Pressure vessels pdf download

Translate PDF. The significantconservatism underupper-shelfconditionsprovidedby the first approach has been quantified in terms of internal pressure and wall-thickness strain gradient. Monotonic crack growth withstood by the assessed component has been evaluated through several elastic-plastic criteria, and leak-before-break and related crack-arrest events have been inferred to be likely the deeper the postulated pre-crackis. Researchresultsindicatethat logarithmicJ-R curve datafitting is more appropriate and conservative than the conventionally used power law in regard to extensive crack propagation.

All rights reserved. Specifically, the The nuclear grade steelASTM ACL3A testedin the as- adoption of the latter approach in assessmentplans of received condition presentedyield strength, Sy, of MPa crack-like defects in PWR vesselshas only recently been and ultimate tensile strength, Su, of MPa at a temper- formally accepted. Charpy impact results indicated an upper- mate the instability conditions for the system performing shelf energy exceeding J for this moderate-strength under fully ductile response following both approaches, low-alloy steel.

In this paper, suchanalytical comparisonis accomplished The cylindrical PWR vessel, with mean radius, R, of by consideringseveral crack types located at the belt-line of mm and wall-thickness, t, of mm is shown an intermediate size PWR pressurized light water cooled schematically in Fig. Tarpani, D. Eight axial radially propagating cracks in the PWR vessel wall. The stress intensity K for outside surface cracks adjusted by conventionally used power law and, conversely, with depth a, and 2a for embedded cracks, axially sited in a logarithmic fit was attempted.

I1 geometric correction terms. JApp is directly related to the The instability estimations according to J50 and JINsr were internal pressure, P, in eqn 3 via SM, the maximum stress defined as intervals, with the upper limit set by a power law peak attained at the inside surface of the vessel wall, by the J-TMAT curve extrapolated from the maximum crack Tresca relation. Experimental J-Au data crack types. This inspection programs by testing small-scale specimens.

VIII crack. Spinelli compatibility between the 2T specimen widths and the vessel wall-thickness, it can be stated that this testpiece provides more realistic failure predictions than does 1T specimens, particularly as concerns axial cracks growing in the radial direction of the cylindrical component, as assessed herein.

Only 2TCT results were therefore used in quantifying linear elastic and elastic-plastic instability con- ditions of the pressure vessel. In Fig. J5,, and JINsT values, whose upper limits are designated B and D, respectively, are also indicated for a particular postulated pre-existent crack. The extended elasto-plastic stress correction term and the Ramberg-Osgood stress-strain relation for the A. All the vessel Fig. On the other hand, J50 predictions integrity. In this sense,deeper cracks may by-pass the and length, 1, surface cracks are much more harmful than instability by arresting owing to vesseldepressurization,or through-cracks.

Also, it is verified that crack depth has at least be prematurely identified owing to leakage. On the much more influence on failure predictions, for both other hand, shallow cracks may suddenly become unstable LEFM and EPFM, than doescrack length. In fact, in dealing without giving prior signs. This reasoningis schematically with shallow embeddedcracks, crack length haspractically illustrated in Fig.

It must be remarked that this no effect on the results. In thesecases,spreadplasticity predicted by linear elasticity-by considering sub-critical effects are anticipated, which are quite beyond the linear crack growth and constant instability stress,which is based elasticity premises. So, LEFM results shouldbe considered on the well-known concept of critical crack depth.

Finally, Figs. Spinelli - - 11 Fig. Ductile crack growth levels at the threshold of vessei instability. Logarithmic fit is more suitable than the traditionally practice, particularly for J-R and J-T extrapolation pro- usedpower law in adjusting J-R curve data points for cedures, when conservative approaches are favored as large relative crack extension levels. The con- long as crack growth levels are outside any JD limits of servatism of the failure predictions supplied by this validity.

Ductile crack initiation Ji is an overly conservative approach to elastic-plastic fracture mechanics. Therefore, J. American Society service in the nuclear power industry. Linear elastic and elastic-plastic fracture mechanics 2. American Society of cracks and embedded cracks in general.

In these Mechanical Engineers, New York, Tarpani, J. The probability of leak-before-ductile instability is Portuguese. In this 4. Analytical failure predictiom 5. Rolfe, T. 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 temperature and low temperature materials are available which can be selected to suit the requirement of every individual need of process industry.

The important materials generally accepted for construction of pressure vessels are indicated here. Metals used are generally divided into three groups as. Low cost Cast iron, Cast carbon and low alloy steel, wrought carbon and low alloy steel. High cost - platinum, Tantalum, Zirconium, Titanium silver. Also, use non-metallic lining such as rubber, plastics, etc.

