Design of pressure vessel

DESIGN OF PRESSURE VESSEL PROJECT REPORT Submitted by\ MIJO JOSEPH VIPIN M VISHNU VIJAY ABSTRACT This project work deals with a detailed study and design procedure of pressure vessel A detailed study[.]

DESIGN OF PRESSURE VESSEL

PROJECT REPORT

Submitted by\

MIJO JOSEPH

VIPIN M VISHNU VIJAYABSTRACT

This project work deals with a detailed study and design procedure of pressurevessel A detailed study of various parts of pressure vessels like shell, closure,support, flanges, nozzles etc Design is carried according to rules of ASME codesection VIII, Division I

The first chapter deals with detailed study of pressure vessel i.e the various materialsused in pressure construction and temperature are mentioned It also deals with thestudy of various parts like flanges, support etc Various methods of fabrication andtesting are also included

The second chapter includes design criteria This is followed by procedure of design,which include design shell and its components, nozzles, reinforcements etc

LIST OF FIGURES

3.9.1 TYPES OF FLANGES 14

Di : inside diameter of the vessel, mm

Do : outside diameter of the vessel, mm

Ri : inside radius of the vessel, mm

Ro : outside radius of the vessel, mm

S : maximum allowable stress, kg/cmA2

E : Joint efficiency, %

T : required the thickness, mm

tn : minimum thickness provided for the nozzle, mm

5

P : design pressure, kg/cmA2

trn : selected thickness for the nozzle, mm

fr : strength reduction factor

Mt : moment at the skirt to head joint, kg-mm W : weight of the vessel H : height of center of gravity

Ab : area with in the bolt circles, mmA2

Cb : circumference of bolt circle, mm

Ba : required area of one bolt, mm

As : area within the skirt, mmA2

Cs : circumference on outer diameter of skirt, mm

Dso outer diameter of the skirt, mm

7

P : design pressure, kg/cmA2

Dsi: inside diameter of the skirt, mm

Fb : safe bearing load on the concrete, kg/cmA2

I : width of the base plate, mm

The equations may be written in the following forms

t = PRi/(SE-0.6P) = Pro / (SE-0.4P)

t = minimum required thickness of the shell exclusively of

Corrosion allowance

Where, t

P= design pressure, or maximum an allowable

working

9

welded -joint efficiency

S maximum allowable

stress

11

Ri inside radius of the shell

Ro= outside radius of the shell

If the thickness of the Shell exceeds 50% of the inside radius, or when thepressure exceeds 0.385SE, the lame equation should be used to calculate the vessel-shell thickness The following forms of the lame equation are given by the code.With the pressure p known.

t = Ri(Vz-l) = Ro (Vz-1)/Vz

Where, z = S.E+P/(S.E-P)

The equation for ellipsoidal head thickness is given by

t = PDi/ (2SE+0.2P) =PDo/ (2SE+1.8P) Where, t = minimum

required thickness of the ellipsoidal head exclusive of corrosion

allowance

P = design pressure, or maximum allowable working pressure

E = welded - joint efficiency

S = Maximum allowable stress

Di = inside diameter of the shell

Do=outside diameter of the shell

CHAPTER 1

INTRODU CTION

Chemical engineering involves the application of sciences to the process industries,which are primarily concerned, with the conversion of one material into another byahemical or physical means These processes require the handling or storing of largequantities of materials in containers of varied constructions, depending upon theexisting state of the material, it's physical and chemical properties and the requiredoperations, which are to be performed For handling such liquids and gases, acontainer or vessel is used It is called a pressure vessel, when they are containers forfluids subjected to pressure They are leak proof containers They may be of any shaperanging from types of processing equipment Most process equipment units may beconsidered as vessels with various modifications necessary to enable the units toperform certain required functions, e.g an autoclave may be considered as high-pressure vessel equipped with agitation and heating sources

Pressure vessels are in accordance with ASME code The code gives for thicknessand stress of basic components, it is up to the designer to select appropriate analytical

as procedure for determining stress due to other loadings The designer mustfamiliarize himself with the various types of stresses and loadings in order toaccurately apply the results of analysis Designer must also consider some adequatestress or failure theory in order to confine stress and set allowable stress limits

15

The methods of design are primarily based on elastic analysis There are alsoother criteria such as stresses in plastic region, fatigue, creep, etc which needconsideration in certain cases Elastic analysis is developed on the assumption that thematerial is isotropic and homogeneous and that it is loaded in the elastic region Thisanalysis is not applicable in the plastic range Under cyclic variation of load causingplastic flow, the material to hardens and the behavior of material becomes purelyelastic This is a phenomenon called shakedown or cessation of plastic deformationunder cyclic loading.

