UNIT II MEASUREMENTS IN POWER PLANTS Measurement of feed water flow, air flow, steam flow and coal flow – Drum level measurement – Steam pressure and temperature measurement – Turbine speed and vibration measurement – Flue gas analyzer – Fuel composition analyzer Flow Measurement Feed water Flow • Orifice and Venturimeter along with differential head sensor Air Flow • Venturimeter Steam Flow • Orifice or Flow Nozzle Coal Flow • Indirect measurement from bunker level • Measurement using load cell • Measurement by belt weighing Level Drum Level • Gauge Glass and DPT Pressure Steam Pressure • Bourdon tube pressure gauge and DPT Material & Range: • Cast Iron upto 16Kg/cm 2 • Carbon steel upto 64 Kg/cm 2 • Stainless steel from 64 to 100 Kg/cm 2 • Molybdenum steel above 100 to 200 Kg/cm 2 Temperature Steam Temperature • RTD and Thermocouples • Range: 200 to 600 ºC • RTD for low temperature range • Thermocouples: Iron-Constantan Speed Turbine Speed • Analog or Digital Tachogenerator (Contact Type) • Magnetic type (Non Contact Type) Vibration Turbine Vibration • Accelerometer or Proximity sensor Gas Analyzer Flue gas analyzer • Oxygen • COx • SOx • NOx Composition Analyzer Fuel composition analyzer • Proximate Analysis • Ultimate Analysis
Transcript
UNIT II
MEASUREMENTS IN POWER PLANTS
Measurement of feed water flow, air flow, steam flow and coal flow – Drum level measurement –
Steam pressure and temperature measurement – Turbine speed and vibration measurement –
Flue gas analyzer – Fuel composition analyzer
Flow Measurement
Feed water Flow • Orifice and Venturimeter along with differential head sensor
Air Flow • Venturimeter
Steam Flow • Orifice or Flow Nozzle
Coal Flow • Indirect measurement from bunker level
• Measurement using load cell
• Measurement by belt weighing Level
Drum Level • Gauge Glass and DPT Pressure
Steam Pressure
• Bourdon tube pressure gauge and DPT Material & Range:
• Cast Iron upto 16Kg/cm2
• Carbon steel upto 64 Kg/cm2
• Stainless steel from 64 to 100 Kg/cm2
• Molybdenum steel above 100 to 200 Kg/cm2 Temperature
Steam Temperature
• RTD and Thermocouples • Range: 200 to 600 ºC
• RTD for low temperature range
• Thermocouples: Iron-Constantan Speed
Turbine Speed • Analog or Digital Tachogenerator (Contact Type)
• Magnetic type (Non Contact Type) Vibration
Turbine Vibration • Accelerometer or Proximity sensor Gas Analyzer
Flue gas analyzer
• Oxygen
• COx
• SOx
• NOx Composition Analyzer
Fuel composition analyzer • Proximate Analysis
• Ultimate Analysis
FLOW MEASUREMENT (For Water, Air and Steam):
Variable Head Type Flowmeters:
• Orificemeter
• Venturimeter
• Flow nozzle
• Dall tube
• Pitot tube
Orificemeter:
• Of the commercially available restriction type primary flow measuring elements, orifice is
the most common
• It is the cheapest and takes the form of a thin plate square edge orifice and is mounted
in between the flanges
• The orifice may be concentric, segmental or even eccentric
• While the concentric one is more common, the segmental or eccentric types are used for
fluids containing solids and these types are to be mounted in such a way that the bottom
of the orifice is flush with the bottom inside of the pipe
• The disadvantage with these types is that standard flow coefficients tables are not
available for them.
• Following figure shows two types of orifice and the machining methods of orifice.
• Type 1 and 2 are very commonly used; “F” is known as the platter
• These two types are easily reproducible and easier to manufacture while type 3 is not.
• Thickness “t” is chosen to withstand the buckling forces.
• Type 1 has also reduced pressure losses
• Flow co-efficient for these types are slightly different and are available for known d/D, F,
t and ɑ.
• Type 3 known as quadrant edged orifice, is used for more viscous fluids where for low
Reynold’s number viscosity correction is usually necessary.
• The design may be made without requiring any viscosity correction.
• Materials for orifice plates are steel, stainless steel, Monel, Phosphor-bronze, Brass,
etc.,
• A general mounting feature of the orifice is shown in the following figure.
• Tappings positions greatly influence the measuring accuracy.
• A downstream tap at vena contracta would generate the largest differential pressure, but
flow rate and d/D ratio greatly influence the location of this tap position.
• Advantages:
o Low cost
o Can be used with differential pressure devices
o Well known and have predictable characteristics
• Disadvantages:
o High permanent pressure loss
o Have changing characteristics because of erosion, corrosion and scaling
o Accuracy is dependent on care during installation
Venturimeter:
• Venturi tubes may be of two different designs: the short recovery cone type and the long
recovery cone type also sometimes known as Herschel standard type.
• Figure shows the line sketch of a venturi with a straight throat of length equal to the
throat diameter (b = d) and the throat tapping is made midway.
