fabrication, inspection, and testing of pressure vessels

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcosemployees. Any material contained in this document which is not alreadyin the public domain may not be copied, reproduced, sold, given, ordisclosed to third parties, or otherwise used in whole, or in part, withoutthe written permission of the Vice President, Engineering Services, SaudiAramco.

Chapter : Vessels For additional information on this subject, contactFile Reference: MEX20204 J.H. Thomas on 875-2230

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Fabrication, Inspection, andTesting of Pressure Vessels

Engineering Encyclopedia Vessels


Saudi Aramco DeskTop Standards

CONTENTS PAGE

EVALUATING FABRICATION DRAWINGS FORACCEPTABILITY................................................................................................... 1

Welding Fundamentals................................................................................. 1

Types of Welded Joints ................................................................................ 6

Groove Welds ................................................................................... 7

Fillet Welds....................................................................................... 9

Plug Welds...................................................................................... 11

Weld Joint Categories ..................................................................... 11

Welding Procedures and Welder Qualification .......................................... 12

Welding Procedures........................................................................ 12

Welder Qualification....................................................................... 16

Acceptable Welding Details ....................................................................... 17

Saudi Aramco Weld Detail Requirements ...................................... 17

ASME Weld Detail Requirements .................................................. 17

Tolerances .................................................................................................. 18

Heads and Shells............................................................................. 19

Plate Thickness ............................................................................... 20

Alignment ....................................................................................... 20

DETERMINING WHETHER VENDOR INSPECTION ANDTESTING PLANS SATISFY SAUDI ARAMCO REQUIREMENTS.................. 22

Methods of Examination ............................................................................ 24

Radiographic Examination (RT) ..................................................... 25

Visual Inspection (VT).................................................................... 26

Liquid Penetrant Examination (PT) ................................................ 26

Magnetic Particle Test (MT)........................................................... 28

Ultrasonic Examination (UT).......................................................... 30



Saudi Aramco DeskTop Standards

Type and Extent of Required Examination ................................................ 33

Pressure Test Plans..................................................................................... 34

Hardness Test Plans.................................................................................... 42

Brinell Hardness Test...................................................................... 44

Vickers Hardness Test .................................................................... 44

Hardness Test Results ..................................................................... 44

Impact Test Plans ....................................................................................... 46

Applicable Codes and Standards ................................................................ 48

SUMMARY........................................................................................................... 49

Work Aid 2B: Procedure for Pressure Test Plans ..................................... 60

Work Aid 2D: Procedure for Impact Test Plans........................................ 65

GLOSSARY .......................................................................................................... 66



Saudi Aramco DeskTop Standards 1

EVALUATING FABRICATION DRAWINGS FOR ACCEPTABILITY

Welding is the most common method that is used for pressure vessel fabrication; therefore,this section focuses on welding. Welding fundamentals and details are discussed to the extentnecessary for a pressure vessel engineer to achieve an adequate knowledge of weldingrequirements as they relate to pressure vessels. Additional welding engineering details arebeyond the scope of this course.

This section also discusses dimensional tolerances which must be applied to pressure vesselcomponents and fabrications. Adherence to relatively stringent dimensional tolerances isnecessary to help achieve quality pressure vessel fabrication and acceptable long termreliability.

Saudi Aramco fabrication requirements supplement those that are contained in the ASMECode, Section VIII, Divisions 1 and 2. Saudi Aramco fabrication requirements are containedprimarily in SAES-D-001, Design Criteria for Pressure Vessels; 32-SAMSS-004, PressureVessels; SAES-W-001, Basic Welding Requirements; and SAES-W-010, WeldingRequirements for Pressure Vessels. Relevant Saudi Aramco and ASME requirements arehighlighted within the topics that are discussed in this section.

This section discusses only Division 1 requirements. Division 2 requirements are generallymore stringent than those that are contained in Division 1. Participants are referred toDivision 2 for additional details as required.

Welding Fundamentals

A weld is defined as a localized union of metal that is achieved in plastic and molten states,with or without the addition of filler metal or the application of pressure. Welding is used inthe fabrication of pressure vessels for both pressure containing parts (for example, shells andheads) and nonpressure containing parts (for example, stiffener rings, lifting lugs, andsupports). Joints that are welded instead of bolted are also sometimes used for pipe-to-equipment connections in situations where the leakage potential of a bolted joint must beeliminated.

The most common welding method is called fusion welding. The fusion welding methoddoes not require any pressure to form the weld. The seam that is to be welded is heated,usually by means of burning gas or through the use of an electric arc which is brought tofusion temperature. Additional metal, if needed, is supplied by melting a filler rod into theweld area. The filler rod is made of a material whose composition is similar to that of thepieces that are being joined. The most widely used industrial welding method is arc welding.Arc welding is the general name that is given to several welding processes that generate theheat of fusion by the use of an electric arc.




An arc welding circuit consists of the following elements:

Power source

Two cables (the electrode cable and the ground cable)

Ground clamp

Electrode holder

Electrodes or rods

Two types of power supplies are used for arc welding: the direct current (dc) generator andthe alternating current (ac) transformer. The choice of power supply depends on theparticular welding that is to be done. Figure 1 shows a typical ac arc welding circuit.




Typical AC Arc Welding Circuit

Figure 1

Regardless of the power source that is used, the electric arc that is produced serves the samepurpose: it produces heat to melt the metal. The two pieces of metal that are to be joined areplaced such that they are nearly touching. The arc from the electrode is directed at thejunction of the two pieces. This arc causes the edges of both pieces to melt. The moltenportions of the pieces flow together along with molten portions of the electrode. As the arccolumn is moved along the joint, the molten material solidifies. The two pieces are thenjoined with a combination of electrode metal and base metal.

The molten pool of weld metal reaches a temperature of approximately 1536C (2800F), andthere is a temperature gradient into the nearby base metal. The portion of the base metal thatis adjacent to the weld and that is affected by the welding heat is called the heat-affected zone(HAZ). Figure 2 shows a typical HAZ.




Heat-Affected Zone

Figure 2

Because the welding heat changes the crystal structure and grain size of the HAZ, a postweldheat treatment (PWHT) may be necessary to restore the material structure to the requiredproperties. The need for PWHT for these metallurgical reasons depends on the materials thatare involved and the service conditions that they are exposed to. As the weld metal and HAZcool from the very high welding temperatures, the thermal contraction that occurs in thelocally heated area is resisted by the cooler base metal that surrounds the locally heated area.This resistance to thermal contraction results in residual stresses that remain in the structure.For thicker plates, these residual stresses must be removed by PWHT. PWHT requirementsbased on stress relief considerations are contained in the ASME Code, Section VIII, and werediscussed in MEX 202.02. PWHT is discussed in more detail later in this module.




Most modern welding electrodes are coated with a flux. As the electrode wire melts, the fluxthat coats the wire burns and produces a gaseous shield around the electric arc. This gaseousshield prevents contamination of the weld by protecting the molten metal from contaminantsthat are in the atmosphere. Figure 3 shows a coated electrode weld deposit and the areaaround the weld.

Coated Electrode Weld Deposit

Figure 3

When the electrode flux melts, part of it mixes with impurities that are in the molten pool andcauses these impurities to float to the top of the weld. When this mixture of impurities andflux cools, it forms a slag. The slag protects the weld bead from the atmosphere and causesthe weld bead to cool more uniformly. The slag also helps to form the contour of the weldbead by acting as an insulator. The slag allows an even heat loss from the local area byinsulation of the weld and HAZ. This even heat loss helps to control the grain structure of themetal. The slag is chipped away after each weld pass before slag is deposited by another weldpass; otherwise, weld defects will be caused. In order to permit later weld inspections, theslag is also chipped away when the metal has cooled after the final weld pass.




Types of Welded Joints

Welded joints are described by the position of the pieces that are to be joined and are dividedinto five basic types: butt, tee, lap, corner, and edge. For design purposes, welds can bedivided into three basic types which call for different design methods. These weld types are:groove, fillet, and plug. Figure 4 shows several examples of welded joints and weld types.

