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