• Other dimensions are shown in the diagrams, and ɑ2 varies from 5º to 15º depending on
the long or short recovery cone type design.
• The short venture is suitable for short obstructed lines.
• It has larger pressure losses than the Herschel type.
• The throat generally is a separate cast body so that it can be easily replaced.
• The venturi is more accurate than the orifice as it has less pressure losses.
• For furthering the accuracy the average pressure differential is measured by replacing
the pressure taps with piezometer rings.
• Its range of operation is very high and because of the constructional shape, has less
wear and abrasion with high reproducibility.
• Discharge coefficient value is larger than that of an orifice and is usually between 0.9
and 1.0
• Advantages:
o Low permanent pressure loss
o For high flow rates
o More accurate than orifice and flow nozzle
• Disadvantages:
o High cost
o Not useful below 76.2mm pipe size
o More difficult to inspect due to its construction
Flow Nozzle:
• It is a variation of the venture in which the exit section is omitted and consequently the
approach section is bell shaped with a cylindrical throat.
• The schematic sketch is shown in figure.
• The ASME standard flow nozzle has an elliptical entrance as compared to the ISA
standard that has a radius entrance.
• Such designs are used for high velocity, > 30m/s, vapour (steam) flow rate.
• A design, sometimes known as critical nozzle venturi is used in special applications such
as critical flow
• It has further been developed to have high differential pressure, low permanent pressure
loss, shorter length and lower weight.
• Advantages:
o Permanent pressure loss is lower than orifice plate
o Useful for fluid containing solids that settle
o Widely accepted for high pressure and temperature steam flow
• Disadvantages:
o Cost is higher than orifice plate
o Limited to moderate pipe sizes
o Requires more maintenance
Correction Factor in Steam Flow:
• The density of steam is a function of both temperature and pressure.
• Measuring errors can occur if the density in operation differs appreciably from the design
figures of the restriction.
• In that event a correction has to be introduced.
• The correction factor for steam when measured in Tons/hr unit is given as
�� = �ρ�ρ� =���� �
• Where ρ�, PO, TO are operating density, pressure and Temperature, and ρ�, PD, TD are
corresponding design values of density, pressure and Temperature
• The correction computation can be built in as an integral part of DPT or separately done
as shown in the figure.
• Input 1: Differential head from orifice (h)
• Input 2: Pressure (PO)
• Input 3: Temperature (TO)
������ = ��ℎ� �
• Scaling and biasing are also necessary to get a standard electrical 4-20 mA signal
corresponding to flow range.
• The input signals are also standard 4-20mA signals for the corresponding differential
head range, pressure range and temperature range.
• The design temperature and pressure are taken into constant K factor.
COAL FLOW:
• Measurement of coal which is fed to boiler as a fuel has to be done for calculation of
boiler efficiency.
• Sometimes this signal can be beneficially used for combustion control purpose also.
• It is not that straight measurement as it is done for liquid or gaseous fuel.
• There are three common methods available
• Indirect measurement from bunker level
• Measurement using load cell
• Measurement by belt weighing
Indirect measurement from bunker level
• Coal is delivered to the power plant by means of truck or rail wagons.
• The user must have sufficient space and mechanical equipment for storing and
handling prior to use.
• The coal is then transferred to coal bunkers from storage location.
• These bunkers are located at an elevation that allows the coal to feed to the boiler
system by gravity.
• By recording the bunker level at regular intervals (say half an hour) and converting
the level difference into volume one may be able to get the average volume flow rate.
• Totalling can be done at the end of the day’s operation and average flow rate for the
day can be found out.
• Though such measurements cannot be used for continuous control purposes, the
information can be very well used for boiler efficiency calculations
Measurement using load cell
• The coal can be fed to hoppers fitted with load cells and then fed to boiler system.
• The hopper will be filled; the weight measurement is taken and then emptied to the
feeding conveyor.
• This cycle can be continuously done and the weight measured is divided by the cycle
time to get the average feed rate.
• This way of measuring is more accurate compared to the one by measuring the
bunker level.
Measurement by belt weighing
• The flow rate of coal cab be measured when it is fed to the boiler system via belt
conveyor.
• Load cell arrangement can be used to measure the weight of the coal in the belt
between two rollers which are designed to float for making the measurement
possible.
• The conveyor speed can be measured and used for calculating flow rate.
• The feed rrate of all belt weighing feeders is a function of the belt speed and the belt
load.
• Belt speed is normally expressed in terms of meter per minute, which belt load is
defined as kilogram per meter of belt.
• Feed rate = Belt Speed X Belt Load
• In case of constant speed belt feeders, the rate is directly proportional to belt load.
LEVEL MEASUREMENT:
• This is one of the very important and critical measurements in a boiler.
• Both local and remote indications of boiler level are always attempted.
Local Indication (Gage Class):
• The basic indication of the drum water level is that in a sight gage glass connected to the
boiler drum.
• The typical arrangement is shown below.
• Since the configuration of the boiler and the distance of the boiler drum from the
operator may not provide a useful ‘line of sight’ indication, the gage glass image is
usually projected with a periscope arrangement of mirrors so that operator may easily
view it.
