Mall Pipe Threads

GEORGE FISCHER …

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4.01

Pipe Threads

Types and Designation 4.02

Comparison of Jointing and FasteningThread Tolerances 4.03

Design and Function of Jointing Threads 4.04

Thread Engagement 4.05

Thread Cutting 4.06

Assembly Method 4.07

Notes on Taper/Parallel Threaded Joints 4.08

Thread Measurement 4.09 - 4.11

Key Data for Threads to ISO 7/1 4.12

Thread Jointing and Sealing 4.13 - 4.14

4 Pages

GEORGE FISCHER …4.02

Fastening Thread to ISO 228-1, BS2779 (BS EN 10227-1)

Internal thread parallel G 11/2

(right-hand)

External thread parallel G 11/2 A

(right-hand)Tolerance class A

External thread parallel G 11/2 B(right-hand)Tolerance class B

Note: LH is added to designate left-handthread, G 11/2- LH

Types & DesignationOver generations many different typesof screw threads have been developedfor particular applicatons, which includefastening components, powertransmission and materials handling.

In the nineteenth century, most factoriesthat needed a fastener would devisetheir own system. Clearly this resulted inall sorts of compatibility problems. TheEnglish mechanical engineer andinventor, Sir Joseph Whitworth (1803-1887) devised a standardised threadingsystem in 1841 to tackle these difficulties.The Whitworth thread form is based ona 55 degree thread angle and roundedroots and crests. See Fig. 4.01.

The Whitworth thread form was selectedfor use as a connecting thread for pipes,which was made self sealing by cuttingat least one of the threads on a taper.This became known as the BritishStandard Pipe thread (BSP Taper or BSPParallel thread). The Whitworth thread isnow used internationally as a standardthread for jointing low carbon steelpipes. See Fig. 4.02.

Pipe threads can be divided into twotypes:

a) Jointing threads - which are pipethreads for joints made pressure tightby sealing on the threads and aretaper external and parallel or taperinternal threads. The sealing effect isimproved by using an appropriatejointing compound (see page 4.13 ).These threads are standardised inISO 7-1 and the new BS EN 10226-1,which will supersede BS 21:1985.

b) Fastening threads - are pipe threadswhere pressure tight joints are notmade on the threads. Both threadsare parallel and sealing is effected bycompression of a soft material ontothe external thread, or a flat gasket.These threads standardised in ISO228-1 and BS 2779:1986. (BS EN10227-1 is being prepared and willsupersed BS2779).

George Fischer malleable iron pipefittings are produced with jointing

threads, taper external and parallelinternal. For special applications, aparallel fastening thread is required. SeeFigure 4.03 of a ‡ union which usesthe two types of thread.

Pipe Thread Designation

The size designation of a pipe thread isderived from ISO 7. See page 3.08 forpipe size designation.

Before a revision of BS21 in 1973 aninternal pipe thread that was specified ,for example , by ‘11/2" BSP. Pl ’ may nowsimply be referred to as ‘Rp11/2’.

Similarly, a taper thread which wasdesignated by ‘11/2 BSP. Tr EXT ’ is nowdesignated by R 11/2.

The method of designating the differenttypes of pipe thread is detailed belowusing thread size 11/2" as an example.

Jointing thread to ISO 7-1, BS EN10226-1

Internal thread parallel Rp 11/2

(right-hand)

External thread taper R 11/2

(right-hand)

Note: LH is added to designate left-handthread, Rp 11/2 -LH

H h

H/6

H/6

r

r 27.5° 27.5°

Pitch P

H = 0.960 491 Ph = 0.640 327 Pr = 0.137 329 P

Basic Whitworth Form

27.5°27.5°

Pitch P

r

H h

90°

r

Axis of screw

Basic form of taper pipe

thread

Fig. 4.01

The George Fischer Union,showing both jointing andfastening threads.

