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Z.F.Sang

L.Li

Y. J. Zhou

Department ofMechanical Engineering,Nanjing University of Chemical Technology,

Nanjing, P.R.China

G. E.0.WideraDepartment ofMechanicaland

Industrial Engineering,Marquette University,

P.O. Box 1881Milwaukee, Wl53201-1881

Effectof GapBetweenPadand Vessel forMoment Loadingon NozzleThe purpose of this paper is topresent astudy ofthe effect of a geometricgap

between the cylindrical shell and reinforcement pad on the local stresses in the areaof the intersection when the nozzle is subjected to moment loading. Experimental and

finite element analyses were performed on two test vessels (four nozzles). A compara-tive study of stresses in the intersection region for different geometric gaps was alsocarried out.

Introduction

A pad-type reinforcement structure is an important type of

local reinforcement inpressure vessels and piping connections because of itssimple form, convenient manufacture,low cost, and rich application experience. As aresult, it is

widely used forapplications with low ormedium pressure,especially those with small fluctuations inpressure and temperature. For the design of the reinforcement pad, the currentdesign code (see, for example, ASME, 1989) statesaspecific

rule. However, only the design method isgiven; it does notprovide the technique for calculating the stresses inthe reinforcement region. Asaresult, the distribution and the magni

tudesofthe local stresses induced by the geometric disconti nuity and the loading due to this reinforcement is not known.As forloads, the design code only considers theeffectofinternal pressure. But, infact, such loadings asaxial thrust,

moments, twist, and shear on the nozzle are usually applied topressure vessels, process equipment, orpiping connections.Besides, designers require that manufacturers keep aperfect

contact between the reinforcement pad and cylinder duringthe fabrication of the vessel toensure that it has the abilityto carry the various loadings as required. But, for a varietyof reasons, perfect contact cannot be kept between cylinder

and pad and, thus,agap results. The effect of the gap on thestresses in thenozzle-reinforcement region is a matterofcommon interest toboth designers and manufacturers.

The stress analysisofthe reinforcement pad structure represents acomplicated prob lem. Research ismostly focused

on the analysis ofspherical shells with radial nozzles. Soli-man and Gill (1978) and Oikawa and Oka (1987) carriedout research on the reinforcement pad structureofsuch shellsby both theoretical and finite element (FEM) analyses. Other

researchers (Chao etal., 1986) also presented valuablere-sults for thedesign andanalysis of the reinforcementpadstructure. Bulletins WRC 107 (1979) and WRC 297 (1984)

are the two most authoritative design documents foranalyzing and calculating the local stresses inthe vessel opening-nozzle region under theaction of external loads, and arewidely used. However, they do notpresent specific design

Contributed by the Pressure Vessels and Piping Division and presented atthe

Pressure Vessels and Piping Conference, Orlando, Florida, July 27-31, 1997,ofTHE AMERICAN SOCIETY OF MECHANICAL ENGINEERS .Manuscript received by the

PVP Division, May 28, 1996; revised manuscript received July 29, 1998. Associ

ate Technical Editor:R.Seshadri.

Fig. 1 Configuration of test vessel No.2

methods when there exists areinforcement structure at thenozzle intersection.

This paper presentsadetailed study of the effect of a geometric gap between the cylindrical shell with radial nozzle and the

reinforcement pad on the local stresses in the area of the nozzle-cylinder-pad intersection under the action of nozzle moments(longitudinal and transverse). The study involves both anexperimental approach aswell as a finite element analysis.Acomparative study of thevariation andmagnitudes of thestresses in the intersection region with different geometric gapsis also carried out.

Journal of Pressure Vessel Technology Copyright1999 by ASME MAY 1999, Vol. 121 / 225

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Table 1 Structural dimensions of the test vessels

Vi-ssel

| N.

D

mm

L

mm mm

T

mm D.T

Nozzle

No.

