , II, III, IV, V for liquid service as per process
requirement shutoff
how to calibate level transmiter ?range -150 to +150.what is the zero and span?
This means that your transmitter is capable of measuring a
differential pressure of 300. Whatever this units of measure
is. You can calibrate the transmitter for any of the
following conditions:
LRV = -150 = 0%
URV = +150 = 100%
Therefore span = 300 UOM (units of measure)
or
LRV = 0 = 0%
URV = +150 = 100%
Therefore span = 150 UOM (units of measure)
or
LRV = -150 = 0%
URV = 0 = 100%
Therefore span = 150 UOM (units of measure)
or
You can also calibrate the tx for any span in between this
300 range.
To find the correct Z/S values for your application measure
the distance from the transmitter to zero position on the
vessel and multiply this with the density of the product you
want to measure (LRV). Do the same with your 100% (URV) value.
Good luck
WHAT IS THE DIFFERENCE BETWEEN A TRANSMITTER & A SMART TRANSMITTER.
SMART transmitter will uses the ,HART communicator for the
purpose of Tx-calibration,while the other normal transmitter
needs an extra care at the time of calibration ,bcz in the
normal we have to use the external air supply for the
pressure application ,,,hence it will be gud to have and use
the SMART tx
We went through various eras or technology time periods in
instrumentation. Believe it or not but measurements of flow
and level had been done by the old Egyptians that build the
pyramids already. They had use for instants a stick with
marks on it to see what the level was in one of their tanks
and various other simple measuring and control devices as well.
We normally are not to concerned with the type of
instrumentation that was used that long ago. We normally say
that instrumentation only started to take off during the
pneumatic era where all measurement and controls were done
by pneumatic and some electrical instrumentation. The world
standard instrumentation signal was also a pneumatic one
then. It was 20 to 100Kpa or 3 to 15Psi. What this means is
that we had to calibrate our pneumatic pressure transmitters
and controllers to give this output of 20 to 100Kpa. You
really had to know instrumentation when you worked with
these pneumatic stuff. We even had pneumatic relays and
pneumatic chart recorders back then.
Later we moved on to electromechanical instruments and some
very simple electronic indicators and chart recorder.
After the transistor and micro processors were developed
things started to move along very fast and we started to see
various electronic instrumentation and transmitters on the
market. I call this the 4-20mA era. These very advanced type
of transmitters had to be pumped up with a small hand pump
as normal, but all you then had to do was make small
adjustments with a small screwdriver to a zero and span pot
to calibrate the transmitter. Very advanced to what we were
use to. These were called the electronic 4 to 20 mA
TRANSMITTERS.
The new electronic world standard signal then started as the
4 to 20 mA signal, but keep in mind that the pneumatic world
standard signal, even today, is still the 20 to 100Kpa signal.
Since then we have moved on to the smart instrumentation era
where we calibrate the transmitters with a HART communicator
and the transmitter itself is actually a small computer that
can even detect a error on itself or tell you if the
calibration you are trying to do is invalid. From there the
technology of SMART TRANSMITTERS. You even get SMART valve
positioners that can accurately detect the erosion of it's
control valve's plug and seat and send a alarm signal to the
CCR that a overhaul on the valve is needed. All just
computer software and programming like anything else in
today's world. Look what you can do with a cellphone today.
Very smart little gadget.
Not to worry they will never be as smart as a instrument tech.
Good luck
difference b/w single acting and double acting actuator by using application only?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.
what is the diffrence between plc&dcs
simply plc is a controller we can control analog and digital
i/o s but i/0 limit is there in plcs.dcs is nothing but
networking of plcs.EX
plc1+plc2+smart instruments+mc baesd instruments all are
controlled by dcs at a time .ic has redundancy also
Explain Split Range Control In Control Valve Briefly.
split range control is which the output of a controller is
split to two or more control valves. For eg,
For 0% controller output,valve A fully open & B fully
closed.
For 25%, Valve A 75% open,B 25% open
For 50%, Valve A& B- 50% open,
For 75%, Valve A 25%open, B-75%open..
For 100%, Valve A fully closed & valve B Fully Open..
Like this, d controller output action can be set depending
upon the application...
In addition wt ANS.1-
in split range control the control signal is split in two
ranges according to the application .for eg,4-20MA.signal
can be used as 4-12 MA. FOR VALVE A OPEN i.e.50%.& 12-20MA
for vlv b open.vice-versa.
which type of rtd is good for any system,2wire,3wire or 4 wire
If the distance between transmitter and RTD is not long,Any
rtd (2,3 or 4 wire)is suitable.