Fig. 4.03

Fig. 4.02

Jointing Thread

Jointing Thread

FasteningThread

Fig. 4.04

Fig. 4.05

GEORGE FISCHER …

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4.03

Combination of Jointing and

Fastening Threads

If a parallel external pipe thread, G(fastening thread to ISO 228-1), isscrewed together with a parallel internalpipe thread Rp, (jointing thread to ISO 7-1), special consideration is required sincethis combination may not assembletogether and does not necessarily leadto a pressure tight joint. (see ISO 228-1section 3 and/or BS21:1985 Table 6).

Internal thread maximum size

External threadminimum size

External threadminimum sizeInternal thread

minimum size

Jointing threads to ISO 7-1

thre

ad

dia

met

er

External threadminimum size

Fastening thread to ISO 228-1

Internalthread

G

Comparison of Jointing andFastening Thread Tolerances

Internal threadminimum size

External threadmaximum size

. . . for tolerance ranges used by ‡

ExternalthreadG ... A

Externalthread

G ... B

Nominalsize

Internal threadmaximum size External thread

maximum size

Internalthread

Rp

External

threadR

Fig. 4.06


Terms relating to Pipe Threads

End of longest permitted internalthread at hand tight engagement

Fitt ing allowance

Wrenchingallowance

Total thread

Gauge length

Major cone

Gaugediameter

d

2- T1 T 1

2+

Complete thread

Useful thread (not less than gauge length plus f itt ing allowance)

Incompletethread

Washoutthread

Vanish cone

Gauge plane

Allowance equivalentto posit ive toleranceon internal thread

sufficient allowance for tightening orwrenching the joint.

● Wrenching Allowance is the lengthof useful thread which is provided toallow tightening or wrenching of thecomponents, beyond the hand tightposition, in order to achieve apressure tight joint.

Taper External Threads, R

The taper has a combined ratio 1:16.The most important parts of the threadare :

● Gauge Length is the length along theaxis from the gauge plane to theend of the thread. This will varywithin a tolerance. The designdimensions ensure that assemby withthe internal thread is always possible, even when the internal thread is atthe minimum diameter.

● Fitting Allowance is the length ofuseful thread required to provide forassembly with an internal thread.Even when the internal thread is atthe maximum diameter, there is stil l

Design and Function ofJointing ThreadsThread shape, dimensions, tolerancesand designations per thread size arespecified in ISO 7-1(CEN 10226).

Internal Threads, Rp or Rc

The length and design of the internalthread must be such to allow the externalthread to be screwed in sufficiently toachieve adequate compression andsealing in the threads, even when theexternal thread is at the maximumpermitted length.

● Washout Thread is the part of thethread which is not fully formed atthe root. This section is not usefulthread and does not contribute to theeffectiveness of the joint.

Fig. 4.07

GEORGE FISCHER …

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4.05

Thread Engagement

Thread engagement is the standarddistance that the pipe may be screwedinto the fitting. This dimension isfundamental to the development of the zmethod (see section 6). The following

41/2 4 3/4 41/2 51/4 41/2 51/2 51/2 67/8 79/16 815/16 11 123/8

6.0 6.4 8.2 9.5 10.4 12.7 12.7 15.9 17.5 20.6 25.4 28.6

23/4 2 3/4 23/4 23/4 23/4 23/4 23/4 31/4 4 4 41/2 5 3.7 3.7 5.0 5.0 6.4 6.4 6.4 7.5 9.2 9.2 10.4 11.5

11/2 1 1/2 11/2 11/2 11/2 11/2 11/2 2 21/2 21/2 3 31/2

2.0 2.0 2.7 2.7 3.5 3.5 3.5 4.6 5.8 5.8 6.9 8.1

71/4 7 1/2 71/4 8 71/4 81/4 81/4 101/8 119/16 1215/16 151/2 173/8

10 10 13 15 17 19 19 24 27 30 36 40

Gauge length(hand tight)

fittingallowance †

wrenchingallowance

averagethreadengagement*

Turns(mm)

Turns(mm)

Turns(mm)

Turns(mm)

Thread Size 1/43/8

1/23/4 1 11/4 11/2 2 21/2 3 4 6

* useful thread = gaugelength plus fitt ing allowance.

