3 Ambient temperature storcge tank design

The section of shell lapPedbehind the angle incteasesthe available cross-sectionarea in length w

Figure 3.67a Typical roofioini

Hand.ai I stanc h ions, plaform supporling brackets

or stiffeneB of any kind musl not be welded acros

'AA- 1he horizonlal Plane of the i

bv the roof and shellmembers

x; the maximum oflplane allowance = 1.5 (tr + 0 / 2

: !ure 3.66 ldeal location fot the cenaoid ofthe compresslon zone area to API

:2-0. (For information onlv, not mandaiory to the BS 2654 and API 690 Codes)

3.8 Frangible roof joint, or weakroof-to-shell joint

3.8.1 lntroduction

'ixed roof tanks which store volatile products will have a mix-

:ufe of product vapour and air in the space between the surface

of the product and the tank roof. This mixture may be in the

'lammable range and, due to malfunction, externalfire or inter--al explosion. there may be a sudden increase in pressure

,vithin the tank which the normal vent devices and emergency

',ents are unable to cope with. Consequently tl^e tank rray be

damaged and this can result in failufe of either the shell-to-bo!iom joint or the roof-to-shell joint.

ln either case such failures are disastrous but the failure of the

shell-to-bottom joint can be particularly horrendous due to the

felease of the stored product over the surrounding area caus-

Lng the attendant ecological and environmental problems.

Of the two types of failure, the roof-to-shell failure is to be pre-

ferred. as this will normally create sufficient free-venting area to

allow the release of the tank over-pressurisation without any

oss of stored product. To increase the likelihood of a preferen-

tial roof-to-shell failu re, some fixed roof tanks can be provided

with a weak rooflo-shell connection, known as a "frangible roofjoint . A typical arrangement of this type of joint is showl in

Figure 3.67b.

3.8.2 Frangible roof joint theory

Assuming a empty cone roof tank, then, as the pressure in the

tank increases above atmospheric pressure, a point will be

feached when the upward force on the roof plating willequalthedownward load due to the weight of the roof plating As ihepressure increases further, the roof plating will tift oif its support

structure and this further increase in pressure is withstood by

lensile membrane forces 'T' in the roof plating (see Figure

3.68). These forces exert a pull at the shell-to-roofiunctlon and

so induce compressive forces in this area

A point will be reached when the upward force due to further in-

crease in pressure, willovercome the downward load duetotheweight of the shell and support structure, and at this pressure'

the floor plating at the tank periphery will start to lift ofi the tank

foundation, as illustrated earlier in Figure 3.52

The floor being allowed to lift off the foundation' can result in

high stresses being set up in the shellto-bottom jointwhich can

"^-"'-;\R@f plat6 not connectedto the roof supporling structur€

Figure 3.67b Typical frangible foof ioint

result in failure ofthe joini. This possibility must be prevented by

designing the roof-to-shelljoint to fail before the shell-to-bottomjoint does. This is accomplished by considering the point at

which the pressure in the tank is such that the floor is just aboutto li11 off its foundatLon.

3.8.3 The maximum compression zone area allow-able

For a roof connection to be considered frangible, the maxlmum

compression zone area allowable must be determined.

The roof plating is assumed to act as a membrane and any

bending effects are ignored, as are any changes in geomeiry,

also th; angle between the slope of the roof and the horizontal

0, is assumed to remain at its design value.

Considering Figure 3.68.

P = internal Pressure

T = membrane force in roof Plating

Wr = weight of roof plating

Figure 3 68 Tensile membrane fotces

Thecentrord ol lh€ composll€ sh€lland rool

area shaLlnol be oulsidelhrs snaded area

STORAGE TANKS & EQUIPMENT 89

3 Ambient tempercturc storcge tank design

Ws = weight of shell and roof support structurewhich is carried by the shell

R = tank radius

€ = angle of the roof slope to the horizontal

Wr and Ws shall have any corrosion deducted.

Note: The above condition assumes that the tank is empty,but the theory is equally valid if the tank contains liquid.When this is the case, then the load due to the weightofthe liquid, which is considered to be effective, (i.e. saywithin 750 mm of the shell), is added to that of the shelland framing.

' However, it is normal practice to design for the worstcondition, which in this case, is when the tank is empty,thus giving a lesser value for the allowable area for thecompression zone for the frangible condition.