But Generally 3 wire RTD only preparred for Industrial
purpose. Most of the Industires & Design document suggest 3
wire RTD is suitable for any application
depend on distance if distance too short its should be two
wire
3 and 4 wire used for long distance
how flame detectors sense the flame
There are 2 types of flame detectors in market viz: IR
(infra red ) and UV (ultravoilet ray).
How to ensure vibration sensor functioning accordingly.With out using an other sensor
By using Vibration Transmitter calibration equipment,fix
the sensor on the calibration equipment.Gap voltage shhould
be adjusted as per requierd.Power on the calibration
equipment and vibration transmitter and increase the
vibration as per range.check the output of the transmitter
in milli amps across millivolts generated as per
vibration.
what do you mean by CJC (cold junction compensation).?i assume u asked for thermocouple .as per seeback effect 2
junctions of the thermocouple should be maintained at
different temperature i.e. hot junction in high temp and
cold jn at zero degree temp.but in practical application it
is difficult to maintain cold jn at 0deg C .so a thermister
is placed in the location where the cold jn. is kept.the o/p
of thermister is used to measure the temp at that location
and is used in calculation to neglect the the effect of temp
at cold jn.
pls dont ask me the calculation,bcause i am also searching
for it.
CJC-IS the cold junction compantion.it is used in
thermocouple.two dissimiler metals joined together it forms
thermocouple.if we apply voltage tn one junction is hot &
another becomes cold Junction.
such Thermocouple generates MV output signal according
to metal characterstics.it gives 0 mv for 0 degree. to
measure actual temperature of the measuring point we should
remove the temp. of measurment envirnment for that the
compansation equals to surrounding temp. is provided and
used as 0mv.
This ref. 0V is generated by Three Methodes-A)Electronic's
0 V ref.,B)Ice bath Methoad,C)by using Peltier cooler .
how can we calibrate RADAR LEVEL transmitter 0% to 100%
You need to be more specific in your questions but I assume
you are referring to the frequency modulation radar (cone
type) and not the wave guide pulse radar (TDR).
This is a very generic procedure, use your radar's vendor
manual to do the calibration.
Take a measuring tape and measure the actual distance from
the cone to the zero position and the 100% position in your
vessel. If it is a closed pressurized vessel use the design
engineer's internal vessel drawings. If all else fail use
the dimensions of the vessel itself to determine what and
where z/100% is on the vessel. Use the vendor manual to see
how to use these measurements. It might be as simple as
entering these values directly as your z/100% calibration
values or you might have to do a small calculation like
maximum possible distance (tank depth) minus z measured
distance equals zero distance, and maximum possible (tank
depth) distance minus 100% measured distance equals 100%
distance.
Also keep in mind radars are distance measuring instruments
and not level measuring instruments and the fact that the
distance measurement is the exact opposite of level in
relation to the total distance measurement. This means if
your tank total distance is 3 meter (assuming tank bottom is
also zero position) and your radar measures a DISTANCE of 2
meters to the top of your product in the tank, the actual
LEVEL in the tank is only 1 meter.
It is easy to look at the disply on the radar and think this
is your actual level measurment. You therefore need to
modify the local display to indicate level in mm or %, as
well as setup your output to level and not distance after
the calibration. The radar will then give out 4mA when the
tank is empty as per example above and 100% as the tank fill
up with product to the point where you have specified 100%
should be. If your output is setup for distance it will
output the exact opposite. Watch out!!
If you need a vendor manual download it from the internet.
Good luck
Difference b/w SCADA & DCSSCADA is an abbreviation for Supervisory Control and Data
Acquisition .
DCS stands for Distributed Control System.
Normally, a SCADA system will get the data from a RTU
(remote terminal unit). A RTU runs independently, except
for some control from the central supervisory system such
as fire fighting, emergency shut-down (i.e water & oil
pipelines). The communication path will be through a GSM,
wireless technology, etc...
For a DCS system, it controls the process as a stand-alone
system. It has the control loops built into it's own
controller. The communication path will be through
something like a LAN high-speed Ethernet, or other
communications network. DCS is a single unit, or a group of
local units
SCADA is nothing but the HMI,which need to communicate with
the controller and then controler will comunicate with the
field instruments.
