Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third
Engineering Encyclopedia
Saudi Aramco DeskTop Standards
SPECIFYING DESIGN REQUIREMENTS
FOR PRESSURE VESSELS
CONTENT PAGE
INTRODUCTION... 6
USE OF SAUDI ARAMCO DOCUMENTS AND THE ASME CODE IN PRESSURE VESSEL DESIGN ... 7
Structure and Scope of the ASME Code, Section VIII, Division 1, Design By Established Rules... 9
ASME Code Structure... 9
ASME Code Scope ... 10
ASME Code, Section VIII, Division 2, Design By Analysis... 11
Saudi Aramco Pressure Vessel Design Data Sheets ... 12
Content of Form 2682 for Division 1 Pressure Vessels... 14
Content of Form 2683 for Division 2 Pressure Vessels... 18
EVALUATING THE ACCEPTABILITY OF CONTRACTOR-SPECIFIED DESIGN CONDITIONS AND LOADINGS... 19
Pressure ... 20 Operating Pressure... 20 Design Pressure ... 21 Temperature ... 24 Operating Temperature... 24 Design Temperature ... 26
Minimum Design Metal Temperature... 26
Other Loadings ... 27 Weight... 30 Wind... 32 Hydrotest ... 36 External Piping... 37 Internal Components... 38
Earthquake Loadings ... 39
Service ... 40
Wet, Sour... 40
Lethal ... 41
EVALUATING THE ACCEPTABILITY OF CONTRACTOR-SPECIFIED PRESSURE VESSEL COMPONENT THICKNESS DESIGN CRITERIA... 42
Weld Joint Efficiency ... 42
Corrosion Allowance... 44
EVALUATING CONTRACTOR-SPECIFIED DESIGN CALCULATIONS FOR PRESSURE VESSEL COMPONENTS... 45
Design for Internal Pressure... 45
Shells ... 46
Sample Problem 1 - Cylindrical Shell Thickness Calculation ... 48
Heads ... 51
Sample Problem 2 - Head Thickness Calculation... 55
Conical Sections ... 56
Sample Problem 3 - Conical Section Thickness Calculation... 56
Design for External Pressure and Compressive Stresses ... 58
Shells ... 60
Heads ... 60
Conical Sections ... 60
Sample Problem 4 - External Pressure Calculation ... 61
Flat Covers ... 67
Quick-Opening Closures ... 67
Small Connections ... 75
Sample Problem 5 - Nozzle Reinforcement ... 76
Nozzle Flange Rating ... 79
Maximum Allowable Working Pressure (MAWP)... 83
Stresses From Local Loads Applied to Nozzle and Attachments ... 85
Allowable Stress Bases ... 86
Calculation Procedures... 87
Shell and Nozzle Attachment Parameters ... 90
Sample Problem 6 Evaluation of Stresses from Local Loads Applied to Nozzles and Attachments ... 93
EVALUATING THE CONTRACTOR-SPECIFIED DESIGN OF PRESSURE VESSEL SUPPORTS ... 95
Vertical Vessel... 97
Column Supports ... 98
Sample Problem 7 - Design of Column Supports ... 100
Skirt Supports ... 108
Horizontal Vessel Saddle Supports ... 111
Design of Horizontal Cylindrical Vessels on Saddle Supports ... 114
COMPLETING SAFETY INSTRUCTION SHEETS FOR PRESSURE VESSELS... 117
Purpose and Use of the Safety Instruction Sheet in Saudi Aramco... 119
Information Covered ... 119
Where to Find Other Information ... 121
SUMMARY... 123
Cylindrical or Spherical Shells Under External Pressure ... 136
Heads and Conical Sections Under External Pressure... 141
Allowable Compressive Stress of Cylindrical Shells ... 147
List of Figures
Figure 1: Design Pressure... 23
Figure 2: Temperature Zones in Tall Vessels... 25
Figure 3: Vertical Reactor with Additional Loads...29
Figure 4: Tower Layout for Determining Effective Diameter for Wind Calculations ... 33
Figure 5: Vortex Shedding... 35
Figure 6: Sample Problem 1... 49
Figure 7: Typical Formed Closure Heads...52
Figure 8: Thickness Transition Between Hemispherical Head and Shell ... 54
Figure 9: Stiffener Rings on Pressure Vessel Cylinders... 59
Figure 10: Sample Problem 4... 61
Figure 11: Factor A... 63
Figure 12: Figure CS-1... 63
Figure 13: Cross-Sectional View of Nozzle Opening... 71
Figure 14: Typical Nozzle Design Configurations... 73
Figure 15: Sample Problem 5... 77
Figure 16: ASME B16.5, Table 1a, Material Specification List (Excerpt)... 80
Figure 17: ASME/ANSI B16.5, Class 150, Pressure-Temperature Ratings (Excerpt) 81 Figure 18: Sample Problem 6... 93
Figure 19: Types of Pressure Vessel Supports ... 95
Figure 23: Horizontal Vessel on Saddle Supports ... 111
Figure 24: Stiffener Rings at Saddle Supports ... 113
Figure 25: Pressure Vessel Safety Instruction Sheet, Form 2694... 118
Figure 26: Weld Joint Categories ... 128
Figure 27: Types of Welded Joints... 130
Figure 28: Maximum Weld Joint Efficiency... 131
Figure 29: Nozzle Loads Applied to a Spherical Shell... 154
Figure 30: Vessel on Column Supports... 158
Figure 31: Vessel Column Configurations and Moments of Inertia ... 160
Figure 32: Allowable Column Compressive Stress... 161
Figure 33: Types of Support Skirts and Skirt-to-Head Welds ... 168
INTRODUCTION
MEX 202.02 discussed materials selection requirements for pressure vessels. Now that the materials have been selected, the mechanical design of individual pressure vessel components can begin. MEX 202.03, Specifying Design Requirements for
Pressure Vessels, describes the use of Saudi Aramco design
requirements and the ASME Code in the mechanical design of pressure vessels. This description includes determination of design conditions and loadings, and the design criteria required for calculating pressure vessel component thicknesses. The pressure vessel component design calculations themselves, as well as the design calculations required for pressure vessel supports, are also discussed.
Saudi Aramco pressure vessel engineers do not perform the detailed mechanical design of pressure vessels. Pressure vessel mechanical design is done by contractors and pressure vessel manufacturers who are employed by Saudi Aramco. Therefore, the role of the Saudi Aramco pressure vessel engineers will typically be to evaluate the designs that are proposed by others. However, in order to perform this design evaluation role effectively, the Saudi Aramco pressure vessel engineer must know the design requirements that the contractors and vessel manufacturers must meet.
USE OF SAUDI ARAMCO DOCUMENTS AND THE ASME CODE IN PRESSURE VESSEL DESIGN
The scope and general use of SAES-D-001, Design Criteria for
Pressure Vessels, and 32-SAMSS-004, Pressure Vessels, were
discussed in MEX 202.02. These Saudi Aramco documents refer to the ASME Boiler and Pressure Vessel Code Section VIII as the basic industry standard that provides the design requirements for pressure vessels that are used by Saudi Aramco. The Saudi Aramco documents supplement the ASME Code as necessary, based on specific Saudi Aramco requirements. Specific design requirements that are contained within these Saudi Aramco documents will be discussed as appropriate in this module.
