A pressure vessel is considered any closed vessel capable of maintaining a differential pressure between the fluids internal and external to the vessel, regardless of shape and dimensions. Because of the danger that exists in manufacturing, testing and operating pressure vessels, the design and development must be regulated by safety standards that are enforced by jurisdictions.
In most countries, pressure vessels must be manufactured to a certain code, and in the United States, that code is the Boiler and Pressure Vessel Code (BPVC) from the American Society of Mechanical Engineers (ASME).The following pressure vessel design guide and resources will help you efficiently optimize your design, before moving on to the actual manufacturing and delivery of a safe and cost-efficient pressure vessel.
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Safety and Codes
The urgency for comprehensive codes in the manufacture and operation of pressure vessels in the USA goes back to the 1800s, and had caused numerous deaths and injuries. Because of these tragedies, the first legal codes based on ASME’s rules were enacted in Massachusetts in 1914.
Who Is Responsible For Pressure Vessel Safety?
You are, if you are involved in any stage of specifying, designing, manufacturing or installing pressure vessels. The underlying objective in pressure vessel design, manufacturing, and operations is to be as safe and as cost-effective as possible. Decisions are made and options are weighed by multiple entities that are responsible for the journey from concept to installation and use. These typically include the following:
The Regulatory Authority, which is the authority in the country, county, state, or province of installation, that is legally charged with the enforcement of the requirements of law and regulations relating to pressure vessels.
The User, who operates the pressure vessel. He or she is responsible to the Regulating Authority for the continued safe operation of the vessel.
The Purchaser, which is the organization that buys the finished pressure vessel for its own use or on behalf of the end-user. This might be the owner/operator or an Engineering Procurement Contractor (EPC).
The Manufacturer, which is the organization that designs, constructs and tests the pressure vessel in accordance with the purchaser’s order and design conditions. Note that the design function may be carried out by the purchaser or by an independent organization.
The Inspection Authority, which is the organization that verifies the pressure vessel has been designed, constructed and tested in accordance with the order and with the standard.
Design Codes Are In Place To Help Avoid Disasters
The purpose of using design codes is to avoid disasters that can affect life and environment. As such, codes have been developed over the years using industry experience and best practices.
While there are several rules developed by countries with recognized technical expertise in the subject, the code that is the most internationally recognized and the most used is Section VIII "Pressure Vessels" of the BPVC of ASME. Other than the BPVC code above, the other most commonly used codes for pressure vessels are below:
- Europe: EN-13445
- Germany: A. D. Merkblatt Code
- United Kingdom: British Standards PD 5500
- France: CODAP
- China: GB-150
The rules developed for the design codes represent years of experience. When used, the code requirements can:
- Communicate design requirements
- Utilize know-how and technology
- Keep equipment costs low
- Reduce insurance costs
- Provide rules for the design of equipment adequate for design
conditions determined by others.
It’s important to state that the codes do not provide rules or guidance for the determination of design conditions. They also don’t provide rules and guidance for the determination of required materials for construction or provide a corrosion allowance. This is left up to the design engineer.
Design and Operating Conditions Impact Safety
It is difficult to know which conditions will ultimately govern the design of a vessel. Typically, higher temperatures are more conservative, as they have a lower allowable stress. However, designers of shell and tube heat exchangers, know that the worst case differential expansion may actually occur at lower operating temperatures. Often a flooded condition may govern when considering seismic effects; but empty may be worse when combined with lateral wind loads.
It’s vital that all pressure vessel designers to also evaluate their designs for toughness and determine if a toughness test is required. The rules of ASME Section VIII may require toughness testing to show the appropriateness of the vessel for service at the designated minimum design metal temperature. The Charpy test is common test used to determine toughness. These rules also provide several options for exemption from toughness testing if certain conditions are met.
ASME BPV Codes
The ASME Boiler and Pressure Vessel Codes are in 12 sections. These 12 sections are as follows.
THE 12 SECTIONS OF THE ASME BPV CODE
ASME Section I: Power Boilers
ASME Section VII: Operation of Power Boilers
ASME Sec II: Materials
ASME Section VIII: Pressure Vessels
ASME Section III: Nuclear Fuel Containers
ASME Section IX: Welding and Brazing
ASME Section IV: Heating Boilers
ASME Section X: Fiber Reinforced plastic Pressure Vessels
ASME Section V: Non-Destructive Examination (NDE)
ASME Section XI: Inspection of Nuclear Power Plants
ASME Section VI: Operation of Heating Boilers
ASME Section XII: Transport Tanks
With focus on pressure vessels, Section VIII is expanded below.
