Bsi bs en 50068 1991 (1999)

EN 50068:1991PageFigure 21 — Necked out opening 22 Figure 22 — Calculation scheme for Figure 23 — Calculation scheme for Figure 24 — Calculation scheme for adjacent branches in longitudi

BS EN 50068:1991

This British Standard, having

been prepared under the

direction of the Power

Electrical Engineering

Standards Policy Committee,

was published under the

authority of the Standards

Board and comes

into effect on

30 September 1991

© BSI 12-1999

The following BSI references

relate to the work on this

standard:

Committee reference PEL/92

Draft announced in BSI News

Amendments issued since publication

Amd No Date Comments

8054 January 1994 Indicated by a sideline in the margin

PageCooperating organizations Inside front cover

version of EN 50068:1991 and its Amendment A1:1993 “Wrought steel enclosures

for gas-filled high-voltage switchgear and controlgear” published by the European

Committee for Electrotechnical Standardization (CENELEC)

National appendix NA gives the constitution of the committees responsible for

UK participation in the preparation of this standard

National appendix NB gives details of International Standards quoted in this standard for which there is an identical or technically equivalent British Standard

A British Standard does not purport to include all the necessary provisions of a contract Users of British Standards are responsible for their correct application

Compliance with a British Standard does not of itself confer immunity from legal obligations.

UDC 621.316.54-213.34-034.14

Descriptors: Enclosure, high-voltage switching device, H.V metal enclosed switchgear and controlgear, pressurized closure, wrought

steel

English version

Wrought steel enclosures for gas-filled high-voltage

switchgear and controlgear

(includes amendment A1:1993)

Enveloppes en acier soudé pour l’appareillage

à haute tension sous pression de gaz

(inclut l’amendment A1:1993)

Kapselungen aus Schmiedestahl für gasgefüllte Hochspannungs- Schaltgeräte und -Schaltanlagen

(enthält Änderungen A1:1993)

This European Standard was approved by CENELEC on 5 March 1990

CENELEC members are bound to comply with the CEN/CENELEC Internal

Regulations which stipulate the conditions for giving this European Standard

the status of a national standard without any alteration

Up-to-date lists and bibliographical references concerning such national

standards may be obtained on application to the Central Secretariat or to any

CENELEC member

This European Standard exists in three official versions (English, French,

German) A version in any other language made by translation under the

responsibility of a CENELEC member into its own language and notified to the

Central Secretariat has the same status as the official versions

CENELEC members are the national electrotechnical committees of Austria,

Belgium, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy,

Luxemburg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and

United Kingdom

CENELEC

European Committee for Electrotechnical StandardizationComité Européen de Normalisation ElectrotechniqueEuropäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B-1050 Brussels

© 1991 Copyright reserved to CENELEC members

Ref No EN 50068:1991 + A1:1993 E

EN 50068:1991

© BSI 12-1999

2

Foreword

At the request of CENELEC Technical Committee

TC 17C, the text of the draft EN 50068 prepared by

TC 17C was submitted to the Unique Acceptance

Procedure (UAP)

The text of the draft was approved by CENELEC as

EN 50068 on 5 March 1990

The following dates were fixed:

For products which have complied with the relevant

national standard before 1991-06-01, as shown by

the manufacturer or by a certification body, this

previous standard may continue to apply for

production until 1996-06-01

This document forms a supplement to

EN 50052 (1986): “Cast aluminium alloy enclosures

for gas-filled high-voltage switchgear and

controlgear” and EN 50064 (1989): “Wrought

aluminium and aluminium alloy enclosures for

gas-filled high-voltage switchgear and controlgear”,

concerning welded enclosures for the same type of

switchgear and controlgear but composed of parts

made of wrought steel It is based on the general

specifications given in HD 358 S2 (IEC 517 (1986)

ed 2) which are however not sufficient to satisfy the

conditions for the service allowance of pressurized

high-voltage switchgear and controlgear.)