The ASME Code, Section VIII, is divided into two main sections: Division 1 and Division 2. Division 1 is used most often by industry. This course concentrates on Division 1. However, it is necessary to understand Division 2 in general, how it differs from Division 1, and where its use might be appropriate for Saudi Aramco applications.
The objective of ASME Code rules, aside from assigning dimensional values, is to establish the minimum requirements that are necessary for safe construction and operation. The ASME Code protects the public by defining the material, design, fabrication, inspection, and testing requirements that are needed to achieve a safe design. Experience has shown that the probability of a catastrophic failure is reduced to an acceptable level by the use of the requirements and safety factors that are contained in the ASME Code.
Pressure vessel codes are also written to be broadly applicable to more than just the refinery and petrochemical industries. Accordingly, these codes cannot anticipate and address every possible design requirement or service application. For example, the ASME Code, Section VIII, Division 1, states that "the user or his designated agent shall establish the design requirements for pressure vessels, taking into consideration factors associated with normal operation, and such other conditions as startup and shutdown." Therefore, the design of pressure vessels for refinery and petrochemical services usually involves factors that are beyond the minimum code requirements. The owner defines what additional factors, beyond normal ASME Code requirements, must be considered in each case.
Additional design factors, not specifically covered in the ASME Code, are vibration, thermal or pressure cycles, corrosion, and erosion. Owners apply supplementary requirements in the design, fabrication, inspection, and testing of pressure vessels that are suitable for their applications. Saudi Aramco's Engineering Standards (SAESs) and Materials System Specifications (SAMSSs) are examples of supplementary owner requirements. However, it is often necessary to supplement even the Saudi Aramco standards to cover design requirements for a particular pressure vessel.
A purchase requisition for a pressure vessel will specify specific design requirements and the appropriate Saudi Aramco Engineering Standards. It will require that the item meet ASME Code rules. The vessel will be inspected before it leaves the vendor's shop by an inspector who is authorized by the ASME Code authorities. The inspector is responsible for ensuring that all Code and other requirements have been met.
Structure and Scope of the ASME Code, Section VIII, Division 1,
Design By Established Rules
The ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Pressure Vessels, is the primary standard that is used by Saudi Aramco for the design, fabrication, inspection, and testing of unfired pressure vessels. When the words "Code" or "ASME Code" are used in this module, they refer to Division 1, unless stated otherwise. A copy of Division 1 is included in Course Handout 1.
ASME Code Structure
The ASME Code, Section VIII, Division 1, is divided into three subsections as follows:
• Subsection A consists of Part UG, the general requirements that apply to all pressure vessels, regardless of fabrication method or material of construction.
• Subsection B covers specific requirements that apply to various fabrication methods used for pressure vessels. Subsection B consists of Parts UW, UF, and UB that deal with welded, forged, and brazed fabrication methods, respectively.
• Subsection C covers specific requirements that apply to several classes of materials that are used in pressure vessel construction. Subsection C consists of Parts UCS (carbon and low-alloy steel), UNF (nonferrous metals), UHA (high-alloy steel), UCI (cast iron), UCL (clad and lined material), UCD (cast ductile iron), UHT (ferritic steel with properties enhanced by heat treatment), ULW (layered construction), and ULT (low-temperature materials).
In addition to these subsections, the ASME Code also contains the following appendices:
• Mandatory Appendices address specific subjects that are not covered elsewhere in the Code. The requirements that are contained in these appendices are mandatory when the subject that is covered is included in the design and construction of the pressure vessel under consideration.
• Nonmandatory Appendices provide information and suggested good practices. The use of nonmandatory appendices is not required unless their use is specified in the vessel purchase order.
ASME Code Scope
The ASME Code scope defines the circumstances where its rules apply. The Code is applicable for pressures that exceed 103 kPa (ga) (15 psig) and through 20 682 kPa (ga) (3 000 psig). At pressures below 103 kPa (ga) (15 psig), the ASME Code is not applicable. At pressures above 20 682 kPa (ga) (3 000 psig), additional design rules are required to cover the design and construction requirements that are needed at such high pressures.
The ASME Code is not applicable for piping system components that are attached to pressure vessels. Therefore, at pressure vessel nozzles, ASME Code rules are applied only through the first junction that connects to the pipe. This junction may be at the following locations:
• Welded end connection for the first circumferential joint, for cases where welded connections are used.
• First threaded joint for screwed connections.
• Face of the first flange for bolted, flanged connections. • First sealing surface for proprietary connections or fittings. The Code is also not applicable to non pressure-containing parts that are welded, or not welded, to pressure-containing parts. However, the actual weld that makes the attachment to the pressure part must meet Code rules. Therefore, items such as pressure vessel internal components, or external supports, do not need to follow Code rules, except for any attachment weld to the vessel.
The ASME Code identifies several other specific items where it does not apply. The exclusions of most interest are:
• Pressure containers that are integral parts of rotating or reciprocating mechanical devices (for example, pump, turbine, or compressor casings).
• Piping systems and their components. ASME Code, Section VIII,
Division 2, Design By Analysis
The overwhelming majority of Saudi Aramco pressure vessels are designed in accordance with Division 1. However, the use of Division 2 is preferred in some applications.
The ASME Code, Section VIII, Division 2, Pressure Vessels,
Alternative Rules, contains requirements that differ from the
requirements that are contained in Division 1. Several of the areas where the requirements between the two divisions differ are highlighted below.
• Stress: the maximum allowable stress for a Division 2 pressure vessel is higher than that of a Division 1 pressure vessel. From the standpoint of general primary membrane stress, a Division 2 vessel is less conservative than a Division 1 vessel for the same design parameters and materials. The Division 2 vessel is thinner, uses less material, and costs less. A Division 2 vessel compensates for the higher allowable primary membrane stress by being a much more stringent design standard than Division 1 in other respects.
• Stress Calculations: Division 2 uses a complex method of formulas, charts, and design by analysis that results in more precise stress calculations than are required in Division 1.
• Design: some specific design details are not permitted in Division 2 that are allowed in Division 1.
• Fabrication and Inspection: Division 2 has more stringent requirements than Division 1.
• Quality Control: material quality control is more stringent in Division 2 than in Division 1.
The areas where Division 2 are more conservative than Division 1 add to the cost of a vessel. The choice between using Division 1 and Division 2 is based on economics. The lower costs that are associated with the use of less material must exceed the increased costs that are associated with the more conservative Division 2 requirements in order for the Division 2 design to be economically attractive. A Division 2 design is more likely to be attractive for vessels that require thicker walls. A wall thickness of approximately 50 mm (2 in.) is a good starting point at which to consider the use of a Division 2 design. The thickness break point is lower for more expensive alloy material than for plain carbon steel, and this break point will also be influenced by current market conditions. A Division 2 design will also be attractive for very large pressure vessels, where a slight reduction in required thickness will greatly reduce shipping weights and foundation load design requirements.