Hencethe upliftforce on the roof plates is given byp r'R2 andthis force is resisted bythe weightofthe shelland support struc-ture Ws.

Then:

p.7r.R2 = Ws equ 3.74

It has already been determined in equation 3.68, that the re-quired compression area at the shell-to-roof junction is givenby:

n.R2A=---l--:-::2 Sc.tan 0

And transposing for p:

2.A Sc tan eO=-'R'Substituting for p in equation 3.74 then:

2 ASctan0 -, ..._xn.K_=vvsR

nence:

^ws2 r.Sc tan 0

equ 3.75

The size and quality ofthis weld is therefore an important factorof the frangible joint. However there does not appear to havebeen very much research done in this area, and this could bedue to difflculties in making meaningful analytical studies oftheinfluence and behaviour of such welds when subjected to thistype of failure mechanism.

The Codes do however require that the peripheral roof plateweld be kept as small as Dossible and in no case shall it belarger than 5 mm. From a practical point of view making theweld size any less than this, can be detrimental in the long term,because experience has shown that in time, this weld suffersfrom the effects ofcorrosion wastage which can eventuallyleadto vapour leaks at the joint.

3.8.5 Formula as expressed in BS 2654

A is expressed in mm2

Ws is given the notation 'T' and is the weight of theshell, shell stiffening and roof framework suFported by the shell but excluding the roofplates, expressed in kilograms.

Sc is expressed in N/mm2 and curb failure is as-sumed to occur at 220 N/mm2, so this flgure isbuilt into the equation.

0 is the slope of the roof at its point of connec-tion to the shell in degrees.

The formula then becomes:

Tx9.807 Tx7.07x10-s

equ 3.76

The area A thus found. is the maximum that can be allowed forihe shell-to-roof compression zone to be considered as a fran-gible joint.

3.8.4 Other factors affecting the frangible roof con-nection

3.8.4.1 Roof slope

ln Section 3.7.2.1 itwas demonstrated that as the roofslope be-comes shallower, the value of 6 decreases and hence the re-quired cross sectionalarea increases. Taken to the extreme, as0 tends to 0', then the required cross-sectional tends to infinity.

Therefore itcan be seen thata shallow slope favours the frangi-ble condition. Both the British and American codes recognisethis and put a limit on the maximum roof slope allowed for a roofto be considered frangible. These limits are given in Sections3.8.5.1 and 3.8.6.1.

3.8.4.2 Size of weld at the roof plate-to-shell connection

During the failure process of a frangible roof, the normal se-quence of events is for the roof to deform, and undergo elasticbuckling.

l\4any creases will appear at the periphery as a reduction in di-ameter occurs and the compression zone will buckle and col-lapse. This causes the peripheral roof plate weld to tear awayfrom its shell mounting and hence the excessive internal pres-sure is relieved.

90 STORAGE TANKS & EQUIPMENT

2 xT x2zo.lan e tan e

Which is as it is shown in Appendix F of BS 2654.

3.8.5.1 Additional requirements to BS 2654

equ3.77

In addition to the restriction in cross-sectional area for theroof-to-shell zone for the frangible condition, the Code requiresthat the following conditions shall also be met, as described inSections 3.8.4.1 and 3.8.4.2:

. The slope of the roof plating at its connection to the shellshall not be more than 1 in 5.

. The peripheral roof plating-to-shell connection weld shallnot be more than 5 mm.

3.8.6 Formula as expressed in API 650

A is expressed in mm2

Ws is given the notation W and is the weight of theshell, shell stiffening and roof framework sup-ported by the shell but excluding the roofplates, expressed in Newtons

Sc is expressed in N/mm'?and cufu failure is as-sumed to occur at 221 Nimm2, (32,000 lbiin')so this figure is built into the equation

0 is the slope of the roof at its point of connec-tion to the shell in degrees

The formula then becomes:

^WW^= 2r"x221 ^ane=

1390 xta" oequ 3.78

Which is as it is shown in clause 3.10.2.5.3 of API 650.

3.8.6.'l Additional requirements to API 650

ln additlon to the restriction in cross-sectional area for theroof-to-shell zone for the frangible condition, the Code requiresthat the following conditions shall also be met, as describedabove in Sections 3.8.4.1 and 3.8.4.2:

. The slope of the roof plating at its connection to the shell

shall not be more than 1 in 6.