SECTION VIII: PRESSURE VESSEL
Division 1 - 15PSIG to 3000 PSIG
Subsection A, General Requirements:
Subsection B, Fabrication Requirements:
Subsection C, Material Requirements:
3 Aspects of ASME’s BPV Code Material Standards
ASME standards for materials are an important factor in calculating a design’s safety. Allowable materials for use are covered in Section II, Part D. The following material properties impact pressure vessel design.
Allowable Stress: For the most part, allowable stresses are determined by four things: tensile strength, yield strength, time dependent properties at higher temperatures (creep), and product form (bolting, welded pipe, etc.).
Additional Material Properties: Yield strength comes from the Y tables in Section II, Part D. Ultimate Strength comes from the U tables in Section II, Part D. Other values, such as density and Poisson’s Ratio values, come from Table PRD in Section II, Part D.
Code Compliant Design Includes Complex Variables
The codes that apply to you and your customers are vital. Insuring that you are following the correct codes determines success and ultimately, future growth. Be sure to include the various wind and seismic codes you need too.
Anyone who designs pressure vessels for food, sanitary, pharmaceutical, or oil and gas industries, designs a pressure retaining component that’s going to be stamped with ASME Code, or wants to design a vessel outside ASME Code or is combining codes should use an ASME pressure vessel design software.
The main issue with pressure vessel design is staying compliant with ASME Code, which is updated every two years. Keeping up to date on all the changes that occur between code years can be complicated, especially when so much goes into the design process, such as nozzles, heads, shells, support structures, flanges, fittings, and materials. Vessel design software eliminates guesswork and saves time since the changes to code are built into the software.
When selecting ASME pressure vessel design software look for quality control procedures to capture and integrate the latest code revisions. The software should generate easy to read report formats that allows for quick verification in pressure vessel design code. It is a good sign when the software is designed by engineers and welding experts who are active in the development of the standards their software reflects.
Authorized Inspector Review of Software Calculations
Authorized Inspectors require that the basis of design calculation used is compliant and proven. Because the safety of pressure vessels is so important, ensuring design meets code is paramount. This can be difficult due to the complex calculations involved in the design process.
Rarely is pressure vessel design done by hand, and at the least, geometry for the required loadings are checked by excel or Mathcad files. While these tools make the designing of a pressure vessel far easier, it’s natural to question whether the software will yield results compliant with ASME Section VIII code. In order to quell these concerns, QA/QC documents will be used to validate the tools and ensure the vessel meets code.
It’s important to note that it’s up to the designers and engineers to guarantee a pressure vessel is up to code. While software and tools can aid in the design, ASME Section VIII states, “The Code neither requires nor prohibits the use of computers for the design or analysis of components constructed to the requirements of the Code.
However, designers and engineers using computer programs for design or analysis are cautioned that they are responsible for all technical assumptions inherent in the programs they use and the application of these programs to their design.”
Frequent Code Changes Require Calculation Updates
Like many other aspects of this industry, codes are constantly adapting and changing. It’s important to stay abreast of the latest code releases and to make sure you’re trained on them as well. Not being compliant or unaware of new codes can negatively impact business and result in unplanned scrap and re-work.
In order to review changes in code, you can look at ASME Section VIII, Division 1 under the summary changes. This will highlight any changes made in the code, as well as give the description of the changes. You can also look for the code year designator next to the code. If you see a number “17,” that means the code was changed in the year 2017, and you can look back in the summary of changes to see what that change was.
If you want to stay abreast of code changes and not be caught by surprise, it’s helpful to be involved in Committee work. These meetings are free and open to the public. They will offer different points of view from inspectors, consultants, and fabricators. These meetings are an excellent way to learn about code changes and be involved in the process.
Sometimes, materials change as well as code. To find material changes, look in Section II, parts C and D, as well as Section IX. These sections will highlight changes in material, and you can be assured you are abiding by any new material requirements.
Feasibility and Quoting
Choosing The Correct Container Is The First Step
Pressure vessels are typically calculated on the principles of thin-walled cylinders. The first step in designing a container is choosing one that is fit for your purpose. The factors influencing this choice are the function of the container, the location, the nature of the fluid to be stored/processed, the temperature, the operating pressure and the ability to store the volume needed by the process.