These specifications are appropriate for pressurized

switchgear enclosures allowing an economic

production without sacrificing aspects of safety For

unusual shapes dictated by electrical conditions

they permit the verification of sound design by proof

tests instead of calculations Nevertheless this

European Standard makes use of many

internationally well acknowledged calculation

rules and the Technical Committee will in

addition pursue the progress in standardization in

CEN/TC 121 and ISO/TC 44 on welding and allied

processes

For the time being reference can only be made to

published International Standards as far as they

are appropriate for the purpose of production of

enclosures to be used in gas-filled switchgear and

controlgear

The present EN has been established as an international specification for the design, construction, testing, inspection and certification of pressurized enclosures used in high-voltage

switchgear and controlgear This standard follows to that extent also Article 2 of the Directive 76/767/EEC

The European Standard contains two normative and one informative technical annexes:

Annex A: Welding procedure and welder performance tests

Annex B: Sample of record form

Annex C: National deviations

List of standards referred to in this standard:

Foreword to Amendment A1

This amendment was prepared by CENELEC Technical Committee TC17C, High-voltage enclosed switchgear and controlgear

The text of the draft was submitted to the voting procedure in March 1993 and was approved by CENELEC as amendment A1 to EN 50068:1991 on July 6, 1993

The following dates were accepted:

— latest date of publication

(IEC 517 (1986) ed 2) Gas-insulated metal-enclosed switchgear

for rated voltages of 72,5 kV and above

ISO 6213:1983 Welding: Items to be

considered to ensure quality

in welding structuresISO 9000:1987 Guidelines for selection and

use of the standards on quality management, quality system elements and quality assurance

ISO/IEC Guide 2:1986 General terms and their definitions concerning

standardization and related activities

ISO 6520:1982 Classification of

imperfections in metallic fusion welds, with explanationsISO/R 373:1964 General principles for

fatigue testing of metals

— latest date of publication

of an identical national standard (dop) 1994-10-01

— latest date of withdrawal

of conflicting national standards (dow) 1994-10-01

Text of amendment

2.1 Type of equipment (page 5)

The first paragraph has to be changed as follows

(new text in bold letters);

of high-voltage switchgear and controlgear,

where the gas is used principally for its

dielectric and/or arc-quenching properties,

with rated voltages.

— 1 kV and up to and including 52 kV and

with gas-filled compartments with design

pressure greater than 3 bar (gauge);

— and with rated voltage 72,5 kV and above.

Annex C (page 67) — only English version

6 Manufacture and workmanship 40

7 Inspection and testing 46

8 Pressure relief devices 53

9 Certification and marking 53Annex A Welding procedure and welder

Annex B Sample of record form 65Annex C (informative) National deviations 67Figure 1 to Figure 4 — Typical connections

between cylindrical and conical shells 11Figure 5 — Design factor ¶ and numerical

Figure 6 — Dished end 15Figure 7 — Dished end with branch 15Figure 8 — Dished end with knuckle and

crown of unequal wall thickness 15Figure 9 — Welded joint

outside 0,6Da: v = 0,75 resp 1,0 16Figure 10 — Welded joint

inside 0,6Da: v = 1,0 16Figure 11 — Welded dished end from

crown and knuckle components

inside 0,6Da: v = 1,0 17Figure 12 — Welded dished end from

crown and knuckle components

outside 0,6Da: v = 0,75 resp 1,0 17Figure 13 — Design factor ¶ for dished

ends of the Klöpper type 19Figure 14 — Design factor ¶ for dished

ends of the Korbbogen type 19Figure 15 — Determination of the

EN 50068:1991

PageFigure 21 — Necked out opening 22

Figure 22 — Calculation scheme for

Figure 23 — Calculation scheme for

Figure 24 — Calculation scheme for

adjacent branches in longitudinal direction

Figure 25 — Calculation scheme for

adjacent branches in a sphere or in

circumferential direction of a cylinder 25

Figure 26 — Branch and pad reinforcement 26

Figure 27 — Weakening factor vA for

openings and branches perpendicular to

cylindrical and conical shells 27

Figure 28 — Weakening factor vA for

openings and branches perpendicular to

Figure 29 — Circular flange joint 29

Figure 30 — Examples of different

Figure 31 — Forces and moment arms

for flanges with O-ring seal 32

Figure 32 — Hubless flange sections

for stress calculation 32

Figure 33 — Flange with untapered

hub section for stress calculation 33

Figure 34 — The factor “A” as a function

Figure 35 — Graph showing in (Dy/Di)

as a function of m = Dy/Di 36

Figure 36 — Forces and moment arms

for flanges with O-ring seal 37

Figure 37 — Flange with large fillet

radius sections for stress calculation 38

Figure 38 — Flange with tapered

hub sections for stress calculation 39

Figure 39 — Flange with flat circular

cover and O-ring seal 40

Figure 40 — Butt welds in plates of

unequal thickness with approximately

coincident middle lines 47

Figure 41 — Butt welds in plates of

unequal thickness with offset of

(not exhaustive; other steels may be used from national standards) 8Table 2 — Design factor ¶ for conical

