Basic Boiler Construction

Front 1

What are shells and drums made of

Back 1

Steel plates rolled to the correct curvature with thickness ranging from 6 mm to 250 mm

Front 2

What is the process of putting shells and drums together

Back 2

If the plate is thick, it is first heated and shaped into half-cylindrical sections. These sections are then welded together to form the complete cylinder. If the plate is thin, it is rolled without heating to form the cylinder from a single sheet. The adjoining edges are then welded. After the shell, or drum, is formed and welded, the ends are closed off by means of heads, or end plates which are also made from steel plates. In the case of a watertube boiler, the heads are made in a dished rather than flat shape. In the case of firetube boilers, the heads must be flat rather than dished in order that the tube ends be securely attached. Because the firetube boiler heads hold the tube ends, they are called tubesheets.

Front 3

What are the 5 basic types of welded joints

Back 3

1. Butt Joint (primary joint used in boiler fabrication)

2. Lap Joint (sometimes used in boiler fabrication)

3. Corner Joint

4. Tee Joint

5. Edge Joint

Front 4

What type of welded joint is this

Back 4

Butt Joint

Front 5

What type of welded joint is this

Back 5

Lap Joint

Front 6

What type of welded joint is this

Back 6

Corner Joint

Front 7

What type of welded joint is this

Back 7

Tee Joint

Front 8

What type of welded joint is this

Back 8

Edge Joint

Front 9

What are the two basic weld types

Back 9

1. Groove Weld (used to make Butt Joints)

2. Fillet Weld (used to make Lap Joints and Tee Joints)

Front 10

What type of weld is this

Back 10

Groove Type

Front 11

What type of weld is this

Back 11

Fillet Type

Front 12

What is a single-welded butt joint

Back 12

When the filler metal is applied from one side only

Front 13

What is a double-welded butt joint

Back 13

When the filler metal is applied to both sides of the joint. Used for thick plates.

Front 14

What type of weld groove is this

Back 14

Square Groove

Front 15

What type of weld groove is this

Back 15

V-Groove

Front 16

What type of weld groove is this

Back 16

U-Groove

Front 17

What type of weld groove is this

Back 17

J-Groove

Front 18

What type of weld groove is this

Back 18

Bevel Groove

Front 19

What type of weld groove is this

Back 19

Double Bevel Groove

Front 20

What type of weld groove is this

Back 20

Double Bevel

Front 21

What type of weld groove is this

Back 21

Double J

Front 22

What type of weld groove is this

Back 22

Double Vee

Front 23

What type of weld groove is this

Back 23

Double U

Front 24

What is a problem that must be surmounted during the welding process

Back 24

Stresses are set up in the metal due to temperature differences existing in the weld area. These stresses may be removed or reduced by heat treatment of the metal of either preheating or postheating.

Front 25

Describe Preheating

Back 25

The heating of the parts in a special oven or furnace prior to welding. Temperature ranges from 80oC to 230oC depending upon the material and thickness.

Front 26

Describe Postweld Heat Treatment

Back 26

Heat the parts after the welding has been completed. In the case of a boiler drum, after welding of the drum joints, the entire drum can be heated to a specified temperature (above 590oC) in a special furnace for a definite period of time. It is then cooled off slowly at a controlled rate until its temperature drops to below 315oC.

Front 27

What does an authorized Inspector check during the fabrication of a boiler

Back 27

- The welding procedures used by the manufacturers are correct

- All welding is carried out by pressure vessel welders who are qualified to perform the required type of welding

- The correct application of heat treatment is used

- Test plates of the same thickness and material as the drum are welded and then subjected to tension and bending tests to verify that the drum welds will safely hold

Front 28

What are some ways that Inspectors examine boilers

Back 28

- radiograph drum welds over their entire length using x-rays or other radioactive material so that any defects within the weld can be seen on an exposed and developed radiographic film or on an electronic video screen

- Use sound waves (ultrasonics) to locate and identify irregularities in a weld which are displayed on an electronic video screen

Front 29

Is riveting still used

Back 29

No, but there are still some older riveted boilers in operation

Front 30

What are the two main types of riveted joints

Back 30

1. Lap Joint

2. Butt Joint

Front 31

Describe riveted lap joints

Back 31

The edges of the plates are overlapped and fastened by one or more rows of rivets. If one row of rivets is used, the joint is single riveted; with two rows, it is double riveted.