. The peripheral roof plating-to-shell connection weld shall

not be more than 5 mm

3.8.7 Difference between Codes

The orincipal difference between the British and the American

Codes isthat BS 2654 allows the slightly steeper roof slope of 1

I in 5, against 1 in 6 to API 650.

The different constants used in equations 3.77 and 3'78 atedue to the tank weight being expressed in kilograms in BS 2654

and in Newtons in API 650.

The maximum allowable cross-sectional area in millimetres

calculated by either equation is found to be the same for a given

set of design parameters.

3.8.8 Conflict of design interests

During the initial tank design stage, the shell{o-roof joint will

have been designed to suit the internal service pressure re-

quirement, as detailed in Section 3 7. The most appropnate

method of providing the required cross-sectional area in the

roof-to-shelljointwill have been established and hence the tank

will be capable of withstanding the compressive forces which

will develop in this area during normal operation of the bnk'

However, it may be necessaryto ensure' that in the event of an

accidental over-pressurisation in the tank' it would be desirable

for the shell-to-roofjoint to fail This may not always be possible

because the compression area built into the tank to satisfy the

operating pressure may be more than that allowed for a frangi-

ble roofjoint, within the strictures of the Code

The likelihood of this conflict occurring and the possible means

by which it can be overcome, will become evident ffom the fol-

lowing Sections.

3.8.8.1 "Service" and "Emergency" design conditions

The maximum cross-sectional area at the compresslon zone

which is allowable by equations 377 and 3.78 for the tank

emergency condition, may be found to be less than that re-

quired to satisfy resistance ofthe internal pressure for the ser-

vice condition calculated by equations 3.68 or 3 71.

When this occurs the tank is deemed not to have a frangibleroofjoint, but this situation may be overcome by providing the

tankwith anchor bolts or straps attached to the lowershellareaofthe tank and secured to a peripheral concrete foundation ring

beam.

3.8.9 Examples of frangible and non-frangible roofjoints

Using the tank shell design illustration given in Section 3 3 2 9,

and issuming a roof slope of 1 in 5, and a roof plate to curb an-gle weld of 5 mm, then further calculations give the following

information:

3.8.9.1 Tank designed for an operating pressure of 7'5mDar

Case AlCase 41 allows for the curb angte to be lapped on to the top ofihe shell, as shown in Figure 3 67a. This arrangement ls gener-

ally adopted for two main reasons;

1) The available area of the compression zone which is re-quired for the tank operating pressure is increased, be-

cause the top of the shell plating behind the angle is alsoincluded in the zone. This is advantageous as it minimises

3 Ambient tempercturc storage tank destgn

the amount of additional area which may have to be pro-

vided by a curb angle.

2r Durinq the erection ofthe tank. lapping the angle directly

up ag;inst the top of the shell plating is a simpler erectionprocedure.

In Case A.1 , the area available from the roof and shell plating is'

on its own, more than enough to satisfy the amount requlred

from equation 3.67 and therefore only the minimum size of an-

gle from Figure 3.58 will be fitted to the tank, in this case a 80 x

80 x 10 angle. Thetotalarea provided in the compresslonzoneisfoundto be5028 mm2. This is more than the allowable area of

4811 mm2, and the roofjoint is therefore considered not to be

frangible.

Case A2

Case 42 allows for the vertical leg of the curb angle to be butt

welded directly on to the top ofthe shell plating as shown in Fig-

ure 3.67b This is a more difficult erection task than that for a

lapped curb angle but can be advantageous when a frangible

roof ioint is required, because the area of the shell-to-roof com-

presiion zone is reduced due to the lesser area of shell plating

being within the zone.

Aoain. it can be seen that the area provided by the shell and

roof is more than enough to satisfy the requirement of equation

3.64, and in this instance, the minimum size curb angle is butt

welded. rather than lap welded to the shell' thus reducing the

area availablefrom the shellbythedepth ofthe angle i.e B0x8

= 640 mm2.

This is enough to reduce the total available compression zone

area to a flgure which is less than the maximum allowed for a

frangible joint and therefore the roofjoint is frangible

CaseAl _ CaseA2

Pressle T5ombar 75dbar

compresson zo.e a@a requned ior ,,7jj nn2 1711mn,

Crrodna,e aoo"oo orlreo olrpt _cp-^ao60rosler Brr-*Flopo.oshel

wh a.d Wc area 35i8 mm? 2878 mm'?

