Basics Of Industrial Instrumentation and Process Control.

This blog aims at providing the Aspiring Minds or Professionals to have answers to their questions relating Instrumentation and Process controls.All questions relating the topics would be answered. Bring it ON......

Thursday, 28 September 2017

Electrochemical Sensors.

September 28, 2017 0 Comments


Electrochemical Sensors
Principle of operation.  

  • Electrochemical sensors work by reacting with the gas of interest and producing an electric signal proportional to the gas concentration.
  • The gas first passes through the capillary diffusion barrier and diffuses through the hydrophobic membrane.This allows specific amount of gas  to react with the sensing electrode to produce sufficient amount of electrical signal and also preventing the leak of electrolyte.
  • The gas that passes through the barrier reacts at the surface of the sensing electrode involving either an oxidation or reduction reaction.
  • A current is generated in the process hence it is called as amperometric gas sensor or a microfuel cell.
Electrochemical Sensor
Importance of Reference Electrode

  • For a sensor requiring  an    electrical driving voltage it is important to have a stable & constant potential at the sensing electrode. In reality the potential at the sensing electrode does not remain constant due to continuous chemical reaction . It causes deterioration of the electrodes over the extended periods of time. To improve the performance of the sensor reference electrode is introduced. This reference electrode is placed in close proximity of the sensing electrode. This reference electrode maintains the value of fixed voltage at the sensing electrode.
  • No current flows through the reference electrode.
  • Current flows between sensing electrode and  counter electrode.
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Friday, 22 September 2017

ATEX Standards

ATEX Standards

September 22, 2017 0 Comments
Atex  standards:
  • Atex stands for atmosphere explosives .
  • As of july 2003 ,organizations in EU must follow the directives to protect employees from exploision risk in areas with an explosive atmosphere.
  • There are two atex  directives (one for the manufacturer and one for user of the equipments.
1.The Atex  95 equipment directive 94/9/EC equipment and protective systems  intended for use in potentially explosive atmospheres.
2.Atex 137 work place directive  99/92/EC ,minimum requirements for improving the safety  and health protection of the workers potentially at risk from explosive atmospheres.
ATEX Coding
EX
European union explosives atmosphere symbol
ii
Equipment group

1.Mining

2.Surface
2
Equipment Category:

M1. Energized

M2. Deenergized.

1. Very high protection.

2. High Protection.

3.Normal Protection
G
Gas

0
1
2
D
20

21

22


Hazardous area zones and equipment categories Hazardous places are classified in terms of zones on the basis of the frequency and duration of the occurrence of an explosive atmosphere.
Gases, vapours and mists For gases, vapors and mists the zone classifications are:
Zone 0:  A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is present continuously or for long periods or frequently.
Zone 1:  A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally.
Zone 2:  A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
Dusts: For dusts the zone classifications are:
Zone 20: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently.
Zone 21:  A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.

Zone 22:  A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
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Trial & Error Method of PID Tuning.

Trial & Error Method of PID Tuning.

September 22, 2017 0 Comments
TRIAL AND ERROR METHOD OF PID TUNING
The Trial and Error method requires a closed loop system, it steps through the system from proportional to integral to derivative.
This method is a divide and conquer approach, first it puts the system into a rough solution from which small tweaks are performed to perfect the response.

  1. To begin, each coefficient of the PID controller is set to zero.
  2. The proportional component is now considered by increasing its value until a steady oscillation is obtained . Scaling the current proportional value down by a factor of two will give the resulting proportional value. Applying this proportional value will dismiss the steady oscillations.
  3. Next the integral coefficient is increased until steady oscillations are again obtained. The present value of the integral coefficient is scaled up by a factor of three and applied to the integral as the final value. This once again sets oscillations off, which brings up the derivative control, this value is increased until for a final time the oscillations are at a constant period and amplitude.
  4. The coefficient of the derivative is then scaled down by a factor of three and applied as the final value for the derivative control. The resulting output may still have some noise associated with it, this must now be tuned by hand with small educated tweak of the different coefficients.
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All about Radar Level Measurements.

All about Radar Level Measurements.