The Division 2 design criteria provide formulas and rules for the more common configurations of shells and formed heads. Requirements include detailed evaluations of actual stresses in complex geometries and with unusual loadings, especially cyclic loads or those that result in localized stresses. The calculated stresses are assigned to various categories and subcategories. These categories and subcategories have different allowable stress values. These allowable stress values are based on multiples of the basic allowable stress intensity value that is specified in Division 2 for the particular material specification. Participants are referred to Division 2 for additional information on the stress categories and subcategories, and their associated allowable stresses.
Saudi Aramco Pressure Vessel Design Data Sheets
Saudi Aramco standard Pressure Vessel Design Data Sheets, also known as Pressure Vessel Data Sheets or Pressure Vessel Design Sheets, are used to:
• Ensure a uniform bidding basis among the various vendors- - Ensure format consistency in the information that is
provided;
- Simplify bid comparisons during pressure vessel purchase.
• Document the as-built details of the vessel.
• Facilitate retrieval and use of design information after the vessel has been installed.
In addition to meeting the purposes that are listed above, the forms are frequently used by operations, inspection, and maintenance personnel after the vessel has been placed in service.
SAES-D-001, Pressure Vessels, refers to other Saudi Aramco documents that must be complied with. Forms 2682 and 2683 are among these documents. Form 2682 is used for Division 1 pressure vessels, and Form 2683 is used for Division 2 pressure vessels. The portion of Form 2682 that contains material selection information was discussed in MEX 202.02. Copies of these forms are contained in Course Handout 3 for reference in subsequent discussions.
The pressure vessel engineer is responsible for completion of the appropriate Design Data Sheet to the extent possible for use as part of the vessel purchase requisition. The portions of the form that must be completed at this time are those that are necessary to define the Saudi Aramco and ASME requirements that ensure a uniform bidding basis. Other portions of the form that do not impact the cost quotation basis, or that are determined later during the detailed vessel design phase, are initially left blank. These blank portions are subsequently completed by the vessel vendor as part of his bid. The completed form from the successful vendor will then be included in the pressure vessel's documentation file. When a vessel is rerated, it is not the Design Data Sheet that is revised but, rather, the Safety Instruction Sheet. The Safety Instruction Sheet will be discussed later in this module.
In most cases, initial data for Forms 2682 and 2683 are provided by engineers who work for a contractor who is employed by Saudi Aramco for a project. Saudi Aramco engineers typically will review the contractor's work to ensure that it is correct. Saudi Aramco engineers fill in the initial data on the forms in cases where no contractor is involved, or when documentation is being prepared for the rerating of an existing pressure vessel to new design conditions.
The sections that follow discuss the overall content of Forms 2682 and 2683. Later sections will focus in more detail on filling out specific portions of Form 2682.
Content of Form 2682
for Division 1 Pressure Vessels
Form 2682, provided in Course Handout 3, is the initial form that is used by Saudi Aramco to specify Division 1 pressure vessels. References to appropriate paragraphs within Division 1 are indicated on the form. The following paragraphs highlight particular sections of this form. Some sections may not need to be initially completed, either because they would not impact the initial bidding, or would clearly be covered by 32-SAMSS-004 or ASME Code requirements. Both 32-SAMSS-004 and the ASME Code are intrinsic parts of the purchase requisition.
• The large open space in the upper middle portion of the form is used primarily for the vessel outline drawing for relatively simple vessels, such as drums. For more complex vessels, the outline drawing with appropriate details is produced on separate vessel design sheets (Form 2526, 2527, or 2528). The drawing shows the overall dimensions and orientation of the vessel, nozzle locations, and additional features needed to define the vessel. This open space may also be used for additional notes or design information.
• The operating conditions of pressure and temperature must be specified. A minimum temperature of less than 0°C (32°F) must be indicated. Temperatures that are below 0°C (32°F) require additional materials and inspection steps to meet Saudi Aramco requirements. A vessel that is in wet, sour or lethal service must be specified. Wet, sour or lethal services (discussed later in this module) require additional items to meet Saudi Aramco and ASME requirements.
• The maximum and minimum design temperatures must be specified in accordance with SAES-D-001 requirements. • The design pressure must be specified in accordance with
SAES-D-001. When the form is first completed, the design pressure is assumed to be equal to the maximum allowable working pressure (MAWP). When the design is complete, the actual MAWP of the vessel is determined on the basis of the nominal vessel thicknesses that are actually supplied. The maximum operating static liquid head and fluid specific gravity must also be specified to permit adequate design. The static liquid head adds to the pressure that is used to determine component thickness, as will be discussed later in this module.
• The material specifications for major vessel components must be specified. These specifications are based on 32-SAMSS-004, as discussed in MEX 202.02. Material selection for vessels in hydrogen service must also consider the potential for hydrogen attack and the Nelson Curve limitations. The hydrogen partial pressure should also be specified on this form, if applicable for the service. • The maximum allowable stress values for the specified
materials should be entered. These values are taken from the Division 1 allowable stress tables, as discussed in MEX 202.02.
• The need for impact testing should be specified. This requirement is based on ASME Section VIII, Division 2, as specified by 32-SAMSS-004, even though the overall vessel design is in accordance with Division 1.
• The need for ultrasonic testing of plate that is over 50 mm (2 in.) thick and of clad plates should be specified in accordance with 32-SAMSS-004.
NOTE: 32-SAMSS-004, which forms part of the purchase requisition, specifies the requirements for impact testing and ultrasonic testing. The information for these items is entered in the upper left corner of the large space provided for the vessel outline drawing. These items may be determined and reported by the vendor. However, it is preferable to specify them here to ensure that they are properly considered in the vendor quotations.
• The required radiography of the shell seams must be specified as either "spot" or "full." The weld joint efficiency that corresponds with the specified radiography is then determined. SAES-D-001 requires full radiography for all butt welds on vessels with a minimum design temperature below 0°C (32°F) or on vessels to be used in wet, sour service. Paragraph UW-11 of Division 1 also specifies other cases that require full radiography.
• The required corrosion allowance must be specified. Corrosion allowance was discussed in MEX 202.02.
• The required wall thickness for internal pressure for the shell and heads must be calculated using the ASME equations that are provided on the left side of the form. If external pressure is a specified design condition, separate calculations must be made and the results entered on the form. The results of the external pressure calculations could require greater wall thickness or the addition of stiffening rings. Internal and external pressure design calculations are discussed later in this module.
Any additional wall thickness that is required by other loadings must be determined by the vendor during the detailed engineering phase of vessel design. However, if the magnitudes of external loads are known, these magnitudes should be specified on this form. Examples of
• The test pressure may be calculated by the vendor.
• The vessel engineer must specify if the vessel must be Postweld Heat Treated (PWHT) for reasons other than those that are specified in the ASME Code. PWHT requirements may be determined by the vendor if only ASME requirements apply for the particular vessel.
• The block that covers the maximum size of shell openings is left blank. ASME rules clearly provide limitations on the size of shell openings and requirements to be met if these limitations are exceeded.