Front 32

Describe butt joints

Back 32

The edges of the shell plates do not overlap but butt together. Cover straps are riveted to the plates covering the seam. They are used for circumferential seams in all riveted boiler shells, but longitudinal seams are nearly always of butt joint construction. Butt joints are stronger and therefore more suitable for longitudinal seams because the force tending to rupture a boiler at the longitudinal joint is greater than that at the circumferential joint. It is possible to bend the plate in the form of a true circle as the ends of the shell plate butt or meet which can't be done with lap joints.

Front 33

Describe boiler tubes made for moderate pressure service

Back 33

Tubes are made from strips of steel, which are
formed into tube shape and the edges welded in an electrical resistance welding
machine. In this method the welding is achieved due to the heat produced when
an electric current flows through the metal at the tube edges.

Front 34

Describe boiler tubes made for high pressure service

Back 34

They are usually seamless and are made by piercing a solid round billet of heated steel to form a rough tube. During piercing the heated billet is spun between rollers as it is forced against the piercing point, forming a rough tube. In the rolling operation a plug the size of the finished tube inner diameter is mounted on a long rolling rod between rollers, which are grooved to the outer dimension of the tube. The roughly shaped tube from the piercing process is then placed between the rollers, which force the tube over the rolling plug, expanding the inner diameter with the plug while reducing the outer diameter between the rollers. The tube is finally finished in a reeling operation where it is spun between smooth rollers while being forced over a smooth reeling plug to achieve final dimensions and smoothness.

Front 35

What is the most common method used to fasten tubes to drums or tube sheets

Back 35

To expand the tube ends into the tube holes in the drum or sheet. This expanding (or
rolling as it is often called) is done by means of an expander, which consists of
three rollers mounted in a cage which fits inside the tube end. A tapered mandrel
fits between the rollers and when the mandrel is turned the rollers are rotated and
forced out against the tube wall thus expanding the tube against the tube hole.

Front 36

Describe watertube boiler tube expansion

Back 36

Watertube boiler tubes are expanded and flared

Front 37

Describe firetube boiler tube expansion

Back 37

Firetube boiler tubes, in addition to the expanding and flaring, have the flared part beaded over against the tube sheet. The beading of the tube end prevents it from being overheated and burned when the boiler is in service.

Front 38

What is an alternative to expansion method of tube attachment used by many high pressure watertube boilers

Back 38

Welded attachments: Boiler drums and headers have stubs welded to them in the factory; the tubes are welded to these stubs during the erection of the boiler in the field.

Some firetube boilers also use welded attachments except that the tubes are expanded both before and after the welding.

Front 39

Why is there a need for a boiler stay

Back 39

Any flat surface exposed to pressure will tend to bulge due to the effect of this pressure. To prevent this bulging from occurring, it is necessary to brace or support the flat surfaces present in the firetube boilers including the tube sheets and waterlegs and the crown sheets on the top of the furnace.

These are not normally required for watertube boilers as they can be built without flat surfaces.

Front 40

Where are diagonal stays used

Back 40

They are welded to the boiler head or tube sheet, and to the boiler shell above the tubes on a firetube boiler to allow access to the area for inspection.

Front 41

What are longitudinal/through stays

Back 41

Long rods extending from the rear tubesheet to the front tubesheet. They are usually fastened to the tubesheets by welding or using nuts and washers. Modern firetube boilers often use through stays which are stronger but allow less access.

Front 42

Where are through stays used

Back 42

The area below the tubes is often supported by through stays.

Front 43

How are stays used in packaged firetube boilers

Back 43

They usually have tubes extending to the bottom of the shell which supports the tubesheets; stays are not required in this area. Due to the need for steam space at the top of the boiler, the upper areas must be stayed.

Front 44

How are stays used in HRT Boilers

Back 44

On their rear tubesheet, the through stays are fastened by means of angle braces which distribute the stress over a large area.