Sometimes, a designer needs to consider alternate methods of pressure vessel design in order to produce an ASME compliant vessel. Two example design methods are The Design by Rule (DBR) and The Design by Analysis (DBA).
DBR is typical design methodology and can be located in the ASME Boiler and Pressure Vessel codebooks in Section VIII, Division I and Part 4 for Division 2, as well as other Pressure Vessel References.
DBA method usually incorporates Finite Element Analysis (FEA) that requires additional expertise in the methods of analysis proposal. ASME BPVC, Section VIII, Part 5 for Division 2 has extensive requirements for using DBA.
Designers sometimes need to focus on both DBR and DBA in pressure vessel design. Sometimes, the DBR method’s scope is too limited for an aspect of the vessel design. Also, sometimes DBA needs to be used to supplement the DBR method in order to check aspects such as cycle life and secondary stresses that may not be considered in the root method.
In many cases, using both DBR and DBA makes life easier for the designer because he/she is able to take advantage of the tools offered by both methods.
Generating Rapid Pressure Vessel Design Quotes
If you want to build in a competitive advantage to your company, it’s important to have the ability to rapidly prototype and quote code compliant pressure vessel design variation. By looking at design options quickly, you can offer different options to a client and figure out the best option for that particular client.
When quoting a prototype for a client, there are several factors to be taken into consideration.
Initial configuration: easily establish initial configuration variations of the design for material costs and fabrication labor estimates. These quick bid responses can be critical in time sensitive projects.
Temperature: Temperature input is entered from a single form and automatically referenced with proper calculations for both internal and external pressure cases.
Pressure: To generate minimum material thickness, according to vessel size and code, the input of pressure is entered from a single form and automatically utilized in calculations for both internal and external psi maximums.
Nozzles: Nozzles can be keyed in by size and the number of nozzles on each component, selecting to include with or without rated flanges.
Heads: Heads of both top and bottom are entered with a pick list, with an option to include the number of nozzles per head. These can be rearranged later in the design process, which saves time.
Shell: Key in the length and diameter and number of nozzles used on the shell. Being able to do all of this in one go is a huge time saver.
Pressure vessels come in several shapes, represented below.
Design Impact of Pressure Vessel Parts
There are also a significant number of pieces and parts that comprise pressure vessels, all of which need to be designed and engineered for your purpose. Below are 3 types of shell head designs in a 3D design module.
Torispherical Shell Head Design:
Flat Shell Head Design:
Hemispherical Shell Head Design:
Avoid Mishaps With Proper Material Selection
Material failure is the leading cause of pressure vessel mishaps. It is essential that the correct material for the application be used.
Pressure vessels, depending on requirements, can be made from a variety of materials, including polymers (plastic), fiberglass, carbon steel and stainless steel. Carbon steel and stainless steel are the most common for industrial applications. Between carbon steel and stainless steel, there are thousands of different alloys and composites that can be used to build pressure vessels, with some being more commonly used or required in certain situations.
Below is a list of pressure vessel materials that are commonly used and why:
Carbon Steel: In addition to limited resistance to corrosion, shock, and vibration, carbon steel possesses a high tensile strength and retains strength at minimal thicknesses.
Stainless Steel: Like carbon steel, stainless steel offers high strength at low thickness and is ideal for tanks and vessels that are exposed to the natural environment (humidity, sunlight, etc.) or high temperatures.
Hastelloy (Brand): Hastelloy® is a nickel-molybdenum alloy brand that is highly resistant to corrosion and because of the molybdenum it is made stronger at high temperatures.
Incoloy (Brand): Incoloy® is relatively easy to fabricate, can be made using the same machines and processes used to make stainless steel, has a high iron content making it a more cost effective, and is designed for high temperature applications.
Nickel Alloy: Despite this complexity and associated cost, nickel alloy has much to offer as a pressure vessel material, providing excellent corrosion resistance, comes in a variety of grades, and protects against thermal expansion. It’s important to note that the manufacturing process for this alloy can be more complex than with other amalgams.
Aluminum: Often considered as an alternative to stainless steel, aluminum is cheaper and much easier to machine than stainless steel. In many instances labor costs may be higher, as some aluminum tank fabrication requires special welding techniques and its lower density typically means an aluminum pressure vessel is unsuitable for extremely high pressures.