(1

mm

t

mm

1

mmT.

mm mm d.D

t./

T

CiP 9

mm

No.1

500 800 400 8 62.5

1 100 4 170 0.2 0.5 Without

padNo.1

500 800 400 8 62.5 2 100 4 170 8 200 0.2 0.5 0

No.2

500 800 400 8 62.5

3 100 4 170 8 200 0.2 0.5 2No.2

500 800 400 8 62.5

4 100 4 170 8 200 0.2 0.5 4

Experimental Details

Test Vessels. Two specially designed and manufacturedtest vessels (No. 1 and No. 2) were used in the experiments.The configuration of vessel No. 2 is shown in Fig. 1. Eachvessel has two diametrically opposite, protruding radial nozzlesof the same size. One of the nozzles of vessel No. 1 was notreinforced, while the other was reinforced with a standard reinforcement pad; there was no geometric gap between the cylindrical shell and reinforcement pad. The two nozzles of vesselNo. 2 were reinforced by reinforcement pads of the same size.The geometric gaps between the cylindrical shell and thesereinforcement pads were 2 and 4 mm, respectively. The specific

structural dimensions of the two test vessels are listed in Table1 and shown in Fig. 2.

Chemical Composition and Properties of the Materials

Used in the Test Vessels. The materials used for each component of the two test vessels are identical. The materials, employed, their chemical composition and mechanical propertiesare summarized in Table 2.

Local Structure of Reinforcement Region. Protrudingnozzle structures with inside and outside fillet welds were usedfor the four nozzles of the two test vessels. The reinforcementpads were located on the outer surface of the cylinders. Thelocal structures and gaps in the reinforcement regions are shownin Fig. 2. In order to keep the gaps uniform during fabrication,four spacers of 2 mm and 4 mm thickness were put under the

pads and arranged evenly around the circumference of the pad;the pads were then welded.

Test Method and Procedure. The electrical resistancestrain gage measurement method was used during the test phaseof the study. The strain gages were installed in the longitudinal(8 = 0-180 deg) and transverse (8 = 90-270 deg) sectionsof the cylinders and nozzles. The gages at each measuring pointconsisted of 90 deg biaxial gages. The locations of the straingages in the nozzle-reinforcement region of vessel No. 2 areshown in Fig. 3. The loads were applied to the ends of thenozzles by a 30T separate-type hydraulic jack. The structure ofthe loading setup and method are illustrated in Fig. 4. The test

( a) Withoutpad ( b) With pad,nogap

( c) With pad, gap=2mm ( d ) Wit h pad, gap=4mra

Fig. 2 Local structure in the nozzle-cylinder-pad region

procedure, and the instruments and data recording employedare summarized in Sang et al. (1995).

Experimental Results. The test results indicate that nomatter what the magnitude of the geometric gap which existsbetween the reinforcement pad and the cylindrical shell, thestresses in the vessel opening-reinforcement pad region, dueto moment loading on the nozzle, possess the following behavior: the stress in the transverse (8 = 90-270 deg) sectionof the cylinder under longitudinal moment ML on nozzle isvery small, it can be ignored; the maximum stress occurs inthe longitudinal (8 = 0-180 deg) section. Similarly, thestress in the longitudinal (8 =0-180 deg) section of cylinder

Nome nc la t ur e

D, = inside diameter of cylinderD = average diameter of cylinder

D] = outer diameter of padT = thickness of cylinder

7*i = thickness of padL = length of cylinder

,d = inside, outside diameter of nozzle (d, = 100 mm, d0 = 108mm)

d = nominal diameter of nozzler = nominal radius of nozzle

t = thickness of nozzle/ = axial length of nozzle

L, = half-length of cylinder8 = angle around nozzle

X = distance from intersection alongcylinder

Y = distance from intersection alongnozzle

ML = longitudinal moment on nozzleMc = transverse moment on nozzle

ak = impact toughness

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Table 2 Materials and properties for the test vessels

