Fabrication of Steel Pipework

8/16/2019 Fabrication of Steel Pipework

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THIRTY SECOND CONFERENCE

F B R I C T I O N O F S T E E L P IP EW O R K

By J. H. BECKLEY

Introduction

The sugar industry is to some extent dependent upon steel tube

for production, and failure of a pipe installation could result in costly

delays. It is important therefore, that the engineers associated with

the industry should have a sound knowledge of the various codes that

have been developed to cover the design and fabrication of pipework.

This paper briefly describes the more important aspects of these codes,

and outlines the various methods used to fabricate steel tube.

Calculating Tube Wall Thickness for Pressure Purposes

British Standard No. 806- Ferrous Pipes and Piping Installations

for and in connection with Land Boilers has been endorsed as Australian

Standard No. B.65. This code covers most aspects of the design of steam

and feed pipework and serves as a basis for the design of other pipework

used for pressure purposes. The formula for calculating tube wall thickness

PD

has been derived from the well-known Barlow's formula of

t c

2s

where t he wall thickness,

=

the design pressure, D the diameter

of the tube, and S the allowable stress.

PL

The formula given in Clause 63 Part

5

of AS.B65 is

t

2

se YP

and the allowable stress S ) at the design temperature, the efficiency of

the longitudinal joint

e)

for the various types of tube and the various

values for

y

are ll clearly set out. The code requires the value of

t

to be multiplied by 1.125 if the tube is to be bent and depending upon

the size and wall thickness of tube, an extra percentage must be allowed

to ensure that ii the tube as ordered is rolled to the minimum wall thick-

ness as allowed by the manufacturing code, then the final thickness

is suitable for the working conditions.

Branch Design

Where a branch is required on a main, allowance must be made

for the weakening effect of the hole which is cut in the main to form

the branch. The method of calculating the strength of a branch pipe is

set out in AS.B66 Clause

79

and where two or more branches are fitted

in close proximity, allowance must also be made for the effect of one

branch upon the other.

As an example of the weakening effect a branch exerts on a main,

a pipe with a

90

unreinforced equal branch has a safe working pressure

of only

70

per cent of the safe working pressure of the straight pipe.

Reinforcement to compensate for the metal cut out of the main to form

the branch can be added (a) by fitting compensating rings, (b) by a

technique developed by Stewarts and Lloyds Limited, known as Triform

Reinforcement .


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9

T

RTY SECOND CONFERENCE

Fig.

l Triform reinforced Tee piece.

Typical details of the latter method using three horseshoe shaped

reinforcing sections welded around the branch axe shown in Figure 1

The fitting of reinforcement to a branch pipe is costly and can often be

avoided by slightly thickening the main and branch pipes.

the design of steam or other hot product mains

is sufficiently flexible to absorb its own expansion

without exerting undue force on equipment to which it connects, or

developing stresses in excess of the allowable maximum in the pipe

itself. Expansion can be absorbed by the use of expansion bends, bellows

and joints, all of these methods requiring special guides and anchors.

If

possible, these special devices should be avoided and the route of the

main selected to allow sufficient flexibility to give trouble free service.

The expansion allowance per

100

feet of pipeline is set out in Table

6

of

AS B65

Stresses and thrusts due to expansion are reduced by pre-

stressing or applying cold-draw to steam mains and other hot product

lines. The minimum amount of cold draw which should be allowed is

50

per cent of the total expansion, but modern practice is to pre-stress

the main to the maximum allowable stress in the cold condition, thus

substantially relieving the stress on the pipework and connecting equip-

ment .in the hot condition.

Several methods are available for the computation of the stresses,

thrusts and bending moments of a main operating at a high temperature.

The method used by Stcwarts and Lloyds is that set out in

a

paper by

Thyer

[l]

This method has been further developed by this company

and has now been incorporated in a computor programme which can

be used to ascertain stresses, thrusts, moments and movements in three

planes in mains having up to six anchor points.