• In modern power plants, closed circuit television is used as shown in following figure to
display the gage glass level in a monitor in the control room.
• Because of the temperature difference between the water in the drum and the water in
the gage glass, the level shown by the gage glass is always lesser.
• The deviation depends also on the boiler pressure, piping and insulation between the
boiler drum and gage glass.
Drum Level Transmitter
• A typical arrangement of a drum level measuring transmitter is shown in the following
figure.
• The transmitter is a differential pressure device in which the output signal increases as
the differential pressure decreases.
• This is because of the decrease in differential pressure for increase in water level.
• Typically the differential pressure range is approximately 60 cms with a zero suppression
of several cms.
• To determine the measuring instrument calibration, the necessary design data are the
location of the upper and lower pressure taps into the boiler drum with respect to the
normal water level, the operating pressure of the boiler drum, and the ambient
temperature around the external piping.
• With these data and the desired range span of the transmitter, the exact calibration can
be calculated by using the standard thermo dynamic properties of steam and water.
• On the high pressure side of the transmitter, the effective pressure equals boiler drum
pressure plus the weight of a water column at ambient temperature having a length
equal to the distance between the two drum pressure connections.
• On the low pressure side, the effective pressure equals boiler drum pressure, plus the
weight of a column of saturated steam having a length from the upper drum pressure
connection to the water level, plus the weight of the column of the water at saturation
temperature having a length from the eater level to the lower drum pressure connection.
TEMPERATURE MEASUREMENT:
Resistance Thermometers
• It is well known that resistance of metallic conductors increases with temperature, while
that of semiconductors generally decreases with temperature.
• Resistance thermometers employing metallic conductors for temperature measurement
are called Resistance Temperature Detector (RTD)
• RTDs are more rugged and have more or less linear characteristics over a wide
temperature range.
• Resistance Temperature Detector
The variation of resistance of metals with temperature is normally modeled in the form:
Rt=R0[1+ α(t-t0) + β(t-t0)2+......] (1)
where Rt and R0 are the resistance values at to
C and t0
o
C respectively;
α, β, etc. are constants that depends on the metal.
For a small range of temperature, the expression can be approximated as:
Rt=R0[1+ α(t-t0)]
For Copper, α=0.00427/ ͦ C
Copper, Nickel and Platinum are mostly used as RTD materials. The range of temperature measurement is decided by the region, where the resistance-temperature characteristics are approximately linear. The resistance versus temperature characteristics of these materials is shown in Figure 1.3, with to as 0oC. Platinum has a linear range of operation upto 650oC, while the useful range for Copper and Nickel are 120oC and 300oC respectively.
Figure 1.3: Resistance-temperature characteristics of metals
Construction:
For industrial use, bare metal wires cannot be used for temperature measurement. They
must be protected from mechanical hazards such as material decomposition, tearing and
other physical damages. The salient features of construction of an industrial RTD are as
follows:
• The resistance wire is often put in a stainless steel well for protection against mechanical
hazards. This is also useful from the point of view of maintenance, since a defective
sensor can be replaced by a good one while the plant is in operation.
• Heat conducting but electrical insulating materials like mica is placed in between the well
and the resistance material.
• The resistance wire should be carefully wound over mica sheet so that no strain is
developed due to length expansion of the wire.
Figure 1.4: Construction of an industrial RTD
RTD measurement circuits:
The resistance variation of the RTD can be measured by a bridge, or directly by volt-
ampere method. There are two variants, viz. the three wire and the four wire systems. These
are essentially used to eliminate the effect of the lead wire resistances that may adversely affect
the measurement.
There are two effects due to the lead wires:
1) They add to the resistance of the Platinum element
2) The resistance of the lead wires may also change with temperature.
These two effects are mitigated or eliminated by either the three or four wire
arrangements.
The lead wires are usually of higher diameter than the diameter of the sensor wire to
reduce the lead wire resistance. In both the three and four wire arrangements, the wires run
close to each other and pass through regions experiencing similar temperature fields. Hence the
change in the resistance due to temperature affects all the lead wires by similar amounts. The
resistances of the lead wires are compensated by a procedure that is described below.
Three wire arrangement for lead wire compensation:
Figure 1.5: Three wire RTD arrangement
Figure 1.5 shows the bridge circuit that is used with three lead wires. The resistances R1
and R3 are chosen to be equal and the same as R0 of the RTD. Two lead wires (labeled 2 and
3) are connected as indicated adding equal resistances to the two arms of the bridge. The third
lead wire (labeled 1) is used to connect to the battery. Thus the bridge will indicate null
(milliammeter will indicate zero) when R2 = R0 when the RTD is maintained at the ice point.
During use, when the RTD is at temperature t, the resistance R2 is adjusted to restore balance.
If the lead wires have resistances equal to Rs2 and Rs3, we have
If the two lead wires are of the same size the bracketed terms should essentially be zero
and hence the lead wire resistances have been compensated for.