† includes wrenchingallowance.

table il lustrates the design threadlengths. The gauge length is the designhand tight position and the fittingallowance is for adjustment andtightening. The figures are for an internalthread to mid tolerance.

Table of Nominal Thread Lengths

Fig. 4.08


Thread CuttingThe usual method of producing pipethreads on low carbon steel tube is tocut them using a threading machine.(Details of the George Fischer pipecutting and threading machines can befound in section 8).

Alternatively, pipe threads can beproduced using hand tools known asstocks and dies.

If a number of pipe threads have to becut, the benefits of using a machineinclude: accuracy which is repeated oneach thread and significant time savings.Once the machine is set for a particularsize, cutting and threading times of 20seconds are not unusual.

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4.07

Assembly MethodThe important dimensional data relatingto thread lengths and their asssembly isdetailed in Fig. 4.08. As an example, thecorrect method of assembling a 1" pipejoint is described.

1. Apply the appropriate jointingcompound (see page 4.13).

2. Engage the threads and tighten tothe hand tight position ( see Fig. 4.09 )which is designed to be at thegauge length.

3. Using a wrench, continue to tighten.The number of turns which arenecessary to achieve a pressure tightseal is the wrenching allowance. Thisis 1 1/2 turns for a 1" connection. (SeeFig. 4.10 ) .

If the size of the internal thread coincideswith the gauge plane on the externalthread, then all of the fitting allowancemay be used for tightening the joint, 2 3/4

turns for a 1" connection. Depending onthe sizes of both threads, small variationsto the number of turns may be requiredto position the outlet of the fitting or toadjust the length of the pipeline.

Fig. 4.09Hand tight position

Fig. 4.10Position after wrenching


A pressure tight joint is achieved by thecompression in the threads resulting fromtightening in the wrenching allowance.This compression and sealing occurs inthe first few turns of the internal thread.As wrenching takes place, the internalthread will stretch to accommodate theexternal taper. This ensures a perfectmetal to metal contact which minimisesthe effects of threads produced atdifferent ends of the permissibletolerances. Variations within thetolerances can occur between threadswhich are factory produced and thoseproduced on site, whether by machine orhand cut.

The metal to metal contact enables thejoint to withstand tensile andcompressive loads and bending stresses.In order to achieve the best sealing effectthe following points should be taken intoaccount:

● threads must be produced to allow thecomponents to be screwed together,without jointing compound.

● the jointing compound should beapplied sparingly, since it is onlyrequired to fil l the inevitable deviationsfrom the theoretical thread profile andthe roughness of the thread surfaces.

● sufficient threads must be available forwrenching, even when the internalthread is at maximum diameter.

● the end of the useful thread should notbe screwed in further than the first fullformed internal thread.

● the washout thread will normallyremain visible after tightening with awrench (typically 1 to 2 threads).Sealing and compressionperformance may be reduced ifattempts are made to seal on theexternal washout thread.

● allowances for thread chamfers maybe necessary (see page 4.10)

Reuseability

Fittings may be reused only if nopermanent deformation has occuredduring the jointing process. If the correct

Notes on Taper/ParallelThreaded Joints

wrenching allowances are adopted,then the stresses in the fitting will bewithin the elastic l imit of the material sothat no permanent deformation occurs.The fitting may then be reused.

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4.09

Gaugeplane Start of

thread

Background

Threads are produced world-wide bymany thousands of factories and alsoindividuals working on site. It is importantto know that the threads on the pipe andfittings are in accordance with theappropriate standard. Measurement ofthreads is known as gauging and iscarried out using a set of standardisedgauges.

Gauges are precision instruments whichshould be regularly checked foraccuracy by a calibration laboratory.They are therefore expensive. Anygauging system must be readilyavailable, quick and easy to use, and beresiliant in a production or siteenvironment.