Titanium: Titanium provides several advantages in salt-water environments, it is resistant to corrosion, it maintains strength and rigidity (even at lower thicknesses), it facilitates more efficient heat-transfer than many other types of metal, and can maintain its structural properties over long periods of time.
Fiberglas: Available in sizes from 4,000 to 42,000 gallons, Above Ground Fiberglass Tanks provide a cost effective and long lasting storage solutions to a wide variety of industries with "Caustic Storage" needs.
Plastic: Polymers (plastics) do have advantages over metals in the construction of vessels. Polymeric structures are usually lighter in weight than metal ones and do not corrode as readily as metals.
However, polymers are not as strong under tensile loads, and they do not resist high temperatures as well as metals do. Moreover, polymers do not conduct heat well, and most do not conduct electricity at all. The lack of thermal conduction can be overcome by adding thermally conductive materials to the polymer formulation.
Parent Material Strength and Joint Efficiency
Once all parts and pieces are assembled for manufacturing, joint efficiency becomes an important factor. Joint efficiency is a numerical value representing a percentage expressed as a ratio of the strength of a welded, brazed, or riveted joint to the strength of the base material.
Without further inspection, it is assumed the welded joint is weaker than the material around it due to potential defects, such as porosity, slag inclusions, and others. Shell thickness and, therefore, weld quantity is increased to account for this reduction in strength.
The Joint Efficiency is determined from the quality of the joint design and the degree of examination. Double welded butt joints are considered the highest quality and full radiography is off course the highest degree of examination. Lower quality joints and lower examination will both result in a lower joint efficiency.
For some design conditions, such as lethal service, the Code requires the designer to specify full radiography. However, when not required, the designer can specify optional radiographic examination to increase joint efficiency and reduce the required thickness of shells and heads.
There are 4 weld joint categories as part of ASME Section VIII Div 1.
- All longitudinal welds in shell and nozzles
- All welds in heads, Hemispherical-head to shell weld joints
- All circumferential welds in shell and nozzles
- Head to shell joints (other than Hemispherical)
- Flange welds
- Nozzle attachment welds
Design Software Tools Have Major Benefits
Selecting a design tool can be a challenge, but it is a very important component of the process. Here are five benefits of pressure vessel design software.
A designer or engineer spending their time, to research and implement code changes and new design requirements, is much more expensive than the cost of a subscription to software that will do that for you.
Designers and engineers are subject to move on from your company at any time, meaning their knowledge goes with them. When you purchase software, you gain a consistent platform that is used to track codes and designs.
Access to an external resource, as a second opinion, helps to ensure decisions are consistent with industry best practices and compliant with Code. Furthermore, engineers will have the benefit of consulting an external resource if there is a problem with a calculation.
Without software, research done across various code books is overwhelming as well as time consuming. With software, this research is always at the ready and reliable.
Authorizing Inspector Familiarity
Inspectors are generally more comfortable with proven software when compared to homegrown spreadsheet macros. Even if you think you’re saving money by not purchasing software, that money may be spent with the Inspection process.
Comprehensive Parts & Pieces Catalog Saves Time
Look for a comprehensive collection of pre-built parts and pieces in the design catalog. This should include Shells, Heads, Nozzles, Flanges, Pipes, Supports, and etc. This means that rather than start from a blank sheet and creating everything by hand, you’ll have a design tool with the necessary components available to complete the design.
You can easily drop dimensions and material properties for every component in the design library, which will lessen the work and risk of making a mistake. Once everything is dropped into the library, you won’t have to worry about tediously looking up every aspect in the ASME Code, because everything in the design library will be code compliant.
Since safety isn’t binary, there’s a safety factor built into the ASME Section 8 code. This means there’s an acting outside decision maker to keep safety codes consistent across the industry, and it prevents designers from straying from the codes put in place.
An important factor in investing in software is the requirement of the engineer to understand how ASME pressure vessel software performs the calculations. Transparent formulas and calculations give the benefit of an external resource to consult, which can protect companies from conflict of interests that may come up.
Multiple code requirements often occur, meaning that designers must make decisions that are not explicitly stated in a single code. This means that sections of different codes must be used and how the codes interact must be understood.
Authorized inspectors must approve that the design is compliant with ASME Code. A concern may be raised if all calculations are being done by hand. Homegrown tools are tougher to prove in regard to compliance, and inspectors are usually more comfortable with commercially supported ASME pressure vessel software. Even if you’ve been using the same calculation for several years, inspectors still must prove each separate design meets code, which can be time consuming and costly for your company.