Four wire arrangement for lead wire compensation:
The four wire arrangement is a superior arrangement, with reference to lead wire
compensation, as will be shown below. Figure 1.6 is the bridge arrangements that are used for
this purpose.
Figure 1.6: Bridge circuit with lead wire compensation (four wire arrangement)
The choice of the resistances is made as given for the three wire arrangement. If the
lead wires have resistances equal to Rs1 - Rs4, we have the following.
Condition for bridge balance in arrangement shown in Figure 1. 6(a)
Condition for bridge balance in arrangement shown in Figure 1. 6(b):
We see that by addition of above two Equations, we get
The lead wire resistances thus drop off and the correct resistance is nothing but the
mean of the two measurements. Since the lead wire resistances actually drop off, the four wire
scheme is superior to the three wire scheme.
Thermocouple:
Principle: Thomas Johan Seeback discovered in 1821 that thermal energy can produce electric
current. When two conductors made from dissimilar metals are connected forming two common
junctions and the two junctions are exposed to two different temperatures, a net thermal emf is
produced, the actual value being dependent on the materials used and the temperature
difference between hot and cold junctions.
e0= a(∆Ɵ) + b(∆Ɵ)2
Where,
∆Ɵ = difference in temperature between hot thermocouple junction and the reference junction of
the thermocouple
a, b = constant (a is usually large as compared with b) [For example: Copper/ Constantan
thermocouple, a=62.1 and b=0.045]
Design of Thermocouple:
It is imminent that the thermocouple output voltage will vary if the reference junction
temperature changes. So, for measurement of temperature, it is desirable that the cold junction
of the thermocouple should be maintained at a constant temperature. Ice bath can be used for
this purpose, but it is not practical solution for industrial situation. An alternative is to use
a thermostatically controlled constant temperature oven. In this case, a fixed voltage must be
added to the voltage generated by the thermocouple, to obtain the actual temperature. But the
most common case is where the reference junction is placed at ambient temperature. For high
temperature measurement, the error introduced due to variation of reference junction
temperature is not appreciable. Such a typical scheme is shown in below figure. Here a
constant voltage corresponding to the ambient temperature is added through the offset of the
op-amp. The thermocouple voltage is also amplified by the same op-amp.
A more accurate method for reference junction temperature compensation is shown in
below figure. Here a thermistor, or a RTD is used to measure the ambient temperature and
compensate the error through a bridge circuit. The bridge circuit is balanced at 0oC. When the
ambient temperature goes above 0oC, the emf generated in the thermocouple is reduced; at the
same time bridge unbalanced voltage is added to it in order to maintain the overall voltage at
the same value.
As referred to above Figure, the cold junction compensation is normally kept along with
the signal conditioning circuits, away from the measuring point. This may require use of long
thermocouple wires to the compensation circuit. In order to reduce the length of costly
thermocouple wires (platinum in some case) low-cost compensating wires are normally used in
between the thermocouple and the compensation circuit. These wires are so selected that their
temperature emf characteristics match closely to those of the thermocouple wires around the
ambient temperature.
Materials:
Theoretically, any pair of dissimilar materials can be used as a thermocouple. But in
practice, only few materials have found applications for temperature measurement. The choice
of materials is influenced by several factors, namely, sensitivity, stability in calibration, inertness
in the operating atmosphere and reproducibility (i.e. the thermocouple can be replaced by a
similar one without any recalibration). Table-I shows the common types of thermocouples, their
types, composition, range, sensitivity etc. The upper range of the thermocouple is normally
dependent on the atmosphere whre it has been put. For example, the upper range of Chromel/
Alumel thermocouple can be increased in oxidizing atmosphere, while the upper range of Iron/
Constantan thermocouple can be increased in reducing atmosphere.
Table-1 Thermocouple materials and Characteristics
PRESSURE MEASUREMENT:
Bourdon Tube:
• Bourdon tube is the oldest pressure sensing element .It is a length of metal tube of
elliptical cross section and shaped into letter “C‟.
• Normally it is used for local indication.
• Bourdon tube pressure gages can be used to measure over a wide range of pressure:
form vacuum to pressure as high as few thousand psi.
• The cross section of the tube is elliptical. When pressure is applied, the elliptical tube
tries to acquire a circular cross section; as a result, stress is developed and the tube
tries to straighten up.
• Thus the free end of the tube moves up, depending on magnitude of pressure.
• A deflecting and indicating mechanism is attached to the free end that rotates the
pointer.
• In bourdon tube the error occurs due to friction in the spindle bearing is called as lost
motion
• Because of compound stresses developed in the tube, actual tip travel is non-linear in
nature.
• However a small travel of tip can be considered to be linear and parallel to axis of link
which is to be matched with a rotational pointer movement is known as multiplication
• Adjustable link is used to adjust the lever so that multiplication occurs
• The approximate linear motion of tip converted to circular motion conversion using lever
and pinion arrangement, a one to one correspondence between them may not occur and
a distortion result, this is known as angularity
• Materials of the Bourdon tube is Phosphor bronze (upto 2000 Psi), Beryllium bronze or
Beryllium copper.