Front 45

Describe staybolts

Back 45

Used primarily on steam locomotive boilers to support the boiler and the firebox. The size of bolts and the number used are determined by ASME Code. In working steam locomotive boilers, loose staybolts are one of the main causes of boiler leakage.

Front 46

Describe function of manholes/manways

Back 46

Allow entrance into drums

Front 47

Describe function of handholes

Back 47

Give access to smaller parts, such as headers and waterlegs.

Front 48

Describe shape of access openings

Back 48

Usually elliptical in shape but may be circular.

Dimensions (elliptical): at least 305 mm x 406 mm

Diameter (circle): at least 380 mm

Handholes: at least 70 mm x 89 mm

Front 49

What is the state of access openings during the operation of the boilers

Back 49

Closed by means of doors or cover plates. These doors, or covers, fit on the inside of the drum or header and are held in place by bolts and yoke pieces. The pressure within operational boilers help to hold the door in place.

Front 50

What do drum connections consist of

Back 50

Nozzles or stubs attached to the drum for connecting various fittings and attachments

Front 51

What kinds of things do drum connections connect to the drum

Back 51

- Safety valve

- Main steam outlet

- Water column

- Feedwater inlet

- Pressure gauge

Front 52

How are connection nozzles/stubs attached to the drum

Back 52

By welding, although on older boilers, they may be riveted. In some cases, threaded connections are screwed into the drum wall via a threaded hole, but these are restricted depending on size and pressure

Front 53

How is Code maintained while using thinner metal in the internal furnace

Back 53

It's currogated

Front 54

What type of furnace design is this

Back 54

Leeds Suspension Bulb Furnace

Front 55

What type of furnace design is this

Back 55

Fox Furnace

Front 56

What type of furnace design is this

Back 56

Deighton Furnace

Front 57

What type of furnace design is this

Back 57

Morison Furnace

Front 58

What type of furnace design is this

Back 58

Purves Ribbed Furnace

Front 59

What type of furnace design is this

Back 59

Brown Furnace

Front 60

What type of furnace design is this

Back 60

Adamson Furnace

Front 61

What type of furnace design is this

Back 61

Ring-Reinforced Furnace

Front 62

Why are adequate foundations and supports required

Back 62

To avoid any movement which would put extra stress on the boiler and its connecting pipework. Also, if the boiler settles at one end, the gauge glass will not five an accurate indication of the water level in the boiler.

For packaged firetube boilers, the boiler room floor must be strong enough to act as a foundation because the boiler is usually mounted on a skid type steel base. The whole assembly can then be placed directly on the floor by an overhead crane. Packaged watertube boilers are also frequently floor mounted.

For large non-packaged type boilers, it is usually necessary to provide special concrete foundations. They may be either top-supported or bottom-supported.

Front 63

Pros of Top-Supporting Large Watertube Boilers

Back 63

- Since these boilers can be 30 m or more in height and the steam drum is the heaviest part, they tend to be top heavy and unstable if bottom supported

- Top support allows expansion downward as the boiler is heated, reducing stress

- The structural steel used to support the boiler can also be used as the boiler house and support for other boiler auxiliaries

- Since these large boilers expand by several cm when heated, top supporting is easier for attachments. Almost all of the connections on the boiler are at the steam drum when top supported, this steam drum is held in a fixed position. If bottom supported, the steam drum moves up and down with heating and cooling. For very large boilers this movement would make attachments difficult.

Front 64

Describe a watercooled wall/waterwall

Back 64

Instead of brickwork, the furnace walls of modern watertube boilers is made of adjacent tubes welded to metal fins to produce a solid panel, which is exposed to the furnace heat and backed by a layer of insulation and then an outer metal casing to protect the insulation.

Front 65

Describe a tangent-tube wall

Back 65

The inside of the furnace wall is made up of tubes which are side by side and touching each other to form a continuous surface. They are backed by a layer of plastic insulation and then a steel casing. Block insulation is then placed against the steel casing and another steel casing forms the outside surface of the wall.

Front 66

Describe a flat stud wall

Back 66

The tubes have flat studs welded to them on each side; these studs fill the space between adjacent tubes. The tubes and studs are backed up by refractory and insulation.