Elastic analysis is therefore in most important method of designing pressurevessel shells and components beyond the elastic limit, the material yields and theplastic region (spreads with increased value of load The load for which this occurs iscalled collapse load rusting pressure

Limit analysis is concerned with calculating the load or pressure at which flow ofjfitructure material occurs due to yielding However, this method is not usually applied

to Resign of pressure vessels When vessels are subjected to cyclic loading, it isnecessary to consider requirements for elastic cycling of the material and the effects ofthis on component behavior In the case of a discontinuity of shape, load may give rise

to plastic cycling Under these conditions, shakedown with occur Maximumshakedown load is twice the first yield load Therefore, an elastic analysis is valid up

to the range of load, under cyclic loading conditions A factor of safety on the stress or

a factor of safety of twenty is applied on the numbers cycles Design stress is accepted

as the lower value

CHAPTER.2 SCOPE

OF THE PROJECT

In sophisticated pressure vessels encountered in engineering construction; highpressure, extremes of temperature and severity of functional performancerequirements pose exciting design problems The word "DESIGN" does not meanonly the calculation of the detailed dimensions of a member, but rather is an all-inclusive term, incorporating:

1 The reasoning that established the most likely mode of damage or failure;

2 The method of stress analysis employed and significance of

results;

3 The selection of materials type and its environmental behaviour

I The ever-increasing use of vessel has given special emphasis to analytical andexperimental methods for determining their emphasis to analytical and experimentalmethods for determining their operating stresses Of equal importance is theappraising the significance of these stresses This appraisal entails the means ofdetermining the values and extent of the stresses and strains, establishing thebehaviour of the material ■involved, and evaluating the compatibility of these twofactors in the media or environment to which they are subjected Knowledge ofmaterial behaviour is required not only to avoid failures, but also equally to permitmaximum economy of material choice and amount used

CHAPTER.3

17

DESIGN CRITERIA 3.1 FACTORS INFLUENCING THE DESIGN

[Regardless of the nature of application of the vessels, a number of factors usuallymust be considered in designing the unit The most important consideration often isthe selection of the type of vessel that performs the required services in the mostsatisfactory manner In developing the design, a number of other criteria must beconsidered such as the properties of material used, the induced stresses, the elasticstability, and the aesthetic appearance of the unit The cost of fabricated vessel is alsoimportant in relation to its service and useful life

3.2 DESIGN OF PRESSURE VESSELS TO CODE SPECIFICATION

American, Indian, British, Japanese, German and many other codes are availablefor design of pressure vessels However the internationally accepted for design ofpressure vessel code is American Society of Mechanical Engineering (ASME)

Various codes governing the procedures for the design, fabrication, inspection,testing and operation of pressure vessels have been developed; partly as safetymeasure These procedures furnish standards by which, any state can be assured of thesafety of pressure vessels installed within its boundaries The code used for unfiredpressure vessels is Section VIII of the ASME boiler and pressure vessel code It isusually necessary that the pressure vessel equipment be designed to a specific code in

order to obtain insurance on the plant in which the vessel is to be used Regardless ofthe method of design, pressure vessels with in the limits of the ASME codespecification are usually checked against these specifications.

