Boiler Drum

7/28/2019 Boiler Drum

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Prof. P. K. Das; Dept. of Mechanical Engineering; IIT Kharagpur

A Course on Thermal Power Plant

Prof. P. K. DasProfessorMechanical Engineering DepartmentIIT Kharagpur


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Function of a Boiler Drum

2

i. Stores water and steam to meet varying load requirement

ii. Part of a circulation loop

iii. Separates of steam and water (three internals and free surface)

iv. Separates of sludge & floating impurities

v. Maintains theppm of dissolved solids by blow down andphosphate dosing

Boiler Drum a standard feature of water tube boilers

Boiler Drum and Drum Level Control Boiler Drum


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3

Boiler Drum asan Input-

Output System

Steam

outlet

Steam,

waterdroplet

Feed water

Water + chemical

Blow down water

Dissolved solids

Precipitates

Down Comer

Water steam impurities

RiserWater, steam,

foam, sludge,

floating

impurities

Boiler Drum asan input output

system



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4

Schematic of a boilerdrum relative positions

of different components



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5

500 MW Boiler DrumInside of a boiler drum tube holes

and baffles



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Typical drum internals

1- Drum;

2- Evaporating tubes;

3- Box;

4- Cyclone;

5- Drainage box;

6- Cover;

7- Steam-washing perforated plate;8- Perforated steam baffle;

9- Feed water distribution box;

10- Steam circulating tubes;

11- Feed water in;

12- Hole in a partition;13- Downtake tubes;

14- Emergency water drainage.



7/42Prof. P. K. Das; Dept. of Mechanical Engineering; IIT Kharagpur


7

Two unique features of drum type boilers

Carry under entrainment of steam bubbles in the

water stream of downcomer, reduces the circulation,

increases the pressure drop in the down comer

Carry over entrainment of water droplets in the steamleaving the boiler drum , dissolved solid gets deposited

in super heater and other downstream components,

creates the problem of over heating and failure.

Carry over is a general term,

associated phenomena are known as priming and

foaming





Priming is the sudden gust of liquid slugs in the steam flow due to rapidchange in steaming rate, increase in liquid level, improper circulation etc.

It not only makes the steam dirty but can also create problems like flow

fluctuation, water hammer, thermal stress.

Foamnig occurs due to the saponification in presence of oil, grease,

floating impurities. It restricts the release of the bubbles as the liquid hasa high surface tension. During bubble rapture droplets are formed which

are also carried away by steam

Apart from mechanical cleaning of steam chemical correction is also

applied.

Tri-sodium phosphate or tannin is injected to induce precipitation of thedissolved salts in the form of sludge.




9

Mechanism of carry over

Liquid excursion by

vapor drag

Carry over bubbleNormal bubble





ABMA Standard Boiler Water Concentrations for Minimizing Carryover




11

Carry Over from Boiler Drum

Mechanical

DrumInternal

Alignments

Faultyscreendrier

Drumlevel

Very highlevels

Foaming

HighAlkalinity

Oil in

water

High TDS inboiler water

Spraywater

Vapor out

High salt content inboiler water

Volatility different foreach salt

Differentquantum ofvapor carry

over

SO3 hashigh

volatility

Getsdeposited in

LP turbine

Other salts invapor carry

over rare

Goes upabove 200

kg/cm2

Carry over effect from drum canbe up to LP turbine blades





12

Gravity Separation of Steam

and Water:

1. Needs large free surface

2. Suitable for low pressureboilers (< 20bar)

3. Low cost

Operation done on steam in a steam drum: Separation,

Washing andDrying





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Gravity separation assisted byinternals like baffles and screens

Baffles are the first level

separator-Impact separations or

separation due to change of

momentum Screen (wiremesh) are

secondary separators- intersects

fine droplets

Baffles and screens


A C Th l P Pl t




14

Cyclone and scrubbers

At high pressure densitydifference between water and

screen diminishes

Cyclone utilize centrifugal forces

Scrubbers are corrugated plate-

helps to remove the last stresses

of moisture

Finally perforated plates or

screens are used at the drum exit


A C Th l P Pl t




15

Steam washing impurities

are removed

Steam is made to passthrough a rain of

Relatively pure incoming feed

water.


A C Th l P Pl t




Saturated steam drier

1 & 5 - Constant- velocity

Channels;

2 & 6 - Perforated plates;

3 - Horizontal chute;

4 - Drainage pipes;

7 - Corrugated plates;

8 - Moisture- trapping pocket


A C Th l P Pl t




Typical external cyclone separator

1- Header;

2- Steam-water mixture in;

3- Whirler;

4- Perforated plate;

5- Steam outlet pipe;

6- Air vent;7- Blow-down;

8- Downtake tubes;

9- Water from drum;

10- Cross-piece

Sometimes steams of high purity is required

Separation inside the steam drum is not adequate

External separators are used






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Guide lines to avoid carry over:

Drum level to be within +/- 50 mm

Maintain silica below recommended value for the pressure

Drum internal alignment to be maintained

FW to be 0.2 to 0.3 mho s/cm

Boiler water solids to be in recommended level

Measure & maintain steam purity within limits FW conductivity to be on line

Boiler water salt control

Dosing of solid salts to be after spray water tap off

Turbine Deposits Guide: Soft & friable, pinkish white, 1st stage blades- easy to remove by

air/water

Sparkling silver, silver gray mainly in 4th and 5th stage blades-

Hard to remove by air/water.