September 22, 2017 0 Comments
What is the principle of operation of Guided Wave Radar ?
  • Radar level measurement technology can be broken down into two different categories; Pulsed and Frequency Modulated Continuous Wave (FMCW). 
  • An advantage with Pulsed Technology is that it requires less processing power. Therefore most two-wire gauges use this technology. 
  • An advantage with FMCW is that higher accuracy can be achieved but more processing power is required and therefore FMCW-radars are typically four-wire. 
  • In Pulsed transmitters the level measurement is a function of the time taken from the radar signal to travel to the surface and back. 
  • In FMCW gauges the transmitter constantly emits a swept frequency and the distance is calculated by the difference in frequency of emitted and received signal.
Are radar gauges safe?
  • Yes 
  •  Radar waves are of no greater intensity than the constant radio, cellular and other communication waves that surround us every day. Furthermore the transmitter is normally placed in a metallic tank that acts as a Faraday’s cage and therefore the radar waves are isolated within the tank.
What is the difference in frequency between Pulsed Technology Guided Wave Radar and Non Contacting Radar?
  • With Guided Wave Radar the pulsed microwave are guided down the tank by the probe, making it less sensitive to disturbances than free propagating microwaves. Pulsed Non Contacting Radar uses a carrier frequency, e.g. 6 Hz  or 26 Hz , to carry the microwave which are radiated into the tank with an antenna.
Why should we use radar technology?
  • The radar signal is  unaffected by the tank content and tank atmosphere, temperature or pressure. The measurement is not influenced by changing material characteristics such as density, dielectric properties and viscosity. Since there are no moving parts the transmitters are  maintenance free. All of the characteristics above make radar a very useful and fast growing level measurement technology.
How does the frequency of the radar affect the measurement?
  • A higher frequency provides a more concentrated narrow beam which can be useful in applications where there are obstacles present in the tank such as man-ways, agitators or heating coils. The downside of high frequency is that the measurement is more affected by vapours, dust and product build up on the antenna, Low frequency radar which have a longer wavelength and wider beam angle, tends to cope better with steam, dust, condensation, contamination and turbulent surfaces.
In what way does the dielectric constant (DC) of the media affect the measurement?
  • Electromagnetic energy is emitted from all radar devices. When the emitted signal reaches a point where there is a change in DC, usually the media surface, some of the signal is reflected back to the transmitter. The amount of energy that is reflected back to the transmitter is proportional to the DC of the media. A rule-of-thumb is that the value of the dielectric constant represents the percentage of energy that is reflected. Thus a DC of eight means that eight percent of the emitted energy is reflected back to the transmitter. Fundamentally media with a higher DC provide stronger return signals and are therefore easier to measure.
Does radar work on foam?
  • The effects of foam on a radar measurement can be difficult to predict. In some applications the foam may dampen out the signal completely while other types of foam may be transparent to the transmitter. The thickness, density and the dielectric constant are factors that need to be considered when evaluating an application with foam.
  • On dry foam the microwaves typically passes through and detects the liquid surface below. On medium type foam the signal can be absorbed or scattered and the results are therefore hard to predict. If the foam is wet the microwaves are often reflected from the foam surface and thereby the foam surface level is measured.
  • The frequency at which the radar operates also affects how foam is measured. Low frequency radar (5 GHz) in general penetrates foam to a larger extent than high frequency (20 GHz) radar. Guided Wave Radar is in general better suited to measure on applications where foam is present, since the radar uses a lower frequency pulse.
Will radio noise or other interference cause problems?
  • The transmitters use a specific narrow frequency and are therefore not prone to disturbances from other sources. It is very uncommon with disturbances and it is rare that the disturbance source operates at precisely the same frequency as the transmitter. Furthermore, the transmitters are often installed in metallic tanks that provide a Faraday’s cage which prevents electromagnetic disturbances from the outside to enter the tank. With Guided Wave Radar, if disturbances are present in tank the coaxial probe are recommended, since the radar signal travels inside of the pipe undisturbed by the interference sources on the outside.
Are there any restrictions on the nozzle height?
  • Since the nozzle, and especially the lower end of the nozzle, can create interfering echoes it is recommended that the height of the nozzle is kept within certain values depending on the type of probe / antenna and type of transmitter that is used. 
Are there any restrictions on the nozzle diameter?
  • In conjunction with the above statement, the nozzle diameter also affects the measurement,since a diameter nozzle that is too small will create disturbance echoes. 
What is purpose of exotic materials?
  • In some applications with high temperatures, or in highly corrosive environment, the probes or antennas need to be made out of exotic materials that can be stand the material stress. 
How viscous can the measured media be?
  • The different probes have different max limits regarding the viscosity of the measured media. The single probes are more suitable for high viscosity media while the coax probe can be used on low viscosity media. The guidelines for the different probes and examples of viscosity are presented below:
How will coating affect the measurement?
  • If coating forms on the probe the measured signal will be weaker. If the media itself has a high dielectric constant some coating is not much concern but if it is a low DC media coating can be a problem. If a twin probe or a coaxial probe is used the coating can cause bridging between the two leads and this will create false echoes that can lead the transmitter to interpret a bridge as the actual level. Single lead is recommended in coating applications. Coating can cause an accuracy influence. Maximum error due to coating is 1-10% depending on probe type, dielectric constant, coating thickness and coating height above product surface.
Will the Guided Wave Radar see sand in the bottom of a vessel?
  • No. Since the sand will be embedded in water which is a high dielectric media (DC~80) the transmitter will only see the water. The same is true for all media that are embedded in water.
How close to the tank wall can the probe be mounted?
  • Like the nozzle the tank wall can also affect the measurement through disturbance echoes. The minimum distance to the tank wall is the same as the distance to any disturbing object that may be present in the tank. If there are obstacles present in the tank the coaxial probe is the best probe to use. If the tank wall is metallic and smooth the probe can be mounted closer to the wall.
What measurement error will an incorrect dielectric constant cause in on an interface level?
  • Errors due to changes in the medium’s dielectric values can be significant. It is calculated by:
  • For example, if the physical thickness is 20” (51 cm) and dielectric varies from 2 to 4:
  • E = ( 20/sqrt(2) ) – ( 20/sqrt(4) ) = 4.14″ = 11 cm

