Pressure vessels are industrial containers designed to hold vapors, liquids, or gases under pressure. These containers are essential in many industries, including oil and gas, chemical processing, and power generation.

Because pressure vessels operate at high pressures, they can be dangerous if not properly maintained. Regular inspections are a critical part of the maintenance process for pressure vessels, helping to identify potential issues before they escalate into serious problems.

This guide provides a comprehensive overview of pressure vessels, covering their definition, inspection requirements, common questions, and how drones can enhance the inspection process.

What is a Pressure Vessel?

While we’ve already introduced pressure vessels as "containers designed to hold vapors, liquids, or gases at a specific pressure," it's important to understand that each country has its own legal definitions and regulations regarding these systems. This is because pressure vessels can pose significant risks if not handled correctly.

To ensure safety, most countries have strict laws governing the design, construction, and operation of pressure vessels. Each vessel also has specific operating parameters known as “design pressure” and “design temperature.” Operating beyond these limits can lead to catastrophic failure, which may result in explosions, fires, or hazardous leaks.

Understanding these limitations is crucial for ensuring the safe and efficient use of pressure vessels in industrial settings.

Common Characteristics of Pressure Vessels

Pressure vessels come in various shapes and materials, but they all share some key characteristics:

• Shape: Common shapes include cylinders, cones, and spheres, which help distribute internal pressure evenly.
• Design: The design must account for the volume and pressure the vessel will contain, with materials chosen based on strength and durability.
• Materials: Steel is the most commonly used material due to its strength and cost-effectiveness, though composites and reinforced concrete are also used in certain applications.
• Corrosion Resistance: Materials must be selected based on the environment and the substances they will contain to prevent degradation over time.
• Pressure: Most pressure vessels operate at 15 psig (pounds per square inch gauge) or higher.

Note: Psig measures the pressure relative to atmospheric pressure. For more information, visit the Energy Education website.

Types of Pressure Vessels

Pressure vessels are used in a wide range of industrial applications. Some of the most common types include:

• Storage Tanks/Vessels: Used to store liquids, often made of carbon steel and available in various sizes.
• Boilers: Designed to generate steam or hot water, typically made of alloyed steel to withstand high temperatures and pressures.
• Heat Exchangers: Used to transfer heat between fluids, commonly found in food processing, chemical manufacturing, and energy production.
• Process Vessels: Used to perform chemical or physical processes on materials, such as mixing, heating, or separation.

Safety Considerations in Pressure Vessel Design

Given the potential dangers associated with pressure vessels, safety is a top priority in their design. Two key safety features include:

• Safety Valves: These valves release excess pressure to prevent over-pressurization, reducing the risk of catastrophic failure.
• Leak Before Burst: This design feature ensures that a vessel will leak rather than explode, allowing for early detection of issues and safer operation.

Pressure Vessel Inspections

Regular inspections are essential to ensure the continued safe operation of pressure vessels. This section covers inspection frequency, procedures, testing methods, and a checklist of what inspectors typically look for during an inspection.

The Elios 2 collects visual data during a pressure vessel inspection

Frequency of Inspections

Most regulatory bodies require pressure vessels to be inspected at least once every five years. Additionally, an initial inspection should be conducted when the vessel is installed and before it is put into service.

What Is Done During an Inspection

During an inspection, inspectors examine both the exterior and interior of the vessel. They may:

• Collect visual data on the condition of the vessel, including welds, joints, and insulation.
• Measure thickness to detect any changes due to wear or corrosion.
• Conduct stress analysis to assess structural integrity.
• Test pressure release valves to ensure proper function.
• Perform hydrostatic pressure tests to evaluate the vessel’s ability to handle pressure.

Types of Pressure Vessel Testing

There are five common non-destructive testing (NDT) methods used in pressure vessel inspections:

1. Visual Testing

Visual inspections involve a detailed examination of the vessel’s surface for cracks, corrosion, or other defects. This is the most commonly used NDT method.

2. Ultrasonic Testing

Ultrasonic testing uses sound waves to measure material thickness and detect internal flaws, making it ideal for identifying hidden defects.

3. Radiographic Testing

Radiographic testing involves using X-rays to detect surface and subsurface defects, providing a clear image of the vessel’s internal structure.

4. Magnetic Particle Testing

This method uses magnetic fields to detect surface and near-surface flaws by identifying disruptions in the magnetic flow.

5. Dye Penetrant Testing

Dye penetrant testing involves applying a fluorescent dye to the surface to reveal cracks or other defects under UV light.