shells and numerical factors

Table 3 — The factor “A” as a function

Table A.2 — Number of test

1 Introduction

This standard covers the requirements for the design, construction, testing, inspection and certification

of gas-filled enclosures for use specifically in high-voltage switchgear and controlgear or for associated gas-filled equipment Special consideration is given to these enclosures for the following reasons

a) The enclosures usually form the containment of electrical equipment, thus their shape is determined

by electrical rather than mechanical considerations

b) The enclosures are installed in restricted access areas and the equipment is operated by experts and instructed persons only

c) As the thorough drying of the inert, non-corrosive gas-filling medium is fundamental to the

satisfactory operation of the electrical equipment it is periodically checked For this reason, no internal corrosion allowance is required on the wall thickness of these enclosures

d) The enclosures are subjected to only small fluctuations of pressure as the gas-filling density shall be maintained within close limits to ensure satisfactory insulating and arc-quenching properties

Therefore, the enclosures are not liable to fatigue due to pressure cycling

e) The operating pressure is relatively low

For the foregoing reasons, and to ensure the minimum disturbance hence reducing the risk of moisture and dust entering the enclosures which would prevent correct electrical operation of the switchgear, no pressure tests shall be carried out after installation and before placing in service and no periodic inspection

of the enclosure interiors or pressure tests shall be carried out after the equipment is placed in service

2 Scope and field of application

Busbars and connections

The scope also covers pressurized components such as the centre chamber of live tank switchgear, gas-insulated current transformers, etc

2.2 Production

The production of the enclosures shall be in accordance with documented welding procedures which shall

be carried out by well trained and supervised welding personnel Where International Standards (ISO or CEN) are not available national standards may be used

NOTE The standard will be revised as soon as possible when ISO or CEN standards covering the various aspects are available.

NOTE Reference should be made to the ISO 9000 series of standards for quality systems.

design pressure (of an enclosure)

pressure used to determine the thickness of the enclosure It is at least the upper limit of pressure reached within the enclosure at the design temperature HD 358 S2 = IEC 517 (1986) ed 2

3.6

design temperature (of an enclosure)

highest temperature reached by the enclosure which can occur under service conditions This is generally the upper limit of ambient air temperature increased by the temperature rise due to the flow of rated normal current HD 358 S2 = IEC 517 (1986) ed 2

NOTE Solar radiation should be taken into account when it has a significant effect on the temperature of the gas and on the mechanical properties of some materials Similarly, the effects of low temperatures on the properties of some materials should be considered.

the materials given in clause 4.

The rules take into account that these enclosures are subjected to particular operating conditions (clause 1)

which distinguish them from compressed air receivers and similar storage vessels

The thicknesses determined by the various equations are minima and therefore the specific nominal thickness shall be increased by the amount of any negative tolerance permitted by the material

In the case of such an enclosure or an enclosure for which calculations are not made, a proof test of the individual housing is necessary before the internal parts are added

Table 1 — Examples of materials (not exhaustive; other steels may be used from national standards)

NFA 36 – 601

[forgings]

A37 – CP A37 – AP A37 – FP A42 – CP A42 – AP A42 – FP A48 – CP A48 – AP A48 – FP

SS14 13 30-01SS14 14 30-01SS14 14 32-01SS14 21 06-01SS14 21 17-01

SS 142333

SS 142343

UN1 5869-75UN1 7382-75UN1 7070-82UN1 8317-81UN1 7660-77[forgings]

When designing an enclosure, account shall be taken of the following, if applicable.

a) The possible evacuation of the enclosures as part of the filling process

For enclosures of this type it is usually necessary to evacuate the air before introducing gas pressure, this ensures purity of the gas The evacuated condition is therefore not an operational condition and in most cases enclosures designed for internal pressure will be suitable for the evacuated condition without buckling

For certain long lengths and large diameters of busbar sections, however, it is possible that the

enclosure will buckle due to external pressure In such cases the design should be checked for external pressure and the enclosure strengthened if necessary Since this is not an operational condition it is not

a matter of safety

b) The full differential pressure possible across the enclosure wall

c) Superimposed loads and vibrations by external effects

d) Stresses caused by temperature differences including transient conditions and by differences in coefficients of thermal expansion

e) Effects of solar radiation

NOTE Pressure stresses due to an internal electrical fault are not considered in the design of an enclosure since after such an occurrence the enclosure would be carefully checked and, if necessary, replaced.