Over the years, different countries havedeveloped a number of slightly differentways to measure the same thread, withsome types of gauge attempting tomeasure more than one of the threadparameters. As a result it is possible toobtain slightly different results dependingon the measuring system used. Thereforea component which is to standard usingone set of gauges may not appear to beto standard using a different method.

This problem is being addressed by theinternational experts from the majorindustrial markets, who are attempting todevise a gauging system which will beadopted world-wide. The result of thisinternational effort will be the revision tothe ISO 7-2 standard, after which thenew European standard BS EN 10226-2will be compiled.

Thread Gauges

Both the jointing threads and thefastening threads are measured withdifferent sets of go and no gostandardised plug and ring gauges. TheUK uses gauges detailed in BS 21 forjointing threads and ISO 228-2 forfastening threads. Since the majority ofpipe threads are jointing threads we willconsider in more detail how they aregauged.

Thread Measurement General purpose pipe threads inaccordance with ISO 7-1 or BS 21 areusually gauged with gauges detailed inBS 21 system B. This uses Taper full formplug and ring gauges and Taper plainplug and ring gauges. (See Fig. 4.12 )

Thread inspection is carried out byassembling the gauge and theworkpiece together. The accuracy of thethreaded component is determined bythe position of the start of the thread,relative to the step in the gauge. Figure bshows gauging of internal and externalthreads at both the maximum andminimum sizes.

Gaugeplane

Gauge plane

Gauge plane Gauge plane

Fig. 4.12Full form, plain plug and ring gauges.

Gauge plane

Ring gauge

Start ofthread

Gaugelength(max)

Plug gauge

Gaugelength(min)

External thread ofmaximum gauge length

External thread ofminimum gauge length

Internal thread (parallelor taper) of maximumsize

Internal thread (parallelor taper) of minimumsize)

Fig. 4.13


It is important to remember that threadinspection using gauges is acomparative test and when checking aparallel internal jointing thread,allowance must be made for thechamfer.

Effect of Thread Chamfer on GaugingResult

Most threads are chamfered duringproduction, to remove any sharp edgesand to ensure ease of assembly. This hasa significant effect on the gauging result,particularly when inspecting internalparallel threads.

The key to understanding this is that themeasuring point is always the start of thethread relative to the step on the gauge.If, for example, a parallel internal threadhas been chamfered with a chamfergreater than 1 pitch, then the start of thethread has been moved to a point insidethe fitting. It is this point inside the fittingthat must be compared with the step onthe gauge. (see Fig. 4.14 )

George Fischer have developed amethod where the measuring pointremains at the front face of the fittingafter applying a correction for the effectof the chamfer. (See the nomogram onthe following page, Fig. 4.15 ) .

Fig. 4.14

Effect of a chamfer on start of thread

Start ofthread

Chamfer < 1 pitch

Chamfer > 1 pitch

Start ofthread

Majorcone

Internal Thread

External Thread

Start ofthread

Chamfer < 1 pitch

Start ofthread

Chamfer > 1 pitch

Minor cone

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4.11

The nomogram is used as follows:

1. Measure the outside diameter DA ofthe chamfer and plot this point on theDA axis.

2. Draw a straight l ine between the DA

plotted point and the pointcorresponding to the size of thethread being inspected.

3. Continue the straigth line to intersectthe n axis and read off the value of n.This is the number of correctionrevolutions to be applied.

4. Correction is carried out by retractingthe plug gauge by n revolutions fromthe hand tight position. At this newposition, the size is measured bycomparing the front face of the fittingwith the step on the gauge.

Example

On a 3/4" fig. 90 elbow a chamferdiameter (DA) of 28.3mm was measured.By drawing a line through the pointsDA=28.3mm and the 3/4" size, andextending the line to intersect the n axis,n=1/4 is read off. The gauge would beretracted from the hand tight position by1/4 turn and the position of the front faceof the fitting compared with the step onthe gauge.