• The required Class, allowable working pressure, and test pressure of flange connections must be specified. Since the Class is specified on the basis of the design conditions, it should not be the factor that limits in the vessel design. Flange Class selection is discussed later in this module. • The block that covers the design loads is typically left
blank, unless specific other loads are known in advance. The vendor or contractor will determine any unspecified loads during the detailed engineering phase, and the vendor will then evaluate the impact of these loads on vessel design.
• The estimated vessel weights and capacity are specified by the vendor as part of detailed engineering.
• The vessel Maximum Allowable Working Pressure (MAWP) will be specified by the vendor after the nominal thicknesses have been determined during detailed engineering. MAWP will be discussed later in this module. • The nozzle schedule, reference mark, size, ASME/ANSI
B16.5 rating, and service must be specified.
• The required material for internal components, flanges, gaskets, and bolting must be specified.
Content of Form 2683 for Division 2
Pressure Vessels
Form 2683, provided in Course Handout 3, is the form that is used by Saudi Aramco to specify Division 2 pressure vessels. Form 2683 is similar to Form 2682 and has comparable sections to be completed. References to appropriate paragraphs within Division 2 are indicated on the form. The previous discussion of Form 2682 applies to similar sections on Form 2683.
An additional requirement for a Division 2 pressure vessel is the potential need for a fatigue analysis. Paragraph AD-160 of Division 2 contains rules that determine the need for a fatigue analysis. These rules are based on the specific parameters that follow:
• The number of pressure and temperature cycles expected. • The magnitude of the expected pressure and temperature
cycles.
• The expected metal temperature differences between adjacent points on the vessel.
If the specified criteria indicate that a fatigue analysis is required, the data that are needed to define the pressure and temperature cycles must be specified. The Design Data Sheet must specify where the cycle data may be found. The vendor then determines the impact that the cycle data have on his design and bid. A fatigue analysis will normally not be required for typical Saudi Aramco pressure vessel applications.
EVALUATING THE ACCEPTABILITY OF CONTRACTOR-SPECIFIED DESIGN CONDITIONS AND LOADINGS
Mechanical engineers are not responsible for specifying either the operating or design conditions for a pressure vessel. Specification of these conditions is the responsibility of the process design engineer. However, mechanical engineers must be aware of the differences between operating and design conditions. Process engineers, on the other hand, must understand the impact that operating and design conditions have on the mechanical design of pressure vessels. The operation and design conditions that are specified should not be overly conservative or liberal.
The mechanical design of a pressure vessel begins with specification of the design pressure and design temperature. These two parameters must be specified together to obtain the correct mechanical design details. Pressure imposes loads on a pressure vessel that must be withstood by the individual vessel components. Temperature affects material strength and, thus, allowable stress, regardless of the design pressure. Some pressure vessels have multiple sets of design conditions that correspond to different modes of operation. For example, during its operating cycle, a reactor may have a high pressure and moderate temperature during normal operation, but it may operate at a much lower pressure and a very high temperature during catalyst regeneration. Both sets of design conditions must be specified because either one or the other operating state may govern the mechanical design of the reactor components.
All pressure vessels must be designed for the most severe conditions of coincident pressure and temperature that are expected during normal service. This requirement is stated in the ASME Code, Section VIII, Division 1. Normal service must include conditions that are associated with:
• Startup.
• Normal operation.
• Deviations from normal operation that can be anticipated (for example, catalyst regeneration or process upsets). • Shutdown.
Pressure vessels must also be designed for other loading conditions and service factors that may apply. These conditions and factors, as well as pressure and temperature, are discussed in the following sections.
Work Aid 1 may be used to assist in evaluating the acceptability of design conditions and loadings.
Pressure
Both operating and design pressure must be considered in pressure vessel design.
Operating Pressure
The operating pressure must be set on the basis of the maximum internal or external pressure that the pressure vessel may encounter. The following factors must be taken into account:
• Ambient temperature effects. • Normal operational variations.
• Pressure variations due to changes in vapor pressure. • Pump or compressor shut-off pressure.
• Static head due to the level of liquid in the vessel. • System pressure drop.
• Normal cleaning and pre-startup activities if other conditions may occur, such as vacuum, that should be considered in the design.
Design Pressure
Generally, design pressure is the maximum internal pressure, in kPa (gauge) or psig, that is used in the mechanical design of a pressure vessel. For full or partial vacuum conditions, the design pressure is the maximum pressure difference that can occur between the atmosphere and the inside of the pressure vessel. The design pressure is applied externally for vacuum conditions. Specific pressure vessels may experience both internal and external pressure conditions at different times during their operation. The mechanical design of the pressure vessel in this case is based on internal or external pressure, depending upon which of these is the more severe pressure condition.
More specifically, the design pressure of a pressure vessel is the pressure that is expected at the top of the vessel. The design pressure is normally based on the maximum operating pressure at the top of the vessel, plus the margin that the process design engineer determines is suitable for the particular application.
A suitable margin must also be provided between the maximum operating pressure and the safety relief valve set pressure. This margin is necessary to prevent frequent and unnecessary opening of the safety relief valve that may occur during normal variations in operating pressure. The safety relief valve set pressure is normally equal to the pressure vessel design pressure.
SAES-D-001 specifies Saudi Aramco requirements for setting the design pressure and considers the possibility of either external or internal pressure conditions. There may be cases where a vessel is not in vacuum service during normal operation or in an upset, but may be subject to steam-out conditions which can cause an external pressure condition. If steam-out is possible, the pressure vessel must be designed for an external pressure of 52 kPa (ga) (7.5 psig) at 149°C (300°F). Work Aid 1 summarizes the procedure for setting design pressure based on Saudi Aramco requirements.
Pressure vessels, especially tall towers, typically will have liquid in them during normal operation. The maximum height of this liquid normally does not reach the top of the vessel. The liquid level that is required for design is specified by the process design engineer.
The weight of the liquid that is contained in the vessel must be considered in the design, as will be highlighted below. The hydrostatic pressure of the liquid must also be considered in the design of the vessel components. Therefore, the pressure that is used to design a vessel component is equal to the design pressure at the top of the vessel, plus the hydrostatic pressure of the liquid in the vessel that is above the point being designed.
Figure 1 illustrates this concept. Work Aid 1A contains equations that are used to calculate the pressure below the liquid level. ; ;; PT = Design pressure at top of vessel. γ = Weight density of liquid in vessel. H = Height of liquid. PBH = Design pressure of bottom head. 20203.F01
The pressure vessel designer must determine the minimum thickness that is required for each vessel section. The thickness calculation must be made for both the design and hydrostatic test cases. The minimum specified wall thickness is then based on the more severe condition. For example, determination of the minimum specified wall thickness of each section of a tall vertical tower must include consideration of the design pressure at the top of the vessel, plus the hydrostatic head that is applicable at the level being designed. Several wall thickness plates are commonly used in a tall, liquid-filled tower. Thicker plates are used in the lower sections because of higher hydrostatic pressure and higher bending moments caused by wind.