For the case of arcing due to an internal fault, reference is made to HD 358 S2 (IEC 517 (1986) ed 2)

5.4 Design pressure

The design is based on the design pressure as defined in clause 3.5.

5.5 Design temperature

The selection of material and the determination of the design stress depend upon the highest wall

temperature which can be expected during service at the design pressure (p).

5.6 Design stress basis

The nominal design strength K shall be selected from the material standard, where

K = yield strength or 0,2 % proof stress at the design temperature.

(For austenitic steels the 1 % proof stress may be used)

The safety factor against yield strength, 0,2 % proof stress or 1 % proof stress, is S = 1,5 Hence it follows the permissible design stress B = K/1,5.

5.7 Calculation of shells, dished ends, openings, screws and bolts

For the purpose of calculation of shells, dished ends, bolts, screws and openings the following specific symbols are used:

di internal diameter of openings and branches mm

tA required wail thickness at openings mm

x distance over which the governing stress is assumed to act mm

h1 height of the straight flange of dished ends mm

EN 50068:1991

5.7.1 Cylindrical shells

The required wall thickness is:

The minimum permissible wall thickness of cylindrical shells is 3 mm

Pi loading of an area (A) with regard to internal pressure N

¶ design factor

n number of screws per flange joint Ps/Pi = —

clamping force per screw = Ps/n N

(Equation 1)

Ps´/P´i

P´

P´s

5.7.2 Spherical shells

The required wall thickness is:

The minimum permissible wall thickness of spherical shells is 3 mm

Figure 1 to Figure 4 — Typical connections between cylindrical and conical shells

(Equation 2)

thickness calculated according to 5.7.3.1 or 5.7.3.2 is to be taken into consideration For the shallow conical

shells with an angle of slope to the axis of the cone Î1 > 70° the wall thickness shall be determined according

to 5.7.3.3 even if smaller wall thicknesses as according to 5.7.3.1 and 5.7.3.2 are found.

In equations 3 and 7 the weld joint factor (v) refers to the circumferential joint and in equation 6 to the

longitudinal joint

If the distance between the circumferential joint and the knuckle is at least 0,5x then the equations 3 and 7

is the weld joint factor v = 1,0.

The minimum permissible wall thickness of conical shells is 3 mm

5.7.3.1 Calculation based on the stress in meridional direction

The required wall thickness is:

The design factor ¶ is to be taken from Table 2 or Figure 5 depending on the difference Ó between the angles of slope of two adjoining shells

and the ratio of the knuckle radius by the external diameter of the shell r/Da

A Shells with knuckle (Figure 1 and Figure 2)

If the wide end of a conical shell is flanged to a knuckle then the wall thickness in the knuckle shall be determined according to equation 3 and shall be maintained away from the knuckle in the conical section over a distance of at least

and along the cylindrical section over a distance of at least 0,5x.

B Shells without knuckle (Figure 4)

Conical shells may be connected with each other or with cylindrical shells by means of welded butt joints providing the following is met:

a) Ó k 30°;

b) joints welded from both sides;

c) the length of the two shells shall be at least 2x according to equation 5.

If deviating from b) the butt joints are to be welded from one side only, then equivalence with joints welded from both sides shall be demonstrated by a welding procedure test

(Equation 3)

(Equation 5)

Table 2 — Design factor ¶ for conical shells and numerical factors cos Î and 1/cos Î

The wall thickness for both shells at the butt joint shall be determined under consideration of the bending stress in the circumferential seam according to equation 3

Figure 5 — Design factor ¶ and numerical factor 1/cos Î

AngleÎ

resp. Ó

¶for a ratio ofr/Da

cos Î 0,01 0,02 0,03 0,04 0,06 0,08 0,10 0,15 0,2 0,3 0,4 0,5

1,21,72,23,35,18,010,7

1,21,62,03,04,77,29,5

1,11,41,82,64,06,07,7

1,11,31,72,43,55,37,0

1,11,21,62,23,24,96,3

1,11,11,41,92,84,25,4

1,11,11,31,82,53,74,8

1,11,11,11,42,02,73,1

1,11,11,11,11,41,72,0

1,11,11,11,11,11,11,1

0,9850,9400,8660,7070,5000,3420,259

1,0151,0641,1551,4142,0002,9203,861

1 cos Î

-EN 50068:1991

5.7.3.2 Calculation based on the stress in tangential direction

The required wall thickness is:

where the weld joint factor (v) is the efficiency of the longitudinal joint and the numerical factor 1/cos Î1

has been taken from Table 2 or Figure 5

The design diameter (Dk) is to be determined according to Figure 1 to Figure 4 and the distance x according

to equation 5

In the case of conical shells connected with each other the wall thickness shall be calculated for each shell with its individual angle of slope to the axis (Î1 or Î2) of the enclosure