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Test Facility



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Collection of carry over



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Blow Down : Intentional drainage of water from the boiler drum tocontrol the level of dissolved solid

Surface Blow down

Removes dissolved solids,

foam, floating impurities

often continuous

Bottom Blow down

Removes sludge and

precipitates mostly

manual

Inadequate blow down - the problem of deposition

Excessive blow down wastage of water, chemical and energy




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24

Impurities in = impurities out

drumtheleavingsteamofqualitydrumthefromflowsteamofrate

necessaryblowdownofrate

drumtheenteringwaterfeedofrate

1*

xSkg

SkgBD

Skg

ppmxppmBDppm

s

fw

carryoversdrumfwfw




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25

Scheme for

automatic surfaceblow down

Possibility of

heat recoveryfrom surface

blow down

stream




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26

Boiler Drum and Drum Level Control Drum Level Control

Drum Level Control :

DesirableIn case of a large change in the level, bring the drum up to the

level at boiler start up and maintain the level at that condition

Steam supply for variable demands changes the water level in the

boiler drum Drum level control is important for both plant protection and

equipment safety

Excessive low level uncover boiler tubes leads to overheating. It

also affects the circulation

Very high level interferes with the process of steam water

separation, leads to loss of boiler efficiency, priming and carry

over. Excessive carry over may lead to surge or even water

hammer

Unique phenomena like shrink and swell are also associated.



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Boiler specifications:-

367.7 kW(500 bhp), 7840.9 kg/h (17,250 PPH)

Operating pressure 1.55 Mpa (225 PSIG)

Boiler Water temperature 476K (397 F)

Boiler is operated with 75% firing rate with 80% efficiency i.e. 5863.6 kg/h

(12,900 PPH) with 4 597.3 kWh (1,56,86,720 Btu) heat input.

Using ON/OFF feed water system (recovery volume of 3% low water capacity

(235 kg) + 2% over capacity(157 kg) = 5% water recovery capacity (392 kg) )

Feed water cycle = 392/5863 hr = 4.01 min

Feed water is supplied at 355.2 K with 261.4 kg/min.

Example




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Resultant impact

Total feed water delivery time = 392/ 261.4=1.5 min, temperature rise= 120 K1.Sub-cooling of water occurs due to mixing of drum water with feed water.

95% of water at 476 K + 5% of water at 355.2 K= 469.7 K of boiler water

i.e. 6.3 K of sub-cooling

2.Steaming pressure falls from 1.55 Mpa (@ 476 K) to 1.34 Mpa (@ 469.7 K) i.e.

13% drop in steaming pressure

3.Steaming pressure falls specific volume changes steamnozzle discharge velocity increases from 20.6 m/s to 23.5 m/s.

2.03 m/s is critical velocity for carry over.

Priming most likely to occur.




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4. To recover steaming pressure, firing rate should be increased by 1640.8 kWh

i.e. 26.76% of total firing rate in 1.5 min.

So boiler would have to fire 75 + 26.76 =101.76 % to recover

5. Steaming Pr. decreases drum water level rises 1.77 to 2.54 cm

wrong signal send to feed water controller shut the

feed 1.77 cm early.

6. Operating steaming pressure reached water level shrinks.




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Prof. P. K. Das; Dept. of Mechanical Engineering; IIT Kharagpur 30

Drum level control

Level measurement

Level control

On-off control

Modulating Control




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Conductivity

probe for level

measurement

Possible actions:

Start or stop a valve

Open or close a valveSound an alarm

Open or close a relay

Two point

conductivity probe




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Capacitance method oflevel measurement




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Float type level indicator

Can be conveniently

used for on-off control




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Differential Pressure cell

Suitable for high pressure

clean water system with a

low conductivity

Diaphragm type sensingelement




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Desirable features of a good drum level control

Operator adjustment of the set point for drum levelCompensation for the shrink & swelleffects

Automatic control of drum level

Manual control of the feed water valve

Bump-less transfer between auto and manual modes

Indication of correct drum level and steam flow

Indication of feed water valve position and feed water flow

Absolute/deviation alarms for drum level

Additionally a good controller should take care ofBlow down variations due to changes in dissolved solids

Variations in feed water supply pressure

Leaks in the steam circuits.




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Options for drum level control

Manual control

Single element control

Two element control

Three element control

Only one analogue input and one analogue output requiredSuitable for single boiler / single feed pump configurations

Suitable for stable loads since there is no relationship between drum level

and steam- or feed water flow

Possible inadequate control option because of the swelleffect




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Limitation of single level control:

Increase of demand decreases steam pressure

Gives rise to expansion of bubbles and drum level swell

Feed water flow rate decreases while an increase of flow rate is

needed to maintain mass balance

Sudden or large load changes gives rise to phasing of shrink and swell

Water level controller looses control

Nuisance of low water trips or water priming and carry over




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Suitable for moderate load

swingAny size of boiler

Uses steam flow and drum

level

As steam flow is verydynamic load change can be

sensed before any change of

drum level

Steam flow is a large variable

can over ride the trim effect ofthe drum level change



B il D d D l C l D l C l


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Limitations of two element control

Can not adjust pressure or load disturbance of the feed water flow

Can not eliminate the phasing interaction between the feed water

flow and drum level as only a slow process of drum level iscontrolled

The second issue can lead to a high subcooling of the drum water

on a large increase of steam demand due to excessive feed water

entry



B il D d D L l C t l D L l C t l


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Three element controller:

Basic two element controller is cascaded to the

feed water flow controller to act as a remote setpoint

Feed water controller (first controller) uses feed

water flow as the process variable and steam flow

as the set point

The set point of the two element controller at just

itself

Minor losses like blow down can be taken care

off

Independent tuning of each controllerCan handle large and rapid load change

Suitable for any boiler size

Must for multiple boilers with a common feed

water heater

No trim correction stroke supervision needed




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Boiler Drum

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