High pressure boilers

HIGH PRSSURE BOILERSprepared by

Sneh Patel

Introduction

Classification of boilers

High Pressure Boilers

Boiler mountings and accessories

Feed water and Feed water Treatment Plant

Draught System

Design of Chimney

Boiler Heat Balance Sheet

INTRODUCTION

According to Indian Boiler Regulation (I.B.R) Act 2007,”Boiler is a closed pressure vessel in which steam is generated with capacity exceeding 25 liters, gauge pressure greater than or equal to 1 kg/cm2, and water is heated at 100oc or above”

Steam boiler is a closed vessel in which heat produced by the combustion of fuel is utilized to generate steam from water, at desired temperature and pressure

DEFINATION

APPLICATIONS OF BOILERS

1. Power generation2. Heating3. Industrial processes

1. Relative position of hot gases and water

• Fire tube boilers (Cochran, Lancashire, Cornish, Locomotive)• Water tube boilers (Babcock and Wilcox boiler)

2. Geometric orientation of boiler

• Vertical boilers (Cochran boiler)• Horizontal boilers (Locomotive boiler)

3. Method of firing

• Internally fired boilers (Lancashire, Locomotive)• Externally fired boilers (Babcock and Wilcox boiler)

CLASSIFICATION OF BOILERS

4. Pressure of steam

• Low pressure boilers (<= 80 bars-Cochran, Lancashire, Cornish, Locomotive)• High pressure boilers (>80 bars-Babcock and Wilcox boiler, Lamont boiler)

5. Method of circulation of water

• Natural circulation boilers (Lancashire, Locomotive, Babcock & Wilcox boilers)• Forced circulation boilers (La-mont boiler, Velox boiler, Benson boiler)

6. Nature of service to be performed

• Land boilers ( Lancashire boiler)• Mobile boilers (or) Portable boilers (Locomotive boiler)

7. Number of tubes in boiler

• Single tube boilers (Cornish boiler)• Multi tube boilers (Locomotive, Cochran Babcock and Wilcox boiler)

HIGH PRESSURE BOILERS

A boiler is called a high pressure boiler when it operates with a steam pressure above 80bar. The high-pressure boilers are widely used for power generation in thermal power plants.Example: Lamont boiler, Benson boiler, loeffler boiler, Babcock and Wilcox boiler

Features of High pressure Boilers

1. Forced circulation of water2. Large number of small diameter tubes3. Higher steam pressure and temperature4. Improved mode of heat transfer and heating5. Pressurized combustion6. Compactness7. High efficiency8. Once through construction

1. La Mont boiler

• It is high pressure, water type boiler

• The water circulation is maintained by centrifugal pump.

• La Mont boiler generates approximately 45 to 50 tonnes of steam per hour at a pressure of 13 bar and atemperature of 500°C.

Advantages of a La Mont Boiler:

1. Small diameter tubes are used, so that high heat transfer rate is maintained.

2. The multiple tubes circuit gives flexibility for suitable location of heat transfer equipments.

2. Benson boiler

• Difficulties of bubble formation is eliminated.

• The transport of Benson boiler parts is easy as no drums are required and majority of the parts are carried to the site without pre-assembly.

• Required smaller floor area.

• The superheater in the Benson boiler is an integral part of forced circulation system therefore no special starting arrangement for superheater is required.

• It has very little storage capacity compared to drum type boiler.

3. Loeffler boiler

• Difficulties of deposition of salt and sediment is eliminated.

• Most of the steam is generated outside from the feed water using part of the superheated steam coming out from the boiler.

• Loeffler boilers with generating capacity of 94.5 tones/hr and operating at 140 bar have already been commissioned.

4. Babcock and Wilcox boiler

• Babcock and Wilcox is a water-tube boiler is an example of horizontal inclined tube boiler it also a High Pressure Boiler.

• The angle of inclination of the water tubes to the horizontal is about 15° or more.

• The pressure of steam in case of cross drum boiler may be as high as 100 bar and steaming capacity up to•27000 kg/h.