DCS(Controller+SCADA) its self have a controller,its
directly commincate to the field instrument.
is here anybody who knows how to calibrate the pressure transmitter?if we have a master transmitter we can connect parrelal
to each other . when master transmitter will show the
pressure,the pt must indicate pressure with aqual to master
transmitter
the first you must see is the transmetter is used as the
loop or indication , if is used as the loop , must avert
operator to put it im manual loop , and if is other case ,
we can calibrate easy without alter the system .Used PID to
determine transmetter function
This procedure can be used to calibrate any type of
instrument in the field on a live plant regardless of it's
type or application. The only variation in the procedure is
what test equipment you will use to calibrate the instrument
with. For instance calibrating a temperature transmitter you
want to take a HART and decade box with you. Calibrating a
pressure , flow or DP transmitter, you want to take a HART
and pressure pump with you like a Druck or hand pump with a
precision gauge on.
The procedure:
Since the transmitter is on line and on a running plant, you
need to determine a couple of things first before you start
working on it. First and most important of all, what does it
do? Is it a control transmitter, a trip transmitter or just
a indication.
Control transmitter means it's output will go to the DCS and
is linked to a PID controller which will give a continues
output, based on this input, to a control element in the
plant plant like a control valve or a heater element. These
are all critical control components that will trip the plant
if you upset their stability.
Trip transmitter means that it is linked to the ESD system
and will start a plant shut down if it is seeing a to high
or low pressure, flow, DP or level depending on the
application you are working on. Just indication means that
it is completely safe to work on and you cannot trip anything.
To make the control transmitter safe to work on you need to
put the DCS PID controller in manual. This will keep the
output of the controller at the current output position and
bypass the output function of the controller. In a fast
moving application the control room operator will send a
field operator with you to control the control valve
manually with it's hand wheel while you work on the control
transmitter.
To make the trip transmitter safe to work on you need to put
a inhibit in the ESD system output trip function of the
transmitter. This is also sometimes refer to as a MOS
(maintenance override switch).
In both cases the control room operators will do this for
you providing that you have all the permit to work and
inhibit permission paperwork in place.
Good luck
how to given power supply of any type two wire transmitter in field?
In the 2-wire transmitter 24VDC power is given. The +ve of
the 24V is connect to +ve of the transmitter, -Ve of the
transmitter is connect to +ve of the Indicator & -ve of the
indictor is connect to -ve of the 24Vdc powersupply.
What is the difference between compensation and extension cable.
Compensation cable is used to compensate or to maintain the
signal that is transmitted by the transducer towards the
reciever.
Extension cable is used for transmitting signals for long
distances
Compensation cable used for thrmocouple application only.
this cable transmit the signal from sensor to tranmitter
without any signal loss, without signal noise.
Extension cable just use for Extension purpose
No difference it is just a different wording for the same
thing. Extension or compensation wire is made from the same
material as the used thermocouple to extend the thermocouple
to the Temperature indicator or controller. So you therefore
get Type K, J, T S ext types of extension wire as well as
Type J, K , T ext types of connectors to join these cables
as well.
Compensation cable is used to compensate or to maintain the
signal that is transmitted by the transducer towards the
reciever.
Extension cable is used for transmitting signals for long
distances
what is thermocouple outputIts the millivolt out put because it works on the emf
generation,which is produced when difference in the
temperature of two junction is there, i,e the current will
flow in the closed loop between the junction
why we use 24v dc in instrument rahter than 230vac????
all loop powered transmitters are capable of
withstanding 10 t0 36v dc ...as per the process requirement
it will vary.. insome industries they are using 12v dc
also.. 24 vdc is used because as per ISA there are some
standards wich will match other process reqirement like
24vdc relays for digital control also wecan use 24dc to
energize tis relays
what is the difference between cold loop and hot loop
SIS instrument LOOP is hot LOOP.
Other normal instrument all is Cold loop
SIS Mean Safety Intergartive System
WHAT IS DIFFERENCE BETWEEN TWO WIRE RTD AND THREE WIRE RTD?
The simplest resistance thermometer configuration uses two
wires. It is only used when high accuracy is not required as
the resistance of the connecting wires is always included
with that of the sensor leading to errors in the signal.
Using this configuration you will be able to use 100 meters
of cable. This applies equally to balanced bridge and fixed
bridge system.
In order to minimize the effects of the lead resistances a
three wire configuration can be used. Using this method the
two leads to the sensor are on adjoining arms, there is a
lead resistance in each arm of the bridge and therefore the
lead resistance is cancelled out.
Serious lead-wire resistance errors can occur when using a
two-wire RTD (see Fig. 3A), especially in a 100 sensor. In
a two-wire circuit, a current is passed through the sensor.