Examples of Welded Joints and Weld Types

Figure 4

Note that, in some cases, a given joint type may employ only one weld type, such as thegroove weld that is used in the butt joint. Other joint types may employ two weld types, suchas the groove and fillet welds that are used in the corner joint.




The choice of the joint and weld type that is to be used in each case depends on the following:

Saudi Aramco and ASME Code requirements.

The geometric relationship between the parts that are being joined and theaccess that is available for welding.

Economic considerations.

Groove Welds

Groove welds are subdivided based on the shape of the edges of the groove welds. Figure 5shows the primary types of groove welds that are used in pressure vessel fabrication. Butt- ortee-type joints with V or double-bevel groove welds are the most common weld joint typesthat are used in pressure vessel fabrication.

Types of Groove Welds

Figure 5




For example, butt-type joints are used to join pressure vessel shell and head plate sectionstogether. Tee-type joints are used to join nozzles to shell or head sections.

Note in Figure 5 that the edges of the pieces that are to be joined are cut from their initiallysupplied straight configuration into some form of bevel. The cut edges are called "edgepreparation." The primary pressure containing welds in pressure vessels must be designed forfull penetration (that is, the weld penetrates through the complete thickness of the metals thatare joined) and for full fusion (that is, the weld metal is completely fused to the base metaland to itself throughout the full thickness). The full penetration requirement is stated in theASME Code, 32-SAMSS-004, and in SAES-W-001. The ASME Code also specifies the fullfusion requirement.

The type of edge preparation that is used depends on the following factors:

The thickness of the parts that are being joined.

The particular welding process that is being used.

Whether the weld will be made in the shop with automatic equipment orwhether it will be made manually.

For thicker plates with access for welding from both sides, double bevel groove welds areused, and the weld is completed from both sides to help ensure full penetration and fusion.The angle of the bevel face is also specified to ensure that the welding electrode has completeaccess to the bottom of the weld area. The bottom of the weld area is called the "root" of theweld. The parts that are being joined are separated by a small distance, called the "root gap."

As the thickness of the parts that are to be joined increases, the width of the open area at thesurface of the weld for a V-groove weld preparation increases because the bevel angle isconstant through the entire thickness. This extra width requires a larger amount of weld metalto make the closure. This extra weld metal increases the cost of fabrication for both materialand labor. The J or U groove-type weld preparations are more frequently used in thickfabrications. With these J- or U-groove weld geometries, the weld root is completelyaccessible, but the total amount of open area that is to be filled with weld metal is reduced incomparison to the V-groove preparation. The weld preparation cost is more for a J- or U-groove weld. However, when thick components are being joined, the total weld cost is lessfor a J- or U-groove due to the reduced actual welding time and material.




The strength of a groove weld is based on the following:

Cross-sectional area that is subject to shear, tension, or compression.

Allowable stress of the weld metal (which is nearly always the same as that ofthe parts that are to be joined).

Stresses in groove welds are computed through the use of standard formulas for tension,bending, and shear. The full penetration groove weld is the most reliable of all weld types.There are no significant stress concentration effects in a full penetration groove weld becausethere are no abrupt geometric discontinuities. The joint efficiency is specified by the ASMECode and depends on the type of weld examination that is used, as was discussed in MEX202.03.

Fillet Welds

A fillet weld has a triangular cross section and joins two surfaces that are typically at rightangles to each other. Figure 6 shows fillet welds at a tee-joint, and defines some relevantterms.

Fillet Weld at a Tee Joint




Figure 6




The size of a fillet weld is specified by the leg length, w, of its largest inscribed right triangle.A 45 fillet weld with legs of equal size is the most common and economical type of filletweld. No edge preparation is required for a fillet weld. This lack of edge preparation lowersthe cost to make a fillet weld. However, the allowable stress of a fillet weld is also lower thanthat of a groove weld. Stress concentrations at the root and toe of a fillet weld can causefatigue failure under cyclic loading conditions. Fillet welds are never used as the primarypressure-retaining weld in pressure vessel construction. Fillet welds are primarily used toattach reinforcing pads, stiffener rings, and other attachments to the main pressure-containingparts.

The stresses in fillet welds are complex because of the eccentricity of the applied load, theweld shape, and stress concentration effects. These stresses consist of shear, tension, andcompression stresses. The stress distribution is not uniform across the throat and leg of a filletweld and varies along the length of the fillet weld. However, practical assumptions are madewith regard to the fillet weld geometry and applied load in order to simplify design.

Where fillet welds are used for attachments to a pressure vessel, SAES-D-001 requires thatthe weld be continuous. A continuous fillet weld is required to prevent the occurrence ofcorrosion between the attachment and the vessel due to corrosive fluid being trapped betweenthe two parts.

Plug Welds

A plug weld is a circular weld that is made through one member of a lap or tee-type joint.Plug weld holes in thin plates are completely filled with weld metal through the entire platethickness. Plug weld holes are typically only partially filled in plates that are about 9.5 mm(3/8 in.) thick and over. Plug welds are most often used in pressure vessel construction to fixa corrosion-resistant strip lining into an existing vessel.

Weld Joint Categories

The ASME Code, Section VIII, Division 1, defines weld joint "categories" by the location ofa joint in a vessel. The joints that are included in each category are designated as CategoriesA, B, C, or D. The Categories are used by the ASME Code in the specification of joint typeand degree of inspection for certain welded pressure-containing joints. Recall that thesecategories were used in MEX 202.03 in the discussion of weld joint efficiency.

As previously discussed, the ASME Code specifies, in Table UW-12, the weld joint types thatmay be used in each Category. The following are examples of specifications in Table UW-12:

Buttwelded joints that are made by double-welding (i.e., welded from bothsides) or by other means which will obtain the same weld metal quality on the




inside and outside weld surfaces may be used for all joint categories. This isthe most commonly used weld type for the weld seams of the main pressurevessel because it results in the best weld joint efficiencies. If a metal backingstrip is used for this weld, the metal backing strip cannot remain in place.

A single-welded butt joint with a backing strip also could be used for all jointcategories in the ASME Code, but such a joint achieves lower joint efficiencies.However, both 32-SAMSS-004 and SAES-W-001 prohibit the use ofpermanent backing strips.

The ASME Code permits the use of a single-welded butt joint without abacking strip for Categories A, B, and C; but the code allows such a joint onlyfor circumferential butt joints that are not over 16 mm (0.625 in.) thick and thatare not over 610 mm (24 in.) in outside diameter. From a practical standpoint,the allowable weld joint efficiency is so low for this type of joint that it istypically not used for pressure vessels in refinery applications.

Economics is a consideration in the determination of what weld joint efficiency and weld typeto use. Higher weld joint efficiencies reduce the required component thicknesses, whichreduce material and fabrication costs. However, these cost reductions come at the expense ofmore expensive weld joint preparations and inspection.

Welding Procedures and Welder Qualification

The achievement of high quality pressure vessel fabrication requires the use of tested weldingprocedures as well as qualified welders or welding machines. The ASME Code, Section VIIIcontains rules for the mechanical design, fabrication, and testing of pressure vessels. TheASME Code, Section IX covers welding procedures and welder qualifications, and the use ofSection IX is specified in SAES-W-001. Section IX is not covered in this section. However,several welding procedure and welder qualification requirements are highlighted in thefollowing paragraphs.

Welding Procedures

The pressure vessel designer determines the basic type and size of weld and the weld jointconfiguration to use in vessel fabrication. The welding engineer, on the other hand, mustspecify exactly how the vessel components are to be welded together, based on the followingparameters:




Material of components

Thickness of components to be joined

Diameter of components to be joined

Position and direction of welding

Type of weld bevel to use (e.g., V, U, J, one side, both sides)

Welding process (including variables such as the welding speed, shielding gas,and flux)

Electrode

DC or AC electric current

Voltage and current levels

Manual or automatic welding

Preheat temperature and, possibly, PWHT procedures

The welding engineer produces a welding procedure that details exactly how the weld is to bedone and considers the parameters that are listed above. Each weld joint type in a pressurevessel has its own welding procedure. When a welding procedure is developed, a welder usesthe procedure to weld a sample piece, and the sample weld is inspected and tested. When thesample weld is approved, the procedure is said to be "qualified": that is, the weldingprocedure has been shown to produce sound welds for the intended application. Pressurevessel fabricators have well established welding procedures that are available for the types ofwelds and materials that they normally use. Therefore, welding procedures do not have to bequalified for every new pressure vessel that is fabricated. Additional welding procedures arequalified only for new welds that the vessel fabricator has not made before.