Name of

PartsMaterial

Chemical Composition % Tension Test Impact TestName of

PartsMaterial

C SI Mn P SAt

MPa MPa %NotchType

Test

Temp. Jfcm8

Head

Cylinder

Pad

16MnR 0.19 0.39 1.48 0.027 0.020 60S 435 27.0 V Ambient22

24

26

Nozzle 20 0.19 0.27 0.46 0.019 0.017 436 267 34.0 V Ambient100

146

154

under transverse moment Mc on the nozzle is very small, itcan also be ignored; the maximum stress for this loadingoccurs in the transverse(0 = 90-270 deg) section. The variation of the stresses in the compression sides in the longitudinal (8 = 180 deg) and transverse (6 = 90 deg) sections ofcylinder and nozzle due to a longitudinal moment ML = 6.27KN-m and transverse moment Mc = 3.14 KN-m, respectively, on the nozzle are shown in Figs. 5 to 8, for each ofthe four nozzles. The abscissas X, Y in the figures are theaxial distances from any point on the cylinder or nozzle tothe point of the cylinder-nozzle intersection. The ordinate isthe magnitude of the test stress.

Finite Element Analysis

Because the purpose of this paper is to determine the localstresses in the opening-reinforcement region, a finite element

analysis with one three-dimensional 8-node brick elementthrough the thickness was also carried out so that, due to thenonsymmetry of the structure when the nozzle is subjected tothe transverse moment, the whole vessel is considered in settingup the FEM mesh. However, when the nozzle is subjected tothe longitudinal moment, only one-half of the vessel is taken.Figure 9 shows the FEM mesh (total of 596 elements). Forcesare applied at every node of the ends of the nozzles such thattheir resultant yields either a longitudinal or transverse momenton the cylinder. Because the bottom of the test vessels wasrigidly clamped by bolts to the base plate, it is regarded as afixed boundary in the FEM analyses. It is worth pointing out

that both the outside and inside fillet welds of the nozzle andcylinder, and the fillet welds of the cylinder and reinforcementpad, are taken into account in these FEM analyses. It is thusseen that the analysis models are not simplified ones, but areconsistent with the actual test models.

1 1 Il > 1

01P

( a) longitudinal section ( b ) transverse section

Fig.3 Location of the strain gages of the vessel No. 2

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Nozzle

PressureSenser

HydraulicCylinder

longitudinal moment M,

Loading frame

Transverse moment M,

3f'ML = 6 27 KN

60 ^ - - - U 2 0 _ l i i - - - - ^ - 24

270

Fig.4 Loading methodFig. 6(a) Stresses of longitudinal section induced byML(nozzle No. 2,0 = 180 deg)

180 150 120 90 60 30

Fig.6(b ) Stresses of transverse section induced byMc (nozzle No. 2,0 =90 deg)

Cylinder X mm

Fig. 5(a) Stresses of longitudinal section induced byML(nozzle No. 10 = 180 deg)

I80 150 120 90 60 30

Fig. 7(a) Stresses of longitudinal section induced byML(nozzle No. 3,0 = 180 deg)

Fig. 5(b) Stresses of transverse section induced byMc (nozzle No. 1,0 = 90 deg)

The deformation in the nozzle-reinforcement region of nozzleNo.4 under a longitudinal momentML =6.27 KN-m and transverse momentMc = 3.14 KN-m are shown in Figs. 10(a) and(b), respectively. Figures 11(a) and (b) illustrate the maximum principle stress variation in the compression sides (8 =180 deg, 9 = 90 deg) of the vessel opening-reinforcement region induced by a longitudinal moment ML = 6.27 KN-m andtransverse moment Mc = 3.14 KN-m, respectively, for nozzleNo. 4 (geometric gap between the cylindrical shell and reinforcement pad is 4 mm), respectively. Figures 11(a) and (b)indicate that the stresses in the pad-reinforced cylinders and

150 120 90 60 3Q

200 200

150 150

100 100

MPa

50

0

-50

r ^ /

Stress

50

0

-50 X-100 -100

> .

-150 -150

-200 -200

Fig. 7(b) Stresses of transverse section induced byMc (nozzle No. 3,0 = 90 deg)

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ISO ISO 120 .