As the temperature of the sensor increases, the resistance
increases. This increase in resistance will be detected by
an increase in the voltage (V = IR). The actual resistance
causing the voltage increase is the total resistance of the
sensor and the resistance introduced by the lead wires. As
long as the lead wire resistance remains constant, it can be
offset and not affect the temperature measurement. The wire
resistance will change with temperature, however, so as the
ambient conditions change, the wire resistance will also
change, introducing errors.
If the wire is very long, this source of error could be
significant. Two-wire RTDs are typically used only with very
short lead wires, or with a 1000 element.
In a 3-wire there are three leads coming from the RTD
instead of two. L1 and L3 carry the measuring current, while
L2 acts only as a potential lead. Ideally, the resistances
of L1 and L3 are perfectly matched and therefore canceled.
The resistance in R3 is equal to the resistance of the
sensor Rt at a given temperatureusually the begining of the
temperature range. At this point, V out = zero. As the
temperature of the sensor increases, the resistance of the
sensor increases, causing the resistance to be out of
balance and indicated at V out. Resistances L1 and L3 in
leads up to tens of feet long usually match well enough for
100 ohm three-wire RTDs. The worst case is resistance offset
equal to 10% of single-lead resistance.
The optimum form of connection for RTDs is a four-wire
circuit (see Fig. 3C). It removes the error caused by
mismatched resistance of the lead wires. A constant current
is passed through L1 and L4; L2 and L3 measure the voltage
drop across the RTD. With a constant current, the voltage is
strictly a function of the resistance and a true measurement
is achieved. This design is slightly more expensive than two
or three-wire configurations, but is the best choice when a
high degree of accuracy is required.
what are the standards used for earthing instruments in the field?
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.
Please explain about FAT? What is the difference between FAT and SAT?
FAT = Factory acceptation test
SAT = Site acceptation test
when any company going to install new instrument control
system,they have to test how its working ? In the FAT shop
they will do configuration setting for all instrumet loops.
according to configuration the system providor will do
function test by simulating signals.
but during the SAT they will check all function with real
Input signals.
I have seen the Temperature element (T/C & RTD) installed in process line on 45 degree position, Why? What it mean? How many types of tapping position for temperature elements?
Normally it will mounted perpendicular to the piping.
might be to avoid loss in pressure or flow.
The temperature probes will be located in straight for the
pipeline medium temperature measurement,
but in the tank it can be only located at side of the tank
at perpendicular or 45 degree position (not straight),
if it is quiet perpendicular mean it can measure only when
the liquid reaches the probe level,
the 45 degree location helps to measure for the wide level
of the liquid in the tank.
Why three wire is using with a RTD?
Resistanc Temperature Detector(RTD)encorporates two basic
concepts
1)Variation of temperature varies the resistance of Pt wire.
2)Variation in resistance affects the balancing of DC
bridge circuit.
For measurement purposes lead wires are brought out to make
external connections.These lead wires have some resistances
which affects the bridge balancing.
Since we can not reduces the resistances of lead wire to
zero.So, either 3 wire or 4 wire connection is employed to
ELIMINATES THE IMBALANCING TO THE BRIDGE CIRCUIT caused by
resistances of lead wires so that an accurate measurement
can be made.These wires(3 or 4) are called compensating
leads.
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
Could anybody pls give ans of boiler 3 element control system?Boiler 3- element control system: (1)Boiler feed water (2)
drum level (3) steam flow
Boiler 3-element system consists of boiler feed water
system, drum level system and steam flow system. working of
this system is based on cascade control
ie, output of one system is use as setpoint of another
what is the basic working principle of I/P and Positioner in any control valve.
I/P is basiclly working on the flapper and nozzle
principle.And the positioner is working on the Forced
balance principle.
/P is input (requirement of process/operator) at which
valve needs to be operated.Positioner gives output
(feedback) to control loop about actual position of valve.
Conclusively-I/P-requirement, Positioner-job done
conformation
I/P Converter , converts 4-20 mA signal from controller
to .2 to 1.0 Kg/cm2 pneumatic signal.
Positioner consist of air supply port , signal input port,
out put port.When positioner get in put signal ,it compare
with stem position and output is generated and signal
boosted sufficiently to operate the valve . It uses force
balance system.
WHAT IS CV OF CONTROL VAVLE
The Cv is actualy the Flow Coefficient of control valves.