Temperature
Both operating and design temperatures must be considered in pressure vessel design.
Operating Temperature
The operating temperature must be set on the basis of the maximum and minimum metal temperatures that the pressure vessel may encounter. The operation and vertical length of some pressure vessels result in large temperature reductions between the bottom and top of the vessel. For example, atmospheric and vacuum pipestill towers are typically very tall and have liquid in the bottom portion and vapor in most of the other sections. The temperature of the liquid in the bottom will be much higher than the temperature of the vapor in the top. It is permissible to specify different operating temperatures at different elevations of such a pressure vessel, as long as the temperatures can be accurately predicted. This approach results in dividing the vessel into sections along its vertical length. Each section is designed for the temperature that it will encounter, rather than for the most severe condition at the bottom of the vessel. Figure 2 shows a tall vessel, and illustrates the range of sections that have different design
; Section 4 (T-Z) Section 3 (T-Y) Section 2 (T-X) Section 1 (T), °C (°F) Support skirt Grade MEX 20203.F04 20203.F02
Sudden cyclic changes in temperature also occur during normal operation and may be associated with only minor pressure fluctuations. In this case, the design is governed by the highest and lowest probable operating temperatures of the material that is in contact with the metal, and by the corresponding pressure at those temperatures.
Design Temperature
The design temperature of a pressure vessel is the fluid temperature that occurs under normal operating conditions, plus an allowance for variations that occur during operation. SAES-D-001 specifies the basis for setting the design temperature, and this basis is summarized in Work Aid 1.
Minimum Design Metal Temperature
A Minimum Design Metal Temperature (MDMT) must also be specified for pressure vessel design. A MDMT is specified to ensure that materials that have adequate fracture toughness are selected for construction. Fracture toughness was discussed in MEX 202.02.
SAES-D-001 specifies the factors influencing the minimum design metal temperature. For most pressure vessels, the minimum design metal temperature equals the minimum design ambient temperature for the construction site. Some services subject pressure vessels to extremely low temperatures during normal operations or process upsets. In these cases, the lower operating temperature must be considered to determine the minimum design metal temperature. For example, if autorefrigeration is possible, it must be considered in the determination of the minimum design metal temperature. Autorefrigeration may occur during startup, shutdown, upset, failure of a piping component, or malfunction of a pressure relief device.
Other Loadings
Paragraph UG-22 of the ASME Code, Section VIII, Division 1, specifies the loadings that must be considered to determine the minimum required shell thicknesses for the various vessel sections. These design loadings are as follows:
• Internal or external design pressure.
• Weight of the vessel and its normal contents under operating or test conditions. This weight includes any additional pressure that is due to the static head of liquid. • Superimposed static reactions from the weight of attached
equipment, such as motors, machinery, other vessels, piping, linings, and insulation.
• The attachment of internal components or vessel supports. • Wind, snow, and seismic reactions.
• Cyclic and dynamic reactions that are caused by pressure or thermal variations, or by equipment that is mounted on a vessel, and mechanical loadings.
• Test pressure combined with hydrostatic weight.
• Impact reactions such as those that are caused by fluid shock.
• Temperature gradients within a vessel component and differential thermal expansion between vessel components. In its simplest form, the design thickness for a pressure vessel shell component may be determined based on design pressure conditions alone. However, the detailed design of a pressure vessel must consider all combinations of pressure, weight, and external and internal loads that may occur in actual operation. The stresses that occur during erection of the vessel at the site must also be considered. It is normal practice to assume that wind and earthquake loads do not occur simultaneously. Therefore, the vessel is designed for the worst of either wind or earthquake.
The directions, types, and locations of stresses that are produced by the various load combinations must be evaluated using appropriate criteria. Some loads, such as wind, will affect the thickness of an entire shell section. Other loads, such as those from piping systems, affect only a local region of the vessel. A complete consideration of all loadings that act on a pressure vessel is a complicated process and is normally done using computer programs. While the Saudi Aramco pressure vessel engineer does not have to calculate the effects of all these loadings, he must be aware of what a pressure vessel designer must do to achieve a correct design.
It is also not normally necessary to calculate all of these loadings before the Pressure Vessel Design Data Sheet is completed, to determine the effect, if any, of the loadings on vessel design. These loadings will generally have, at most, a localized effect on vessel design and should not be a major factor in distinguishing one vendor's bid from another. Vendors know that they will have to consider wind, hydrotest, piping, and internal loads in their final designs, and they typically will provide cost allowances for these considerations in their bids as necessary. The Pressure Vessel Design Data Sheet has an area where it may be indicated whether these loads were considered.
Figure 3 shows a typical vertical reactor with additional loads applied.
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Pipe connection with imposed forces and moments
Catalyst bed with liquid holdup Support grid Grid support welded to shell Wind Earthquake 20203.F03
32-SAMSS-004 requires that the following load conditions be used in the design of pressure vessels and their supports:
• Full wind or seismic load combined with all dead loads except operating liquid. This is the case for the condition when the vessel is erected but not operating and when it is exposed to the design wind or seismic load (whichever is greater).
• Internal design pressure or external pressure loads, plus total operating weight (vessel in erected condition, trays, operating liquid) and full wind or seismic load. This case is for the condition when the vessel is under normal operation and is exposed to the design wind or seismic load (whichever is greater).
• Test pressure combined with hydrotest weight (weight of vessel in erected condition and weight of test water), plus wind or earthquake loads reduced by 40%. This case is for the condition when the vessel is being hydrotested in the field. It is assumed that the hydrotest would proceed if the wind velocity is up to the specified value but would not be done at higher wind velocities or during an earthquake.
Weight
The weight and location of the following exterior and interior attachments must be considered in determining the dead weight load that acts on a pressure vessel:
• Attached equipment and piping • Catalyst bed supports
• Fireproofing • Insulation • Platforms • Trays
The dead weight load must also include the weight of the vessel contents, such as liquid, catalyst, and inert balls, as well as the weight of the vessel shell itself. Weight load calculations must be made both for the operating fluid in the vessel, for the maximum design liquid level, and for the case with the vessel completely filled with water for hydrotest. The hydrotest calculations must be made for both the shop and field testing. Loads that act eccentrically to the vessel axis are resolved into forces and moments that act along the vessel axis. Examples of eccentrically applied loads are the weight of platforms that are attached to the side of a tower or the weight of a heat exchanger that is bolted to a nozzle on the side of a tower. These dead weight loads result in longitudinal compressive stresses in the vessel shell and supports. CSE 110 provided procedures for the calculation of weight loads. The general approach is as follows:
• Calculate the metal volume of the shell, heads, and support. This calculation takes into consideration the geometry of the individual components.
• Calculate the weight of metal based on the volume that is found.
• Determine the internal volume of the vessel shell and heads.
• Calculate the weight of water for the hydrotest based on the total volume of the vessel.
• Calculate the weight of operating liquid in the vessel based on its maximum fill height.
• Determine the weight of internal components, appurtenances, and insulation.
• Calculate the total operating and hydrotest weights of the vessel.