DA

1/4 gaugerevolution

DA = measured diameter of the thread chamfer

n = number of correction revolutions for the plug gauge

example.:D

A = 28,3 mm

Fig. 4.15 Nomogram (to be reproduced only with the express permission of George Fischer)

Theoretical chamfer diameter DA on the fitting front face

for

Rp

1" t

o 2

"

21 / 2"

to 4

"

1 / 4"

to 3 /8

"1 / 2

" to

3 /4"


Ke

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fo

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ISO

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, BS

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an

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es

to

ISO

65

, BS

13

87

Thre

ad

siz

e1 / 8

1 / 43

/ 81 / 2

3 / 41

11 / 41

1 / 22

21 / 23

45

6N

om

ina

l dia

met

er D

N6

810

1520

2532

4050

6580

100

125

150

9.72

813

.157

16.6

6220

.955

26.4

4133

.249

41.9

1047

.803

59.6

1475

.184

87.8

8411

3.03

013

8.43

016

3.83

0

0.90

71.

337

1.33

71.

814

1.81

42.

309

2.30

92.

309

2.30

92.

309

2.30

92.

309

2.30

92.

309

2819

1914

1411

1111

1111

1111

1111

43/ 8

41 / 243

/ 441 / 2

51 / 441 / 2

51 / 251 / 2

67/ 8

79/ 1

6815

/ 16

1112

3 / 812

3/ 8

46

6.4

8.2

9.5

10.4

12.7

12.7

15.9

17.5

20.6

25.4

28.6

28.6

±0. 9

±1. 3

±1. 3

±1. 8

±1. 8

±2. 3

±2. 3

±2. 3

±2. 3

±3. 5

±3. 5

±3. 5

±3. 5

±3. 5

23/ 4

23 / 4

23/ 4

23 / 4

23/ 4

23 / 4

23 / 423

/ 431 / 4

44

41 / 25

5

71 / 87

1 / 471 / 2

71 / 4

87

1 / 481 / 4

81 / 410

1 / 811

9/ 1

612

15/ 1

615

1 / 217

3 / 817

3/ 8

11 / 21

1 / 211 / 2

11 / 2

11 / 21

1 / 211 / 2

11 / 22

21 / 221 / 2

331 / 2

31 / 2

10. 2

13. 5

17. 2

21. 3

26. 9

33. 7

42. 4

48. 3

60. 3

76. 1

88. 9

114.

313

9.7

165.

10.

032

0.04

20.

054

0.06

70.

085

0.10

60.

133

0.15

20.

189

0.23

90.

279

0.35

90.

439

0.51

9

2.0

2.3

2.3

2.6

2.6

3.2

3.2

3.2

3.6

3.6

4.0

4.5

5.0

5.0

6.2

8.9

12. 6

16. 1

21. 7

27. 3

36. 0

41. 9

53. 1

68. 9

80. 9

105.

312

9.7

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0.30

0.62

1.25

2.04

3.70

5.85

10. 1

813

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22. 1

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51. 4

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132.

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8.94

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0.37

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585

1.01

81.

379

2.21

53.

728

5.14

08.

710

13. 2

1218

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0.40

0.64

0.84

1.21

1.56

2.41

3.10

3.56

5.03

6.42

8.36

12. 2

16. 6

19. 8

2.6

2.9

2.9

3.2

3.2

4.0

4.0

4.0

4.5

4.5

5.0

5.4

5.4

5.4

5.0

7.7

11. 4

14. 9

20. 4

25. 7

34. 4

40. 3

51. 3

67. 1

78. 9

103.

512

8.9

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0.19

0.47

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1.74

3.27

5.19

9.29

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620

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35. 3

648

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84. 1

313

0.59

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0.10

20.

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0.32

70.

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0.92

91.

276

2.06

63.

536

4.88

98.