3.3 DEVELOPMENT AND SCOPE OF ASME CODE

In 1911, American Society of Mechanical Engineering established a committee toformulate standard specifications for the construction of steam boilers and otherpressure vessels This committee reviewed the existing Massachusetts and Ohio rulesand eonducted an extensive survey among superintendents of inspection departments,Engineers, fabricators, and boiler operators A number of preliminary reports wereissued and revised A final draft was prepared in 1914 and was approved as a code andcopy righted in 1915

The introduction to the code stated that public hearings on the code should be heldevery two years In 1918, a revised edition of the ASME code was issued In 1924, thecode was revised with the addition of a new section VIII, which represented a newcode for unfired pressure vessels

3.4 THE API-ASME CODE

In 1931, a joint API-ASME committee on unfired pressure vessels was appointed

to prepare a code for safe practice in the design, construction, inspection and repair ofunfired pressure vessels

3.5 SELECTION OF THE TYPE OF VESSEL

19

The first step in the design of any vessel is the selection of the type best suited forthe particular service in question The primary factors influencing this choice are,

i The operating temperature and pressure

ii Function and location of the vessel

iii Nature of fluid

■ iv Necessary volume for storage or capacity for processing

It is possible to indicate some generalities in the existing uses of the commontypes of vessels For storage of fluids at atmospheric pressure, cylindrical tanks withflat bottoms and conical roofs commonly used Spheres or spheroids are employed forpressure storage where the volume required is large For smaller volume underpressure, cylindrical tanks with formed heads are more economical

3.6 TYPES OF PRESSURE VESSELS

3.6.1 OPEN VESSELS

Open vessels are commonly used as surge tanks between operations, as vats forbatch operations where materials be mixed and blended as setting tanks, decarters,chemical reactors, reservoirs and so on Obviously, this type of vessels is cheaper thancovered or closed vessel of the same capacity and construction The decision as to

whether or not open vessels may be used depends up on the fluid to be handled andthe operation.

3.6.2 CLOSED VESSELS

1 Combustible fluids, fluids emitting toxic or obnoxious fumes and gases must bestored in closed vessels Dangerous chemicals, such as acid or caustic, are lesshazardous if stored in closed vessels The combustible nature of petroleum and itsproducts associates the use of closed vessels and tanks throughout the petroleum andpetrochemical industries Tanks used for the storage of crude oils and petroleumproducts and generally designed and constructed as per API specification for weldedoil storage tanks

[3.6.3 CYLINDRICAL VESSELS WITH FLAT BOTTOMS AND CONICAL

OR DOMED ROOFS.

The most economical design for a closed vessel operating at atmosphericpressure is the vertical cylindrical tank with a conical roof and a flat bottom restingdirectly on the bearing soil of a foundation composed of sand, gravel or crushed rock

In cases where it is desirable to use a gravity feed, the tank is raised above the ground,and columns and wooden joints or steel beams support the flat bottoms

3.6.4 CYLINDRICAL VESSELS WITH FORMED ENDS

Closed cylindrical vessels with formed heads on both ends used where thevapour pressure of the stored liquid may dictate a stronger design, codes are

21

developed through the efforts of the American petroleum Institute and the AmericanSociety of Mechanical Engineering to govern the design of such vessels Thesevessels are usually less than 12 feet in diameter If a large quantity of liquid is to bestored, a battery of vessels may be used.

3.6.5 SPHERICAL AND MODIFIED SPEHRICAL VESSELS

Storage containers for large volume under moderate pressure are usuallyfabricated in the shape of a sphere or spheroid Capacities and pressures used in thesetypes of yessels vary greatly for a given mass; the spherical type of tank is moreeconomical for large volume, low-pressure storage operation

3.6.6 VERTICAL AND HORIZONTAL VESSELS

In general, functional requirements determine whether the vessel shall be vertical

or jjiorizontal Eg Distilling columns, a packed tower, which utilizes gravity, requirevertical installation

Heat exchanges and storage vessels are either horizontal or vertical If the vessel to beinstalled outdoor, wind loads etc, are to be calculated to prevent overturning, thus jhorizontal ismore economical However, floor space, ground area and maintenance requirements should beconsidered.

3.6.7 VESSELS OPERATING AT LOW TEMPERATURE RANGES

Pressure vessels constructed in such a manner that, a sudden change of sectionproducing a notch effect is present, are usually not recommended for low temperature rangeoperations The reason is that, they may create a state of stress such that the material will beincapable of relaxing high-localized stresses by plastic deformation, therefore, the materialsused for low temperature operations are tested for notch ductility

Carbon steels can be used down to 60 degree C Notch ductility is controlled in such asmaterials through proper composition steel making practice, fabrication practice and heattreatment They have an increased manganese carbon ratio Aluminium is usually added topromote fine grain size and improve notch ductility

Ductility of certain materials including carbon and low alloy steels is considerablydiminished when the operating temperature is reduced below certain critical value is usuallydescribed as the transition temperature, depends upon the material, method of manufacture,previous treatment and stress system present Below transition temperature, fracture may takeplace in a brittle manner with little or no deformation Whereas, at temperatures above thetransition temperature, fracture occurs only after considerable plastic strain or deformation.