Saudi Aramco welding procedure requirements are contained in SAES-W-001, Basic WeldingRequirements. Several of these SAES-W-001 requirements that go beyond the ASME Codeare highlighted as follows:

Welding procedures must be submitted to Saudi Aramco for review andapproval prior to the start of work. This review and approval procedure avoidsthe potential problem caused by welds being made by means of unacceptableprocedures and by the need to then determine whether these welds can beaccepted or whether they must be remade.

A weld map, drawing, or table that specifies exactly where each weldprocedure will be applied must be provided by the vessel manufacturer. Thisinformation simplifies the review process, helps ensure consistency between




procedure and weld, and assists maintenance personnel should repairs oralterations be required later.




Additional requirements are also specified for the test coupon, procedurerequalification requirements, procedure variables, documentation, and approvalrequirements.

Preheat and PWHT requirements were discussed in MEX 202.02 and must be specified in thewelding procedure. Saudi Aramco preheat and PWHT requirements are specified in SAES-W-010 and are contained in Work Aid 1.

The ASME Code contains the temperature and hold time requirements when PWHT is neededfor stress relief considerations. These ASME Code PWHT requirements are based onmaterial type and thickness, as specified in Paragraph UCS-56 for carbon and low-alloysteels. The following parameters (based on the ASME Code, Section VIII, Division 1) mustbe controlled during PWHT:

The minimum PWHT temperature and the minimum holding time attemperature are specified based on the material P-No. and thickness.Acceptable PWHT procedures are also specified. These requirements ensurethat adequate stress relief will occur.

Heatup and cooldown rates must be controlled within specified limits in orderto avoid excessive local thermal stresses within the vessel during PWHT. Forcarbon and low-alloy steels, these heatup and cooldown rates are as follows:

- The furnace temperature must not exceed 427C (800F) before thevessel or vessel part is placed in it.

- Above 427C (800F), the heatup rate must not be more than 222C(400F)/hr divided by the maximum metal thickness of the shell or headplate, in inches. In no case can the heatup rate exceed 222C(400F)/hr.

- During heatup, the maximum temperature variation in the portion of thevessel that is being heated must be limited to 139C (250F) in any 4.6m (15 ft.) length.

- During the temperature hold period, the maximum difference intemperature between any two parts of the vessel that is being heatedmust not exceed 83C (150F).

- The furnace atmosphere must be controlled to avoid any excessivesurface oxidation of the vessel.




- Above 427C (800F), cooldown must be done in a closed furnace orcooling chamber at a maximum rate of 278C (500F)/hr divided by themaximum metal thickness of the shell or head plate in inches. In nocase can the cooldown rate exceed 278C (500F)/hr. From 427C(800F) down, the vessel may be cooled in still air.

- Except as permitted for P-No. 1, Groups 1 through 3, and P-No. 3,Groups 1 through 3 materials, vessels which have received PWHT mustreceive an additional PWHT after any weld repairs have been made.The concern here is that the repair welding may defeat the benefits ofthe original PWHT. Weld repairs may be made to these materials afterthe final PWHT without doing another PWHT provided that thefollowing conditions are met:

+ The repairs are made before the vessel hydrotest.

+ The PWHT is not required for service reasons.

+ The size of repair is within specified limits.

+ Specified inspections are made.

It should be noted, however, that SAES-W-001 requires that PWHT bedone after all repairs are completed.

As noted earlier, the ASME Code specifies PWHT based primarily on stress reliefconsiderations. PWHT may be required based on process service considerations as well,since welded components are prone to cracking in certain process environments. SAES-W-010 requires that PWHT be done on vessels in specific process services. These PWHTrequirements are summarized in Work Aid 1.

Welder Qualification

A qualified weld procedure specifies how the weld is to be made. However, the actual weldswill be made either by men or machines. An unqualified welder or defective machine resultsin a poor quality weld, even if a qualified welding procedure is used. Therefore, theindividuals or equipment that actually do the welding must be tested to confirm that they havethe capability to carry out the procedure. The result of these qualifications and tests is thatqualified welding procedures are performed by qualified welders.




The ASME Code requires that welders and welding operators that are used to weld pressure-containing parts and to join load-carrying nonpressure parts to pressure parts be qualified inaccordance with Section IX of the ASME Code. Other requirements apply for less criticalwelds. Methods must also be established that relate the specific welder to his work and thatpermit test records to be maintained.

Acceptable Welding Details

All pressure vessel welds, including the welds that attach heads, nozzles, small fittings, andnonpressure components to a shell, must conform to requirements that are specified in theSAESs, 32-SAMSS-004, and the ASME Code. Details that are used for the primarycircumferential and longitudinal welds were discussed earlier in conjunction with weld jointcategories. Other Saudi Aramco and ASME Code weld detail requirements are highlightedbelow.

Saudi Aramco Weld Detail Requirements

Saudi Aramco specifies weld detail requirements in 32-SAMSS-004 and SAES-W-010.These requirements are contained in Work Aid 1. The paragraphs that follow elaborate ontwo of these requirements.

For welded connections, a 6 mm (1/4 in.) NPS weep hole is required in eachnozzle reinforcing pad, saddle wear plate, or attachment pad that covers a weldseam. The weep hole permits later pressure testing of the pad attachment weldsand also provides a vent during welding.

Support skirts are to be welded to vessel heads (with the exception ofhemispherical heads) so that the centerlines of the skirt plate and the straightflange of the head line up. This alignment eliminates any additional localstresses that may be caused by eccentric application of the vessel weight loads.The weld that attaches the skirt is to have no undercut. This lack of undercutminimizes local stress intensification effects and the potential for fatigue failureunder cyclic loading.

ASME Weld Detail Requirements

Work Aid 1 summarizes two locations of ASME Code weld detail requirements. Theparagraphs that follow provide additional comments about several of the ASME requirements.Further information related to these and other weld details is contained in the ASME Code.




Thickness of a pressure vessel head sometimes differs from the thickness of the shell it isattached to, such as when a hemispherical head is attached to a cylindrical shell. Thetransition between the component thicknesses must be made gradually in a taper in order toavoid an excessive local stress. The head-to-shell weld will typically be made in thecylindrical shell. However, the weld can also be located within the taper. Head-to-shellthickness transitions are illustrated in Figure 16 in Work Aid 1.

An intermediate head is attached to the inside of a cylindrical shell when the intermediatehead separates two sections of the vessel. The butt weld between shell sections also attachesto the head, and a fillet weld is also located between the head and shell. The ASME Codepermits elimination of the fillet weld if there is no access and if the service is noncorrosive.However, the fillet weld should generally be used for all refinery applications to avoid thepotential for accelerated corrosion due to process fluid getting between the head and shell.The attachment of an intermediate head to a cylindrical shell is illustrated in Figure 16 inWork Aid 1.

In some cases, a nozzle neck that has a weld-end may be attached to a pipe that is thinner.This attachment between components of different thicknesses could occur if extra thicknesswas included in the nozzle neck for reinforcement or if the pipe and nozzle materials and/orallowable stresses differ. In such a case, the nozzle neck must be tapered to the pipethickness. Tapers of similar thickness are also used to join shell sections that are of differentthicknesses. Shell thickness and nozzle thickness tapers are illustrated in Figures 15 and 17respectively in Work Aid 1.

Stiffener rings may be attached to the vessel shell by continuous, intermittent, or acombination of continuous and intermittent welds. Intermittent welds must be placed on bothsides of the stiffener and may be either staggered or in-line. The ASME Code specifiesacceptable spacing, size, and length of the welds. Stiffener ring attachment weld options areillustrated in Figure 18 in Work Aid 1.