Due to the critical nature of designing and manufacturing pressure vessels, that are compliant to all relevant codes, it’s vital for manufacturers to have software that simplifies the complexities involved. For thirty years, the engineers at CEI have worked to create the tools and software that are needed to make safe and cost-efficient pressure vessels.
They have delivered comprehensive software tools that are used by industry professionals at all stages of design and manufacturing, starting with the initial stage of quoting a new project through evaluating long-term pressure vessels reliability.
Testing Pressure Vessels
Hydro testing is a pressure test that fills the pressure vessel with water, while removing the air and pressurizing the system up to 1.3 times the designed pressure limit (adjusted for differences in the design state and the test state). Hydro tests are beneficial because once it’s completed, designers know if the vessel can handle the pressure that it’s designed to withstand.
Integration with Third Party Tools Is Critical
Need to send your design out to a format for exchange with other tools and vendors? Take a look at the best way to do this and adopt the most open and popular standard available. Be sure to understand your needs for integration as it can be as simple as exporting to PDF or as complex as a computer aided manufacturing (CAM) interface.
Listed below are the three major benefits of using pressure vessel software for manufacturing:
1. Integration of 3D Design Capability
By combining modeling software with ASME code support, engineers can save significant time.
2. Rapid Modeling of Material Properties
This software allows designers to quickly and easily adapt design based on client wants and needs. Engineers can work through various material thickness and properties easily, which saves both time and money, as well as meeting ASME code.
3. Design for Lowest Cost of Ownership
When engineers consider the impact of material properties, nozzle placement and design, manufacturing processes, and the additional loads imposed on the nozzles by attached piping early in the design stage, the overall operational lifetime cost of the pressure vessel is reduced. The best way to construct a cost-effective pressure vessel is to ensure its design and manufacture is correct the first time.
ASME Stamp Requires Compliance for Design, Manufacturing, and Installation
When a pressure vessel is ASME stamped, all stages of design, construction, inspection and testing are performed in accordance with the provisions of the code. In addition, a representative of the ASME Code will witness certain points during the above stages.
A fundamental principle is that the ASME Stamped pressure vessel must receive an inspection by an authorized “third party" during all stages to verify compliance with the applicable requirements of the Code. A signature by an authorized third party, certifying that the container has been manufactured in accordance with the requirements thereof, is a key step for the acceptance of boilers and pressure vessels, especially by several bodies involved in the legalization process.
Besides inspecting pressure vessels in the stages mentioned above, authorized inspectors can also supervise the installation procedures at site. After the equipment has been placed into service, an authorized inspector can periodically inspect the compliance with legal requirements defined by local regulations on boilers and pressure vessels.
In addition to all the requirements, any stamped boiler or pressure vessel must comply with all aspects of the Code, meaning it must be designed, fabricated, and inspected by a manufacturer holding an Authorization Certificate issued by ASME.
Even when a pressure vessel is designed per the ASME Code, the equipment, itself, may not be ASME stamped. Equipment that has the ASME stamp is usually associated with quality level, and, therefore, safety. ASME stamped equipment requires thorough documentation management and stringent inspection procedures. The stamp and associated processes are generally the requirement of the customer to insure safety and reduce liability.
It is essential that those involved with pressure vessels and pressure vessel design and manufacture are up to speed with the latest codes, rules, and regulations. It is also essential that purchasers specify pressure vessel code compliance, and that the end user operates and maintains the pressure vessel to code and in a safe manner. Safety is paramount since it lowers cost, reduces insurance and above all, saves lives.
Much has been said about software license models. A one-time purchase seems easy, but as time passes, software needs to be updated. With a subscription model, you’re always assured of the latest revision of the software, and the latest code changes are included.
The tools that you select work better when you have superior support. If your support person is a phone call or chat bot away, that will help to improve your business and problem resolution time.
By choosing the best material for your project, educating yourself on the appropriate codes, assembling your pressure vessel to industry standards, and using the correct software, the manufacturing of pressure vessels can be a safe and efficient process.
If you are in need of ASME pressure vessel software consider using CEI’s DesignCalcs software. Designed by engineers for engineers, DesignCalcs ensures all of your designs are up to the latest codes. With the design library you can save time and have pressure vessel designs ready for an audit and stamp test. If you need any more information feel free to reach out to our pressure vessel software experts here at CEI.