5.7.3.3 Shallow conical shells (:1 > 70°)

The required wall thickness is:

If the distance between knuckle and circumferential joint is at least:

then the weld joint factor is v = 1,0.

Provided the requirements for shells without knuckle are met (see 5.7.3.1), conical shells with different

angles of slope may be joined by butt welds This requires a knuckle radius of r = 0 in equation 7.

5.7.4 Dished ends subject to internal pressure

C Hemispherical dished ends

Other configurations for dished ends may be used with appropriate calculation or proof test

Up to a wall thickness of 50 mm the height of the straight flange (h1) need not exceed 150 mm With hemispherical dished ends no straight flange is required

Shorter straight flanges (h1) are acceptable provided the circumferential joint between dished end and cylindrical shell is non-destructively tested to the same extent as a fully strengthened butt joint with a design stress level equal to the permissible design stress level.

If a dished end is welded together from crown and knuckle components, the joint shall be at a sufficient distance from the knuckle The distance shall be regarded to be sufficient, if in the case where

a) crown and knuckle are of different thickness (see Figure 8)

b) crown and knuckle are of equal thickness (see Figure 6)

x = 3,5t for dished ends of the Klöpper type

x = 3,0t for dished ends of the Korbbogen type

with a minimum however of at least 100 mm

NOTE When determining the transition from the knuckle to the crown of a dished end the starting point shall be the internal

diameter With thin-walled dished ends of the Klöpper type the transition is about 0,89Di, and with thin-walled dished ends of the

Korbbogen type it is about 0,86Di These factors get less as the wall thickness increases.

(Equation 12)

Figure 6 — Dished end

Figure 7 — Dished end with branch

Figure 8 — Dished end with knuckle and crown of unequal wall thickness

EN 50068:1991

5.7.4.2 Percentage of the permissible design stress level in joints

A weld joint factor v = 1,0 may be applied, if the extent of testing corresponds to that specified for a design

stress level equal to the permissible design stress level or if the dished end is made from one plate

A weld joint factor v = 1,0 may also be applied in the case of welded dished ends (with the exception

of hemispherical dished ends) regardless of the extent of testing provided the welded joint intersects

the crown area of 0,6Da (see Figure 10 and Figure 11) If the welded joint does not intersect the

crown area of 0,6Da, the weld joint factor is v = 0,75 or v = 1,0 according to the extent of testing

(see Figure 9 and Figure 12)

5.7.4.3 Weakening due to openings and branches (Figure 7)

Openings in the crown are of 0,6Da of dished ends of the Korbbogen and Klöpper type and in hemispherical

dished ends are to be checked for adequate reinforcement according to 5.7.5 without taking into account

the design factor ¶ In the case of pad reinforcement the pad shall not go beyond 0,8Da with dished ends of

the Klöpper type and 0,7Da with dished ends of the Korbbogen type

Figure 9 — Welded joint outside 0,6Da : v = 0,75 resp 1,0

Figure 10 — Welded joint inside 0,6Da : v = 1,0

For openings outside the crown area of 0,6Da increased design factors ¶ according to Figure 13 and

Figure 14 apply Where the ligament between adjacent openings is not entirely within 0,6Da then the minimum width of the ligament shall be at least equal to the sum of half the opening diameters

measured on the distance between the centres of the openings

5.7.4.4 Calculation rules

The required wall thickness of the crown is:

The required wall thickness of the knuckle is:

The design factors ¶ for dished ends (with or without openings and branches) are to be taken from;a) Figure 13 for dished ends of the Klöpper type

b) Figure 14 for dished ends of the Korbbogen type

as a function of t/Da and di/Da

Figure 11 — Welded dished end from crown and knuckle components inside 0,6Da : v = 1,0

Figure 12 — Welded dished end from crown and knuckle

components outside 0,6Da : v = 0,75 resp 1,0

(Equation 13)

(Equation 14)

EN 50068:1991

The curves with di/Da > 0 apply for reinforced openings in the whole crown and knuckle area The design factor ¶ = 1,1 applies regardless of the wall thickness to hemispherical dished ends in a distance of:

adjacent to the welded joint

With dished ends of the Klöpper type and Korbbogen type subject to internal pressure an additional check

is required to make sure whether the knuckle is adequately dimensioned to prevent elastic buckling (the

formation of folds in the knuckle) This is the case where the buckling pressure (pB) according to Figure 15

is equal or higher than 1,5 times the design pressure (p).