Tolerances

Pressure vessel components are designed for specified dimensions through the use ofprocedures and equations that were discussed in MEX 202.03. The actual fabrication of theindividual components and the completed vessel must match the dimensions that were used inthe design calculations within relatively small tolerances. These small tolerances are requiredfor the design to be valid and for it to have the reliability that the ASME Code intends.




The ASME Code specifies acceptable dimensional tolerances for specific situations. Thisspecification also includes allowable alignment tolerances between components that are beingwelded together. Excessive misalignment between welded components can result in poorquality welds, local stress intensification effects that were not considered in the design, and areduction in long-term weld reliability. Saudi Aramco generally accepts the ASME Codetolerance requirements without additions.

Heads and Shells

The following list summarizes the primary dimensional tolerance requirements for heads andshells based on the ASME Code, Section VIII, Division 1.

Cylindrical, conical, and spherical shells that are under internal pressure mustbe substantially round and must meet the following requirements:

- The difference between the maximum and minimum inside diameters atany cross section is not to exceed 1% of the nominal diameter at thecross section. Since all the design equations are based on circular crosssections, deviations beyond this value would introduce higher localstresses that were not accounted for in the design calculations.

- When the cross section either passes through an opening, or within adistance of one inside diameter (I.D.) from the opening measured fromits center, the permissible diametral difference stated above may beincreased by 2% of the opening I.D.

Cylindrical, conical, and spherical shells that are under external pressure mustmeet the same dimensional tolerances noted above, plus additional dimensionaltolerances that are specified in Paragraph UG-80 of the ASME Code. Theseadditional requirements account for local geometric discontinuities, whichreduce the buckling resistance of a shell. Participants are referred to theASME Code for details.

The inner surface of a torispherical, toriconical, hemispherical, or ellipsoidalhead cannot deviate outside of the specified shape by more than 1-1/4% of Dand cannot deviate inside the specified shape by more than 5/8% of D. D is thenominal outside diameter of the vessel shell at the point of attachment. Theknuckle radius cannot be less than the specified value.




A hemispherical head or any spherical portion of a torispherical or ellipsoidalhead that is designed for external pressure must meet additional tolerances thatare specified in Paragraph UG-81 of the ASME Code. This requirement is dueto the influence that geometric shape has on the buckling characteristics of ashell.

The difference between the maximum and minimum diameters of head skirts isto be limited to a maximum of 1% of the nominal diameter.

Plate Thickness

For plate material that is ordered, it must be specified that the material is to be no thinner thanthe required design thickness. If plate is furnished with an undertolerance of no more than thesmaller of 0.25 mm (0.01 in.) or 6% of the ordered thickness, it may still be used at the fulldesign pressure for the thickness ordered.

In the extreme case, this degree of permissible plate thickness undertolerance permits at mosta 6% overstress in the vessel component. This amount of overstress will still be well below alevel that could cause a failure. From a practical standpoint, there will be slight variations inplate thickness so that the entire plate would not be this thin. In addition, the allowablestresses are based on minimum permissible material strength properties, and the material willtypically be stronger than these minimum permissible material strength properties. Therefore,permitting a nominal plate thickness undertolerance of up to 6% is well within reasonablesafety margins.

It should also be noted that, except for certain special provisions that are noted in ParagraphUG-16, the ASME Code requires that the minimum thickness for shells and heads, after theyare formed, shall be 1.6 mm (1/16 in.) exclusive of any corrosion allowance. This minimumthickness requirement results in a basic degree of mechanical integrity of the vessel regardlessof the actual design loads.

Alignment

As noted earlier, the alignment between two parts that are being welded together must bewithin a reasonable tolerance in order to achieve an acceptable weld. The list that followshighlights several ASME Code requirements for alignment.

Plates that are to be welded together must be fitted, aligned, and retained inposition during the welding operation. This procedure keeps the parts frommoving during welding.




Any tack welds that are used to achieve alignment must either be removedwhen they are no longer needed, or their ends must be ground and the tackweld incorporated into the final weld. Tack welds must also be made usingqualified welding procedures. If qualified welding procedures are not used, arelatively poor quality tack weld could be the initiation point of a weld failure.

Alignment at edges that are to be buttwelded must have a maximum offsetwithin the limits that are shown in Figure 7, based on weld joint category. Thethickness, t, is the nominal thickness of the thinner edge at the joint.

Joint Categories

t, in. A B, C, & D

Up to 1/2, incl. 1/4t 1/4t

Over 1/2 to 3/4, incl. 1/8 in. 1/4t

Over 3/4 to 1-1/2, incl. 1/8 in. 3/16 in.

Over 1-1/2 to 2, incl. 1/8 in. 1/8t

Over 2 Lesser of1/16t or 3/8 in.

Lesser of1/8t or 3/4 in.

Edge Alignment in Butt Welds

Figure 7

Any offset within the allowable tolerances must be fared at a 3:1 taper over thewidth of the finished weld. Additional weld metal may be added at the edge ofthe weld to meet this requirement. This 3:1 amount of taper minimizes theeffects of local stress concentrations.




DETERMINING WHETHER VENDOR INSPECTION AND TESTING PLANSSATISFY SAUDI ARAMCO REQUIREMENTS

Overall inspection of completed pressure vessels includes an examination of the following:

Welds

Base material specification and quality

Dimensional requirements

Equipment documentation

This section discusses only the methods and extent of required weld examinations.

A good weld combines a good design with the execution of a qualified procedure by aqualified welder. However, the ultimate quality of the actual welds that are made in apressure vessel may not be acceptable for a variety of reasons. The pressure vessel designeris responsible for specification of the type and extent of weld examination that is required inorder to ensure that acceptable welds are achieved. The most common weld defects for whichwelds are examined are as follows:

Poor weld shape due to part misalignment.

Cracks in welds or heat-affected zones (HAZ) of the base metal.

Pinholes on the weld surface.

Slag inclusions or porosity in the form of voids.

Incomplete fusion between weld beads or between the weld and the base metal.

Lack of penetration or an insufficient extent of penetration of the weld metalinto the joints.

Undercut, an intermittent or continuous groove that is located adjacent to theweld and that is left unfilled by weld metal.

Several of these common weld defects are illustrated in Figure 8.




Typical Weld Defects

Figure 8




The presence of defects reduces the strength of the weld below the requirements of the designcalculations, reduces the overall strength of the fabrication, and increases the risk of failure.Weld inspection must be performed in a manner that will detect unacceptable defects and thatwill not damage the vessel material. This type of inspection is called nondestructiveexamination, or NDE.

Radiographic weld examination, weld joint efficiency, and ASME Code requirements havealready been discussed. For example, a spot radiographic examination produces a weld jointefficiency of 0.85 in a full-penetration butt weld. A 100% radiographic examination producesa weld joint efficiency of 1.0 in a full-penetration butt weld. In practical terms, a weld jointefficiency of 1.0 means that there is greater assurance that high weld quality is achieved, thatthere is no difference in quality between the weld and the base metal, and that the vessel partsmay, therefore, be fabricated from thinner material. Main seam pressure-containing welds arenot the only ones whose quality must be assured. Welds that connect nozzles or majorstructural components to vessel shells must also be of high quality. The sections that followdiscuss radiographic and other forms of weld inspection, the types of defects that they candetect, and the extent of required examination.

After a pressure vessel has been completely fabricated, it must be pressure-tested before it isconsidered safe for operation. The objective of a pressure test is to bring the vessel, undercontrolled conditions, to an internal pressure that is high enough to demonstrate itsmechanical integrity. Later sections discuss pressure test requirements in more detail.

Weld hardness tests may be required prior to fabrication and after the welding of vesselcomponents, based on service considerations and the vessel material. Weld hardness must bekept below specified maximum values in order to decrease the potential for weld cracking incertain process environments.

If the material is not exempt from impact testing in accordance with Division 2 requirements,Charpy impact tests must be made to confirm that the material has adequate fracturetoughness prior to fabrication. The need for this impact testing must be included as part of thevessel vendor's fabrication plans.

