ZJE - 65 1970

J. Koebto, J. JMbtiyr

PRETIGHTENING OF LARGE PRESSURE VESSEL FLANGE JOINTS

$KODA Concern ИЩЕМ POWER PUNTS DIVISION, NffOMUTION CENTRE

PIZE* - CZECHOSLOVAKIA

62l.039*536o2 ZJE - ф 621,772 1 9 7 0

Jo Kuohta, J» Šnobcrger

PfíSTIOHTEHIliG OF URGE PHSSSUHE VJSSSEL ПАКОВ JOINTS

SKODA concern

WJCLSAR РДОВЙ PLANTS DIVISION, PLZSÍÍ - CZECHOSLOVAKIA

DíFORiíATICK С 8 Ш В

J, Kuchta, Je Šneberger

PRiSTICanEKISG OF LARGE PI&SSUHE VESSUL If'UK&S JOilíTS

SUKKAKT

Pr-e tightening of Large Pressure Vessel Flange Joints

Calculationss design of auxiliary devices and certain

experiments during the solving of the pretightening of free

flange joint with 5500 mm pitch diameter and 42 bolts with

200 mm thread diameter and 4000 mm in length, have been

carried out»

The angles of rotation of the nuts for the method cf

cimultaneous pretightening of several bolts under the condition

of equal final forces in all bolts were established©

The bolts w«re stressed by means ox three hydraulic jacks

and in other experiments by means of thermal preheating^

Pretightening and releasing of the bolts in the actual

flange joint was checked by the detailed strain gauge measurement

of each bolt* This experiment was repeated several times with

mild rubber and also with metal sealing between the flanges*

Measured values are compared to theoretical solution*

~ 4 -

i.i£П ух s/mbol

В. ~ r a l i a s of t h e centre, of g r a v i t y of t.Jie fre-r : iang< С""*

5 • • radius of the centre of gravity of the head/shell fiangr-

e - lever агтп of the bolt ioroe h « free flange height b - free flange width (3 er.iric angle between two bolts (£ « half--angle cf a section

m - number of ,iacks

n ~ number of bolts Reined to one .jack

d+ ~ width of the bearing sarfac J index the nut

P *- belt cross - section

1+ - bolt effective 1-wtb V .-.lu..iii»g washers

a - ^iJ r, CJ. ojLie bolt thread

fci - elongation cf the end of the bolt IT— i 11 О П »» 3Ci - flange flexibility under the bolt Й— i v. У - radial deformation under the bolt 3^ i _r

clt ft

w- - free flange axial compression unaer the bolt Ы— i ш. « flange rotation ?ngle under the belt u— i M <-• single torsional moment J"k - moment of inertia in torsion of the weak.end flange ^y^z moments of inertia of the weakened flange to the

axes y, z Jg " moment of inertia of the head /shell flange to the

axis ¥ G - shear- modulus

Б - nodulus of elasticity

<-д "^ ъ-а

le Introduction

Pressure vessel flange joints belong to original design elements0 Producing of prestressing in these flange joint did not represent any troubles up to this time and it can be said it has been meastered well* However, the rapid development in nuclear pressure vessels has drawn the designer s attention to this design task again0

New effective pretightening processes are searched which would satisfy the challenging requirements on reliability and uniformity of prestressing with minimum consumption of time ani costs0

The flange joints of the first Czechoslovak HWR full scale model pressure vessel is the typical example of such a large joint о There was our task to solve the pretightening device in principle and to propose the suitable pretightening process*

2 0 Model vessel flange .joint

Main dimensions of this joint are shown in Fig* Z.0 The joint consists of two free flanges which are tightened by 42 bolts, each bolt having two main threads M 210 x 6 mm and one auxiliary thread M 180 x 6 mm* A hole of 40 mm dia* has been drilled through the whole length of each bolt»

Both upper and lower nuts are designed so that the tensile stress along their profile with uniform distribution of stress on each thread may be reached*, Both nuts are set up on spherical washers which exclude coarse additional bending stresses in bolts*

" O '•'-*

Рог hydrostatic testing pressure 8.5 kp/cm the pres stres sing force 4-75 Mp in 1 bolt and minimum dispersion of individual bolt fn-pcss round the mean level are required0

The pretightening is complicated дет»е by the lack of spaces e0 go the distance oetween two neighbouring nuts were 25 mm only0

5o Pretignt-ening device

Jr. principle the task can be solved by three wayss a/ mechanical pretightening b/ thermal pretightening e/ hydraulic pretightening