Refer to CSE 110.02 for the specific procedures to use and sample problems. Note also that the Pressure Vessel Design Data Sheet has an area where the vendor fills in the estimated vessel weights for three cases: empty with internal components, operating condition, and filled with the test medium.
Wind
Wind loads are imposed on all pressure vessels that are located outside of buildings. Wind loads induce stresses in the vessel shell components. However, wind loads are only a significant design consideration for tall, vertical pressure vessels. Wind load evaluation requires consideration of the two concepts that are discussed in the paragraphs that follow.
Pressure applied across the pressure vessel surface due to wind velocity produces forces along the vessel length. These forces
produce bending moments along the length of the vessel that, in turn, produce additional longitudinal stresses in the vessel shell. These longitudinal stresses are in tension on the windward side of the vessel and in compression on the leeward side. Wind loads also cause vertical vessels to deflect. This vessel deflection must be kept within reasonable limits in order not to adversely affect process operations, for example by causing a nonuniform liquid flow across distribution trays or by creating an unsafe situation for personnel who may be on the vessel. The following must be considered in the design of pressure vessels for wind loads:
• The pressure vessel data sheet specifies the design wind velocity to be used for pressure vessel design. It must also be assumed that a field hydrotest can take place if there is a wind up to 60% of the design wind speed.
• The effective vessel diameter, De, considers such factors
as insulation, piping, platforms, and ladders, and determines the vessel area that is exposed to the wind. This determination is used to calculate the forces and bending moments that are imposed on the vessel. Figure 4 shows a generalized tower layout and illustrates how items that are attached to the vessel effectively increase the diameter that is exposed to the wind.
Caged ladder Insulation thickness Distance between platforms Insulation thickness Pipe OD Vessel OD Platform 20203.F04
Figure 4: Tower Layout for Determining Effective Diameter for Wind Calculations
CSE 110.02 provided procedures to calculate the wind loads that are imposed on a tall tower. The general approach is as follows:
• Determine the gust factor based on the height of the vessel.
• Determine the height correction factors for various elevations along the total height of the vessel.
• Determine the shape factor for the vessel. • Determine the equivalent vessel diameter.
• Calculate the lateral shear forces that are applied to the vessel at various elevations along the total height.
• Calculate the bending moment that is applied to the vessel due to the shear forces. The maximum moment occurs at the base.
Refer to CSE 110.02 for the specific procedures to use, and for sample problems. Note that these procedures may be used to calculate the applied loads; however, do not use these procedures to calculate the stresses in the vessel that result from these loads. This stress calculation is typically done by the vendor during detailed engineering.
Note that the Pressure Vessel Design Data Sheet has a location where an indication may be made of whether the design includes wind loads. In most cases, this item will either be left blank or answered "No." Wind design calculations are typically not done to complete this sheet. Therefore, any component thicknesses specified on the Pressure Vessel Design Data Sheet will not include consideration of wind. However, this omission should not be interpreted to mean that wind does not need to be considered by the vendor in his final design.
The dynamic effect of vortex shedding occurs when wind flows past a circular pressure vessel and when the air behind the vessel is no longer smooth. A region of pressure instability occurs where vortices are shed in a regular pattern. The vortex shedding alternates from one side of the vessel to the other. These vortices cause an alternating force to act perpendicular to the wind direction and causes the vessel to vibrate. The concept of vortex shedding is illustrated in Figure 5.
Wind Tower cross section Shed vortex 20203.F05
Figure 5: Vortex Shedding
When the vortex-shedding frequency coincides with the mechanical natural frequency of the vessel, mechanical resonance of the vessel occurs. Mechanical resonance causes an increase in vibration amplitudes, and fatigue failure of vessel sections can eventually result. The vessel must be designed so that its natural frequency is high enough to avoid resonant vibration. The parameters that affect this phenomenon are wind velocity, vessel diameter, and vessel height.
Hydrotest
Pressure vessels are required to pass a hydrostatic pressure test. In the hydrotest, the vessel is filled with water, either in the shop or in the field, and pressurized to the prescribed test pressure. The vessel, its associated support, and its foundation must be designed for the weight of test water, as discussed earlier.
A vertical vessel is often designed to be tested horizontally in the shop. In this case, the following must be considered:
• It may be necessary to retest the vessel in the field after repairs or modifications are made.
• During normal operation, the vessel may be accidentally filled with liquid or solids.
Given the above considerations, a vertical pressure vessel is normally designed so that it may also be tested in the installed position, even if the original shop hydrotest is in the horizontal position.
The hydrotest verifies the structural adequacy of the pressure vessel. The hydrotest also provides some mechanical stress relief before the application of service conditions, as long as the test temperature is above the vessel material's ductile-to-brittle transition temperature. MEX 202.02 discussed the ductile-to-brittle transition temperature.
Paragraph UG-99 of the ASME Code, Section VIII, Division 1, provides procedures to determine hydrotest pressure. Saudi Aramco supplements these requirements in 32-SAMSS-004,
Pressure Vessels, and in procedures that are contained in the
Pressure Vessel Design Data Sheet. These procedures are completely discussed in MEX 202.04.
The Pressure Vessel Design Data Sheet has a location where an indication may be made of whether the design includes hydrostatic test loading. In most cases, this item will either be left blank or answered "No." Stresses that result from hydrotest weight loads will generally not be calculated when this sheet is completed. Therefore, the specified component thicknesses will not reflect the hydrotest. The effect of hydrotest weight load on component thickness, if any, will be determined by the vendor during detailed engineering.
External Piping
Most pressure vessel nozzles have piping systems attached to them. The nozzles act as anchor points for the piping systems and absorb forces and bending moments that are imposed on the vessel by the piping system. These forces and bending moments are caused by weight and differential thermal expansion of the piping.
Piping systems produce additional stresses in the pressure vessel nozzle and adjacent areas of the shell. These additional stresses are localized and diminish away from the nozzle. The detailed design of the nozzle and adjacent shell must be strong enough to maintain these stresses within allowable limits. To accomplish this stress control, the following are sometimes necessary:
• Increase the nozzle thickness.
• Increase the nozzle reinforcement pad size.
• Use a thicker section of vessel shell in the local area.
Nozzle design modifications that are required to accommodate piping loads are not necessary in most cases. Nozzle and shell designs are generally strong enough to accommodate design pressure and to absorb piping loads. However, special attention should be paid to very low-pressure applications and large-diameter nozzles (over 600 mm [24 in.]). In the low-pressure case, the vessel shell may be relatively thin and the nozzle may not need reinforcement for pressure. Thus, there may not be adequate inherent strength to absorb the additional piping loads. In the large-diameter nozzle case, piping reaction loads from differential thermal expansion may be high.
The evaluation of loads imposed on pressure vessel nozzles will be discussed further in a later section of this module.
Here again, the Pressure Vessel Design Data Sheet may indicate if piping system reaction loads have been considered in the design. This item will typically be left blank or answered "No." Loads from attached piping systems will typically not be available when the sheet is first completed since the piping systems have not yet been designed. Therefore, the specified component thicknesses do not consider these piping loads. The vendor will typically be supplied with piping loads later, and he will determine their effect on vessel design then.