Methods of Examination

The five primary weld NDE methods are as follows:

Radiographic examination (RT)

Visual Inspection (VT)

Liquid penetrant examination (PT)

Magnetic particle test (MT)

Ultrasonic examination (UT)




The choice of which weld examination method or methods to use depends on the weld qualityrequired of the joint, the position of the weld, the material to be joined, and the particulardefects that are expected. These weld NDE methods are discussed in the paragraphs thatfollow.

Radiographic Examination (RT)

The most important NDE method is radiographic examination. In radiographic examination,a ray is emitted from a controllable source, penetrates a test specimen, and leaves an image ona strip of film that is mounted behind the test specimen. The major advantage of RT is that itproduces a permanent record of the examination on film. Figure 9 shows a typical setup forRT examination.

Typical RT Setup

Figure 9




Any change in density of the weld metal shows on the film as a dark spot. Flaws such as gaspockets, slag inclusions, incomplete penetration, or cracks that are located anywhere throughthe weld thickness are readily detected. RT examination is most effective in the detection andidentification of small flaws, but RT is not practical for complex shapes such as tee junctionsbecause the results of the examination are difficult to interpret. RT examination is mosteffective in the examination of buttwelded joints, such as longitudinal and circumferentialjoints in pressure vessel shells.

RT examination is a relatively expensive method due to the high equipment cost and requiredsafety precautions. When RT examination is done, access to the area is restricted to essentialpersonnel, and the operators are located behind protective shields in order to minimizeoperator exposure to the rays that are emitted.

Visual Inspection (VT)

A thorough visual inspection is usually satisfactory for minor structural welds, such as thosethat connect insulation support rings to a vessel shell. All weld surfaces that will be examinedby more extensive means are first subject to VT. Visual weld inspection involves measuringthe weld and noting any areas of obvious surface porosity, slag inclusions, weld undercut, oroverlap. The VT provides an overall impression of weld quality and helps to locate areaswhere additional NDE should be performed.

Liquid Penetrant Examination (PT)

A liquid penetrant examination is used to detect weld surface-type defects. Defects which aPT examination may detect are cracks, seams, porosity, folds, inclusions, shrinkage, or anyother surface defect. PT examination is used for both ferrous and nonferrous materials. Themajor limitation of PT examination is that it can only detect imperfections that are open to thesurface. It cannot be used as the only examination tool for critical pressure-containing welds.PT is often used as the first and only step up from VT for relatively minor structural-typewelds. In some cases, PT examination is done on intermediate weld passes for critical weldsin order to detect and repair defects early before an entire weld is made. PT is often done onthe weld root pass to ensure that the first weld pass is sound. PT is also often used after thefinal weld pass to find flaws that go through the weld surface, after which another inspectionmethod is used to search for internal defects.

PT is relatively simple and is less expensive than RT, MT, or UT. The basic steps of a PTinspection are as follows:




(1) Surface Preparation and Cleaning: All surface coatings, such as paint andcontaminants, must be completely removed since they could prevent the entrance ofpenetrant into the metal and could also prevent the identification of the flaw. Solventsare commonly used for surface preparation.

(2) Penetrant Application: Liquid penetrant solutions have high fluidity, low viscosity,and high reliability to permit penetration into defects by capillary action. The liquidpenetrant solutions also contain a fluorescent or visible dye to mark potential defectareas. Spraying is a common means of solution application. Adequate liquidpenetration into any flaws generally takes 10 to 30 minutes, after which excesspenetrant is removed.

(3) Removal of Excess Penetrant: Excess penetrant must be removed from the surface bywiping the surface with a clean cloth or equivalent. The penetrant must still be liquidat this stage rather than dried, or the entire process must be started again. Theobjective is to remove the penetrant from the weld surface without removing anypenetrant that seeped into weld defects.

(4) Development: After excess penetrant has been removed, developer is immediatelyapplied to make the flaws readily visible. By acceleration of the capillary bleed-outprocess, the developer helps detect penetrant that is retained in surface flaws.Development emphasizes the presence of a flaw by causing the penetrant that isretained in it to spread over a larger area. Development also acts as a color-contrastingbackground for the dye or fluorescent penetrants.

(5) Inspection and Evaluation: After development, the weld is inspected. Inspection isdone in normal light when visible dye penetrants are used and in ultraviolet light whenfluorescent dye penetrants are used. With either type of penetrant, both true and falseindications may be revealed.

The standard true flaws that are indicated by PT include cracks, pits, and porosity. A largecrack appears as a solid line of some width and becomes apparent soon after developerapplication. A cold-shut crack is an undersurface crack that bleeds to the surface. A cold-shutcrack appears as a line of dots and comes to the surface a few minutes after the developer isapplied. Porosity indications appear as dots and come to the surface almost immediately afterdeveloper application.




False or nonrelevant indications are not caused by surface flaws. The primary reasons thesefalse indications occur are poor PT application procedures or rough weld surfaces. Theresults of the PT are evaluated to determine if the flaws are real, to determine their extent andexact nature, and to determine if repairs are needed.

Magnetic Particle Test (MT)

The MT examination can detect cracks, porosity, and lack of fusion at or near the surface offerromagnetic materials. Flaws that are up to 6 mm (1/4 in.) beneath the weld surface can bedetected. MT depends on the magnetic properties of the material that is inspected and cannotbe used on nonmagnetic materials. MT is frequently employed on the root and final weldpasses or every 6 mm (1/4 in.) of weld buildup for critical welds where RT inspection is notpractical (such as for nozzle attachment welds).

MT examination is based on the magnetic lines of flux (or force lines) that can be generatedwithin a test piece. These force lines are parallel if no defects are present. If there is a defect,a small break in the force lines appears at the defect location. In MT examination, ironpowder is applied to the surface and then the test piece is magnetized. If there are no defects,the iron powder is aligned in straight lines along the North-South magnetic flux lines. If thereis a defect, the iron powder alignment is disturbed and flows around the defect. Figure 10shows schematically how the iron powder is distributed at a defect during an MTexamination.




Sub-Surface Defect Along Magnetic Lines of Flux

Figure 10




Ultrasonic Examination (UT)

Ultrasonic examination is frequently used to detect subsurface flaws, such as laminations orslag inclusions, that may be present in thick plates, welds, castings, or forgings. UT is oftenused to confirm that high weld quality is obtained in pressure-containing joints that cannot beRT examined. A heavy wall thickness pressure vessel typically employs 100% RTexamination of the primary longitudinal and circumferential joints. Unless specially designednozzles are used, the nozzle attachment welds cannot be reliably RT examined, becausenozzles are typically tee joints. UT inspection may be used to ensure that the nozzleattachment welds are equal in quality to the primary vessel joints that were RT examined.

In UT examination, sound waves are generated by a power source and applied to the testpiece through a transducer. Figure 11 shows a pulse echo ultrasonic examination system.

Pulse Echo UT System

Figure 11




In the system shown in Figure 11, the sound waves pass through the test piece and arereflected back to the transducer either from the far side of the test piece or from a flaw that islocated at an intermediate position within the test piece. By careful calibration, the UToperator knows if a flaw has been detected and knows its location and its size.

Figure 12 shows a through-transmission UT system. It uses two transducers, one to transmitthe sound waves and the other to receive them. In this case, if a flaw is present, the flawblocks the reception of the sound waves from the receiving transducer.

Through-Transmission UT System

Figure 12




Figure 13 summarizes the types of nondestructive examinations, the defects typically foundby each, and the advantages and limitations of each process.

NDE TYPE DEFECTSDETECTED

ADVANTAGES LIMITATIONS

Radiographic Gas pockets, slaginclusions, incompletepenetration, cracks

Produces permanentrecord.Detects small flaws.Most effective for butt-welded joints.

Expensive.Not practical forcomplex shapes.

Visual Porosity holes, slaginclusions, weldundercuts, overlapping

Helps pinpoint areas foradditional NDE.

Can only detect what isclearly visible.

Liquid Penetrant Weld surface-typedefects: cracks, seams,porosity, folds, pits,inclusions, shrinkage

Used for ferrous andnonferrous materials.Simple and lessexpensive than RT, MT,or UT.

Can only detect surfaceimperfections.