BOILER MOUNTINGS & ACCESSORIES

Mainly mountings are required and essential to a Boiler :

1. Safety valve2. Two water level indicators3. Pressure gauge4. Fusible plug5. Steam stop valve6. Feed check valve7. Blow-of cock8. Man and mud hole

BOILER MOUNTINGS

1.SAFETY VALVE 2. WATER LEVEL INDICATOR

3. FUSIBLE PLUG 4. PRESSURE GAUGE

5. BLOW OFF COCK 6. FEED CHECK VALVE

7. STOP VALVE

1. AIR PREHEATER 2. ECONOMISER

3. SUPER HEATER

BOILER ACCESSORIES

FEED WATER SYSTEM

• Feed water is water being applied to the feed water heater or to the boiler and consists of both make-up water and condensate

• BFW is 95 % condensate and 5 % makeup water

• All natural waters contain varying amounts of suspended and dissolved solids as well as dissolved gases (O2, CO2).

• The type and amount of impurities in fresh water vary with the source (lake, river, well)

• Impurities in water are of importance when water is to be used for steam generation.

• For higher-pressure boilers, feed water must be pre treated to remove impurities.

Impurities in Water and its Effect on Boiler

Boiler Water Composition with respect to Operating Pressure

FEED WATER TREATMENT PLANT

1. Clarification2. Filtration3. Softening Lime-soda ash softening Ca(0H)2 + Ca(HC03)2 → 2CaC03↓ + 2H20 Ca(0H)2 + MgS04 + Na2C03 → CaCO3↓ + Mg(0H)2↓ + Na2S04

Ion Exchanging Softening: CaEx + 2NaCl → Na2Ex + CaCl2

4. Dearation: N2H4 + O2 → 2 H2O + N2

DRAUGHT SYSTEM

The difference of pressure for to maintaining the constant flow of air and discharging the gases through the chimney to atmosphere is known as draught.

Types of draught system

1. Natural draught (produced by chimney)

2. Artificial draught

a. Steam jet draught (produced by jet of steam)

• Forced draught• Induced draught

b. Mechanical draught (produced by fans)• Forced draught• Induced draught

1. NATURALDRAGUHT

P1=Pa+WgH

P2=Pa+WaH

P=P2 – P1

Advantages and Limitations of Chimney / Natural DraughtAdvantages:

It does not require any external power for producing the draught. The capital investment is less. The maintenance cost is nil as there is no mechanical part.

It has long life.

Limitations:• Natural draught produced by chimney is hardly 10 to 20 mm of water under the normal atmospheric and flue gas temperatures.

• The available draught decreases with increase in outside air temperature and for producing sufficient draught, the flue gases have to be discharged at comparatively high temperatures resulting in the loss of overall plant efficiency. And thus maximum utilization of Heat is not possible.

•The chimney has no flexibility to create more draught under peak load conditions because the draught available is constant for a particular height of chimney of the plant.

2. ARTIFICIAL DRAUGHT

2.1 FORCED DRAUGHT

2.2 INDUCED DRAUGHT

2.3 BALANCE DRAUGHT

Losses in the air-gas loop systemThe total draught losses in the air and gas loop system are given by,

ht = hv + hb + he + hd

ht = Total draught loss in cm of water headhv = Velocity head in cm of water headhb = Fuel bed resistance equilent to cm of water headhe = Head loss in equipmenthd = Head loss in duct and chimney

Measurement of Draught:

•The draught losses in different parts of the boiler plant are measured in mm of water with the help of manometers.

DESIGN OF CHIMNEY

1. Height of Chimney

2. Diameter of Chimney

3. Efficiency of Chimney

HEAT BALANCE SHEET OF BOILER

Heat supplied % age Heat Expenditure (K J) % age

(K J) (in approx.)Gross heat 100 (a) Heat utilized in steam generation 78.00

supplied (b) Heat carried away by flue gases 12.00

(c) Heat utilized in evaporating and superheating 4.750

the moisture fuel and water vapour formed

due

to burning of hydrogen of fuel. 3.00

(d) Heat loss by incomplete combustion 1.500

(e) Heat carried away by excess air 0.500

(f) Heat carried away by carbon nash 0.250

(g) Heat uncounted for such as radiation and error

etc.

Total 100 Total 100

HEAT BALANCE SHEET (Basis 1 Kg of low grade fuel)

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