• Though the C-type tubes are most common, other shapes of tubes, such as helical,
twisted or spiral tubes are also in use.
• “C” shaped tubes are normally used when pressure is less than 60 bar
• “Elliptical” or “Spiral” tubes are used when pressure is greater than 60 bar
Capacitive Type Transducer
• Principle:
• Capacitive transducers are used for measurement of pressure by converting the
pressure into a displacement.
• The output of a parallel plate capacitor depends on the gap between its movable and
fixed plates.
• Due to pressure, if the gap between the plates is altered, its capacitance also changes.
This change in capacitance becomes a measure of pressure.
• Construction:
• This transducer consists of a pair of parallel plates with the middle plate (diaphragm)
moving with pressure.
• The diaphragm is usually metal or metal coated quartz and is exposed to the process
pressure on one side and to the reference pressure on the other.
• Depending on the type of pressure, the capacitive transducer can be either absolute,
gauge or DPT
• Stainless steel is the most common diaphragm material used but for corrosive service,
high nickel steel alloys, s
• Tantalum also is used for highly corrosive and high temperature applications.
• Silver diaphragms can be used to measure the pressure of Chlorine, Fluorine and other
halogens in their elemental state.
• Working:
• Due to applied pressure, the diaphragm gets deflected.
• The deflection of diaphragm causes a change in capacitance that is detected by a bridge
circuit.
• This circuit can be operated in either a balanced or unbalanced mode.
• In balanced mode, the output
varied to maintain the bridge a null.
• Therefore, in the balanced mode, the null setting itself is a measure of process pressure.
• When operated in unbalanced mode, the process pressure measuremen
the ratio between the output voltage and the excitation voltage.
• The output voltage eo measured as the difference of voltage e
capacitors formed with this movable plate is approximately given by
Where x is the displacement of the diaphragm and d is its diameter
• Capacitance pressure transducer are widespread in part because of their wide
rangeability, from high vacuums in the micron range
This transducer consists of a pair of parallel plates with the middle plate (diaphragm)
The diaphragm is usually metal or metal coated quartz and is exposed to the process
and to the reference pressure on the other.
Depending on the type of pressure, the capacitive transducer can be either absolute,
Stainless steel is the most common diaphragm material used but for corrosive service,
high nickel steel alloys, such as Inconel or Hastelloy, give better performance.
Tantalum also is used for highly corrosive and high temperature applications.
Silver diaphragms can be used to measure the pressure of Chlorine, Fluorine and other
halogens in their elemental state.
Due to applied pressure, the diaphragm gets deflected.
The deflection of diaphragm causes a change in capacitance that is detected by a bridge
This circuit can be operated in either a balanced or unbalanced mode.
In balanced mode, the output voltage is fed to a null detector and the capacitor arms are
varied to maintain the bridge a null.
Therefore, in the balanced mode, the null setting itself is a measure of process pressure.
When operated in unbalanced mode, the process pressure measuremen
the ratio between the output voltage and the excitation voltage.
measured as the difference of voltage e1 and e
capacitors formed with this movable plate is approximately given by
e0 = e1-e2 = Ex/d
is the displacement of the diaphragm and d is its diameter
Capacitance pressure transducer are widespread in part because of their wide
rangeability, from high vacuums in the micron range to 10000 Psi (70 MPa)
This transducer consists of a pair of parallel plates with the middle plate (diaphragm)
The diaphragm is usually metal or metal coated quartz and is exposed to the process
Depending on the type of pressure, the capacitive transducer can be either absolute,
Stainless steel is the most common diaphragm material used but for corrosive service,
uch as Inconel or Hastelloy, give better performance.
Tantalum also is used for highly corrosive and high temperature applications.
Silver diaphragms can be used to measure the pressure of Chlorine, Fluorine and other
The deflection of diaphragm causes a change in capacitance that is detected by a bridge
voltage is fed to a null detector and the capacitor arms are
Therefore, in the balanced mode, the null setting itself is a measure of process pressure.
When operated in unbalanced mode, the process pressure measurement is related to
and e2 across two
Capacitance pressure transducer are widespread in part because of their wide
to 10000 Psi (70 MPa)
SPEED MEASUREMENT:
DC Tachometer generators:
• It consists of a small armature which is coupled to the machine whose speed is to be
measured.
• This armature revolves in the field of a permanent magnet.
• The emf generated is proportional to the product of flux and speed.
• Since the flux of the permanent magnet is constant, the voltage generated is
proportional to speed.
• The polarity of output voltage indicates the direction of rotation.
• This emf is measured with the help of a moving coil voltmeter having a uniform scale
and calibrated directly in terms of speed.
• Following figure shoes a DC tachometer generator.
• A series resistance is used in the circuit for the purpose of limiting the current from the
generator in the event of a short circuit on the output side.
• Advantages:
o Direction is indicated directly by polarity of the output voltage
o Output voltage is typically 10mV/rpm and measured with DC voltmeter.
• Disadvantages:
o Commutator and brushes required periodic maintenance.
o The input resistance of meter should be very high as compared with output
resistance of generator. This is required to limit the armature current to small
value. If the armature current is large, the field of the permanent magnet is
distorted giving rise to non-linearity.