Ben

Both hot and cold processes are used to form bends in tube. AS.1365

lists the permissible minimum radii to which the various sizes of tube

may be bent, making allowance for the reduction in wall thickness as

covered by the Code. Tubes can be bent to a smaller radius, but additional

wall thickness must be provided to allow for the greater thinning which

takes place in a tight radius bend.

For hot bending the pipes are tightly packed with sand or river

gravel, which supports the pipe wall during the bending process and

thus prevents collapse or deformation of the tube. The arc length of the


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THIRTY SECOND CONFERENCE 1965

ST TION RY

FORMER

Fig. 4 Compression bending.

groove. To resist the side thrust a shaped slipper is used and as with

compression bending deformation is prevented by the circular shape

in which the tube is held. For small radii bends down to one and a half

to two times the bore diameter additional support is required and a

mandrel is held in the bore of the tube located so that it supports the

outer surface of the bend and maintains the circular cross section through

the bend. Tight radius bends for superheaters are bent on this type of

machine and they are also used for the manufacture of tubular furniture.

e 5 shows general details of this system of bending.

ROT T I

ENDING

ORM

Fig. 5 Tension bending.

Bends can also be formed using either the fully gussetted technique

or the cut and shut type bend. In the fully gussetted type angular seg-

mental sections are cut and welded together to form a bend of the required

angle. The

minimum

requirements are that the angle between adjacent

segments does not exceed30 and the width of the throat of each segment

is not less than one and a half inches. This type of bend is used

for large diameter rolled and welded pipes and care should be taken

to ensure that the location of the longitudinal weld in the various segments

is staggered to avoid the formation of crucifix welds. The cut and shut

process gives a better form to the bend and typical details of the pre-

paration of the pipe are given in Figure 6.

Gussetted bends are permitted by the Pipe Welding Code CB.15

for pressures up to 250 pounds per square inch but at this pressure

great care would have to be taken to ensure that complete penetration

of the weld between the segments is obtained and the requirements

of the Code in regard to circumferential buttwelds must be observed

in welding this type of bend. Bends of this type are normally used for

low pressure systems. Gussetted and cut and shut type bends are avoided


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TH IRTY SECOND CONFERENCE

I 9

@ POINTS AT WHICH HWT IS APPJ~Q

MbXlMUM CHANGE OF

LINE AT EACH CUT 3 ° bPPWX

SUITABLE HOLE DRILLED AT THE END f C ?

VI W

AFTFH

BENOIMii

Fig

6 Cut

and

shut

bend.

in high pressure pipework where small radii can be obtained by the use

of hot forged long or short radius elbows manufactured in accordance

with BS.1640.

Buttwelding elbows formed to this standard are manufactured

with

a

radius equal to the bore for short radius elbows and one and a

half times the bore for long radius elbows. These bends are formed by

process which avoids thinning on the back of the bend and are suitable

for pressures equal to straight pipe of similar dimensions.

Methods of oining Pipes

Various methods of joining pipes are adopted, the three most common

being by screwed connection, flanges or by welding. Other types of

joints such as Victaulic and Viking Johnson joints using rubber rings

for the sealing medium are also employed, various grades of rubber

being used for different purposes.

Screwed joints are usually adopted for low pressvres and temp-

eratures but flanges and welding can be used for

ll

conditions for which

the tube or flange is designed. BS.10, which is now entitled Flanges

and Bolting for Pipes, Valves and Fittings , and gives dimensions of

ll

standard flanges, has recently been extensively revised and has been

endorsed s Australian Standard B.52.

This code now includes a flange rating table whereby the allowable

pressure for all flange tables decreases as the temperature increases and

vice versa. For example, Table 'H' flanges are now suitable for 250

pounds per square inch at 80Q°F, but the allowable pressure drops to

115 pounds per square inch at 900°F, and rises again to 500 pounds

per square inch for temperatures up to 450°F.