Non Contacting Radar Questions

What is the principle of operation for non contacting radars?
  •  The transmitter constantly emits a swept frequency signal and the distance is calculated by the difference in frequency between the emitted and received signal at any point in time. One advantage with FMCW is that higher accuracy can be achieved. The level of the liquid is measured by short radar pulses which are transmitted from the antenna at the tank top towards the liquid. When a radar pulse reaches a media with a different dielectric constant, part of the energy is reflected back to the transmitter. The time difference between the transmitted and the reflected pulse is proportional to the distance, from which the level is calculated.
How close to the tank wall can the non contacting transmitter be mounted?
  • The minimum distance to the tank wall depends on which antenna is used. With a more concentrated beam (larger antenna), the closer to the tank wall the device can be mounted. 
What is the difference between a cone and a horn antenna?
  • It is just different names for the same type of antenna.
  • I will soon be posting  technical information on how a radar level transmitter be mounted with problem regards entering the total range of the level to be measured.. Stay Tuned.


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Thursday, 21 September 2017

Installation of Flow-meters.

Installation of Flow-meters.

September 21, 2017 0 Comments
Installation of Flow meters.
We as a instrumentation Engineers are many times in a situation where we have to mount a new instrument such as flow meters, level indicators etc. This post is regarding the upstream and downstream distances required for mounting of flow meters. A flow meter when mounted incorrectly can lead to erratic reading. A must for all the Instrumentation Engineers.
Vortex flow meter.
  • These  meters work on the phenomenon of vortex shedding that takes place when fluid or gas meets an obstacle downstream termed as bluff body.
  • These meters can be installed vertically or horizontally.
  • For liquid measurements the meter must be full at all times.
  • Vortex flow meters must be often smaller than line size. Installation should keep the meter full at all times. i.e zero flow.
  • For steam measurements design process piping so that the meter remain full ,with no entrapped air.
  • The meter can be installed in any orientation without affecting accuracy.
  • Avoid installation in loops such condition may cause water hammering at start up due to trapped condensation.
Upstream and downstream distance .
  • Using k factor correction upstream 10d
  • Downstream 5d
  • Without k factor ,35d & 5d.
  • Vortex flow meter doesn’t require the piping arrangement whats more critical is the piping to remain full and that flw profile is completely free of any distortions..
  • This usually requires a couple of control valves used to reduce pipes and keep the meter full and generous amount of upstream  straight run piping.
  • Between upstream of control valve and the meter 30 d the piping arrangement is required and between meter and  downstream valve there should be straight run equaling atleast 5 x the piping diameter..
  • Pressure transmitter can be mounted at atleast 4x diameter and temperature transmitter at 6x the diameter.



