boiler auxiliaries performance

7/31/2019 Boiler Auxiliaries Performance

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Boiler Auxiliaries Performance

Many steam generating units are operating day after day with

efficiencies at or near test condition values. Any unit can operate at

these efficiencies with the proper instrumentation , reasonableauxiliaries, proper maintenance program of instrumentation and

auxiliary equipment and proper operating instruments. A standard

procedure should be established by logging the consumption of

individual auxiliaries for different loads.The auxiliaries those need to be monitored for their performance are :

Fans and pumps (CC pumps if provided)

Fuel burning equipment (coal mills, burners)

Wind box dampers (air,fuel,overfire)

Air Preheaters

Dust collecting equipment (ESP)

Soot blowers


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Boiler Auxiliaries Performance

There are essentially two different kinds of fans , 1) The centrifugal fan

in which gas or air accelerates radially outward in a rotor from heel to

tip of blades, discharging into surrounding scroll casing. And 2) The axial

flow fan in which the gas or air is accelerated parallel to the fan axis,

similar to the ordinary desk fan, but with a casing added to develop

static pressure. The axial fans can be a single stage or multiple stage fan.

Both the types of fans are used in practice though axial fans withvariable blade pitch control are preferred for their part load efficiency .

The fans are called Primary Air(PA), Secondary Air(SA), Induced

Draft(ID) fans , Gas Recirculation (GR) fans, Seal Air fans, Scanner Air

fans and Igniter fans depending on the application of their use. FGD

units are provided with booster fans to boost up the pressure.

Fan control : To meet varying requirements of the system, common

methods of fan output control are damper control, variable-speed

control and blade pitch control in case of axial flow fans.


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Fan Performance

Fan Efficiency : The energy used by a fan impeller goes into bearing

friction, flow work, compression, velocity head, and heating. It is

generally assumed in fan work, where the pressures are usually less

than 25.5 cm of water plenum or vacuum, that the fluid is

incompressible. An ordinary manometer reading on the inlet and outlet

of a fan gives the static pressure static pressure relative to theatmosphere. Their difference is the working static pressure of the fan.

Similarly an efficient pitot tube turned into the air stream furnishes an

indication of the sum of static pressure and velocity head on the

attached manometer. This is called total pressure, or dynamic pressure.Static fan efficiency = Static air hp/ Shaft hp.


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Fan PerformanceFan performance is best expressed in graphical form. These functionalrelations are the fan characteristic curves. In these curves capacity incfm is shown horizontally as the independent variable and head (staticpressure), shaft horse power and static efficiency are dependentvariables plotted vertically. Fan speed is constant. Since fan operationfor a given capacity must match single values of head and horsepoweron the characteristic curves, a balance between fan static pressure andsystem resistance is required. If the system resistance for a givencapacity is less than the head indicated on the fan characteristic curve,additional variable flow resistance, such control damper, must be addedto the system. Varying the operating speed to yield a family of curves,will change the numerical performance values of the characteristics.

However, the nature of the curves remains substantially unaltered.Performance at different speeds for the same efficiency can be relatedby the following :

1. Capacity is directly proportional to speed.

2. Head is directly proportional to speed squared.

3. Power output is directly proportional to speed cubed.


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Fan PerformanceSystem resistance along with the fan static pressure characteristics at

various speeds, both as function of volumetric flow rate are plotted. If

the fan operates at constant speed, any output less than that shown at

the intersection of the system resistance and specified rpm curves must

be obtained by throttling the excess fan head. This results in wastage in

power that can be avoided by using a variable speed drive.

Backwardly curvedblade wheels are generally selected for forced draft

service because the high speed is suitable for standard motor drive. The

power demand is self limiting, and the static efficiency is high. These

fans may be satisfactorily operated in parallel.

Induced draft fans operate in gas of much higher temperature and may

handle gases laden with dust. Forwardly curvedblade wheels run at thelowest speed to develop a given pressure, hence are frequently chosen

for induced draft service so that the centrifugal stresses in the wheels

will be least. The forward curvature reduces the blade depth, but gives a

large inlet opening for the gas. conditions.


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Fan Performance

The rising horsepower and pressure characteristics of this type usually

render it unsuitable for parallel operation , for it can overload its driver

under abnormal working conditions.