The minimum permissible wall thickness of dished ends is 3 mm

The stress distribution due to internal pressure in dished ends of the Korbbogen and Klöpper type is determined by means of the elastic analysis (see Annex A)

5.7.5 Openings in cylindrical, conical and spherical shells subject to internal pressure

5.7.5.1 General

These rules apply for circular openings in cylindrical, conical and spherical shells subject to internal pressure and for their compensation

These rules cover ratios of di/Da k 1,0

NOTE Experience has shown that in the case of great diameter ratios, stresses occur in the cross section perpendicular to the enclosure axis (gusset area).

The results available from research work in this field are not sufficient as to establish general design rules Generally, these stresses

do not result in failure as long as no creep occurs and damage by cyclic loading is not expected.

Additional external forces and moments are not covered by these rules and are therefore to be considered separately

For conical shells these rules apply only if the wall thickness is determined by the circumferential stress.These rules permit plastic deformations of up to 1 % at highly stressed local areas during pressure test Therefore openings shall be designed carefully Abrupt changes of geometry should be avoided

5.7.5.2 Compensation methods

A Increased wall thickness of the shell

Openings are reinforced by increasing the wall thickness of the shell (see Figure 16 and Figure 17)

B Local increase of the wall thickness

Openings are reinforced by means of a set-in or set-on ring or pad (see Figure 18 and Figure 19)

C Increased wall thickness of the branch

Openings are reinforced by increasing the wall thickness of the branch (see Figure 20 and Figure 21)

(Equation 15)

Figure 13 — Design factor ¶ for dished ends of the Klöpper type

Figure 14 — Design factor ¶ for dished ends of the Korbbogen type

Figure 16 — Increased thickness of a cylindrical shell

Figure 17 — Increased thickness of a conical shell

Figure 18 — Local increase by reinforcement rings

Figure 19 — Pad reinforcement

EN 50068:1991

5.7.5.3 Design and finish of openings

Openings are as far as possible to be arranged at a distance of at least x = 3t away from welded joints In

the case of openings in or adjacent to welded joints non-destructive testing of the welded joint in the area

of the opening shall be possible so that in case of doubt the soundness of the joint can be verified

Openings with sharp edges should be avoided

The throat thickness a) of a fillet weld of a pad reinforcement should be at least half of the pad

thickness (0,5h according to Figure 19) In the case of branch reinforcements the thickness of the

load bearing welded joint shall be at least equal to the thinner wall thickness of the connection

The materials of the shell to be reinforced and of the reinforcement itself shall be as far as possible of equal ductility When the design strength of the reinforcement is lower than the design strength of the shell this

shall be considered in the calculation according to 5.7.5.4 The inverse case shall not be applied in the

calculation

5.7.5.4 Weakenings

The weakening effect of openings is generally compensated for by the weakening factor (vA) For branches

perpendicular to the shell the factors vA given in Figure 27 and Figure 28 are adequately precise The wall

thickness tA in these figures is identical with the required wall thickness

NOTE The weakening factor presumes the ability of a structure to shake-down plastic deformations in the elastic range

Thin-walled shells with large openings do not match with this presumption due to their high membrane stress in relation to the bending stress.

If the nominal design strength (K) of the reinforcement is smaller than that of the shell to be reinforced

then for pad reinforcements the cross section of the reinforcement, and for nozzle reinforcements the

branch wall thickness, shall be reduced accordingly before determining the weakening factor vA

from Figure 27 and Figure 28

Figure 20 — Branch reinforcement

Figure 21 — Necked out opening

5.7.5.5 Calculation schemes

Openings may also be compensated according to the following relation:

which is based on the consideration of the equilibrium between the pressurized area and the load bearing cross section The wall thickness obtained from this relation shall be not less than that of the unpierced

shell For equation 16 the pressurized area Ap and the load bearing cross section AB shall be determined from Figure 22 to Figure 25 where