All these ways were appreciated from various points of vieVu

a/ Mechanical pretightening « Here the p.restressing with loaded nut by means of torsional moment is meant<> The main problem is rooted not in the design of the needed torque device but before all how to prevent the bolt not to be twisted by the memento

The further disadvantage lies in the fact that for the specified torsional moment the axial bolu force depends on the lead angle and on the fricition coefficients between threads nad between nuts andwashers0 These coefficients are varible and depend on a lot of factors, before all on the change of specific pressure between frictional surfaces* High-class quality and precise shop tolerances of these surf aces are needed and thus the question of costs comes forward,,

Besides, the dependence of axial force in bolt on the

fidction coefficients results in considerable dispersion

of £4;-::ces round the nean level and if stronb uniformity is

required, the neasurement of force in each bolt will be

аесевяагто Thus, this way is not acceptable for pretightening

of large flange Joints aid has not been applied to our fall scale model» b) thermal pretigfrfceaing «_ The requested elongation is

achieved by thermal expansion of the heated bolt* It is necessary tnat the пез&гиш temperature of the preheated bolt should not reach che point when the mechanical properties of bolt material could changec

Besidess with prolongation-of the time of preheating9 the threaded part is sore and more preheated and much attention for the thread shop tolerances must be paid* On the other hand, id о thermal preheat pretightening

is very simple, the main a:\vanta.ntage lying in possibility to get the desired elr»ngat; ya in all b- lts at once* Therefore9

this way has boan also included into our test program» HoweverSJ

the detailed results are not presented in this article© Concisely the main conclusions:

If the uniform force level is to be attained, it is necessary that all starting passive gaps in the joint should be eliminated*. The gap elimination in jointfc with two free flanges can be guaranted only when certain starting force level is present о We found out that variable starting fore* level 5 to 10% of full progressing would be needad for ensuring the neeo^ary rt motal * to metal1* contact for all parts of the joint* It i.z evident that under these circumsta»e#i the thermal pretigh'^iin? process o ^ o t be used at all*

ta J5 **

t>/ Hydraulic nretigfotening - Needed elongation is generated by polling the bolt mechanically by means of hydraulic jackg Two principles in the jack design, may be useds the cylinder with a plunger or the closed annular membrane«> We think that problem ox sealing of i he expected large pressures is the decisive о While with the cylinder the problem of sealing of large dimensions is the question,

• with the membrane this problem is solved automatically о

A hydraulic membrane jack has been developed in «

£ВЭМ«=#огкя, see Figo 2* The jack consists of a franie with formed "bed" for the membrane, upper supporting matrix^ pulling head, nut rotation device, hand pump and a special annulus membrane welded from two pieces0

During the pumping the membrane rest partly upon the frame, partly through the supporting matrix upon the pulling head© The pulling head is screwed on the auxiliary thread of the bolto The rotation of the free nut. is performed by a simple device, eonsisting of hand-powered little gear, which drives by means of Galls chain the gear system on the upper edge of the nutí» There is a no~ niue on the little gear on which the exact nut rotation

angle is read0

With regard to production costs it was decided to use

3 á&cks6 Thusj each jack operates consecutively on 14

bolts* la Pigo 3 there is a general view.of flange joint

with jacks prepared to the pretighteningo

4* Determination of nut rotation angles

The appointment of nut rotation angles for consecutive

hydraulic pretigxtening process will be more difficult

than in the cast of simultaneous thermal pretightening

process*

u 4 t»

To calculate needed nut rotation angles we applied a

^eaeral method /L l/e

A simplified supposition that the joint is axially

symmetric is madea Further, flange and bolt mat ©rials are

homogeneous, isotropic, the Hook's law holds, strains are

negligible in comparison to the flange dimensions, the

contact of bearing surfaces is perfect and stablea Besides9

zexo starting force level is considered*

In accordance to the total number of bolts the number

"m" of jacks is appointed so that equal number of bolts

"n*1 is joined to each ,jack

All flanges are considered to be elastic rings* Free

flanges are deformed in such a manner that their cross-

sections are rotated, owing to axial forces are compressed

and owing to radial reactions are as well radially def ormed*

Head and shell flanges on the contrary are not either rotated

nor axially compressed - only their radial deformations are

considered»

The uniform support in the flange contact point is

substituted by a series of imaginary single supports9 nubmer

of which equals number of bolts, .i. e« m x n» The angle

between supports equals (h , i» e* is the same as between the bolts»

The applied forces are represented by elongations Q£

bolt end |ги> -Ц* The designation of dimensions is in the Fig0 4* The free flange, having the mean radius Rf rotates owing to single moments Mf relative angle of which equals

The angle of flange rotation at bolt ifi- i i

vi _ ц .3ČL m s -2EL - Y 7ti *

- 10 -where #iis the flange flexibility

*

The unit radial reactions I call forth a radial deformation of the ceaterline in the place of single supports. This ra­dial deformation of the fres flange is espressed Ъу

, . _ s 3 пь4

j-o and of the head /shell flange by

.5 m-4 ^-тЬг^*^-21»1) f «bere

Ф Н - - b - s t í -J-II-) *Ь£]*е* ••) +-fc[T- (•••)! *Ч~) The foregoing formulas are derived in /L 2/.