Vessels with formed heads are commonly fabricated from low carbon steel wherever corrosion and temperature considerations will permit its use because of the low cost, high strength, ease of fabrication and general availability of mild steel. Low and high alloy steel and non-ferrous metals are used for special service. Steels commonly used fall into two general classifications. Steels specified by ASME code. Structural grade steels, some of which permitted by ASME.

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 flat plates as closures except for small diameters. A variety of formed heads is used for closing the ends of cylindrical vessels.

These include flanged only heads, flanged and shallow dished, torispherical, elliptical, hemispherical and 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 with standard dished heads, whereas vessels that require code construction are usually equipped with either the ASME - dished or elliptical dished heads.

The most common shape for the closure of pressure vessels is the elliptical dish. Most chemical and petrochemical processing equipment 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 internal pressure are the elliptical dished head ellipsoidal head with a major to minor axis ratio equal to 2.

These include flanges for closures, nozzles, manholes and hand holes and flanges for 2- piece vessels, supports platforms, etc,. Flanges may be used on the shell of a vessel to permit disassembly and removal, for cleaning of internal parts. Flanges are also used for making connections for piping and for nozzle attachments of opening. A great variety of type and sizes of 'standard' flanges are available for various pressure services.

These flanges are called 'companion flanges', because they are usually used in pairs. Forged steel flanges are manufactured in the following standards types for all pressure ratings. Welding neck flanges differ from other flanges in that, they have a long, tapered hub, between the flange ring and the welded joint.

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 the strength of the flange. These flanges are recommended for the handling of costly, flammable or explosive fluids, where failure or leakage of the flange joint might disastrous consequences.

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. These flanges have about the same ability to withstand pressure without leakages as the slip in flange, which is less fhan that of the welding neck flanges.

For these reasons, these flanges should not be used for connections where, severe bending stresses exist. The principal advantage of these flanges is that the bold holes are easily aligned and this simplifies the erection of vessels of large diameter and usually stiff piping. Theses flanges are also useful in cases where, frequent dismantling for cleaning or inspection is required, or where it is necessary to rotate the pipe by swiveling the flange..

It can be connected instantly without welding. The only disadvantage is that possibility of leakage. In this application, a valve followed by blind flange is frequently used at the end of line to permit addition of line while it is 'on stream'. Openings in a cylindrical shell, conical section or closure may produce stress concentrations, adjacent to the opening and weaken that portion of the vessel. In order to minimize such stress concentrations, it is preferable that the opening be circular in shape.

As a second 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 shapes are permissible if the vessel is tested hydrostatically. If the opening in a closure of cylindrical vessel exceed one-half the inside diameter of shell, 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 addition to providing the area of reinforcement, adequate welds must be provided to attach the metal of reinforcement and the induced stresses must be evaluated. Vertical vessels are supported by brackets, column, skirt, or stool supports, while saddles support horizontal vessels. The choice of type of support depends on the height and diameter of the vessel, available floor space, convenience of location, operating variables, the size of jjhe vessel, the operating temperature and pressure and the materials of construction.

Brackets of lugs offer many advantages over other types of supports. They are inexpensive, can absorb diametrical expansions by sliding over greased or bronze plates, jfcre easily 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 is usually by fillet welds should be able to transfer the load safely from vessel to support and that, the support should be strong enough to withstand the load of the vessel.

The skirt is usually welded to the vessel because the skirts are not required to withstand the pressure in the vessel; the selection of material 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 external pressure for the skirt, unlike for the shell, but the stresses from dead weight and from wind or seismic bending moments will be maximum.

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 a curved or hooked end embedded in the concrete will bond to the embedded surface of the steel. Each method has certain advantages for particular types of equipment. However, fusion welding is the most important method. The size, shape, service and material properties of the equipment all may influence the selection of the fabrication method.

Gray iron casting have been widely used for the mass production of small pipe fittings and are used to a considerable extent for large items such as cast iron pipe, heat exchanger shells and evaporator bodies because of the superior corrosion resistance of cast iron as compared with steel. Large diameter vessels cannot be easily cast, and the strength of gray iron is not reliable for pressure vessels service.

The vessel diameter is still limiting because of a problem in casting. Alloy cast steel vessels can be used for high-temperature and high-pressure installation. Vessels with wall thickness greater than 10cm ire often forged.

Other special methods of shaping metal such as pressing, spinning and rolling of plates are used for forming closures for vessel shells. Riveting was widely used prior to the improvement of modern welding! 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 welded today. Machining is the only method other than cold forming that can be used to exact tecure tolerances.

