CHAPTER 1 Stresses in Boiler Shells Q. Sketch a double butt strap joint for a multi-tubular tank boiler. State why th is must be the strongest jo int in th e shell. A. Co nsider a th in cylindrical shell subjected to an internal pressure. Th is sets up stresses in the circumfere ntial and longitudinal axes which can be calcul ated as follows: CIRCUMFERENTIAL STRESS Longitudinal seam -- Fig. I Stress in the lon gitudinal seam If pressure acting upon the circumference is resolved into hor izontal compo ne nts. th e resulting horizontal force = pressure x project ed area. Th is is resisted by th e stresses setup in the longitudinal joint. Th en for equilibrium conditions: Hori zontal forces to left = Horizontal forces to right Hor izontal forces to left = Resisting force in longitud inal joi nt Pressure x projected area = Stress x cross sect. area of joint Pr ess. x dia. x length = Stress x 2 x thickness x len gth Pr essure x diameter S .. di I ·· --0------,:-:--0---- = tress 10 longitu ina j oint 2 x thickn ess LONGITUDINAL STRESS ~ < , Area of circ umfe rential = :~"~d~ seam ..... ,- .....-'-:-Po -,- Q)Area of end plate ;::==9 _/ Fig.2 Stress in the circu mfere ntial Circumferentia l seam seam STRESSES IN BOI LER SHELLS The force acting upon th e e nd pla te is resisted by the stress se t up In the circu mfe ren tial jo int. Then for equilibrium co nd itio ns : Horizontal forces to left = Horizontal forces to right Pr essure x e nd plat e area = Resisting fo rce in circumfe re ntial jo int Press. x : x di am et er' = Stress x cross sect. a rea of jo int Pr essu re x di am eter = Stress in circumfere ntial jo int 4 x thickness Thus it follow s th at longitudinal joint stress is tw ice the circumfere ntial joi nt stress . Longitud inal joint (the strongest joint ~T'"~. in the she~ {~~~-- \ l"" I~ I \ \ / l~~ i r c u m fe r e n t i a l and / 1> /,/// end joints (only need \ -, , " / be half as strong as '--.. the longitudinal joint) Fig. 3 Rivel ed joi nts in a boile r she ll T hen re arranging the shell form ula in te rms of pressu re Max. stress x 2 x thi ckness Max. pressure = - --- -:-:------ - ­ di am eter Thi s pres sure will be reduced by the efficiency of the joint subjected to the grea tes t stress. Max. stress x 2 x th ickness Max . wo rking press. = . x Joi nt efficiency d iam et er It has been sho wn th at the joint subjected to the great est stress will be the longitudinal jo int ; the strength of th is joi nt th er efore gove rns the allowable wo rking pressure, and so the strongest type of riveted jo int used in the boil e r is used for this joint. See Fig. 4 on facing page . Q. D iscuss the need fo r co mpe nsa tion for holes cut in th e shell of a boi le r. State the regul at ions co nce rn ing th is compen sation. Show methods of com pe nsa tion th at can be used. Sketch a manhole door, an d show the po sition of these doors in the she ll of a Scotch boiler. Wh y mu st a door cut in the cylind rica l portion of the she ll be placed in a certai n way? 2 ST RESSES INBO ILE R SH E LLS Alternate rivets omitted from End plate the outer rows '"'_, ~"""""_~ ) Outer strap) I ,/ o o o o Rivet on o <i:. of the o joint o o o o o o o o " ) ;' Outer strap Shell plate Fig. ~ Det ail of rive ted loogitudinal jnin t A. A ny mat er ial cut fro m the she ll will weaken it by an exte nt related to th e amo unt cut out. In gen er al any holes cut in the shell, with a d iameter great e r tha n:­ 2· 5 x plate thi ckn ess + 70 mm mu st be pr ovided with co m pe nsa tio n for th e loss of stre ng th due to th e mat erial cut ou t . So me forms of co mpensating pad s are shown in Fig, 5, Leak detecting Riveted construction hole ">. " ' , { c om p e n s a tin g , ,_~_ _ - " ~ I ~ pa d -- - -""\ ",,_ ~ ~ . ) I' '" "S hell " --- ~ Fig, 5 Co mpe nsa ting pads RIV et Welded construction The larges t holes cut in th e she ll include th e manholes, and where these are cut in the cylindrical portion of th e she ll they mu st be arr anged with th eir minor axis parallel to th e longitudinal ax is o f the boiler. T his is due to th e st ress acting up o n the longitudinal seam be ing twice that ac ti ng up on the circumferential seam , Thus th e she ll mu st not be weake ned more th an necessary alo ng its lo ngitu d inal ax is. Longitudinal axis , subjected to the ~ greatest stress ___,':$: ~ --... -: - Major axis along line Manhole cut from shell of least stress in such a manner as to Fig,6 A rra ngement of manhole in weaken it by the least a Scotch bo iler she ll amount possible 3 STRESSES IN ROlLER SHELLS Stud (screwed into door) ./.