3.6.8 VESSELS OPERATING AT ELEVATED TEMPERATURE.

Embrittlement of carbon and alloy steel may occur due to service at elevated temperature

In most instances, brittleness is manifest only when the material is cooled to jK>omtemperature This inhibited by addition of molybdenum and also improve tensile and creepproperties Two main criteria in selecting the steel elevated temperature are metallurgicalstrength and stability Carbon steels are reduced in their strength properties due to rise intemperature and are liable to creep Therefore, the use of carbon steel is generally limited to500dege C

The SA-283 steels cannot be used in applications with temperatures over 340degreC.The SA-285 steels cannot be used for services with temperature over 482degreC However,both SA-285 and SA-285 SA-212 steels have very low allowable stress, at higher temperature

3.7 MATERIAL SPECIFICATION

Plain carbon and low alloy steels plates are usually and where service condition permitbecause of the lesser cost and greater availability of these steels Such steels may me fabricated

by fusion welding and oxygen cutting if the carbon content does not exceed 0.35%.vessels may

be fabricated

Vessel may be fabricated of plate steels meeting the specification of SA-7, SA-113, Grade A,

B, C&D, provided that,

1. operating temperature is between -28degreeC&360degreeC

2. The plate thickness does not exceed 1.5cm

3. The vessels does not contain lethal liquids and gases

4. The steel is manufactured by the electric furnace or open hearth process

5. The material is not used for unfired steam boilers

One of the most widely used steel for general purpose in the construction of

■ressure vessel is SA-283, Grade C This steel has good ductility and forms welds andmachines easily It is also one of the most economical steel suitable for pressure vessels.[However, its use is limited to vessels with plate thickness not exceeding 1.5cm

For vessels having shells of grater thickness SA-285 Grade C is most widely used Himoderate pressure applications In case of high pressure or large diameter vessels, highstrength steel may be used to advantage to reduce the wall thickness SA-212, GradeB is wellsuit for such application and requires a shell thickness of only 79% of that required by SA-285,Grade C This steel also is fabricated but is more expensive than other steels

Now, many new series of materials like low alloy, high alloy steels, high temperatureand low temperature materials are available which can be selected to suit the requirement ofevery individual need of process industry.

The important materials generally accepted for construction of pressure vessels areindicated here Metals used are generally divided into three groups as

carbon and low alloy steel

Nickel, Copper and their alloys, Lead

3. High cost - platinum, Tantalum, Zirconium, Titanium silver

Materials mentioned (2 & 3) groups are some times used in the form of cladding orbonding for materials in group (1) Also, use non-metallic lining such as rubber, plastics, etc

Vessels with formed heads are commonly fabricated from low carbon steel wherevercorrosion and temperature considerations will permit its use because of the low cost, highstrength, ease of fabrication and general availability of mild steel Low and high alloy steel andnon-ferrous metals are used for special service

Steels commonly used fall into two general classifications

1 Steels specified by ASME code

2 Structural grade steels, some of which permitted by ASME.

3.8 CLOSURES FOR PRESSURE VESSELS

All formed heads are fabricated form single circular flat plate by spinning by drawingwith dies in a press Although the cost of heads formed from flat plates involves additional cost

of forming, the use of formed heads as closures usually more economical than the use of flatplates as closures except for small diameters

A variety of formed heads is used for closing the ends of cylindrical vessels Theseinclude flanged only heads, flanged and shallow dished, torispherical, elliptical, hemisphericaland conical shaped heads For special purposes, flat plates are used to close a vessel opening.However flat heads are rarely used for large vessels