Magnetic Particle Cracks, porosity, lackof fusion

Flaws up to 6 mm(1/4 in.) beneath surfacecan be detected.

Cannot be used onnonferrous materials.

Ultrasonic Subsurface flaws:laminations, slaginclusions

Can be used for thickplates, welds, castings,forgings.May be used for weldswhere RT not practical.

Equipment must beconstantly calibrated.

Summary of NDE Types

Figure 13

Complete discussion of the primary NDE methods that are used in pressure vessel fabricationrequires the investigation of many different techniques, procedures, and equipmentpossibilities for each method that was described. Such a complete discussion of NDE isbeyond the scope of this course.




Type and Extent of Required Examination

The type and extent of examinations that are required for pressure vessel welds are specifiedby Saudi Aramco requirements and by the ASME Code. Requirements that are contained inSection VIII, Division 2 tend to be more stringent than Division 1 requirements. Participantsshould refer to Division 2 for details when required.

The ASME Code also specifies inspection procedures and acceptance criteria which must befollowed. Work Aid 2A summarizes the steps which may be used to confirm that vesselvendor inspection plans meet Saudi Aramco and Division 1 requirements. The followingparagraphs elaborate on several of these inspection requirements.

SAES-W-001 requires that any pressure-containing weld that will not behydrotested must be 100% radiographed. Such a situation is rare for pressurevessels. However, this situation could occur in very large field-fabricatedvessels where the foundation has not been designed for the total water weightand where the vessel is not completely filled with water for field pressuretesting.

32-SAMSS-004 requires UT examination as follows:

- All plates that are over 50 mm (2 in.) thick must be UT-examined. Asplates get thicker, they are more prone to internal laminations whichcould be detrimental to vessel integrity after subsequent fabrication isdone. For example, if attachment welds are made in the vicinity of alamination, weld shrinkage stresses could cause a lamination to openfurther. Such opening of a lamination could also occur during plateforming.

- All plates that are over 50 mm (2 in.) thick must be 100% UT-examinedfor a distance of 150 mm (6 in.) back from a nozzle weld preparation orother cut-out. The presence of a lamination in these areas could lead topoor quality welds and/or high local stresses that were not considered inthe vessel design calculations.

- Clad steel plates must be UT-examined. UT examination is done toensure that there is an acceptable bond between the cladding and baseplate.




Pressure Test Plans

All pressure vessels that are designed to ASME Code requirements must be pressure testedafter fabrication and inspection in order to demonstrate their structural integrity before theyare placed into operation. The pressure test is made at a pressure that is higher than thedesign pressure. This excess pressure provides a safety margin since the vessel componentstress levels during the test will be higher than the stress levels which will occur duringoperation. The objective of the pressure test is to bring the vessel to a high enough internalpressure, under controlled conditions, to demonstrate its mechanical integrity. Successfulcompletion of the pressure test signifies that the vessel is acceptable for operation.

Pressure tests are typically made using water as the test medium because of the relative safetyof water compared to a pneumatic test. The ASME Code permits performance of a pneumaticpressure test as an alternative to a hydrostatic test under certain circumstances. However, 32-SAMSS-004 specifies that all vessels except those in refrigerant service must be hydrotested.Vessels in refrigerant service must be either hydrotested and dried or must be pneumaticallytested. SAES-A-004, Pressure Testing, prohibits a pneumatic pressure test without writtenapproval from the Chief Inspection Engineer. SAES-A-004 also specifies generalrequirements for pressure testing.

Since the hydrostatic test will almost always be used, only the hydrostatic test will bediscussed. Participants are referred to the ASME Code for pneumatic test requirements.

32-SAMSS-004 requires that the pressure at the top of the vessel be determined by the rulesof Paragraph UG-99 (c) for Division 1 vessels or by the rules of Paragraph AT-301 forDivision 2 vessels. These ASME Code rules require that the hydrostatic test pressure at thetop of the vessel be calculated by multiplying the "calculated test pressure" for each elementby 1.5 and by reducing the value by the hydrostatic head on that element. The "calculated testpressure" for each element is determined based on the appropriate equation for MAWP thatwas discussed in MEX 202.03, the nominal component thickness which includes corrosionallowance, the appropriate weld joint efficiency, and the material allowable stress at testtemperature. The Pressure Vessel Design Sheet (Form 2682 or 2683 for Division 1 orDivision 2 vessels, respectively) also contains the required equations. Work Aid 2B may beused to help determine if the hydrotest pressure that is specified by a vendor is correct. Useof this pressure level for the hydrotest permits potential use of the vessel up to its MAWPwithout the need for a new hydrotest.




Form 2682 contains areas that directly relate to determination of the required hydrotestpressure. Refer to the copy of Form 2682 that is contained in Course Handout 3 and note thefollowing:

Hydrotest pressures must be calculated for the shop test with the vessel in thehorizontal position, for the field test with the vessel in the final position andwith uncorroded component thicknesses, and for the field test with the vessel inthe final position and with corroded component thicknesses.

The basis for calculation of the initial test pressure for the vessel in the shop isthe lower of the pressure calculated for the shell or the pressure calculated forthe heads.

The shop hydrotest pressure must also consider the permitted hydrotestpressure of any flanged connections. The calculated hydrotest pressure cannotexceed the test pressure of the flanged connections.

SAES-D-001 and 32-SAMSS-004 require that, during the pressure test, the stress at anysection of the vessel cannot exceed 90% of the material minimum specified yield strength(MSYS), based on use of the design weld joint efficiency (E). The stress in the vessel islimited to 90% of the MSYS to ensure that there is an adequate safety margin beforepermanent deformation in vessel components can occur. Also recall that a 48 kph (30 MPH)wind must be considered to act during the field hydrotest, based on 32-SAMSS-004requirements.

The requirement to design the vessel for field hydrotest introduces a complication, especiallyfor tall towers. A tall tower is typically designed for a specific liquid level (design high liquidlevel) as part of its normal design conditions. However, the specific gravity of the designliquid is normally less than 1.0, and the design high liquid level is usually much lower thanthe top of the tower. During a field hydrotest, water at a specific gravity of 1.0 is used, andthe tower is filled to the top. The larger specific gravity and fill height of hydrotest waterresults in a higher weight and hydrostatic head load than occurs during normal operation.Therefore, to withstand the hydrotest loads in some cases, thicker plates are required for lowersections of the tower than would be required for the operational loads.




There are situations where one pressure vessel may have two or more individual sections thatare separated by intermediate heads. Each vessel section must typically be designed for aseparate hydrotest. Each separate hydrotest could affect the intermediate head design becausethe head is exposed to a higher weight and hydrostatic test pressure than would occur duringnormal operation.

The paragraphs that follow summarize additional general hydrostatic test requirements thatare based on the ASME Code, Section VIII, Division 1.

If visible, permanent distortion of the vessel occurs during hydrotest, theASME Authorized Inspector has the right to reject the vessel. Permanentdistortion should not occur as long as the design is correct and the test pressuredoes not exceed the value that was calculated on the basis described above.

Pressure chambers of combination units that are designed to operateindependently must be hydrotested as separate vessels: that is, each chambermust be tested without pressure in the other chambers. In addition:

- If the common elements are designed for a higher differential pressurethan the MAWP's of the adjacent chambers, then the hydrotest of thecommon elements must subject them to at least 1.5 times their designdifferential pressure, corrected for the effect that design temperature hason material allowable stress. The allowable stress correction is equal tothe ratio of the material allowable stress at test temperature to theallowable stress at design temperature.

- If the common elements are designed for the maximum differentialpressure that can occur, and if this pressure is less than the higherpressure in the adjacent chambers, then their common elements musthave a hydrotest pressure that is at least 1-1/2 times the differentialpressure that is marked on the unit, corrected for temperature.

After testing and inspection of the common elements, the adjacent chambersare then hydrotested simultaneously. Care must be taken to limit thedifferential pressure between the chambers to the pressure that is used whentesting the common elements.




All joints and connections must be inspected after application of the hydrotestpressure. This inspection must be made at a pressure that is not less than 2/3 ofthe test pressure.