Axial flow designs are found in forced draft service.

Fan testing : It is difficult to obtain consistent data from field tests of

fans installed in flue and duct systems because straight sections are

seldom long enough to eliminate flow disturbances from such things asbends, change in section and dampers. Structural arrangements at the

fan entrance and discharge also materially affect field performance

results. Manufactures, therefore, generally are reluctant to guarantee

fan performance except on the basis of factory tests on a test blockarranged for measuring under ideal conditions. However plant

engineers may perform fan tests as acceptance test. The pitotstatic

equipment is generally employed, and there should be some provision

for test openings. Experience indicates that the velocities in the ducts

and breechings vary across the flow, so that the area should be divided


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Fan Performance

Into small elements. Rectangular sections are divided into small squares

and readings taken at the centre of each. It is the square roots of

velocity head readings that are arranged for the calculation of flowvelocity, not the readings themselves. On tests where it is not possible

to hold the rpm constant, corrections to the readings must be made

before comparing results with constant speed guarantees. If N is the

rated speed and Nx is the test speed, multiply test volume by N/Nx, testpressure by (N/Nx) square and test powers by (N/Nx) cube before

comparing with the ratings.

Fan Safety Factors : To make sure that the fans will not limit a boilers

performance, margins of safety are added to the calculated or net fan

requirements to arrive at satisfactory test block specification. These

margins are intended to cover conditions encountered in operation that

can be specifically evaluated.


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Fan Performance

For example, variation in fuel ash characteristics or unusual operatingconditions may foul heating surfaces. The unit then requires additionaldraft. A need for rapid load increase or a short emergency overload

often calls for overcapacity of the fans. The customary margins to allowfor such conditions are 15 to 20 % increase in the net weight flow of airor gas, 15to 20% increase in net head, and 25F increase in the air or gastemperature at the fan inlet.

General performance requirements for force draft fans.Reliability : Modern boilers must operate continuously for long periods(up to 18 months) without shut down for repairs or maintenance. Thefan must be well balanced, and the blades so shaped that they will notcollect dirt and disturb this balance.

Efficiency : High efficiency over a wide range of output is necessarybecause boilers operate under varying load conditions.

Pressure : Fan pressure should vary uniformly with output over thecapacity range. This facilitates damper control and assures minimum

disturbance of air flow when minor adjustments to the fuel burningequipment change the system resistance.


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Fan Performance

Capacity : When two or more fans operate in parallel, the pressure out-

put curves should have characteristics similar to the straight blade or

backward curve blade fans in order to share the load equally near the

shut off point.

Horsepower : Motor driven fans require self limiting horse power

characteristics, so that driving motor cannot overload. This means that

the horsepower should reach a peak and drop off near the full load fan

output.

ID fans : Induced draft fans has the same basic requirements as forced

draft fan except that it handles high temperature gas which may contain

erosive ash. Flat, forward curved and occasionally backward curvedblades with less curvature are used. Excessive maintenance from

erosion is sometimes avoided by protecting casing and blades with

replaceable wear strips. Bearings, usually water cooled have radiation

shields on the shaft between rotor and bearings to avoid overheating.


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Axial fan-Fixed pitch, adjustable pitch, variable pitch

Performance control characteristics

Areas of constant efficiencies run parallel to the boiler resistance

line-high efficiency over a wide boiler load range

Control range is very large both above and below the maximumefficiency

The lines of constant blade angle are actually individual fan curves-

as the curves are very steep, change in resistance produces very

little volume change.As the blade angle can be adjusted from minimum to maximum

flow change is nearly linear


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Stall Characteristics

Axial flow fans can experience stall

Stall is the aerodynamic phenomena which occurs

when a fan operates beyond its performance limit

Flow separation occurs

Fan operation becomes unstable- normal

performance curve not followed

Extended operation in the stall region should beavoided

Unpredictable flow vibration can damage rotating

blades


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The angular relationship between the air flow impinging on theblade of a fan and the blade itself is known as the angle of

attack. In axial flow fan, when this angel exceeds a certain

limit, the air flow over the blade separates from the surface and

centrifugal force then throws the air outwards, towards the rimof blades. This action causes a build up of pressure at the

blade tip, and this pressure increases until it can be relived at

the clearance between the tip and the casing. Under this

condition the operation of the fan becomes unstable, vibration

sets in and the flow starts to oscillate. The risk of stall increases

if a fan is oversized or if the system resistance increase

excessively.