For pressures not covered by the ASME code, the vessels are often equipped withstandard dished heads, whereas vessels that require code construction are usually equippedwith either the ASME - dished or elliptical dished heads The most common shape for theclosure of pressure vessels is the elliptical dish Most chemical and petrochemical processingequipment such as distilling columns, desorbers, absorbers, scrubbers, heat exchangers,pressure surge tanks and separators are essentially cylindrical closed vessels with formed ends

of one type or another

As mentioned above, the most common types of closures for vessels under internalpressure are the elliptical dished head (ellipsoidal head) with a major to minor axis ratio equal

to 2.0 : 1.0 and the torispherical head in which the knuckle radius is equal to 6% or more of theinside crown radius (ASME standard dished head)

3.9 FLANGES AND FLANGED FITTINGS

A variety of attachments and accessories are essential to vessels These include flangesfor closures, nozzles, manholes and hand holes and flanges for 2- piece vessels, supportsplatforms, etc,

Flanges may be used on the shell of a vessel to permit disassembly and removal, forcleaning of internal parts Flanges are also used for making connections for piping and fornozzle attachments of opening

A great variety of type and sizes of 'standard' flanges are available for various pressureservices The flanges designated as "American Standards Association (ASME) B 16.5 - 1953"are used for most steel pipelines over 3.8 cm nominal pipe sizes These flanges are called'companion flanges', because they are usually used in pairs Forged steel flanges aremanufactured in the following standards types for all pressure ratings

3.9.1 TYPES OF FLANGES

3.9.1.1 WELDING- NECK FLANGES

A sectional view of a welding - neck flange is shown Welding neck flanges differ fromother flanges in that, they have a long, tapered hub, between the flange ring and the weldedjoint This hub provides a more gradual transition from the flange ring thickness fo the pipe -wall thickness, thereby decreasing the discontinuity stresses and consequently increasing thestrength of the flange These flanges are recommended for the handling of costly, flammable orexplosive fluids, where failure or leakage of the flange joint might disastrous consequences

3.9.1.2 SLIP-ON FLANGES

The slip-on types of flanges are widely used because of its greater ease of aligned inwelding assembly and because of its low initial cost The strength of this flange as calculatedfrom internal pressure considerations is approximately 2/3rd that of a corresponding welding-neck type of flange The use of this type of flange should be ' limited to moderate services,where pressure fluctuations, temperature fluctuations, vibrations and shock are not expected to

be severing The fatigue life of this flange is approximately l/3rd that of welding - neck flange

3.9.1.3 LAP JOINT FLANGES

Lap joint flanges are usually used with a lap-joint stab These flanges have about thesame ability to withstand pressure without leakages as the slip in flange, which is less fhan that

of the welding neck flanges In addition, these flanges have the disadvantages of having onlyabout 10% of the fatigue life of welding neck flanges For these reasons, these flanges shouldnot be used for connections where, severe bending stresses exist

The principal advantage of these flanges is that the bold holes are easily aligned andthis simplifies the erection of vessels of large diameter and usually stiff piping Theses flangesare also useful in cases where, frequent dismantling for cleaning or inspection is required, orwhere it is necessary to rotate the pipe by swiveling the flange

LAPPED FLANGE BLIND FLANGE

SLIP ON FLANGES

D -~—: -~J

Figure: types of flanges

3.10 NOZZLES, OPENINGS AND REINFORCEMENTS

Nozzles and openings are necessary components of pressure vessels for theprocess industries Openings in a cylindrical shell, conical section or closure may producestress concentrations, adjacent to the opening and weaken that portion of the vessel In order tominimize such stress concentrations, it is preferable that the opening be circular in shape As asecond choice the openings may be made elliptical, as a third choice they may be made around

An around opening has two parallel sides and two semicircular ends Openings of other shapesare permissible if the vessel is tested hydrostatically

If the opening in a closure of cylindrical vessel exceed one-half the inside diameter ofshell, the opening and closure should be fabricated Others require reinforcement Small sizes

of openings welded or brazed to a vessel do not require reinforcement

■ In the case of shell, opening requiring reinforcement in vessel under internal pressurethe metal removed must be replaced by the metal of reinforcement In addition to providing thearea of reinforcement, adequate welds must be provided to attach the metal of reinforcementand the induced stresses must be evaluated

Materials used for reinforcement shall have an allowable stress value equal to or greaterthan of the material in this vessel wall except that, when such material is not available, lowerstrength material may be used; provided, the reinforcement is increased in inversed proportion

to the ratio of the allowable stress values of the two materials to the ratio of the two materials

to compensate for the lower allowable stress value of any reinforcement having a higherallowable stress value than that of the vessel wall.