Welded on flanges are now widely used and the methods of welding

the flange to the tube are given in Clause 77 of AS.B65. The four main

methods being

:

Type

l Welding neck type flange where the flange is secured to

the pipe by circumferential weld. The end of the flange

is machined to match the pipe bore and outside diameter.

Type

2 Face and back welded on flanges.

ype 3

Bore and back welded on flanges.

These three types of flanges are suitable for all design and pressure

conditions.


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9 IRTY SECOND CONFERENCE 1965

Type 6. Bore and rear fillet welds--(slip on type flanges)-These

flanges are suitable for all d and pressure conditions

covered by British Standard Flange Tables up to and

including 750 pounds per square inch and Table

'H'.

Figure 7 shows the above types of flanges.

r P ~ I

~JLE-2

TLP ~ i s 6

Fig. 7 Types

o

welded flanges.

Five other variations of these welding preparations are also given

in AS.B05 and show the welding preparation for positional welds, and

for limited temperature and pressure conditions. Normally the faces of

flanges Tables 'E' to 'F' inclusive are machined with a flat surface, but

for flanges over Table 'F', a raised face with a gramophone finish is

employed.

Oxy-acetylene welding and manual metallic arc welding are now

used' for pipework for all operating pressures and temperatures. A welded

joint gives maintenance free service and

is

adopted in major power

stations operating at pressures of up to 2,450 pounds per square inch

at temperatures of 1,050°F. Entire steam mains including the connections

to valves, boilers and turbines are all welded.

Acceptable methods for preparing pipe ends for welding and the

special requirements for all welding of pressure pipework are given in

Australian Standard No. CB.15. A plain buttweld is used for most low

pressure installations, but buttwelding using a backing ring is preferred

for higher pressures and temperatures. A straight 35 bevel with inch

toe, is used for pipes up to approximately inch thickness, but

a

'J'

shape welding preparation is employed for thicker pipes to reduce

the amount of weld metal which is to be deposited. Figure

of some of the various types of welding preparations. Whilst the backing

Fig. 8 Tube

end preparation

or butt

weld.


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1965

THIRTY SECOND CONFERENCE 97

ring is not essential for high pressure welds, it helps to ensure that complete

root penetration is achieved, guarantees alignment of the pipe bores,

and minimizes the possibility of cracking of the base run of weld.

A

requirement of the Code for Pipe Welding, CB.15 is that the

bores of adjacent pipes should match exactly. The permissible variations

are shown in Table 1

T BLE I--Permissible variations in bores o adjacent pipes for welding

-

.

Maximum permissible

Nom inal bore difference in internal Maximu m ou t of

inches diame ter alignment at th e bo re

inches inches

U p t o and including

4

O ver 4 up t o and including

1 6 :

ver 10 up t o and including 4

.

Manufacturing tolerances generally allow a greater bore variation

than that given above and it is usually necessary to match the bores

of adjoining pipes either by machining or expanding.

ranch Welds

In a branch, welds must conform to the requirements of the

Australian Code for pipe welding, CB.15. For high pressure work, two

methods of attaching are adopted,

a

set on ,

b

set in . Sectional views

of these two types of welding are shown in Figure

9.

The root gap and

the angle of bevel are designed to enable full penetration to be achieved,

but where possible an internal sealing run should be applied. The set on

type of welding also allows the weld to be carried out using a temporary

backing ring which can be machined out after welding, thus ensuring

complete penetration of the branch weld.

For low pressure pipework, the end of the branch may be cut to

follow the profile of the main pipe and square to the centre-line, a bevelled

weld preparation not being required. An internal sealing weld is parti-

cularly desirable in this type of branch welding preparation.

Fig. 9---Weld prepa rations for branches.


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9 TH RTY SECOND CONFERENCE 1965

Prior to any welding being carried out preheat as shown in Tables.

I1 and I11 should be applied.

Il-Preheating temperatures for gas welding

-

p

r

Minimum preheating temperature

Thickness

l

..