ML= 6. II KN m

Fig.8( a) Stresses of longitudinal section induced byML (nozzle No. 4,6 = 180 deg)

180 ISO 120 90

\

/ " j X MC= J. M KN

T O =-270'

^ .- -3J 60 90 ^^ * 1 210 240 270 300

X

/

pad Cylinder X mm

Fig.8(b) Stresses of transverse section induced by M 0 (nozzle No. 4,6 = 90 deg)

nozzles induced by moments on the nozzle possess an obviouslocalized behavior.

Comparisons and Conclusions

1 Table3shows the comparisons between the experimentaland FEM analysis results. The stress ratio referred to in this

Elliptical head

Nozzle

Weld of reinforcement

Outside weld

Nozzle

Fig.9 Meshes of FEM analysis

Journal of Pressure Vessel Technology

Fig.10( a) Longitudinal deformation

Fig. 10(b) Transverse deformation

table is the maximum stress in the cylinders or nozzles dividedby the nominal stress induced by the moments on the nozzles.

The stresses in Table 3 are the measured values. They arenot extrapolated to the junction, and as a result they are lower

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Fig. 11(a) Stresses induced byML(nozzle No. 4, 0 = 180 deg)

m 3 2 . ^ 5

n r ^~ I B

H-5-4' '

05

[_[ -8.5167

H*~ ^074

H - -55 851H - -4H 588H - -85 345

H - -yy 105H - -H0 8 6

-104 .62 -11 8.37

- -152.15

Fig. 11(b) Stresses induced byMc (nozzle No. 4,0= 90 deg)

than those from the FEM analysis. From this table one canseethat the maximum stress islocated in the transverse sectionofthe cylinder. The analyses further show that the maximum stressoccurs outsideof the cylinder-pad weld region.It iscausedbythe higher circumferential bending induced by the stiffeningeffects of the reinforcement pad and welds and the discontinuityof the geometric shapes.

2 Both the experimental and FEM analysis results indicatethat thestiffening effect of the reinforcement pad isobvious,whether thereis a geometric gap between thecylindrical shelland reinforcement padornot.Itgreatly reduces the stress con

centration at the edgeof the opening.3 The experimental and FEM analysis results indicate that

the stresses in thecylinders andnozzles possess an obviouslocalized behavior, nomatter what the numerical valuesofthegeometric gaps are. The attenuation cycleofstressinthe nozzleis approximately 30mm, which is2.1yrt; thatin thecylinderis approximately 120 mm, which is 2.1yRT.

4 The stressesin thecompression sides (8 = 180deg approximately or9= 90 deg) and the tension sides(9 = 0degor 9 =270deg) under moments MLorMcon thenozzleareobviously antisymmetric about thecentral lineof thenozzle,no matter what thenumerical values of thegeometric gapsbetween the cylindrical shells and reinforcement pads are. Thatis tosay, theabsolute values of the stresses are thesame,thevariations are the same, but the signs of the stresses are opposite.

Itis for this reason that this paper only presents the stressesinthe compression sides.

5 The stressesinthe transverse sectionofthe cylinders(orreinforcement pads) induced by transverse moment Mc aremuch larger than the stresses in the longitudinal section ofthe cylinders (orreinforcement pads) induced by longitudinalmomentML.The stress ratioof these two sections inducedbya unit momentonthe nozzlesarelisted inTable4.

6 For a loading ofmomentson thenozzles, theeffectofthe geometric gap between the reinforcement pad and cylinderon the local stressesinthe reinforcement region isnot obvious.Thatis tosay, thestressesin thecylinders andnozzles witha

Table 3 Comparisons of results

Vessel

No.

Nozzle

No.

GeometricGapjmm

Section

Moments

ott Noafc

KN-m

Maximum Stress Mpa Stress Ratio*

Vessel

No.

Nozzle

No.

GeometricGapjmm

Section

Moments

ott Noafc

KN-m

Test FEM Test FEMVessel

No.

Nozzle

No.