AC Tachometer generators:
• In order to overcome some of the difficulties mentioned above, AC tachometer
generators are used.
• The tachometer generator has rotating magnet which may be either a permanent
magnet or an electromagnet.
• The coil is wound on the stator and therefore the problems associated with commutator
are absent
• The rotation of the magnet causes an emf to be induced in the stator coil.
• The amplitude and frequency of this emf are both proportional to the speed of rotation.
• Thus either amplitude or frequency of induced voltage may be used as a measure of
rotational speed.
• When amplitude of induced voltage is used as a measure of speed, the circuit of figure is
used.
• The output voltage of AC tachometer generator is rectified and is measured with a
permanent magnet moving coil instrument.
Variable Reluctance Type (Magnetic Pickup)
• A metallic toothed rotor mounted on the shaft whose speed is to be measured.
• A magnetic pickup is placed near the toothed rotor.
• The magnetic pickup consists of housing containing a small permanent Magnet with a
coil wound round it.
• When rotor rotates the reluctance of the air gap between pickup and toothed rotor
changes giving rise to an induced emf in the pickup coil.
• This output is in the form of pulses, with a variety of waveshapes.
• The frequency of the pulses of induced voltage will depend upon the number of teeth of
the rotor and its speed of rotation.
• Since the number of teeth is known, the speed of rotation can be determined by
measuring the frequency of pulses with an electronic counter.
• Suppose the rotor has T teeth, the speed of rotation is n rps and number of pulses per
second is P.
• Number of pulses per revolution = T
• Hence speed, n = ����������������� !���"#��#$ =
% &�' = % �60&�*
• A typical rotor has 60 teeth.
• Thus if the counter counts the pulses in one second, the counter will directly display the
speed in rpm.
• Advantages
o Simple and rugged in construction
o Maintenance free
o Easy to calibrate
o Information from this device can be easily transmitted.
VIBRATION MEASUREMENT
• A schematic diagram of a seismic transducer is shown in figure.
• It is called a seismic accelerometer
• The mass is connected through a parallel spring and damper arrangement to a housing
frame.
• The housing frame is connected to the source of vibration whose characteristics are to
be measured
• The mass has tendency to remain fixed in its spatial position so that the vibrational
motion is registered as a relative displacement between mass and housing frame
• This displacement is sensed and indicated by an appropriate transducer.
• Potentiometer, LVDT, piezoelectric crystals can be used to measure displacement.
FLUE GAS ANALYZER
• Following components are to be monitored in flue gas such as oxygen, Carbon
monoxide, carbon dioxide, Nitrogen dioxide, Nitrogen monoxide, Suplhur dioxide and
sulphur oxide
Oxygen Analyzer:
• Various methods of oxygen analysis are
o Paramagnetic oxygen analyser
o Electroanlaytical method
o Zirconia analyser
o Westing House Oxygen analyser
• Paramagnetic oxygen analyser:
• Among ordinary gases, oxygen, nitric oxide and nitrogen dioxide are paramagnetic. Most
gases are slightly diamagnetic and repelled out of the magnetic field. But oxygen is more
paramagnetic than nitric oxide or nitrogen dioxide.
• Oxygen has the property of being paramagnetic in nature i.e., it does not have a strong
magnetism as permanent magnet but at the same time it is attracted by the magnetic
field.
• Most gases are however slightly diamagnetic i.e., they are rippled out of a magnetic
field.
• The above setup is known as Hays-Magno thermo oxygen recorder.
• It uses magnetic property of oxygen along with thermal conductivity for the measurement
of oxygen in the gas.
• The gas sample which is to be analysed is passed across the bottom of the gas cell that
contains an electrically heated wire.
• A strong magnetic flux from a permanent magnet is directed across the wire.
• Due to magnetic flux, the oxygen is pulled into the region around hot wire and it is
heated by wire.
• Oxygen losses magnetic susceptibility in inverse proportional to the square of the
absolute temperature.
• The flow of gas is proportional to the amount of oxygen present in the setup around the
hot wire.
• The hot wire is then cooled and its resistance is decreased.
• This resistance wire is connected to one of the arms of Wheatstone bridge.
• Thus the change in resistance value is a measure of the oxygen present in the sample.
(NOx) Nitrogen monoxide or Nitric Oxide (NO) and Nitrogen dioxide (NO2) Analyzer:
• Different types of NOx analysers are :
o NDIR and UV analysers
o Chemiluminescent Method
o Gas chromatographic method
o Electro chemical devices
• Chemiluminescent Method
• The concentration of NOx
• Interaction of NO with O3
• NO2 produced by this reaction is an excited state and reverts to the ground state with the
emission of radiant energy.
NO +O3 ->NO
NO2+ ->NO
• The emitted radiation is received by a photomultiplier tube whose output is amplified and
fed to a recorder.
• The intensity of radiation is proportional to the amount of
• Direct determination of
Because NO2 react with ozone slowly.