The unit bolt forces S are transmitted to the flange by the nut and cause the axial compression of the flange, amount of nhish is derived in /L 3/t

Belative strain of a bolt having the length 1 caused by the unit force is . . '

In the beginnig of the pretightening procese the nuts touch the upper surface of the free flange. Sjr preetreeeiag of the first group of the bolts (here H°~lt 15, 29) about & , the

- il

forces A<S, andA4X* to Л ^ arise /Pigu 5 Л The other-bolts are not pretightened and therefore it holds

ь&~ -•• - д45п*0

From the condition of equal ax ia l displacements a t the point Ax i t follows

For rad ia l displacements of the point B^ i t holds

-ДА -JL*. **4Y4 [*****ФЯ+ Oí [v^vn^lif]

Similar deformation conditions may be arranged also for points B^/i к 2, 5, о о о п/« Зу the pretightening of the second group of bolts /here U^ 2, 16, 30/the forces ^ ^ 2 and' also До Si arise9 because the bolts in the first group are already

prestressed© The products A«S3* ^ZS4» »«»>^»} Д ^ п are equal to zero again0 Zhus9 by consecutive presstressing of next groups of bolts, axial and radial conditions of deformation for points L f L f 1 г 1 to a can be arranged* Introducing signs AjSi - *% f j and Aj*i s €fi& we can arrange a system of equations, which corresponds to the unit prestressing of bolts J& * j n in the first cycle

- 12 -

( nn) £nj -U044

-ь^ч ^[т^+^Р+м5 у *<jf*og*n)

The significance of the individual matrices

[tWj^Hl'

W • • • » • • • .

г»ч-п*ч*«|(ЙГ

(«ni)» >4

^i К. «i at: .

• • • • • • • •

-13

(*j»)>

• -, *пм

3t, ^ 31* у &

*lH •*!-*! *«-*!•• -,1Сп.т

The sign í<j+fi) tj+n) represents a vector, components of which

are equal to zero except for U°- ( j + n) which is equal to

lo Solving this system for j ~ 1 to n the elements &j for 3 £ř i besides elements £"j can be found© The coefficients čij represent a change of radial reactions daring the prestressing of 3-bolts about tj » 1 cm and have no immediate use© The coefficients oij represent a change of the axi-al force Si in i-bolta during the prestressing of j-bolts about & s 1 cnio Because for j < i the coefficients

$ij -s 0, it is possible to arrange a triangular matrix (iijj from elements Sij for the first cycle*

Solving the system

the bolt elongations |$ are determined^ Ingles u^, which are neceasar for caching the requested forces {^ in all bolts at the end of the first pretightening cyele, av* established!

щ *• Jm> Ф Зв©

- 14 -

The force in i-bolts S. after j-bolts prestressing can be established from formula

This force can reach temporary a value which is higher than final forces Ri •

In the second and next cycles already prestressed bolts are pretightened. This means that oij Ф 0 for ,j i and the matrix for calculation of ^ is square • In horizontal lints of this matrix however the fame elements repeat itself, for which it holds o ^ = e - at the same time, so that the second and further lines can be obtained by the cyclic replacement of the elements in first line* This line has already been established in the I cycle»

Solving the system

the values of }L and 0^ for the second, and if need be also for next cycles | may be obtained» These second and next cycles used to be complementary to the first cycle in order that the reverted force level may be reached. The forces £,£ «ad P2i etc may be either constant or have different magnitude in each bolt.

