Industrial Instrumentation and Control (I&C)
An informative blog for sharing views and knowledge about different industrial instruments and control technologies that is being used today.
Wednesday, February 23, 2011
LIMIT SWITCHS COMBINATION
In measuring a process variable 2 conditions
are measured one is high limit of process variable and other is low limit of process variable
In normal condition of a process variable both the high or low limit switchs are used in closed condition
Because during the normal behavior of a process variable if there is any abnormality (i.e. breakup in the wire of switch) then that state must be detected
Therefore the high limit switch’s combination is used in normally closed combination. While low limit switch is used in normally open combination
INTERFACE LEVEL MEASUREMENT
What is interface level measurement?
Ans: The level measurement between two liquids having different specific gravity
How interface level is measured?
There are different methods of interface level measurement
1. interface level measurement using level troll
2. interface level measurement using capacitance probe
3. interface level measurement using DP transmitter
Interface level measurement by using level troll
There are two methods of interface level measurement with level troll
a) using service fluids (both)
b) using the water
a) Using the service fluids
First of all the the level troll is 100% filled with the liquid having low specific gravity.
Then out put of transmitter is set to 0% (4ma/3 psig).
After that the level troll is 100% filled with the liquid having heavier specific gravity and the out put of transmitter is set to 100% (20ma/15psig)
By this transmitter would measure the interface level
b) using the water
In this method level calculation is made as follows
Water level for low density liquid= S.G of liquid*total height of level troll
Fill up the level troll about the level obtained by the formula then set the out put of transmitter for 0% (4ma/3psig)
After that obtain the level of water for the high density liquid
After level for high density liquid= S.G of liquid*total height of level troll
Fill up the level troll about the level obtained by formula and set the transmitter out put for 100 %( 20ma/15psig)
Interface level measurement by using capacitance probe
How capacitance probe measure the level?
Ans:
To know about capacitance probe u must know about the working of a capacitor
A capacitor consist of two parallel plates separated by a dielectric (insulator)
The capacitance of capacitor depends upon the height, distance of the plates and amount of dielectric between the plates
You can vary the capacitance of capacitor by varying dielectric between the plates
This method is used in level measurement
A probe is inserted into the tank containing dielectric liquid and the tank wall is referenced as the 2nd probe
As well as liquid level increase or decrease the capacitance (between probe and wall of the tank) will also increase or decrease
Interface level with the capacitance probe can be also measured
Two liquids having different specific gravity will also have a different dielectric constant therefore by using capacitance probe the probe will measure a interface between dielectric liquid’s level (capacitance) which can be denoted as interface level
NOTE: that capacitance probe must always be 100% submerged with either high, low density liquid or with the both liquids other wise it will measure two interface levels one between air and low density liquid and the other between low density and high density liquid
Friday, February 18, 2011
Sunday, December 5, 2010
CASCADE CONTROL
Cascade control system
In some process two or more process variables effect equally on the behavior of each other where on is controlled with the control combination of another
For these kind of process the cascade control is used
Cascade control is a type of control terminology in which a process variable is
controlled by control combination of two process variables
In cascade control two process variables are controlled with a single final control element
In this control system one controller is recognized as master controller and the other as slave controller
The master controller is given a setpoint by operator and its out put becomes the setpoint of the slave controller and output of the slave controller goes to the final control element
Therefore any change of any process variable creates the change in final control element position therefore process is controlled by control combination of both process variables
Following a type of cascade control is given
(in this system temperature is controlled by controlling the flow)
Here burner temperature controller is used as master controller and fuel flow controller is used as slave controller where temperature controller’s out put becomes setpoint of the flow controller and finally out put of flow controller goes on the fuel flow control valve
Hence controlling the flow of fuel the burner’s temprature is controlled
Saturday, November 13, 2010
Hart communicator's function
Hart communicator’s mostly used functions in calibration of a device
LSV= lower set value:
The minimum range value of device, it can measure
USV=upper set value:
The maximum value of device , it can measure
LRV=lower range value:
The minimum value to which device has to read it as 0%
USV=upper range value
The maximum value to which device has to read it as100%
Sensor zero trim:
To zero the sensor value (means which output is generating sensor to zero it)
D/A trim=digital to analog trim:
To zero the out put of transmitter,to generate the 0% out put at that state of the device
Wednesday, November 10, 2010
calibration of transmitter
Calibration of a pressure transmitter
Calibration with analog apparatus
Equipment
Pressure Transmitter range (0 to 200 psig )
Pressure source range (0 to 250 or more than)
Multi meter
24vdc power supply
Set range of transmitter to required range (0 to 200 psig)
Connect multi meter in series of power supply to the transmitter
Make graduation of the range of transmitter as follows
(for 0 to 200 psig range)
Applied pressure | Multi meter output | %age |
0 psig | 4 ma | 0% |
50 psig | 8 ma | 25% |
100 psig | 12 ma | 50% |
150 psig | 16 ma | 75%0 |
200 psig | 20 ma | 100% |
First adjust the zero (with zero screw) that multi meter must read 4ma
If multi meter is reading less then 4 ma then increase the out put by clockwise moving to the zero set screw
And if multi meter is reading more than 4 ma then decrease the out put by moving the anticlockwise to zero set screw
After adjusting the zero apply 50 psig pressure (as u made graduation) to the transmitter where multimeter must show 8ma if not then adjust it from span screw
After adjusting check
100 psig = 12ma.
150 psig = 16ma
200 psig = 20ma respectively
after adjusting all the readings check two or more time to all readings if no changes occur then your transmitter is calibrated
Calibration with HART communicator (Highway addressable remote transducer)
Equipment
Pressure Transmitter range (0 to 300 psig ) “must be Hart enabled”
Pressure source range (0 to 350 or more than)
Hart communicator (275/375)
24vdc power supply
Procedure
Connect the Hart communicator with the transmitter
Connect its terminals across the 250 Ohms resistor connected in series of power supply to the transmitter
When Hart communicator detects your device then go into its setup
Adjust the range through the option lower/upper range value
Set lower range value to zero (0) and upper range value to 300 psig
Set the zero by zero trim option (given pressure must be zero)
Apply 300 psig pressure to the transmitter and set it the 100% range value of transmitter by span trim option
After this setting check the 0 and 100% out put by connecting also multi meter in the series of the power supply
At 0 psig pressure multi meter must read 4ma
Apply the 300 psig where multi meter must read 20ma
Repeat this practice two or three times your transmitter will be calibrated when it matches its output reading match to the input