AB = AB0 + AB1 + AB2

The maximum length of the load bearing cross section to be considered in the calculation shall not exceed

b according to equation 18 for shells and ls according to equation 20 for branches The protrusion of a

branch may be considered in the calculation only up to a length of k 0,5 ls

The requirements specified for pad reinforcements and for reinforcements by increased wall thickness of the branch should be observed

If the nominal design strength K1 or K2 of the reinforcement is lower than the nominal design strength K

of the shell to be reinforced, then the dimensions shall comply with:

It should be pointed out that the required wall thickness tA is to be determined by iteration

(Equation 16)

(Equation 17)

Figure 22 — Calculation scheme for cylindrical shells

l´s

EN 50068:1991

A Increased wall thickness of the shell

The weakening factor vA for openings according to Figure 16 and Figure 17 shall be taken from the

lower curve of the Figure 27 and Figure 28 (ts/tA = 0) depending on the ratio:

Figure 23 — Calculation scheme for spherical shells

Figure 24 — Calculation scheme for adjacent branches in

longitudinal direction of a cylinder

B Pad reinforcement

If the wall thickness (t) of the cylinder or sphere is lower than the required wall thickness tA at the

opening, then the opening is adequately compensated, if the wall thickness tA is available round the opening over a width of:

In the calculation the wall thickness tA may be taken into account only up to twice the wall thickness (t)

of the shell The thickness (h) of the external pad reinforcement shall therefore not exceed the value t,

(except of set-in reinforcement rings) Internal pad reinforcement should be avoided as far as possible

The width (b) of the pad reinforcement (see Figure 19) may be reduced to b1 provided the thickness of

the pad (h) is increased to (h1) according to:

b1 · h1 U b · h

The limits laid down above should however be observed

C Increased wall thickness of the branch

For reinforcements according to Figure 20 and Figure 21 the weakening factor vA shall be taken

from Figure 27 and Figure 28 depending on the ratio of the wall thickness ts/tA and the related opening diameter:

The wall thickness of protruding branches according to Figure 20 (type c) may be reduced by 20 %

provided the protrusion is U ts (see Figure 26)

The ratio of the wall thicknesses should be

The required length ls of the branch is:

The length of the branch may be reduced to ls1 provided the wall thickness ts of the branch is increased

to ts1 according to:

but taking into account also equation 19

Figure 25 — Calculation scheme for adjacent branches in

a sphere or in circumferential direction of a cylinder

EN 50068:1991

D Increased wall thickness of the shell and of the branch

Pad and branch reinforcements may be applied in combination for the compensation of openings (see Figure 26) For the calculation of this kind of reinforcement the requirements specified in B and C shall be applied together

Branches or sockets in series joined to the shell by fully penetrated welds with wall thicknesses calculated for internal pressure only may be designed in a simpler manner with a weakening factor determined by:

where:

L = centre distance of branches or sockets

For branches or sockets joined to the shell by partially penetrated welds di should be replaced by da in equation 23

5.7.6 Screws and bolts in circular flange joints

5.7.6.1 General

These rules apply for the design and manufacture of steel screws for such enclosure flange joints which are not subjected to significant extra forces due to connected piping This category contains screws and bolts for inspection hole covers, blind flanges, removable covers and flange joints

These rules do not apply to pipe flanges where the screws are subjected to higher loads; nor do they concern screws and bolts for manhole covers and the like where the principal sealing force is the pressure acting on the cover The general provisions are, however, applicable to such screws and bolts as well

5.7.6.2 Design rules for circular flanges

The loading of the sealed area with regard to internal pressure only shall be determined by:

Figure 27 — Weakening factor vA for openings and branches

perpendicular to cylindrical and conical shells

EN 50068:1991

Figure 28 — Weakening factor vA for openings and branches perpendicular to spherical shells

The total screw clamping required for satisfactory sealing is determined by:

For flanges with O-ring seals m = 1,2 may be used.