Additonalarea rea! red -1807 '1167

L se ected curb a.gle size I 8ox80x10RsA 80x80xl0Rsa

Selected curb ang|e afea 1510 mm'? 1510 mmz

..rr.* I .,r. '*ls totalarea provLded suilicient?

13608s kg 136089 kg

lr,,laximum area a lowed iorirangible

ls lhe oofto nl ffang ble?

3.8.9.2 Tank designed for an operating pressure of20 mbar

Cases Bl and 82

At this higher pressure the required compresslon zone area

has significantly increased from 1711 mm2 to 7570 mm'?.

Following what was learned from case 42, the selected curb

angle size of 150 x 150 x 18 for Case 81, is butt-welded to the

tank shell as shown in Figure 3.67b However, it can be seen

that in doing this, the loss of shell area leaves a deficit of 152

mm, (7570-7418) in the area required for operation, and this is

not acceptable.

Case 82 is calculated in the same way as Case B1 except that

the larger angle size of 200 x 200 x 16 is used and the conse-quent increase in the cross-sectional area ofthe angle gives an

acceDtable totalarea forthe compression zone required forop-erational purposes.

STORAGE TANKS & EQUIPMENT 91

3 Ambient temperaturc storage tank design

For both Cases 81 and 82 however the area of the compres-

sion zone is far in excess ofthe maximum allowed for a frangi-ble roofjoint.

Compression zone area reqlired for

case B1

--l

CAeBz

_ 20.00 mbar

Curbangle lapPed or butted to shell?

2318 mm': 1918 mm2

5652Add tronalarea requ red 5252.32

!:r*t"djy9j!q:l!1Selected curb angre area

i50 x 150 x 18 RSA

s100 mm'

,oqr?oirI r$

Ls lotal area Pmvide suffclenl?

I19634lg 140426 kg

[,lax dum area alowed lorlrangblejoni

lslhe roof lointfrangble?

. o*ulLt!-

Case 83

From the previous Cases B1 and 82 it was found thai for this

oarticular tank size and its attendant design parameters there

was no advantage in butt-welding the curb angle to the shell

Case 83 therefore is based on lap welding the curb angle as

shown in Figure 3.67a. lt can be seen from the results that in do-

ing this the inclusion ofthe additionalarea oftheshell plate be-

hi;d the curb angle atlows a smaller angle size of 150 x 150 x 15

to be used, and the combination gives an adequate overall total

area in the comPresslon zone.

However, as before in the previous cases, this area is wellin ex-

cess of that allowable for a frangible roofjoint.

3.8,10 Tank anchorage - a means to frangibility

The tank in Case 83 meets the Code requirement for having

sufficient cross-sectional area in the roof-to-shell compression

zone for operating conditions But under an emergency over

pressure condition, this area is too great to ensure that the

;ooflo-shell joint is frangible and therefore may not fail under

this extreme condition. This could cause the shell-to-floor rim of

the fank to lift off the foundation and the resulting distortion in

this area could cause this joint to fail rather than the

roof-to-shell joint.

This occurrence can be prevented by anchoring the tank to a

suitably designed concrete ring beam which forms a part ofthe

92 STORAGE TANKS & EQUIPMENT

tank foundation. Three methods of anchorage are illustrated in

Figures 3.69 (a), (b) and (c).

3.8.10.1 Ensuring a frangible roof connection using an-cnorage

Apart from the frangibility consideration, anchorage may also

be required due to the following conditions;

. The operating pressure causing uplifr ofthe tank.

. The overturning effect on the tank of the prevailing wind

. Instability of the tank caused by seismic action.

These instances are discussed in Section 3.9 and Chapter 15

or26, butfornoW the means of designing anchorageto ensure

a frangible roofjoint will be considered as follows:

3.8.1 0.2 Determining anchorage requirements

Where a roofis deemed notto befrangible. then the pressure at

which it would fail has to be determined. This is done by trans-posing equation 3.69 or 3.71 depending upon which code is be-

ing used, and thus determining a failure pressure p

Takino the case for the British Code then from equation 3 69:

o=4 Jc t1n J*s.77

1r-

Failure is considered to occur at a compressive stress Sc of 220

N/mm'z.

Hence failure Pressure

o=44 A:tan o+0.77.tr

Remember that in the British Code p is in mbar.