Close tolerances are required for the mating parts of the equipment. Flange faces, bushings, and bearing surfaces are usually machined in order to provide satisfactory alignment. Laboratory and pilot plant equipment for very high-pressure service is sometimes machined for solid stock, pierced ingots and forgings. This method of construction is virtually unlimited with regard to size and is extensively used for the fabrication and erection of large size product equipment in the field.

There are two types of fusion welding that are extensively used for fabrication of welds. The gas welding process in which a combustible, mixture of acetylene and oxygen supply the necessary heat for fusion 2.

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

In the case of hydrostatic test, the vessel must be subjected to a hydrostatic test pressure at least equal to one and a half times the maximum allowable pressure at the test 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. The pneumatic test pressure should be at least 1. In conducting pneumatic test, the pressure in the vessel should be gradually increased to not more than half the test pressure.

Following these, the pressure should be reduced to maximum allowable pressure and held there for a sufficient length of time to permit inspection of vessel. The "proof test" can be used to establish the allowable working pressure in Vessels that have parts, which the stress cannot be computed with satisfactory accuracy.

The pressure is raised and the vessel is inspected for signs of yielding indicated by flaking or strain lines in the wash. The vessel is first observed. Strain gauge measurements may be used in non destructive testing. The Vessel rating at the test temperature is equal to one-half the pressure producing this jpennanent 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 opposed reference points in symmetrical structure.

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 welded pipes can also be used. Parts of vessels formed are connected by welded or riveted joints. Design procedure is primarily based on fabrication by welding. The modification empirically shifts the thin wall equation to approximate the "Lame" equation for thick- walled vessel's shown above.

An inexperienced welder or welder using inferior materials, incorrect procedures can fabricate a vessel that has a good appearance but has unsound joints, which may fail in service. Thus, it is essential that the welding variables be controlled in order to produce sound joints in the equipment. A number of codes and standards have been published for the puipose. The American welding society AWS established the basic standards for quantifying operators and procedures.

For practical purposes, therefore the rules for qualifying welders and welding procedures are essentially the same in the various codes and standards. Each fabrication shop should establish welding procedures best suited to its need and its equipment. To meet the welding standards previously mentioned, it is not necessary that, regardless of the procedures used, the welded joints must pass the qualification tests for welding procedures.

To meet welding standards, welded joints must be tested to determine tensile strength, ductility, and soundness. The required tests for the welding procedures specified by API standard 12C involved the following.

For Groove Welds: 1. Free bend test for ductility I 3. Side bend test for soundness i B. For fillet welds: 1. Fillet weld - soundness test The minimum results required by the tests such as those listed above are described in detail in the various codes.

A few representative requirements are: a. In the various soundness tests, the convex surface of the specimen is examined for the appearance of cracks or other defects. If any cracks exceeds 0. The selection of the type of joint depends upon the service, the thickness of the metal fabrication procedure and code requirements. The following figure is a diagram from the API-ASME code for unified pressure vessels which illustrates some of the types of welded joints used in the welding of steel plates for the fabrication of pressure vessels.

This may be result of metallurgical discontinuities and residual stress. The code rules make allowance for these factors by specifying joint efficiencies for various types of welds with and with out stress relief and radiographing. The designs are permitted some option in the selection of the kind of weld joint to be and in whether or not, the welded joints must be radio graphed. In addition, vessels having a shell thickness greater than 1. In addition, steel greater than 2.

The increase in joint efficiency may be used if these steels are heat treated at over x C. The code gives the temperature and describes the procedures to be used in thermal stress relieving. Radiography examination is required for double welded butt joints. If the plastic thickness is greater than 2.

It has been found from experience that, an allowance may be made for such Weakness by introducing a "joint efficiency factor E" in the equations. This factor is llways less than unity and is specified for a given type of welded construction in the prarious codes. The thickness of the metal, C allowed for any anticipated corrosion is then added to the calculated required thickness, and the final thickness value rounded off to the nearest nominal plate size of equal or greater thickness.

In no case, shall the temperature of surface of metal exceed the maximum temperature listed in the stress tables for materials not exceed the maximum temperature limitation specified elsewhere in ASME section VIII. The thickness computed for the applicable loadings in UG plus the thickness added for corrosion allowance in the connections.

Smaller of the following. For vessels under internal pressure only, the thickness required for pressure for the shell at the location. Where the nozzle neck attached to the vessel, but on no case less than the minimum thickness specified for the material in UG - VIII, div 1.