• So to increase the speed of the reaction,
reactions or converters.
• The chemiluminescent reaction with ozonator is specific for NO.
• The sample containing NO
converts NO2 to NO.
• Analysis of the sample by chemiluminescen
• Analysis of another sample without passing through the thermal converter gives only
NO.
• The difference of the above two
measured by a phototube and read out.
• Advantages
o High sensitive photometry
o Continuous analysis
Chemiluminescent Method
The concentration of NOx can be best analyzed by chemiluminescent method.
3 (Ozone) generates NO2 and oxygen.
produced by this reaction is an excited state and reverts to the ground state with the
emission of radiant energy.
>NO2+ + O2
>NO2 + Hγ
The emitted radiation is received by a photomultiplier tube whose output is amplified and
The intensity of radiation is proportional to the amount of Nitric Oxide (NO).
Direct determination of NO2 by this chemiluminescent method is a slow process.
react with ozone slowly.
So to increase the speed of the reaction, NO2 must be reduced to NO by using catalytic
reaction with ozonator is specific for NO.
The sample containing NO and NO2 is passed through a thermal converter which
Analysis of the sample by chemiluminescent gives total NOx.
Analysis of another sample without passing through the thermal converter gives only
The difference of the above two analyses corresponds to NO2 in the air sample which is
measured by a phototube and read out.
High sensitive photometry
Continuous analysis
method.
produced by this reaction is an excited state and reverts to the ground state with the
The emitted radiation is received by a photomultiplier tube whose output is amplified and
Nitric Oxide (NO).
s a slow process.
must be reduced to NO by using catalytic
is passed through a thermal converter which
Analysis of another sample without passing through the thermal converter gives only
in the air sample which is
• Disadvantages
o Requires ozone generator
o NO2 catalytic reduction is necessary
CO (Carbon monoxide):
• NDIR spectroscopy is the standard method to estimate Carbon monoxide.
• This method is used for continuous analysis of Carbon monoxide based on the capacity
of CO to absorb the IR radiations
• NDIR consists of a sample cell and a reference cell, two IR sources, ccells and a detector
• The reference cell is filled with a non absorbing gas such as Nitrogen and the sample cell is continuously flushed with the sample containing CO which absorbs radiation at 4.6µm
• The detector consists of two compartmefilled with CO
• The IR radiation is produced from a hot filament and is passed alternatively through a sample and a reference cell with the help of optical chopper.
• The radiation after passing through the two celby a pressure sensitive diaphragm.
• The reference cell passes almost all of the IR energy onto one part of the detector cell while a varying amount of IR energy which is inversely proportional to the CO concentration passes through the sample cell and reaches the other detector part.
• Since more radiation enters the reference cell side of the detector, the diaphragm is moved towards the Sample side cell of the detector.
• The resulted distortion of the diaphragm isamplified and recorded.
SOx (SO & SO2):
Requires ozone generator
catalytic reduction is necessary
spectroscopy is the standard method to estimate Carbon monoxide.
This method is used for continuous analysis of Carbon monoxide based on the capacity
of CO to absorb the IR radiations
NDIR consists of a sample cell and a reference cell, two IR sources, c
The reference cell is filled with a non absorbing gas such as Nitrogen and the sample cell is continuously flushed with the sample containing CO which absorbs radiation at
The detector consists of two compartments seperated by a thin metal diaphragm and
The IR radiation is produced from a hot filament and is passed alternatively through a sample and a reference cell with the help of optical chopper.
The radiation after passing through the two cells reaches a detector cell which is divided by a pressure sensitive diaphragm.
The reference cell passes almost all of the IR energy onto one part of the detector cell while a varying amount of IR energy which is inversely proportional to the CO
tion passes through the sample cell and reaches the other detector part.
Since more radiation enters the reference cell side of the detector, the diaphragm is moved towards the Sample side cell of the detector.
distortion of the diaphragm is converted to an electrical signal which can
spectroscopy is the standard method to estimate Carbon monoxide.
This method is used for continuous analysis of Carbon monoxide based on the capacity
hopper, two filter
The reference cell is filled with a non absorbing gas such as Nitrogen and the sample cell is continuously flushed with the sample containing CO which absorbs radiation at
nts seperated by a thin metal diaphragm and
The IR radiation is produced from a hot filament and is passed alternatively through a
ls reaches a detector cell which is divided
The reference cell passes almost all of the IR energy onto one part of the detector cell while a varying amount of IR energy which is inversely proportional to the CO
tion passes through the sample cell and reaches the other detector part.
Since more radiation enters the reference cell side of the detector, the diaphragm is
converted to an electrical signal which can be
• In the coulometric analysis, the SO2 electro generates free bromine or iodine from their solutions.
• This free bromine or iodine is used to give a measure of SO2.
• In the bromo coulometric analysis, air containing SO2 is drawn continuously through an electrolytic cell which contains acidified bromine solution and two sets of electrodes as shown in the figure.
• The indicator , reference set of electrodes are used to detect the bromine concentration while the other set comprising the generator, auxiliary electrode is used to generate bromine which is necessary to maintain the balance.