-.0 Fig. 7 Manhole door Q. Discuss the reasons for the limitation of pressure imposed upon tank type boilers. A. With reference to the thin shell formula: Pressure x diameter Stress = . 2 x plate thickness Thus it can be seen that if the stress in the material is to be kept within fixed limits (as is the case with boiler material) then, if the pressure or diameter increases, the plate thickness must also Thickness o change if the ratio is to remain constant. Fig. H Therefore if boiler pressure is increased, either the boiler shell diameter must decrease, or the boiler scantlings increase; the latter leading to increased cost and weight. In order to accommodate the combustion chamber, smoke tubes, etc., no great reduction in the shell diameter of tank type boilers is possible, and thus very thick shell plates would be required for high pressure. The furnace must also be considered, as its thickness must be kept within certain limits to prevent overheating. However, its diameter cannot be reduced too much, otherwise difficulties in burning the fuel in the furnace would arise. For these reasons the maximum pressure in tank type boilers is limited to about 1750 kNrrn". Q. Show the reason for the staying of any flat surfaces in a pressure vessel. How can the use of stays be avoided? A. When a force is applied to a curved plate as shown in Fig. 9, internal forces are set up which enable the plate to withstand the force without undue distortion. The bursting stress can be resolved Fo~ce into perpendicular components, \ t one of which will oppose the force. ~ The surface will bend until this com­ Bursting stress ponent balances the pressure. It will ~~ %:.--------- (acts perpendicular --"~I"'"'"'i--..-, to any radius) then be found that the surface is in \ I the form of an arc of a circle. \ \ -.1.. Component of stress to When the pressure acts upon a flat \ : balance the force \, plate, it will tend to bend the plate until equilibrium is obtained. Thus Fig. 9 Stress in a curved plate to prevent undue distortion the flat 4 STRESSES IN BOILER SHELLS 10 l-orce (due hlr,c (due 10 pressure) pr essure) N" balancine force (u ntil plate bends sufficie ntly to provide it) Balancingforce Fig. 10 provided by stay plate must be very thick or supported by some form of stay. It follows that if the use of flat surfaces can be avoided in the design of a pressure vessel there will be no need to fit internal stays. Thus pressure vessels are often given hemispherical ends but, if this is not possible, any flat surfaces must be stayed or of sufficient thickness to resist the pressure without undue distortion. o-'------!D Hemisphericalend plates Flat end plates no internal stays required internal stays must be Fig. II fitted to support them Q. State the regulations relating to the materials used in boiler construction. To what tests must these materials be subjected? A. The Department of Trade and the classification societies, such as Lloyds, have very strict rules governing the dimensions and materials used in the construction of pressure vessels, so that they can withstand the forces set up by pressure and thermal effects. The DOT require that carbon steel used in the construction of pressure vessels is to be manufactured by open hearth, electric, or pneumatic processes such as LD. Kaldo, etc . These may be acid or basic in nature. To ensure the materials used are of uniform quality within the requirements laid down. tests are carried out on samples of material. Some of the materials are as follows: Shell plates for riveted construction , and 2 steam space stays are to have a tensile strength between 430-560 MN/m , with a percentage elongation of not less than 20 per cent on a standard test length. In the case of shell plates for welded construction the strength requirements lie between 2 400-450 MN/m • Plates which have to be flanged have lower strength requirements.