Pressure Vessel Inspection Checklist

Here’s a list of items typically checked during a pressure vessel inspection:

External Inspections—What to Check

• Inspect external coverings like insulation and coatings for damage.
• Look for signs of leakage on the vessel’s exterior.
• Check mountings for proper expansion and contraction capabilities.
• Examine the vessel and its connections for deformations, cracks, or gouges.
• Inspect nuts, bolts, flange faces, and surfaces for corrosion or defects.
• Check shell surfaces and heads for blisters, bulges, or deformations.
• Examine welded joints for cracks or other issues.

Internal Inspections—What to Check

• Inspect the interior for cracks, blistering, corrosion, or other defects.
• Check threaded connections for proper engagement.
• Ensure openings leading to fittings or controls are unobstructed.
• Verify that special closures are adequate and secure.
• Examine areas of high stress for signs of wear or cracking.

For a printable version, you can copy this list into a document and add check boxes next to each item.

Common Questions About Pressure Vessels

Here are some frequently asked questions about pressure vessels, along with detailed answers:

• At what pressure does a vessel become a pressure vessel?
• What is the difference between an unfired and a fired pressure vessel?
• What regulatory bodies oversee the construction and inspection of pressure vessels?
• What are the pressure vessel standards around the world?

At What Pressure Does a Vessel Become a Pressure Vessel?

A vessel is generally considered a pressure vessel if it holds vapors, gases, or liquids at a pressure of 15 psig or higher. However, this threshold may vary depending on local regulations.

What Is the Difference Between an Unfired and a Fired Pressure Vessel?

An unfired pressure vessel is one that is not directly exposed to a heat source, often used for cooling or heating fluids through indirect means. A fired pressure vessel, on the other hand, receives direct heat, such as in boilers or thermal oil heaters.

What Regulatory Design Codes Pertain to Pressure Vessels?

Regulatory codes vary by country, but some of the most widely recognized ones include:

• Australia – Australian Standards
• Canada – CSA B51
• European Union – Pressure Equipment Directive
• North America – ASME Boiler and Pressure Vessel Code
• Japan – JIS (Japanese Industrial Standard)

What Are the Pressure Vessel Standards Around the World?

Some of the most common international standards include:

• AD Merkblätter (Germany)
• ASME Section VIII (USA)
• API 510 (North America)
• BS EN 13445 (Europe)
• CODAP (France)
• EN 13445 (Europe)
• EN 286 (Simple pressure vessels)

Five Ways Drones Can Help with Pressure Vessel Inspections

Drones have revolutionized the way pressure vessel inspections are conducted. By using drones, inspectors can safely collect visual data from inside a vessel without having to enter the confined space themselves.

The Flyability Elios 2, for example, is designed to navigate tight spaces and continues flying even after collisions, making it ideal for inspecting complex pressure vessels. Its advanced features, such as flight stabilization and high-quality imaging, make it a powerful tool for inspectors.

Here are five key benefits of using drones in pressure vessel inspections:

1. Safety

Drones eliminate the need for human entry into potentially hazardous environments, significantly improving worker safety.

2. Cost Savings

Using drones reduces the need for scaffolding and manual labor, lowering inspection costs. Drones also speed up the process, minimizing downtime for the vessel.

3. Access

Drones can reach difficult-to-access areas, such as welds and burners, providing detailed views that would be challenging to obtain manually.

The Elios 2 inspecting a gas tank

4. Better Data

Drones can capture high-resolution images and video, creating a detailed record of the vessel’s condition. This data can be reviewed later for trend analysis and long-term monitoring.

5. Environmental Benefits

By reducing the need for frequent manual inspections, drones can help companies conduct more frequent checks, leading to improved safety and reduced emissions. A study found that increased inspection frequency could reduce CO2 emissions by up to 649 metric tons annually.

 

The Future of Pressure Vessel Inspections

The future of pressure vessel inspections looks promising, with emerging technologies like drones and advanced sensors transforming the industry. These tools are making inspections faster, safer, and more accurate than ever before.

The Growing Adoption of Drones

Drones are becoming increasingly popular in the field of pressure vessel inspections. Their ability to access hard-to-reach areas quickly and safely makes them an invaluable asset for inspectors. Companies are adopting drones not only for safety reasons but also for cost and time efficiency.

With the ability to conduct inspections more frequently, drones enable proactive maintenance and early detection of potential issues. This shift is particularly beneficial in environments where human entry is risky or impractical.

Recent Technological Advances May Diversify Drone Use Cases

New advancements in drone technology, such as SLAM (Simultaneous Localization and Mapping), are expanding the possibilities of remote inspections. Drones equipped with these features can create real-time 3D maps, helping inspectors locate defects with precision.

In addition, drones can now be fitted with various sensors for ultrasonic thickness testing, thermal imaging, and optical gas imaging, among others. These capabilities are opening new doors for more detailed and accurate inspections.

Software improvements are also playing a key role in managing the large amounts of data collected during inspections. These tools are streamlining the process, making it easier for inspectors to analyze and act on the information gathered.

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