GeometricGapjmm

Section

Moments

ott Noafc

KN-mCylinder Nozzle Cylinder Nozzle

No. 1

No.1Without

pad

8 = 180 ML=6.27KN-m -347 -260 -466

(Cylinder)

1.82 1.36 2.44

No. 1

No.1Without

pad8=90 Mc=3.14KN-m -311 -250 -350

(Cylinder)

3.27 2.63 3.68

No. 1

No.28 = 0

With pad

6=180 ML=6.27KN-m -148 -200 -282

(Nozzle)

0.77 1.05 1.48No. 1

No.28 = 0

With pad6=90 Mc=3.1 4KN- m -165 -155 -160

(Cylinder)

1.79 1.63 1.68

No.2

No.38 = 2

8=180 ML=6.27 KN-m -135 -210 -247

(Nozzle)

0.68 1.10 1.93

No.2

No.38 = 2

6=90 Mc=3.14KN -m -170 -135 -155

(Cylinder)

1.84 1.42 1.63

No.2

No.4 8 = 4

8=180 ML=6.27KN-m -130 -230 -274

(Nozzle)

0.65 1.21 1.43No.2

No.4 8 = 48=90 Mc-3. 14KN -m -150 -125 -132

(Cylinder)

1.63 1.32 1.39

"Stress ratio = cr/{ML{c)/Zb)

where

a = maximum stress of experiment or calculation, MPa

ML(C) = longitudinal (transverse) moment on nozzles, KN-m

Zb = section modules of binding, mm3

_ n(dl-df)

32d

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Table 4 Test results of the transverse-longitudinal stress ratio

Vessel No. Nozzle No.Geometric Gap

8mmTransverse-longitudinal Stress Ratio

(o /M c ) /(o /MJ

No. 1No. 1 Without pad 1.79

No. 1No. 2 8 = 0 2.27

No.2No. 3 8 = 2 2.51

No.2No.4 8 = 4 2.31

certain size of geometric gap between the reinforcement padand cylindrical shell are not obviously larger than those withouta geometric gap.

Acknowledgment

The support of the Pressure Vessel Research Council andin particular Ed Weis, Chairman of the Piping and Nozzles

Committee, and Kam Mokhtarian, Chairman of the Shell Intersections Subcommittee, are gratefully acknowledged.

ReferencesASME Boiler and Pressure Vessel Code, 1989, Section 8, Division 1, American

Society of Mechanical Engineers, New York, NY.

Chao, Y. J., Wu, B. C, and Sutton, M. A., 1986, "Ra dial Flexibil ity of Welded-Pad Reinforced Nozzles in Ellipsoidal Pressure Vessel Heads," International

Journal of Pressure Vessels and Piping, Vol. 24, pp. 189-207.Mershon, J. L Mokhtarian, K., Ranjan, G. V., and Rodabaugh, E. C, 1984,

"Local Stresses in Cylindrical Shells Due to External Loading on Nozzle

Supplement to WRC Bulletin No. 107," WRC Bulletin No. 297.

Oikawa, T. And Oka, T., 1987, "A New Technique for Approximating the

Stress in Pad-Type Nozzles Attached to a Spherical Shell," ASME JOURNAL OF

PRESSURE VESSEL TECHNOLOGY, Vol. 109, pp. 188-192.

Sang, Z. F., Li, L Qian, H. L., and Widera, G. E. O., 1995, "Behavior of PadReinforced Cylindrical Vessels Subjected to Axial Thrust on Nozzle," ASMEPVP-Vol. 318, Joint ASME/JSME Pressure Vessels and Piping Conference, p. 1.

Soliman, S. F. and Gill, S. S 1978, "Radial Loads on Pad-Reinforced Nozzles

in Spherical Pressure VesselsA Theoretical Analysis and Experimental Investigation," International Journal of Pressure Vessels and Piping, Vol. 6, pp. 451-

472.Wichman, K. R., Hopper, A. G and Mershon, J. L 1979, "Local Stresses in

Spherical and Cylindrical Shells Due to External Loading,"WRC Bulletin No. 107.

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