The metal temperature during hydrotest should be maintained at least 17C(30F) above the minimum design metal temperature but not over 49C(120F). The minimum metal temperature is specified to minimize the risk ofbrittle fracture. The test pressure must not be applied until the vessel and waterare at about the same temperature.

Vents must be provided at all vessel high points (based on test position) topurge possible air pockets while the vessel is filled with water.

Sample Problem 1: Calculation of the Required Hydrotest Pressure

The horizontal drum that is described in Figure 14 is being supplied to Saudi Aramco.

Determine the required hydrotest pressures for the new vessel in the shop and field. Alsodetermine the required service test pressure with the vessel corroded. Work Aid 2B may beused to help solve this problem.




Design Temperature - 500FDesign Pressure - 125 psigHead and Shell Material - SA-516 Gr. 70

- Allowable Stress = 17,500 psi- Yield Stress = 38,000 psi

Corrosion Allowance - 0.125 in.E = 0.85 for shell weldsANSI Class 150 flanges, hydrotested to 450 psig

Drum designed to be completely water filled. Longitudinal stress in the shell due to weightplus pressure does not govern the specified shell thickness, and it may be assumed that thespecified thickness is correct.

Sample Problem 1

Figure 14




Solution:

Determine Initial Test Pressure Basis, Pc.

Shell

Sc = 17 500 psi T = 0.75 in.

E = 0.85 R = 60 in.

Heads

Sc = 17 500 psi T = 0.75 in.

E = 1.0 D = 120 in.

Therefore, the shell is the limiting component and is the basis for determining testpressure.




Answer:

; the smallest of Ps (PE or PD, as applicable).

Determine test pressure of bolted flange connections, PNT.

PNT = 450 psig in this case, as was stated in the given information. PNT can be foundin ANSI/ASME B16.5 for each flange Class.

Determine shop hydrotest pressure at top with vessel horizontal, PSH.

h = 10 ft. (height of test water in vessel with the vessel horizontal).

Solution:

Answer:

Determine the field hydrotest pressure with the drum new and in the final position.




Solution:

Answer:

Therefore, test pressure at the top = 274 psig (the higher of C or D). Note that the shop andfield test pressures with the vessel new are equal in this case since this is a horizontal drumand since the hydrostatic head pressures from the water are equal. This would not be the casefor a tall tower which is hydrotested horizontally in the shop but vertically in the field.

Confirm that the stress in the limiting component does not exceed 0.9 of the minimumspecified yield strength times the weld joint efficiency, E.

Solution:

Since the shell is the limiting component, we only need to check its stress:

Note that the pressure used is at the bottom of the limiting component.

Answer:

Since S < 34 200 psi, the hydrotest pressure is acceptable. Note that E was accounted foralready in the equation.




Determine service test pressure at the top of the vessel with the vessel corroded andconfirm that the stress in the vessel does not exceed 0.9 of the minimum specifiedyield strength times E.

Service Test Pressure :

P = 192 psig at shell bottom

T = 0.75 - 0.125 = 0.625 in., corroded thickness

Solution:

Answer:

S = 21 865 psi < 0.9 38 000 = 34 200 psi

Therefore, the vessel is acceptable.

Hardness Test Plans

Hardness in metals is defined as the ability to resist the penetration of the metallic surface.Hardness can be readily measured, is directly proportional to material strength, and isinversely proportional to ductility and toughness. Therefore, the harder a material is, the moreprone it will be to cracking or brittle fracture. Hardness is affected by material chemistry andmicrostructure and is thus greatly influenced by the heat of welding. Therefore, there will bea variation in hardness across the base metal, weld metal, and HAZ of welded componentsdue to local variations in chemistry (especially carbon content), welding technique, andpreheat temperature. Welding procedure changes and PWHT are often done to reduce weldhardness when hardness reduction is needed due to material and/or service considerations.

The Brinell, Vickers, and Rockwell methods are the most common approaches that are usedto measure hardness. It is also possible to convert hardness measurements that are takenusing one method into equivalent values on the other hardness measurement scales.Discussion of such conversions is beyond the scope of this course. The paragraphs that




follow briefly describe the Brinell and Vickers hardness measurement methods, since SaudiAramco requires these methods.




Brinell Hardness Test

In the Brinell test, a steel ball that is 10 mm (3/8 in.) in diameter is pressed into the surface ofthe metal with a load of 3 000 kg (6 614 lb.). The diameter of the impression that is made inthe metal surface is then measured through the use of a special microscope. The diameter ofthe impression is converted to the Brinell hardness number (BHN) by consulting a table. Forexample, soft iron is about 100 BHN, and file-hard steel about 600 BHN. A portable Brinellhardness tester, which uses a much lighter weight to make the indentation, is used forhardness testing of production welds in a pressure vessel.

Vickers Hardness Test

The Vickers hardness test employs a similar principle as the Brinell test in that the Vickerstest expresses the results in terms of the pressure under the indentor and uses the same units.However, the indentor is a diamond that is shaped as a square pyramid, the loads are lighterand vary between 1 and 120 kg (2.2 - 265 lb.), and the impression is measured using amedium-power compound microscope. The Vickers method is more flexible and isconsidered to be more accurate than either the Brinell or Rockwell methods. However, theequipment is more expensive, and the other methods are faster for production work.

Hardness Test Results

The ASME Code does not have weld hardness testing requirements or limitations. However,SAES-W-001, SAES-W-010, and 32-SAMSS-004 have weld hardness testing requirements.Since the requirements that are contained in the two welding SAESs are more complete andnewer, this course will focus on these SAESs. SAES-W-010 specifies when hardness testingmust be done for pressure vessels and also specifies the acceptable hardness limits. SAES-W-001 focuses more on the procedural aspects of hardness testing.

Work Aid 2C provides a checklist which may be used to help confirm that vessel vendorhardness test plans meet Saudi Aramco requirements. Hardness test requirements arespecified for both the welding procedure qualification welds and the vessel production welds.The paragraphs that follow elaborate on several of these hardness test requirements.

The Vickers hardness test procedure must be used for welding procedurequalification welds for vessels that are in sour service except for specifiedexemptions. However, the exemptions must still comply with all NACE MR-01-75 hardness levels and test requirements. Hardness testing is required forthis application since hard welds are prone to cracking in sour service.




One of the exemptions from hardness testing is if all vessel internal surfaces areclad or weld-overlaid with austenitic stainless steel or nickel-based alloys. Inthis case, the cladding or overlay shields the ferritic base plate from the sourfluid. Partial or complete strip lining, partial cladding, or partial overlay are notexempt from hardness testing.

Another exemption from hardness testing is if the weld procedure is used onlyfor external structural attachments and the vessel wall at the attachment point isat least 25 mm (1.0 in.) thick. In this case, the weld HAZ will not extend to thevessel inside surface and thus will not be affected by the sour fluid.

The weld procedure qualification welds must also be hardness-tested forvessels in any service if the wall thickness is greater than 38 mm (1.5 in),except for vessels that are made from austenitic stainless steel or nickel-basedalloys, and except when the weld procedure is used only for external structuralattachment welds. The concern here is that the high heat inputs that arerequired to make heavy welds could cause the welds to be hard and more proneto cracking under service loads.

PWHT may be required for specific materials and/or thicknesses in order to meet the hardnesslimits that are specified by Saudi Aramco.

SAES-W-010 also requires hardness testing of the production welds for all vessels that are insour service, regardless of material, to ensure that the production welds consistently haveacceptable hardness levels. The Brinell hardness test is used in this case because it isgenerally quicker and less expensive for production hardness testing than the other hardnesstesting methods.

SAES-W-001 specifies hardness testing procedural requirements as follows:

Hardness testing of welding procedure qualification coupons must conform toStandard Drawing AB-036386 (W), Hardness Testing for Welding ProcedureQualifications.




Production weld hardness testing, when specified, must meet the followingrequirements:

- Testing must be done with a portable hardness tester (TeleBrinell orapproved equivalent). The Brinell scale must be used unless anotherscale is approved by Saudi Aramco.