о» 15«»

^о Application

The described method was used twice at tne opportunity of pretightening of the full scale model vessel*

For calculation of the nut rotation angles it was very important to know the magnitude of the bolt force decrease after relieving the oil pressure in jack* This decrease8 called jack~effect, was experimentally investigated in special testing facility with a single bolt*, The bolt was prestressed to a certain level? the nut having been rotated up to the end by little torque 100 kpm and then the pulling force (the oil pressure) relieved» Such a cycle was repeated on a higher iorceo

In Fig о б the bolt force when the pressure is present and the force after pressure relieving is plotted vse jack

4

oil pressure*. It can be seen that the jack-effect rate was 36% at first and finally stabilized itself on the rate about 12%o Similiar history of the jack-effect; was found in course of the second testing cycle, see Figo 7-

The supposition can be expressed that such a force decrease can take place only in the threaded part of the bolt о The first-usually large-decrease is obviously caused by plastic strains the thread roots and by adapting of bearing surfaces,, In successive strain hardening the effect of plastic strains deereasef and the jack-effect is infuenced by the elastic strain only0 The jack-effect may be inf uenced also by the special farm Of the nut profile* The nuts with variable cross-section ere доге flexible in radial direction than other common nuts*

We x-éepee wtd the jack-effect in real pretightening process by enhancement od demanded force level about 15&»

In order to veri fy the whole applied p*etághteaing Ше<я?7*

- 16 -the forces in all bolts were measured by means of electric strain gauges.

6. Flange joint first pretightening

Starting conditions for first pretightening were very-complicated. The head was not sealed by the metal compensator, which is shown in Pig. 1, but by a auxiliary ring with fourd rubbe 0- rings, Unfortunately the diameter of these 0-rings was not 0 9 mm as prescribed but due to a mistake during the assembly 0 10 mm Q-ring had been used. Thus, the passive gaps in the flange joint were exceedingly large*

In order to ensure the metal-to metal contact a very high mean starting force level was necessary, see Fig* 8 and 9« These forces were introduced into equations as starting forces in individual bolts. Now the calculated, t* e» theoretical with 15% enhancement, angles were applied to the nuts* Desired préBtrěssing of 475 was to be set up in each bolt and the pretightening was to be finished. As a result an essentially lower force level was reached and on the tcp of it rather waved.

Because the reached level was considerably lower than 475 Mp, it was decided to perform next two pretightening cycles. Nut rotation angles were determined by a quite simple proportional law» The increase of forces in bolts N« 1, 151 29 due to 1° nut rotation angle were measured and according to these sensitivities the angles for other remaining 1£ bolts of the group were established so that the mean level Ш> Mp at the ©nd of the third cycle and about 500 Цр ftt'-"th#::ead of the fourth cycle might be reached.

Three groups of reasons which caused the discrepancy in theory tb m^aeuremeut comparison may be seen in detailed analysist

us Цр ^ a) Jack- effect underratinge - It is obvious that not the

stabilized magnitude of 15% ought to have used but the higher staring magnitude corresponding to the jack --effect of new hitherto not loaded bolts*

b) bffect of inequality in starting force levele - Seme local variations of forces in individual bolts in the I cycle ( 0-55 Mp) cannot be placed to debit of the theory but were caused by rough irregularities in staring force level© Actually first level was established by consecutive application of 100 kpm torque to all nuts* It is obvious that this moment was not sufficient for elimination of oil gapsG With this experience the better starting level was reached in the II model pretightening as it will be shown in the next paragraph0

c) Theory imperfections e - The rigidity characterized by coefficients ojí proved to be an important factor*» In

st the calculation of the I cycle, when consecutively the

number of prestressed bolts increases, no periodic

relations hold between this coeficients (triangular

matrix)e Only when bolt3 H& " n *• are tightened we receive

theoretical coefficients d«i tOon »&wi which repeat periodically in each bolt and are the same for

all next cycles«

Graphic representation of the coefficients and also

of real coefficients in all 4 cycles taken from the 3 rd

group of bolts (bolts 2?£ 29v 4a) is shown in Fig* 10*

From this figure a certain consequent increase of rigidity

after each cycle may be seen with the trend to a constant

value»

The above - mentioned jack-effect of new bolte takes a

- IB -st

certain part in a low flange joint rigidity during the I and

II u d pretightening cycle. But this is not the main cause. When

investigation of the permanent rubber sealing deformations and

the analysis of flange displacements have been msde?, the

incorrect diameter of rubber O-rings seems to be the main cause

of the rigidity change. These rings influenced the flange

rigidity till their deformation was constant. Considering that

a change of 0,1 mm in bolt elongation represents as much as

20 Mp in the bolt force and comparing this sensitivity to the

whole axial oisplacements of the flange joint, the above

presented elucidation can. be regarded as to be justified^

The rigidity coefficients «ij are derived from theoretical

dependence between forces and deformations in specified flange

joint. It has been found that share of individual joint

members in the summary rigidity and therefore in the nut

rotation angles is not respected well by the theory. While

the theory supposes rather high increase in bolt forces after

bolt N— " n" pret lightening, in reality very slight increase

and even some decrease occurs in many bolts, see Fig« 10»

The imperfections in theoretical presumptions lead to the st occurence of "waves" in calculated angles of the I cycle and

consequently, like á "copy*, to the occurence or Waved force

level.