It actually measures the friction experienced by a fluid as
it moves through some pipe / valve. The standard definition
of Cv is:
"the flow of water through a valve at 60 oF in US
gallon/minute at a pressure drop of 1 lb/in2."
coefficient valve
one cv equal the flow of one u.s galleon(3.8 litter)per minutes of water at 60 F under a pressure drop of value of 1psi.
what is difference between RTD , thermo couple and thermistor in general ?? (except temperature co-efficient )
in general RtD and thermister is passive transducer and
thermocouple is active transducer
RTD & Thermocouple are positive temp. Coefficeint and thermistor is negative temp . Coff..
with t/c we can measure upto 1700deg. but with RTD 600
degree is accurable. in t/c only the tip is sensing point
wihre in RTD the average value will be taken.
Why we use 4 to 20 mA, (This is International Standard) instaed of this can we use 4-16, or 4-24 or 3- 15 or any other range
4-20 mA signals are possible to divide equally, 0-4 mA as
close loop checking.
We use 4-20 mA range as a standard bcoz we know very well
when there is no input i.e. output must be 4 mA otherwise
instrument is giving wrong output(i.e. instrument not
having proper calibration).
There are basically 2 reasons of using 4-20ma
1. we can know whether the instrument is working properly
or not. we can distinguish "live zero" and "dead band". if
there is problem in instrument then also it is going to
show zero and if there is no supply then also it will show
zero so we can differentiate this two.
2. 4-20ma is linear with 3-15psi .
We use 4-20 mA range as a standard bcoz we know very well
when there is no input i.e. output must be 4 mA otherwise
instrument is giving wrong output
the 4mA zero is termed a "live zero". If 0mA was the zero
point there would be no way of telling if the transducer
was reading zero or in a fault condition eg. an open loop.
with a 4mA zero an open loop would drive the indicator eg.
a recorder into a minus figure (-25%). The 4 - 20mA scale
is to give a calibration points for 0-25-50-75-100%.
Because the start point is 4 the steps are 4mA.First the question is why can't we use 3~15mA or other
range, here also we can check live zero and also we can go
step current like 3,6,9,12,15(i.e 0%,25%,50%,75%,100%).ok
The reason is the transmitter voltage is 24Vdc , here we
can load max of 600 ohms, for this voltage & load better
range is 4~20.
Why 4 ~ 20 is better since we can convert easily 1~5Vdc by
put 250 ohms.All the controllers and DCS , PLCs are operated
by the input of 1~5Vdc (external or Internal convertion)
Which standard is used for calibration of Control Valves ?
0.2 bar - 0%, 0.4 - 25 %, 0.6 - 50 %, 0.8 - 75% and 1bar -
100%
what r the different types of instrumentation cables? give their specification?
Instrumentation cables such as Pair,Tried,Individual etc.
All made up of Aluminium polyester tape and ATC drain having Tin-copper
IN Industry Instrumentation Cables faciliate Smooth Communication Low Level Signal from Electronic Transmitters.
Hence they are speciliasied and the specificiation are
1.Triad
2.Pair
3.Individual.
This Type ensures smooth communication of very Low level signal from transmitter to control room & also effectively Cut the cross communication & noise.
what do we mean by hook up drawing. what r the information we get from it?
hook-up drgs provides detialed information about how to
install an instrument in proper manner..
it also includes the details of Bill Of Materials..
It is necessary document for project engineering as well as
maintenance dpt..
what is the working principle of vortex flowmeter
vortex shedding phenomenon
An obstruction in a fluid flow creates vortices in a
downstream flow. Every obstruction has a critical fluid
flow speed at which vortex shedding occurs. Vortex shedding
is the instance where alternating low pressure zones are
generated in the downstream.These alternating low pressure
zones cause the obstruction to move towards the low
pressure zone. With sensors gauging the vortices the
strength of the flow can be measured.
dear friend,
in vortex flometer, it is natural phenomenon that if you
put an obstructionin the path of flow, automaticaly vortex
are generated.in this case the important point to be
observed is that the number of vortexes generated are
directly propotional to the velocity of the flowing fluid
but the distance between vortex to vortex remain constant,
while we have the formula
v=F.lemda..... where the lemda is defiened as the distance
between vortexes which is constant.
and velocity is directly propotional to frequency.
where we can use several types of sensors to detect the
vortexes, such as piezo electric type sensor. when a
vortex will hit to the sensor the pulse is generated and by
calculating the number of pulces we can determine the
velocity and then flow.
YOU can physically observe this case.. ... but it is to be
needed to go near river and search the place where the
stones are up than water level. sit there and observe the
phenomenon
it is better to fo that place where the depth of water is
not more than 6 inch and flowing speed is not much fast
...... cisit that place daily and observe that when the
flowing speed is more that day the more vortexes are
generated