413

13.0

5018

.699

0.49

0.77

1.02

1.44

1.87

2.93

3.79

4.37

6.19

7.93

10.3

014

.50

17.9

021

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Key Data for Threads to ISO 7/1

Fig. 4.16

GEORGE FISCHER …

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4.13

Thread Jointing and Sealing

Principles

In a threaded connection the threadsprovide the mechanical strength butshould not be relied upon to give apressure tight joint. The metal to metalcontact between threads will resist loadsfrom pipeline expansion and contraction,in addition to any bending momentswhich may be applied to the joint.

A pressure tight joint is achieved by usinga jointing compound or thread sealantwhich will act as a fil ler for gapsbetween the threads and also fil l anyroughness on the threaded surface.

Types of Jointing Compound

A wide and varied number of jointingcompounds exist and it is only intendedto make some general observations andcomments in this section.

Care should be taken when selecting

a thread sealant to ensure it meets the

specification requirements of each

installation.

Most thread sealants consist of a viscousliquid or paste which is applied to thethreads before screwing together. Thenotable exception is PTFE tape, which ismade from pure PTFE material and has awide application.

Selection of a Thread Sealant

A number of jointing compounds areused as general purpose thread sealantsbut the wide variety of pipeworkapplications has led to the developmentof thread sealants for specificapplications.

For example:

● steam

● marine

● potable water

● natural and LP gases

● compressed air

● fuels

Before selecting a sealant it is necessaryto be aware of the type of environment

that the joint will have to endure.

Some of the main points forconsideration are:

● operating temperature andpressure of system

● the fluid in the pipe and itscharacteristics

● exposure to vibration

● exposure to thermal shock(expansion and contraction)

● health and safety aspects (e.g.toxicity, flammability)

Other factors which may affect theselection of jointing compound:

● setting or curing times beforesystem may be pressurised

● ability to dismantle connections -ease of maintenance

● size of thread

● ease of application

● shelf l ife

Consideration of these factors will assistin selection of the most appropriatejointing compound for a particularapplication. Confirmation of suitabilityshould be addressed to themanufacturer concerned and theirrecommendations strictly followed.

Method of Application

Reference should always be made to themanufacturers instructions for the exactmethod of application. Meanwhile, thefollowing guidelines il lustrate some of thegeneral principles.

The threads to be jointed must be cleanand dry, free from rust, dirt, oil andgrease. If a joint is being re-made thenany previously used jointing compoundmust be removed.

Pastes

The jointing compound is normallyapplied to the external thread using abrush. Apply the reinforcing material ifrequired and the joint is then screwedtogether.


Reinforcing Material

Dependent on manufacturersinstructions it may be necessary to use areinforcing material in conjunction withthe jointing compound.

This is traditionally hemp which is woundevenly around the threads, starting at thefront of the thread and wound in thedirection of the threads towards thewashout.

Note: Hemp is not permitted for use onhot and cold water according to the UKWater Byelaw Scheme, although a hempsubstitute is available from somesuppliers.

For threaded joints above 2" the use ofreinforcing material is generallyconsidered essential, mainly due to thelarger gaps between the threads whichrequire fil l ing and sealing.

PTFE Tape

This material is used as a generalpurpose thread sealant. The tape isapplied to the external thread by startingat the front of the thread and winding ina clockwise direction towards thewashout. This ensures that the sealingmaterial is drawn into the threads as thejoint is made.

A slight tension to the tape, during theapplication should be maintained whichwill help to achieve the correctpositioning.

The number of turns required issomewhat dependent on the threadtype and thread quality, although as ageneral guide use a single wrap up to 2"threads and double or multiple wrapsabove 2".

Standards and Specifications

With any piping installation it is importantto take account of any standards orrequirements which have been detailedby the specifier. This is particularlyimportant when dealing with potablewater and gas installations.

All jointing compounds in contact with

potable water must be approved by

the UK Water Byelaw Scheme.

Many mechanical service installationswill have the jointing compoundspecified and there may be some slightvariations between different contracts.

Most requirements for thread sealantsare detailed in BS 6956, JointingMaterial and Compounds. The newEuropean standards are currently beingintroduced as part of the BS EN 751series.

Mall Pipe Threads

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