- The weld must be ground to a smooth flat surface for testing. Thissmoothing is required to obtain an accurate hardness measurement.

- Hardness indentations must be made at or near the middle of the weldbead. This indentation location will give an average hardness readingthat is a composite of the weld metal, base metal, and HAZ (consideringthe size of the indentor that is used).

- Hardness retesting may be performed within specified limits if theoriginal hardness test results are too high.

The Saudi Aramco hardness testing requirements ensure that weld hardness is considered inthe weld procedure qualification tests when appropriate. The hardness tests then confirm thatthe hardness of the actual production welds is acceptable for sour service.

Impact Test Plans

Material toughness, brittle fracture, Charpy impact testing, and the ASME Code exemptioncurves were discussed in MEX 202.02. Also recall that 32-SAMSS-004 requires that SectionVIII, Division 2 exemption criteria and impact test procedures must be used for both Division1 and Division 2 pressure vessels. Therefore, once it is determined that the material cannot beexempt from impact testing, the vendor's impact test plans must be based on both Division 2and Saudi Aramco requirements.

Work Aid 2D contains a checklist which may be used to confirm that vendor impact test plansmeet Saudi Aramco requirements. The paragraphs that follow highlight several SaudiAramco and Division 2 requirements. Participants are referred to Division 2 for additionaldetails. Unless otherwise noted, the stated requirements are from Division 2.

Requirements for impact test procedures and apparatus are specified.




Each set of impact tests must consist of three specimens of a specified size.Unless otherwise specified, plate specimens may be oriented with the specimenlength parallel to the final direction of rolling. With this orientation, impactenergies will be measured in the direction in which the plate will tend to betougher.

Certified impact test reports by the materials manufacturer are acceptableprovided that either of the following conditions are met:

- The specimens are representative of the material that was delivered andthe vessel fabrication will not reduce the impact properties of thematerial.

- The materials from which the specimens are removed are heat treatedseparately such that they are representative of the material in thefinished vessel.

This approach ensures that the basic materials are acceptable before they arriveat the vessel vendor and provides earlier materials quality control. The material"as-tested" must then be confirmed to be equivalent to the material "as-fabricated."

As an alternative to this confirmation, the vessel manufacturer may do theimpact testing.

Minimum required Charpy V-notch impact energy values are specified. Thesevalues are stated both as the average value for the three specimens and theminimum value for any one specimen. Acceptable impact energy values arespecified as a function of tensile strength, and higher values are required as thetensile strength increases. Higher strength steels are more prone to brittlefracture than lower strength steels, all other parameters being equal. Therefore,higher strength steels must achieve higher impact energy values in order tohave adequate fracture toughness.

32-SAMSS-004 requires that the impact test temperature must be 18C (32F)below the minimum design temperature that is specified on the Pressure VesselDesign Data Sheet Form 2682 or 2683. This approach provides an extramargin of safety against brittle fracture. When impact testing is required, itmust include the base metal, weld metal, and HAZ.




SAES-W-001 contains the following items related to impact tests:

- For any flux, Gas Metal Arc Welding (GMAW) electrode or for anyFlux Cored Arc Welding (FCAW) electrode that is used for weldingprocedures with impact toughness requirements, the specified brand,type, and maximum size used for the Procedure Qualification Record(PQR) must be used in the actual fabrication.

- Welding procedures with impact test requirements must be submitted bythe Inspection Department to the Consulting Services Department forfinal review and approval. This approach provides additional review toensure that material toughness is adequately considered in the weldprocedure.

Applicable Codes and Standards

The codes and standards that apply to the evaluation of vendor inspection and testing planswere discussed throughout the prior sections of this module. These codes and standardsinclude the following:

ASME Code, Section VIII (Division 1 or 2, as appropriate)

ASME Code, Section IX

SAES-D-001, Pressure Vessels

SAES-W-001, Basic Welding Requirements

SAES-W-010, Welding Requirements for Pressure Vessels

32-SAMSS-004, Pressure Vessels

Work Aid 2 may be used to help determine whether vendor inspection and testing plans meetSaudi Aramco requirements.




SUMMARY

This module discussed welding fundamentals, types of welded joints, welding procedures andwelder qualification, and acceptable welding techniques. It has also identified the acceptablewelding details and fabrication tolerances for pressure vessel components that are containedin the Saudi Aramco SAESs and SAMSSs, and in the ASME Code. These requirements willenable the Participant to correctly evaluate fabrication drawings for acceptability. The secondsection of this module discussed methods of examination; type and extent of requiredexamination; pressure, hardness, and impact test plans; and the applicable codes andstandards which will enable the Participant to determine whether vendor inspection andtesting plans satisfy Saudi Aramco requirements.

This module completes discussion of the requirements that must be met for a new pressurevessel. MEX 202.05 will discuss maintenance and repair requirements for existing pressurevessels.




WORK AID 1: STEPS, DRAWINGS, AND CODE (SAES-W-001, SAES-W-010) ANDPROCESS REQUIREMENTS FOR EVALUATING FABRICATIONDRAWINGS FOR ACCEPTABILITY

This Work Aid may be used to evaluate fabrication drawings for suitability, and takes theform of a checklist with appropriate references. Figure references that are noted are in theASME Code, Section VIII, Division 1, unless stated otherwise. Reference should be made tothe classroom copies of the ASME Code, Section VIII, Division 1 in Course Handout 1, andSAES-W-001 and SAES-W-010 in Course Handout 2, in conjunction with this Work Aid.

1. Vendor fabrication drawings are to reference the ASME Code, Section VIII, Division1, and 32-SAMSS-004, Pressure Vessels. This reference should generally besufficient confirmation that the vendor intends to follow the fabrication requirementsthat are contained in these documents, unless there is specific information to thecontrary. For example, dimensional tolerances will typically not be shown on vendordrawings, but must meet ASME requirements.

2. Shell and head pressure-containing welds are to be full penetration and full fusion.

3. Shell and head joint details are to be consistent with weld joint categories that are usedin the vessel design in accordance with Figure UW-12.

4. Nozzle attachments are to be full-penetration welds.

5. Nozzles and manholes that extend into clad vessels are to be in accordance withStandard Drawing AB-036367 (Reference Course Handout 3).

6. Reinforcing pad attachment welds are to be at least 50 mm (2 in.) from head-to-shellwelds.

7. A 6 mm (1/4 in.) NPS weep hole is to be in all reinforcement or other plates that arewelded to the shell.

8. Support skirts are to be welded to the vessel heads such that the skirt centerline alignswith the head straight flange centerline. This requirement is not applicable tohemispherical heads.

9. Welds that are greater than 3 mm (0.125 in.) thick shall be made in at least two passes.




10. Internal and external supports, support rings, pads, and structural brackets that areattached to the vessel shall be seal-welded all around.

11. Welded attachments are to be continuously seal-welded to the vessel.

12. Thickness transition details are to meet ASME Code requirements. Refer to theASME Code, Section VIII, Division 1 as follows:

Figure UW-9 for shell transitions.

Figure UW-13.1 for head-to-shell transitions.

Figure UW-13.4 for nozzle neck attachment to thinner pipe.

Figures 15 through 17 provide excerpts from these figures.

Typical Shell Transitions

Figure 15




Typical Head-to-Shell Transitions

Figure 16




Nozzle Neck Attachment to Thinner Pipe

Figure 17




13. Nozzle attachment details shall be in accordance with Figure UW-16.1, except thatonly details that use full-penetration welds are acceptable. Typical nozzle attachmentdetails from this figure were illustrated in MEX 202.03.

14. Stiffener ring attachment details shall be in accordance with Figure UG-30. SeeFigure 18.

Stiffener Ring Attachment

Figure 18




15. Weld procedure requirements shall be as follows:

A weld procedure is required for each weld type and shall meet ASMErequirements.

A weld map or equivalent is to be provided that relates each weld in the vesselto the required weld procedure.

Weld procedure test requirements s

fabrication, inspection, and testing of pressure vessels

Documents

saudi aramco requirements

saudi aramco weld

welding procedures

visual inspection

methods of examination

ultrasonic examination

welding fundamentals

technical material