• The oxidation of SO2 and the reduction in bromine concentration reaction is, 2H2O + SO2 + Br2 -> SO4
2 - + 4H+ + 2Br –
• This oxidation, reduction develops a potential between the indicator and reference electrode, and this voltage is compared to a reference voltage.
• The difference between the two voltages is sensed by the other electrode system causing an electric current to flow between them and generating sufficient bromine to maintain the original concentration. According to the reaction:
2Br - ->Br2 + 2e
• This current flow necessary to maintain the proper balance is a measure of the SO2 concentration in the air.
Hydrocarbon:
• Thermal Conductivity Analyzer or Katharometer
• It is based on the principal that all gases have the ability to conduct heat in varying degrees.
• This difference in heat conduction can be used to determine quantitatively the composition of a mixture of gases.
• It uses a heated filament as a sensing element which is placed in the path of emerging gas stream.
• The heated element may be fine platinum, gold, tungsten wire or a semi conducting thermistor.
• The amount of heat lost from the filament by conduction to the detector wall depends on the conduction to the detector wall depends on the thermal conductivity of the gas.
• R1 and R2 is measuring and reference chamber
• R3 and R4 are constant resistance
• Initially the calibration is done by passing a reference gas.
• After calibration reference chamber is filled with reference gas and measuring chamber
is exposed to analyzing gas.
• Due to change in thermal conductivity, the bridge gets unbalanced.
• The deflection in galvanometer is a measure of thermal conductivity of a gas.
Note:
� Electrostatic Precipitator or fabric filters remove particulate matter and flue gas
desulfurization removes the sulphur dioxide produced by fossil fuels (coal)
� Nitrogen oxides emission are reduced either by modification of the combustion process
to prevent their formation or by catalytic reaction with ammonia or urea (to produce N2
rather than NOx)
FUEL COMPOSITION ANALYSER:
• Common tests are which are used to find the commercial value of the coal are proximate
analysis and ultimate analysis
Proximate Analysis:
• Proximate analysis indicates the percentage by weight of the Fixed Carbon, Volatiles,
Ash, and Moisture Content in coal.
• The amounts of fixed carbon and volatile combustible matter directly contribute to
the heating value of coal.
• Fixed carbon acts as a main heat generator during burning.
• High volatile matter content indicates easy ignition of fuel.
• The ash content is important in the design of the furnace grate, combustion volume,
pollution control equipment and ash handling systems of a furnace.
• The proximate analysis of most coals indicates the following ranges of various
constituents
Constituents Moisture Ash Volatile Matter Fixed Carbon Percentage 3 to 30 % 2 to 30% 3 to 50% 16 to 92%
• Significance of Various Parameters in Proximate Analysis
a) Fixed carbon:
Fixed carbon is the solid fuel left in the furnace after volatile matter is distilled off.
It consists mostly of carbon but also contains some hydrogen, oxygen, sulphur and
nitrogen not driven off with the gases. Fixed carbon gives a rough estimate of heating
value of coal
b) Volatile Matter:
Volatile matters are the methane, hydrocarbons, hydrogen and carbon monoxide,
and incombustible gases like carbon dioxide and nitrogen found in coal. Thus the volatile
matter is an index of the gaseous fuels present.
Volatile Matter
• Proportionately increases flame length, and helps in easier ignition of coal.
• Sets minimum limit on the furnace height and volume.
• Influences secondary air requirement and distribution aspects.
• Influences secondary oil support
c) Ash Content:
Ash is an impurity that will not burn. Typical range is 2 to 30%. Ash present in the
coal is of two forms as fixed and free ash. The fixed ash present in the coal comes from
the original vegetable matter and it cannot be removed from coal before burning the
coal.
The free ash comes with the coal in the form of clay, shales and pyrites. The free
ash can be reduced or removed by mechanical processing of coal such as washing and
screening, but the presence of fixed ash is more or less unavoidable.
Ash
• Reduces handling and burning capacity.
• Increases handling costs.
• Affects combustion efficiency and boiler efficiency
• Causes clinkering and slagging.
d) Moisture Content:
Moisture in coal exists in two forms as inherent and free moisture. The inherent
moisture is the combined moisture and that is held in the pores of the coal. The
percentage of inherent moisture is determined by heating the coal to 110 ºC in the
current of nitrogen. The inherent moisture is never removed from the coal used for
power plants as it is costly procedure.
The free moisture is defined as moisture present in the coal which can be
removed just by exposing the coal to the natural air flow or by drying with the help of air
at 50 ºC
Moisture
• Increases heat loss, due to evaporation and superheating of vapour
• Helps, to a limit, in binding fines.
• Aids radiation heat transfer.
Ultimate Analysis:
• The ultimate analysis indicates the various elemental chemical constituents such as
Carbon, Hydrogen, Oxygen, Sulphur, etc.
• It is useful in determining the quantity of air required for combustion and the volume and
composition of the combustion gases.
• This information is required for the calculation of flame temperature and the flue duct
design etc.
• The ultimate analysis of most coals indicates the following ranges of various constituents