3.11 SUPPORTS FOR VESSELS

Cylindrical and other types of vessels have to be supported by different methods Verticalvessels are supported by brackets, column, skirt, or stool supports, while saddles supporthorizontal vessels The choice of type of support depends on the height and diameter of thevessel, available floor space, convenience of location, operating variables, the size of jjhevessel, the operating temperature and pressure and the materials of construction

Brackets of lugs offer many advantages over other types of supports They areinexpensive, can absorb diametrical expansions by sliding over greased or bronze plates, jfcreeasily attached to the vessel by minimum amounts of welding, and easily leveled or shimmed

in the field Lug supports are ideal for thick-walled vessels, but in thin-walled vessels, this type

of support is not convenient unless the proper reinforcements are used or many lugs are welded

to the vessel

It is also necessary to ensure that, the attachment of the support to the vessel, which isusually by fillet welds should be able to transfer the load safely from vessel to support andthat, the support should be strong enough to withstand the load of the vessel

3.11.1 SKIRT

Vertical vessels are normally supported by means of suitable structure resting on areinforced concrete foundation This support structure between the vessel and the j&undationmay consist of a cylindrical shell termed as skirt The skirt is usually welded to the vesselbecause the skirts are not required to withstand the pressure in the vessel; the selection ofmaterial is not limited to codes The skirt may be welded directly to the bottom dished head,flush with the shell or to the outside of shell There will be no stress from internal and externalpressure for the skirt, unlike for the shell, but the stresses from dead weight and from wind orseismic bending moments will be maximum.

3.12ANCHOR BOLT

The bottom of skirt of vessel must be securely anchored to the concrete !foundations bymeans of anchor bolts embedded in the concrete to prevent over turning from bendingmoments induced by seismic and wind loads

The concrete foundation is poured with adequate reinforcing steel to carry tensile loads.The anchor bolts may be formed from steel rounds threaded at one end and usually with acurved or hooked end embedded in the concrete will bond to the embedded surface of thesteel

3.13METHODS OF FABRICATION

Process equipment is fabricated by a number of well-established methods such asfcsion welding, casting, forging, machining, brazing and soldering and sheet metal forming.Each method has certain advantages for particular types of equipment However, fusionwelding is the most important method The size, shape, service and material properties of theequipment all may influence the selection of the fabrication method

Gray iron casting have been widely used for the mass production of small pipe fittingsand are used to a considerable extent for large items such as cast iron pipe, heat exchangershells and evaporator bodies because of the superior corrosion resistance of cast iron ascompared with steel Large diameter vessels cannot be easily cast, and the strength of grayiron is not reliable for pressure vessels service Cast steel may be used £>r small diameterthick walled vessels Further more, because of its higher strength and greater reliability ascompared with cast iron; it is more suitable for high-pressure service where metal porosity is

not a problem The vessel diameter is still limiting because of a problem in casting Alloy caststeel vessels can be used for high-temperature and high-pressure installation.

: Forging is a method of shaping metal that is commonly used for certain vessel jpartssuch as closures, flanges and fittings Vessels with wall thickness greater than 10cm ire oftenforged Other special methods of shaping metal such as pressing, spinning and rolling of platesare used for forming closures for vessel shells

Riveting was widely used prior to the improvement of modern welding !fcchniques, formany different kinds of vessels, such as storage tanks, boilers and a verity |tf pressure vessels

It is still used for fabrication of non-ferrous vessels such as copper and aluminium However,welding techniques have become so advanced, that even these materials are often weldedtoday

I Machining is the only method other than cold forming that can be used to exact tecuretolerances Close tolerances are required for the mating parts of the equipment Flange faces,bushings, and bearing surfaces are usually machined in order to provide satisfactoryalignment Laboratory and pilot plant equipment for very high-pressure service is sometimesmachined for solid stock, pierced ingots and forgings

3.13.1 FUSION WELDING

It is the most widely used method of fabrication for the construction of steel vessels.This method of construction is virtually unlimited with regard to size and is extensively usedfor the fabrication and erection of large size product equipment in the field There are twotypes of fusion welding that are extensively used for fabrication of welds These are,