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Stall is aerodynamic phenomenon which occurswhen a fan operates beyond its performance limits

and flow separation occurs around the blade.
http://4.bp.blogspot.com/_4O4AQvDj16o/SjEr90hRqgI/AAAAAAAAAVE/Zy9CVErKHaM/s1600-h/blade+stall.jpg


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Stall prevention

When axial fans are sized properly and the resistance curve is

parabolic chances of stall is less

Possibility of stall increases when the fan is over sized compare to

volume capacity, System resistances increases significantly or fans

are operated improperly

Noise in a FanSingle tone noise is generated when the concentrated flow

encounters a stationary object after leaving the rotating blade

passage.

The distance between the blades and the stationary objects affectsthe sound

The blade passing frequency and its first harmonic is most dominant.

Broad band noise is produced by the fluid passing through the fan

housing, contains a range of frequencies


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Mill Performance

The power consumption of coal mill should be low to minimize

operating cost. The main factor of performance depends on (a) the

grindability of coal. (b) Surface moisture of coal. (c) the fineness of thegrind needed.

Grindability : It is a term used to measure the ease of pulverizing a coal

in comparison with a standard coal chosen as 100(unity) grindability.

Hardgrove index is most often used to measure the grindability of acoal. The method is based on the premise that the work done in

pulverizing is proportional to the new surface produced. A definite

amount of grinding energy is applied is applied to a prepared sample in

a miniature pulverizer (Hardgrove Grindability Machine) and the new

surface is determined by sieving. A coal is harder or easier to grind if its

grindability index is less or greater. If the coal has grindability index of

50, it would require twice the grinding power of the standard coal to

produce a specified particle size.


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Mill Performance

Fineness : the fineness of the pulverized coal needed for

successful burning depends largely on the ratio of volatile

matter(VM) to the fixed carbon. Since the carbon burns slower the

VM, low VM coals must be ground finer to expose more surface to

permit more coal in a given residence time in the furnace. The

finer the grind more the power needed. Moisture makes the coal

particle adhere to each other, defeating the purpose of grinding.

The coal samples for checking fineness are collected by using bag

collector or a cyclone and the results of sieve analysis are plotted

on a Rosin and Rammler chart. The mill fineness checks are done

periodically to ensure good performance of the mills


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ESP Performance

Efficiency of ESP is defined as the ratio of mass of all particles retained

by collector (ESP) and the mass of all particles entering collector.

The Collection efficiency of ESP is related to the time of particle

exposure to the electrostatic field, the strength of the field, and the

resistivity of the dust particle.

Larger collection-surface areas and lower gas-flow rates increase

efficiency because of the increased time available for electrical activityto treat the dust particles.

An increase in the dust-particle migration velocity to the collecting

electrodes increases efficiency

It is easier to collect large particles than smaller particles.

Care must be taken to maintain the gas temperature entering the

precipitator at or above the design temperature, otherwise local cold

spots result in condensation, fouling and rapid corrosion.


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ESP PERFORMANCE - PROBLEMS OF ESP

BROKEN WIRES

ELECTRODE FAILUREGAS DISTRIBUTION

ALIGNMENT PROBLEMS

CORROSION

BROKEN INSULATORSELECTRICAL SUPPLY PROBLEMS

RAPPING SYSTEM PROBLEMS

HOPPERS CHOKING


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Boiler auxiliaries performance

Fuel air damper will Maintain the length of the flame. Modulate in

proportion to fuel burner pressure or speeder speed.

Auxiliary air damper will maintain the furnace to wind box DP.

Over fire dampers are used to control NOx

Bottom auxiliary damper is used to control unburnt carbon falling in

bottom ash hopper

RAPH should give the designed secondary and primary air temperatures

at all loads and should ensure that the outlet flue gas temperatures are

maintained without exceeding the air leakages across seals.

The excessive O2 is the indication of airheater leakages and differential

pressure measurement across APH on air and flue gas sides indicate the

fouling of airpreheaters.

boiler auxiliaries performance

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