Carbon steels Alloy steels

Ill-Preheating tempe ratures for metal-arc welding

p

I TT

U p t o and including 11/16

. .

100°C (211°F)

Ov er 11/16 up t o including 718 . 200°C (392°F)

Ov er 718

. . .

300°C (572°F)

Parent metal

Carbon steel (up to 0.2 6% )

.

200°C (392°F)

200°C (392°F)

300°C (572°F)

Carbon steel (0 .2 6- 0. 40 x)

.

Chromium-molybdenum steel

Thickness

up to and including

14

Ov er 314

all thicknesses

all thicknesses

Preheating temp.

(minimum)

Following welding stress relieving is carried out at the following

temperatures

or carboa steels-within the range of 600-650°C.

or alloy

steels-within the range of 630-660°C.

These temperatures are maintained for approximately one hour per

inch of pipe thickness with a minimum of half an hour.

Pipework 44 inches outside diameter or less may be normalized i

conditions of access make stress relieving impossible. Normalizing temp-

eratures are

carbon

steels--within the range of 900-950°C.

alloy

steels-within the range of 925-975°C.

This temperature is maintained for not less than two minutes and

can be measured by means of one or more thermocouples an optical

pyrometer or temperature responsive crayons. The width of the zone

heat treated at the specified temperature extends at least half an inch

on either side of the weld. Immediately after normalizing the joint is

covered with a suitable or asbestos cloth to ensure slow and even

cooling. Full details of heat treatment with specific requirements

are given in Clause 330 of the Pipe Welding Code CR.15.

Pressures and temperatures of steam generating equipment used

in the sugar industry are not generally sufficiently high to require the

use of alloy tube. The codes we have referred to for most of the foregoing

data also contain full information to cover the fabrication of high tensile

carbon tube as well as the various types of alloy material. The welding

preparation for welding flanges to alloy tube is slightly different to that

required for carbon steel a greater depth of welding being required

but for branch and buttwelds welding preparation is similar to that

used for carbon steel. The pre-and post-heat treatment of the high carbon

and alloy steels is most important and the requirements of the code

must be rigidly followed when fabricating these materials.


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1 TH IRTY SECOND CONFERENCE

1965

between the rubber and the steel, it is also usual to sand blast the bore

to remove all rust or

mill

scale immediately, prior to the application of

the adhesive solution to the pipe bore.

Personnel engaged in the design of pipework will agree that one of the

most diacult aspects of this work is to arrange for adequate simple

supports. Hot product lines are subject to expansion and allowance

must be made for movement in arranging the length of hanger rods,

where vertical expansion will take place, springs must be provided.

Where movements greater than two inches are anticipated constant

load supports can be obtained which are designed to give

an

even

supporting efIort over a range of movement of up to approximately

18 inches.

Anchors in a main subject to expansion should generally be welded

to the pipework and take the form of special brackets which can be

bolted to the building steelwork. Use of U bolts is not always satisfactory

as they have a tendency to slip. Pipe clips should be made from sub-

stantial flat sections and fully designed to carry the required load and

hanger rods should include a means for adjustment of length and also

be fitted with spherical washers to obtain even support when angularly

displaced due to expansion. Holes in supporting structures should be

large enough to accommodate angular movement of hanger rods.

The foregoing remarks are intended as a guide to those responsible

for pipework installations. The relevant Australian and British codes

cover all aspects of this work and provided the full requirements of these

codes are followed, no trouble should be experienced in the fabrication

and installation of industrial pipework.

of the Standards Association of Australia to print

extracts from the Australian Codes B.65- Ferrous Pipes Piping

Installations for and ifi connection with Land Boilers , and

CB.15-

Pipe Welding , is atefully acknowledged.

l]

Thyer

A. M.: 1938.

Trans.

Inst.

Eng.

Aust. Vol.

XIX.

Stewarts and Lloyds Australia) Pty. Ltd.,

Sydney,

N.S.

W .

Fabrication of Steel Pipework

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