Internal Components
Pressure vessel internal components, such as tower trays and reactor catalyst bed supports, are examples of components that are typically supported from the vessel shell. The weight and bending moment loadings that are associated with pressure vessel internal components induce stresses in the vessel shell and support attachment welds to the shell. These stresses must be kept within allowable limits, and the internal components and their associated supports must be designed by the pressure vessel vendor.
32-SAMSS-004 requires that all internal and external supports, support rings, pads, and structural brackets that are attached to the vessel be seal-welded all around. This seal-welding prevents any corrosion between the shell and the attachment. The evaluation of loads that are imposed by attachments to a pressure vessel shell or head will be further discussed later in this module.
The Pressure Vessel Design Data Sheet may be used to designate whether loads due to internal components were considered in the specified design. This item will typically be left blank or answered "No." The vendor will typically determine the effect of loads due to internal components as part of his detailed design.
Earthquake Loadings
Earthquake loadings on a vessel result from a sudden, erratic, vibratory motion of the ground that supports the vessel. The vessel responds to this motion. The main factors that cause vessel damage are the intensity and the duration of the earthquake motion. The forces and stresses in the vessel shell are transient, dynamic, and complex. Accurate evaluation of earthquake forces and the vessel stresses that these forces cause requires computer analysis.
To simplify vessel design procedures, the vertical component of the earthquake motion is normally disregarded. This approach is acceptable since most structures have enough excess strength in the vertical direction to be considered earthquake resistant. The horizontal, or lateral, earthquake forces are then reduced to equivalent static forces that act on the vessel.
Earthquake design relies mainly on experience and observation. It is based on the performance of structures that have been previously subject to earthquakes. Structures that are built in seismic risk zones must be designed to withstand a minimum horizontal shear force applied at the base of the vessel in any direction. The shear force is translated into equivalent static forces through the height of the vessel. The static forces are used to calculate the shear forces, bending moments, and resulting stresses through the height of the vessel.
The simplified equation that follows may be used to estimate the lateral seismic force at the base of the vessel:
V = ZIKCSW
Where: V = Lateral seismic force at the base of the vessel, kg (lb.).
Z = Seismic probability coefficient for the site.
I = Importance factor; assume I = 1.0 for a pressure vessel.
K = Arrangement factor; assume K = 2.0 for a pressure vessel.
T = Fundamental period of vibration of the vessel, assuming a uniformly loaded cantilever beam fixed at the base.
S = Site-coefficient based on soil characteristics, assuming S = 1.5 unless an exact value is known.
W = Total dead weight load of the vessel and contents above the plane being considered, kg (lb.).
CSE 110.02 contains a simplified procedure to calculate the earthquake loadings on a tall, single-diameter tower. This procedure involves calculation of the shear force and overturning moment applied at the base. When earthquake is a design consideration, the calculated loadings are used to determine the resulting stresses in the shell.
Service
The pressure vessel service can affect material selection, fabrication, and inspection requirements. Therefore, the service must be specified on the Pressure Vessel Data Sheet. In addition, it also must be specified whether the vessel is in wet, sour service or in lethal service. Material selection requirements based on service considerations were discussed in MEX 202.02. Fabrication and inspection requirements based on service considerations will be discussed in MEX 202.04.
Wet, Sour
In-service cracking at welds is possible in a wet, sour process environment. This is called stress corrosion cracking and is caused by the combined action of tensile stress and corrosion in the presence of water and H2S. Wet, sour service must be
specified, when applicable, for the vessel vendor to know that additional Saudi Aramco material specification and inspection
Lethal
The owner must identify to the vendor whether the pressure vessel will contain a lethal substance. The ASME Code Section VIII, Division 1, defines a lethal substance as:
"Poisonous gases or liquids of such a nature that a very small amount of the gas or of the vapor of the liquid mixed or unmixed with air is dangerous to life when inhaled. For purposes of this Division, this class includes substances of this nature which are stored under pressure or may generate a pressure if stored in a closed vessel."
It is very rare for a refinery service to be in this category. When the lethal service category does apply, the ASME Code requires additional measures to increase vessel quality. These measures include 100% radiography of all butt welds, PWHT of carbon and low alloy steel materials, and restrictions on the use of certain carbon steel material specifications. The vendor must consider these requirements in his cost quotation.
EVALUATING THE ACCEPTABILITY OF CONTRACTOR-SPECIFIED PRESSURE VESSEL COMPONENT THICKNESS DESIGN CRITERIA
Two additional pressure vessel design criteria are required before vessel component thicknesses can be calculated (by the vendor) and in turn evaluated for acceptability (by Saudi Aramco engineers). These additional design criteria are:
• Weld joint efficiency • Corrosion allowance
Weld Joint Efficiency
Weld joint efficiency (E) is used to account for the quality of a welded joint and for the concentration of local stress. Determination of a stress concentration factor takes into consideration the fact that the stress in a localized region of a component or structure may be higher than would be calculated if normal static analysis were used. This higher local stress is due to local material or structural discontinuities. Stress concentration in welded joints arises from the following factors: • The geometry of the weld itself.
• The metallurgical structure of the weld with respect to the base metal.
• Weld defects, such as slag inclusions, shrinkage cracks, or porosity.
The last two factors are functions of the procedure that is used to make the weld. The first factor is the main source of stress concentration and can be controlled by the vessel design engineer. The net effect of the three stress concentration factors is to reduce the fatigue strength or efficiency of the weld. Weld joint efficiency must be considered in the vessel design.
Paragraph UW-12 of the ASME Code, Section VIII, Division 1, specifies the weld joint efficiencies to be used in the formulas for pressure vessel component thicknesses. These efficiencies depend on the type of weld joint design that is used and on the degree of weld radiographic examination that is made. Types of welded joints and weld inspection methods will be discussed in MEX 202.04. For our purposes here, it is only necessary to know that the efficiency of a weld joint is determined by the type of weld joint that is used and by the extent of its radiographic inspection. The ASME Code also defines weld joint categories based on the location of a joint in a vessel. The Code then specifies, by example, the weld joint designs that may be used in each category. MEX 202.04 discusses weld joint types and inspection further.
32-AMSS-004 requires 100% joint efficiency for all hydrogen, wet sour, lethal, cyclic and unfined steam drum services. 100° weld efficiency is also required for hydrocarbon, steam, amine and caustic services above 120°C (250°F).
Figure 26 in Work Aid 2A is excerpted from the ASME Code, Section VIII, Division 1, and identifies pressure vessel weld joint categories.
Figure 28 in Work Aid 2A is also excerpted from the ASME Code and defines weld joint efficiencies based on the type of weld (shown in Figure 27 of Work Aid 2A) and degree of radiographic examination. Figure 28 shows that the weld joint efficiency decreases as the degree of radiography decreases for a given type of weld joint. Note from Figure 28 that the direction of weaker weld joint designs is vertically downward and that lower weld joint efficiencies correspond with weaker weld joint designs.