Orifice type Flow meter.
  • For an orifice  plate ,the straight run required depend upon  the beta ratio and the type of upstream components in a pipeline.
  • If using an standard integral orifice plate of 0.4 beta ratio straight run requirement of 20 straight pipe diameters and 10 d downstream.
  • If using a conditioning orifice plate  of beta ratio 0.4 then 2-6 diameters.
If straightening vanes not used:
  • If collars ,flanges, wide open gates, reducers or bends, elbows or tees which are all in the same plane-10xdiametrs.
  • Angle turns out in two planes 50x diameters.
  • Pressure regulator ,control valves or similar apparatus-75 x diameters.
If straightening vane is used:
  • 6 diameters upstream and 5 diameters downstream.

Magnetic flow meter requirements

  • Always pipe a electromagnetic flowmeter so it is full of liquid.
  • The pipe configuration should avoid a build up or accumulation of any secondary phase(solid or air).any entrained air should be carried by meter flow.
  • You can install magmeters in horizontal lines but best practice call for installation in  vertical lines with upward flow.
  • The general rule of thumb for straight piping is 5 diametr upstream and 3 diameters downstream.
  • Avoid downward flows.
  • Avoid installing flow meters on top as these give rise to air bubbles.
  • Always install control valves and shut off valves downstream of flow meters.
  • Never install flow meters on suction side of pump.

Coriolis type Flow meter.

  • Coriolis mass flow metrs measure mass directly and offer few installation limitations. These flow meters are insensitive to swirl and velocity profile distortion.
  • Install flow metrs so they remain full of liquid and air cannot get trapped inside the tubes.The most desirable is the vertical installation but installation in horizontal piping is also acceptable.
  • Installation wher fluid flows in downward direction is not acceptable.
  • Good practice call for upstream installation of strainers,filters,air vapor eliminators as necessary to remove all undesirable secondary phases and air bubbles.
  • Install control valves downstream to increase backpressure on them so as to decrease the probablity of cavitation and flashing.

Turbine flow meter requirements
  • Typical manufacture specification for turbine flow meters call for straight pipe lengths of 10-15 pipe diameters upstream and 5d downstream .Additional pipe recommendations include 20 pipe diameters for 90 – degree elbow be fitter stainer or thermowell .
  • 25 pipe diameter for a partially open value
  • 50 pipe diameter for two elbows in different planes or if flow is spiraling.
  • Straightening vanes can reduce the length of straight pipe otherwise required.
  • Install control valves only on downstream side of meter.

Plusses & minuses.
  • Moderate cost ,very good at low viscosity fluids,moderate viscosity and stedy flow rate.
  • Turn down is very good.
  • Reliable if put in clean fluids especially with lubricity.
  • Not good for steam.

Thermal Flow Meter.

Upstream obstruction
Minimum straight pipe requirements
Without flow conditioner
With flow conditioner
Single bend preceded by >9 diameters of straight pipe
15d
3
Pipe size reduction in straight pipe run
15d
3
Multiple bends in plane with <9 diametrs of straight pipe between them
20
9
tees
30
10
Multiple bends out of plane
40
5

Plusses and minuses:
  • Not applicable to abrasive fluids they can damage the sensor.
  • Fluids that coat the sensorcan alter the relationship between thermal; properties of fluid and the measurement and adversely affect the measurement.extensive coating can render the sensor useless and can increase the maintenance.
  • Not suitable for applications with fluids that have varying composition and unknown components.
  • Aerosols and dropletys can cause the meter to read erratic.this is because large amount of thermal energy is used to heat dropletsis interpreted  as high flow signal.
  • Not good for steam.
  • Dirty fluids should be avoided.
  • Two phase fluids to be avoided.
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