Similarly, for the American Code, from equation 3.71.

o= 1.1 A tanoro.o8.th'D"

Forthe American Code, failure is considered to occur at a com-

pressive stress of 221 N/mm2.

The constant 1.1 in equation 3.71 is calculated using a allow-t t'!

- 1.1able stress of 137.5 N/mm' e.g. -

This has to be recalculated using thefailure compressive stress

of 221 N/mm/ and the new constant is '1! 'r,125

Failure pressure is therefore

p = 1.77.# t"n o * o.os. r'.

In the American Code p is in kilopascals - (1 kPa =10 mbar)

3.8.10.3 Worked examPle

Consider the tank depicted in Section 3 3.2.9.

This tank is 30 m diameter, has a roofslope of 1: 5, a roof plate

thickness of 5 mm and compression zone details as given in

Section 3.8.9.2 for Case 83.

Anchorage is io be provided using bolb

Using the BS Code for this example, then the failure pressure

will be:

4.44 x7818 x0.2 ^ -- .=U./a XO- 1s'

= 34.43 mbar

= 3.443 kNi m'?

This pressure acting on the roofofthe emptytankwillproduce a

uplift of:

equ 3.79

equ 3.80

Compress on zone area required ior ope€tion

Curb ang e lapped orbltted lo shelt

150 | 150 x 15 RsA

ls lotalarea Prov de sufiicieot?

[,lax m!m area a lowed for irang ble]oini

s lh€ rooiioinlfrang ble?

Figufe 3.69a Anchotage using bolts

Figure 3.69b Anchorage using siraps

3 Ambient temperaturc storcge tank desryn

UP=" R'P

=nx152 x3.443

= 2433.71 kN

The weight of the tank shell, stiffening and roof structufe given

in case 83 is 139041 kg which equates to 1363 55 kN

Then the net uplift = 2433.71 -1363.55 = 1070 16 kN

The BS Code requires anchors to be spaced around the tank

circumference at a minimum of 1 m and a maximum of 3 m

In this case a 3 m spacing will be used and hence the number ofbolts required is;

30xn ^,.^3

This is rounded up to 32.

However, as there are 12 plates per shell course, then 36 an-

chors will be selected, giving 3 per plate and thus clashes be-

tween anchor brackets and vertical shell course butt welds will

be avoided.

The load per bolt due to the over-pressurisation uplift will be

1070 16 :zg.t3 ttt36

The BS Code also requires anchors to have a minimum cross-sectional area of 500 mm2. This equates to a bolt core diameterof 25.33 mm and hence a overall bolt diameter of 30 mm will be

selected, which has an actuat core stress area of 561 mm'? (this

excludes any corrosion which may be required).

The stress in each bolt due to the over-pressurisation uplift willbe

29.73 x 1000

561

= 53.0 N/mm'?

The BS Code states that the allowable tensile stress in the an-chorage shall not exceed 50% of the specified yield strength, or33.33% of the minimum tensile strength of the anchorage ma-terial, whichever is the lowesi.

Taking medium strength steel having a minimum tensilestrength of 430 N/mm'? and yield of 255 N/mm2 for this diameterof bolt, then the allowable tensile stress would be 127.5 N/mm'?.

The selected bolt size is therefore acceptable.

3.8.10.4 Further design check

From above it can be seen that the tank can be subjected to apressure greater than its design pressure i.e. 34.58 mbar in-

stead of 20 mbar The original tank design must therefore be

checked to ensure that the allowable stress in the shell (equa-

tion 3.7) is not exceeded. This is accomplished by transposingS, the allowable stress and t in equation 3.7.

3.8.1 0.5 Other anchorage considerations

The anchorage design here is only catering for the uplift due toover-pressurisation and it must be borne in mind that this mayhave to be combined with any anchorage requirements whichmay be found to be necessary to stabilise an overturning mo-ment on the tank due to wind loading which is dealt with in

Section 3.9.

3.8.11 API 650 Code - anchor requirements

3.8.11.1 Minimum bolt diameter

The minimum anchor bolt diameter should not be less than 25mm, plus a corrosion allowance of at least 6 mm, giving a mini-mum diameter of 31 mm. This is similar to that given in the BS

'rllllttll{ | I ll

lrn caseswheElheanchorborbarc

5

):

I

FigLre 3.69c Combinalion usrrg slrap ard bolld'lchotage

STORAGE TANKS & EQUIPMENT 93