The majority of pressure vessels use a Type 1 joint design. A Type 1 joint design has a weld joint efficiency of either 0.85 or 1.00; these values correspond with either spot or full radiographic examination. Later discussion of the ASME Code calculation formulas will show that the required shell and head thicknesses increase with decreasing weld joint efficiency. Work Aid 2A summarizes how to evaluate the acceptability of the specified weld joint efficiency. The degree of radiography and corresponding joint efficiency are specified on the Pressure Vessel Design Data Sheet.
Corrosion Allowance
Corrosion was discussed in COE 103 and COE 105. Corrosion, erosion, or abrasion causes the components of a pressure vessel to thin during the operating life of the vessel. In order to compensate for this thinning, components must have their thicknesses increased over those that are calculated based on the ASME Code design formulas. Internal corrosion/erosion-resistant linings are sometimes used as an alternative to the use of greater component thicknesses.
Process design and materials engineers typically specify the corrosion allowance for pressure vessel components. These allowances are based on determinations of the expected corrosion rate for the vessel material in the anticipated process environment. The expected corrosion rate is multiplied by the design life of the vessel (normally 20 years) to determine the corrosion allowance that is to be used in the vessel design. The expected corrosion rate is a major factor that influences material selection. A high corrosion rate for a material in a particular service requires a large corrosion allowance. This larger corrosion allowance requires thicker components and increases the cost of the pressure vessel. It is often possible to use a higher-alloy material that has a lower corrosion rate and corrosion allowance in the same service. In many cases, the greater cost per pound of the higher-alloy material is offset by the ability to use thinner components, and therefore, less material. Saudi Aramco minimum corrosion allowance requirements for carbon steel were discussed in MEX 202.02. The required corrosion allowance must be specified on the Pressure Vessel Design Data Sheet to permit the vendor to determine the required component thicknesses.
Work Aid 2B summarizes how to evaluate the acceptability of the specified corrosion allowance.
EVALUATING CONTRACTOR-SPECIFIED DESIGN CALCULATIONS FOR PRESSURE VESSEL COMPONENTS
This section discusses the ASME Code calculations for the following pressure vessel components:
• Shells • Heads • Conical sections • Flat covers • Nozzles • Nozzle flanges
The calculations required to determine the stresses that result from local loads that are applied to nozzles and attachments to the vessel will also be discussed in general terms.
The ASME Code, Section VIII, Division 1, requires the minimum thickness of shells and heads to be 1.6 mm (0.0625 in.) for most applications, regardless of calculation results. This is the thickness after the vessel components are formed and before corrosion allowance is added. This minimum thickness requirement provides a basic level of mechanical strength for the pressure vessel, even if the calculations indicate that the vessel may be thinner for the design loads that are actually imposed.
Design for Internal Pressure
This section discusses calculation of the wall thickness of shells, heads, and conical sections under internal pressure. Refer to Work Aid 3A for the ASME equations that are required to perform the calculations.
Shells
The idealized equations for the calculation of hoop and longitudinal stresses, respectively, in a cylindrical shell under internal pressure are as follows:
σθ = Prt and σ1 =Pr 2t
These equations assume a uniform stress distribution throughout the thickness of the shell. Since this is an idealized state, the ASME Code formulas have been modified to account for non-ideal behavior. For hoop stress, the formula is:
σθ =Pr tE1+
0.6P E1
Where: P = Internal design pressure, kPa (psig) r = Inside radius of the vessel, mm (in.) t = Vessel thickness, mm (in.)
E1 = Weld joint efficiency for a longitudinal joint
Rearranging this equation and substituting S (allowable stress, kPa [psi]) for σθ yields:
t= Pr SE1− 0. 6P
The formula that follows applies when the thickness required to resist the longitudinal stress due to internal pressure must be calculated:
t = Pr 2SEc + 0.4P
Longitudinal stress can govern the design of particular sections in a pressure vessel. Longitudinal stress can govern when loadings other than internal pressure induce longitudinal stresses that are greater than one half of the hoop stress that is due to internal pressure. In these cases, the longitudinal stress that is due to these other loads is added to the longitudinal stress due to internal pressure. The total combined longitudinal stress is then limited to the maximum allowable stress of the vessel material at the design temperature.
The most common example of where longitudinal stress can govern the design of a vessel component is when wind load on a tall tower causes a bending moment. This bending moment creates a longitudinal bending stress in the cylindrical sections and increases at lower tower elevations because of the greater tower length on which the wind pressure acts. The wind load sometimes requires that the thickness of the lower tower sections be increased beyond the thickness that is required for internal pressure alone, in order for the longitudinal stresses due to wind plus internal pressure to be acceptable.
The thickness of a spherical shell will be approximately half the thickness of a cylindrical shell for the same design conditions, material, and diameter. Refer to Work Aid 3A for the ASME Code shell thickness equations.
When the required thickness for internal pressure must be determined, the specified corrosion allowance must first be added to the new vessel inside radius so that the corroded vessel inside radius is used in the equations. The thickness calculated using these equations must then be increased by the specified corrosion allowance in order to arrive at the minimum required new vessel thickness.
Note that the Pressure Vessel Design Data Sheet has an area where the thickness calculation equations are summarized.
Sample Problem 1 - Cylindrical Shell Thickness Calculation
The pressure vessel described in Figure 6 will be used in this and subsequent Sample Problems. Hand calculations are used in the solution of all Sample Problems, Exercises, and Evaluations in this course to assist in understanding the design concepts and parameters that are involved. However, computer programs are typically used for these calculations on the job. Saudi Aramco engineers often use the CODECALC computer program for these calculations.
The geometry and design data of a vertical cylindrical pressure vessel are specified in Figure 6.
Cost estimates are being prepared for this vessel. It is your job to estimate the required component thicknesses. What are the minimum required thicknesses for the two cylindrical sections?
4'-0" 6'-0" Hemispherical 2:1 Semi-Elliptical 60'-0" 10'-0" 30'-0" DESIGN INFORMATION
Design Pressure = 250 PSIG Design Temperature = 700°F Shell and Head Material is SA-515 Gr. 60
Corrosion Allowance = 0.125" Both Heads are Seamless Shell and Cone Welds are Double Welded and will be Spot Radiographed The Vessel is in All Vapor Service
Cylinder Dimensions Shown are Inside Diameters
MEX 20203.F06
Solution
Since the welds are spot radiographed, E = 0.85. S = 14 400 psi for SA-515/Gr. 60 at 700°F.
Use Work Aid 3A for this solution. 6 ft. - 0 in. Shell r = 0.5D + C = 0.5 × 72 + 0.125 r = 36.125 in. tp = Pr SE1− 0.6P = 250× 36.125 14 400× 0.85 − 0.6 × 250 tp = 0.747 in. t = tp + c t = 0.747 + 0.125
t = 0.872 in. required including corrosion allowance 4 ft. - 0 in. Shell r = 0.5 × 48 + 0.125 r = 24.125 in. tp = 250× 24.125 14 400× 0.85 − 0. 6 × 250 tp = 0.499 in. t = 0.499 + 0.125