1. The gas welding process in which a combustible, mixture of acetylene andoxygen supply the necessary heat for fusion

2. The electric arc welding process, in which the heat of fusion is supplied by anelectric arc Arc welding is preferred because of the reduction of heat in theweld material, reduces the oxidation and better control of deposited weld metal

3.14 PRESSURE TESTING OF CODE VESSELS

All pressure vessels designed to code specification except those exempted because ofsmall size must be tested hydrostatically, pneumatically or by means of "PROOF TEST"

In the case of hydrostatic test, the vessel must be subjected to a hydrostatic testpressure at least equal to one and a half times the maximum allowable pressure at the testtemperature Following the application of the test pressure, all joints and connections snust beinspected with the vessel under a pressure not less than 2/3'd of the test pressure [Althoughwater is used in this test, any non-hazardous liquid may be used below its boiling temperature

If the vessels are designed so that they camiot safely be filled with water (as in the case

of tall vertical towers design to handle vapours pneumatically), testing may be used Thepneumatic test pressure should be at least 1.25 times the maximum allowable pressure at thetest temperature In conducting pneumatic test, the pressure in the vessel should be graduallyincreased to not more than half the test pressure There after the test pressure should be raised

in increments of 1/10th of test pressure until the test pressure is reached Following these, thepressure should be reduced to maximum allowable pressure and held there for a sufficientlength of time to permit inspection of vessel

The "proof test" can be used to establish the allowable working pressure in Vessels thathave parts, which the stress cannot be computed with satisfactory accuracy In one procedure

of this test, all areas of probable high stress concentration are painted with a wash of lines oranother brittle coating The pressure is raised and the vessel is inspected for signs of yieldingindicated by flaking or strain lines in the wash The vessel is first observed.

Strain gauge measurements may be used in non destructive testing In this case thepressure is increased in increments of 1/10th the test pressure, each increment

I followed by relaxation of the pressure, until a permanent strain of 0.2% is reached The

Vessel rating at the test temperature is equal to one-half the pressure producing thisjpennanent strain A modification of the strain gauge measurement procedure is also kennitted

by the code This method involves the use of measuring gauges at diametrically opposedreference points in symmetrical structure

In another version of the proof test, a sample used is tested to destruction and I

identical vessels are rated at the test temperature at l/5th the pressure at which the tested vessels

is failed

CHAPTER.4 DESIGN PROCEDURE 4.1 DESIGN

OF SHELL AND ITS COMPONENTS

iijhe pressure vessel considered here is a single unit when fabricated However, for the Jonvenience of design, it is divided into the following parts J (1) Shell; (2)head or cover; (3) nozzles; (4) support;

Most of the components are fabricated from plates or sheets Seamless or weldedpipes can also be used Parts of vessels formed are connected by welded or riveted joints

In designing these parts and connections between them, it is essential to take r intoaccount, the efficiency of joints For welded joints, the efficiency may be taken

as 100% if the joint is fully checked by a radiograph and taken as 85%, eve if it is

checked at only a few points If the radiographic test is not carried out 50 to 80%, Iefficiency is taken Efficiencies vary between 70 to 85% in the case of riveted joints

All these are made for pressure vessels operating at pressures less than 200kg/kmA2.Design procedure is primarily based on fabrication by welding

4.1.1 DESIGN OF CYLINDRICAL SHELLS UNDER PRESSURE

The equation for determining the thickness of cylindrical shells of vessels underinternal pressure are based upon a modified membrane-theory equation The modificationempirically shifts the thin wall equation to approximate the "Lame" equation for thick-walled vessel's shown above

'4.2 WELDING STANDARDS

r

The success of fabrication by welding is dependent upon the control of the weldingvariables such as experience and training of the welder, the use of proper materials, andwelding procedures An inexperienced welder or welder using inferior materials, incorrectprocedures can fabricate a vessel that has a good appearance but has unsound joints, whichmay fail in service Thus, it is essential that the welding variables be controlled in order toproduce sound joints in the equipment A number of codes and standards have been publishedfor the puipose

The American welding society (AWS) established the basic standards for quantifyingoperators and procedures These standards of qualification form the basis of most of the