It may be stated that the cause of this disagreement

does not lie in composition of the final system of equations,

but already in composition of the formulas for Щ^{^,Щ • We had not the opportunity up to this time to rectify these formulas with respect to measured relations in the real flangt joint* In oar case it was necessary to find quickly the more truthful way of calculation of the angles for our model vessel For this purpose a few variants of calculation was performed. We have found: 1, 2?he more convenient flange joint rigidity can be reached

w-. 19 " if longer bolts are taken into aocouo.t - in our case about 20>So 2о The right angles oan be reached if the requested for^e*

are enhanced by a rate 1^5 /áack-sffect included/*

Tbsr* xii'vledge has been considered in the next pretigb-

tenins °£ 'ЬЬэ mccU-Л vessel flange joint о This second pretigh-tening worked wit:: deformed threads on bolts and nuts and vras performed th:- vessel э?«а sealed by a metal compensating

7^ Flange joint second pretightening

Before this pretightening has been initiated9 full elimination of all gaps and consequently the uniform starting force level was reached by iollov/ing ways 1) Six bolts regularly round the perimeter were prestressed

to the force of 2>0 Mp* 2) Remaining 36 nuts were rotated by the torque of 100 kpm«

3) Six prestressed bolts were relieved and their nuts were rotated by 100 kpmc •The starting force level reached by this way is shown

in Figo 11 о A certain non~>unif ormity may be seen especially in the vicinity of six prestressing bolts« But such differences are very small, are fully defined and may be respected in the proper calculation»

We decided to get the requested level 475 Цр la the course of one pretightening cycle» With respect to experiences gained in the first pretigjitening the needed angles were calculated* They are shown graphically in Fig* 12 bearing the sign CYCLE Ie

- 20 -

The forces, measured when these angles were applied,

are plotted in Fig. 11. At the first sight a considerable

dispersion of forces is seen, the forces being spread in

5 waves regularly on the perimeter. The mean level 440

Mp was reached.

Such results must be indicated as unsatisfying. It is obvious that corrections made up to this time, i. é.

taking longer bolts and higher requested force level into

accounts and neither the gap elimination nor the uniform

starting force level, do not guarantee the right pretigh-

tening in course of one cycle. For all that, we think

that all imperfections can be supressed. Before all, one

must insist on the elimination of all gaps and on the

attaiDment of uniform starting force level. It is necessary

to exceed the fictitious bolt length even more and to

enhance the requested mean force level in the joint.

The force dispersion and typical waves round the mean

level could be suppressed by considering the proper

coefficients Sij as shown in Fig. 13. In this figure

coefficients measured during the joint second pretigh-

tening and theoretical coefficients after the corrected

theory are denoted. It seems that all positive coefficients

oij will have to be neglected during the calculation,

( i. e. to presume that prestressing of a specified bolt

has no influence on remote bolts) and also that real

influence on near-by bolts must be well known.

The joint second pretightening was finished by next

cycle, in whie- the great force dispersion from the st I cycle ou0 to have been rectified. During this rectifying cycle the force level 510 Mp has been reached, see Fig. 11.

._» 21 v*

8; Gonclasions

The application of the described method allowed to point

oat the possi&lity to pretighten the large flange joint in

one cycle by means of a three hydraulic jacks0 Here the angles

of nut rotation are determined theoretically in advance on

the oasis of material and geometrical bh»J:({kt&Tf.št±;~ of the

flange joint.

This process$ which has been used the first time In

fact, was compared to real experiment and, in spite of all

imperfections, has been found to be very promising* We are

convinced that it is quite real to find right relations

between forces and deformations and thus to adapt this method

rJ ;he general - purpose useо We believe we shall scon have enough of possibility to

devote ourselves to this interesting method*

References /L - 1/ - Je Dvořák: Rozloženi sil ve šroubech velkých pří­

rubových spojů*/Force distribution in large flange joint bolts/* Strojírenství 11, H- 8, 1961 - SHTL Erague

'TJ - 2/ - Biezeno-Grammels (Dechnische Dynamik « Springer j Berlin 1939

/L « 3/ « Papkovičt Teorija uprugosti /ÍDheory of elasticity/ Moscow 1939

Fig, 1 — Modal prt»aur# vessel flang» joint

- 23 -

fig». 2 - Hydraulic M A I M I jack deeijn. 1 - IMM 2* eeekrAa» J- ufftr tuppertiaf

ж tri к 4- pvlHftf bft*4 5» gear pat on the mrt

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