The force on each screw or bolt is:

where:

n = number of screws per flange joint

In determining the screw forces differences in the coefficients of thermal expansion and differences in temperature between flange and screws or bolts shall be taken into account

5.7.6.3 Calculation of screws and bolts

The design area (Ab) of the screw thread is determined by:

With the safety factor S = 1,5 against yielding of bolt material this equation becomes:

If the screw or bolt temperature can be established mathematically or empirically, this temperature shall

be the design temperature However, the minimum design temperature shall be 20 °C even if the

calculation gives a lower value

Figure 29 — Circular flange joint

(Equation 25)

(Equation 26)

(Equation 27)

(Equation 28)or

EN 50068:1991

5.8 Calculation of integral flanges and flat circuit covers

For the purpose of calculation of integral flanges and flat circuit covers the following specific symbols are used:

c

D3 diameter of the section through the flange where the radial bending stress is

M1 bending moment per unit length of the mean circumference at junction between

M2 radial bending moment per unit length in flanges N·mmtotal radial bending moment in flanges N·mm

M3 bending moment per unit length of the mean circumference at junction between

Pa total axial force by internal pressure acting on shells or hubs N

Pp total force by internal pressure acting on the annual area inside the gasket N

r fillet radius at junction between flange and shell or hub mm

5.8.1 General

These rules deal with circular flanges permanently attached to cylindrical shells The flange is assumed to

be made in one piece with the shell or joined to it by welding The flange may be joined directly to the shell

or via a hub which may be tapered or have constant wall thickness

Figure 30 shows without sealing details examples of some different flange types which shall not be considered to be standard

Thus the flange in Figure 30 a) may have a hub of the same wall thickness as the shell and be welded to the shell In Figure 30 d) the flange itself may be welded to the hub

The following rules may also be used to design flange joints to dished ends

5.8.2 Flanges with flat sealing surface and O-ring seal (Figure 31)

The total bending moment (M) of the external forces on the flange shall be calculated according to:

where Pa and Pp are obtained from the equations 31 and 32

NOTE This calculation is based on the assumption that the cross section of the O-ring is smaller than that of the groove.

Figure 30 — Examples of different flange types

t3 maximum wall thickness of tapered hubs mm

Ba axial stress by internal pressure acting on shells or hubs N/mm2

Bb bending stress in shells, hubs or flanges N/mm2

B1 total maximum stress in shells or hubs at junction with flanges N/mm2

B3 total maximum stress in shells at junction with hubs N/mm2

EN 50068:1991

(Equation 31)(Equation 32)

Figure 31 — Forces and moment arms for flanges with O-ring seal

Figure 32 — Hubless flange sections for stress calculation

5.8.3 Flanges with long untapered hub (Figure 33)

These rules also deal with hubless flanges joined to cylindrical shells as shown in Figure 32

The term “long untapered hub” refers to a hub where:

with

and

A Stress in section I-I

The bending moment M1 in section I-I per unit length of the circumference at diameter Dm1 for hubless

flanges of Dm2 respectively for flanges with long untapered hub is calculated according to:

with

where Ù is to be replaced by the corresponding subscript (1 or 2)

The factor A is to be taken as a function of K1 and K2 from Table 3 or from Figure 34 The values of K1and K2 are calculated according to:

The logarithm ln of the numeral m is to be taken from Table 4 or from Figure 35 as a function

EN 50068:1991

When m = Dy/Di k 1,5 then the value of ln m can be calculated with great accuracy by means of:

NOTE The equation 40 is also to be used for values of m = Dy/Di approaching 2,5 without great error in the bending moment M1.

The factor A is always less than 1 The figures in Table 3 are therefore decimals; 95 means 0,95

and 096 means 0,096

NOTE The values of the factor A in Table 3 and in Figure 34 apply provided that the flanges have screw holes; an average weakening factor of 0,8 has been inserted If exceptionally there are no screw holes (e.g clamp rings), K2 determined by

equation 39 is to be multiplied with 1,25 to compensate for the weakening factor.

The total maximum stress B1 in section I-I as the sum of the bending stress through the moment M1

and the axial tensile stress through the internal pressure (p) is obtained as follows:

with

and

where:

tE = wall thickness in section I-I of the shell or hub

5.8.4 Flanges with short untapered hub ( Figure 33)

The term “short untapered hub” refers to a hub where:

with ¶2 according to equation 34

A Stress in section I-I

The bending moment M1 is calculated according to equation 36 for a hubless flange, i.e one joined

directly to the shell having the wall thickness t1 This value of M1 is signified by M11

Then bending moment M1 is calculated according to equation 36 for flange with long untapered

hub having the wall thickness t2 This value of M1 is signified by M12

The bending moment M1 in section I-I per unit length of the circumference is calculated

with M11and M12 according to:

The total maximum stress B1 in section I-I is given by equation 41 with the wall thickness t2

æ ö = ln 1 c( + ) = c

1 c/2+ -

Bb1=6 M1/t2E