Pressure vessels are industrial containers designed to hold vapors, liquids, or gases.

Because pressure vessels usually hold materials at a high pressure they can be dangerous if not properly maintained. Regular inspections are a critical part of the maintenance process for pressure vessels, helping to reveal potential problems before they develop further.

In this guide to pressure vessels we cover what pressure vessels are in detail, the requirements for pressure vessel inspections, common pressure vessel questions, and how drones can help with inspecting them.

What Is a Pressure Vessel?

Although we have already provided a definition of pressure vessels in the first sentence of this article—they’re “containers designed to hold vapors, liquids, or gases at a specific pressure”—each country has strict definitions of what they are, so it’s worth spending some more time defining them.

The reason for these narrow definitions is because pressure vessels are dangerous. 

To help mitigate the dangers they pose, almost every country in the world has laws regarding how pressure vessels are designed, how they’re built, and how they can be operated.

In addition to general regulatory requirements for pressure vessels, every individual pressure vessel has specific operating limitations, called its “design pressure and design temperature.” 


If a pressure vessel is operated beyond the pressure or temperature it was designed to handle, the result could be the catastrophic failure of the unit. In the worst case scenario, operating outside of a vessel’s design could lead to fires, poisonous gas leaks, or even explosions, all of which could pose an extreme danger to anyone working in the surrounding area.

Common Characteristics of Pressure Vessels

Here are some common characteristics of pressure vessels:

  • Shape. While a pressure vessel could hypothetically be made in several different shapes, the most common shapes used are cones, spheres, and cylinders. 

  • Design. Regardless of the vessel’s shape, the minimum mass of a pressure vessel scales along with the volume it contains and pressure it will be put under. This mass is inversely proportional to the strength and weight ratio of the vessel’s construction material, and the minimum mass required goes down as strength goes up.

  • Construction materials. Since they have to withstand extremely high pressures, pressure vessels must be incredibly strong. For this reason, the most common material used to make them is steel. Pressure vessels can also be made from composite materials, like filament wound composite, or concrete with cabling wrapped around or within the vessel to provide the tension needed to resist internal pressure.

  • Corrosion resistance. If a pressure vessel will be used in a scenario that may produce corrosion, this must be taken into consideration in choosing the material.

  • Pressure. Most pressure vessels are designed to operate at 15 psig* or above.

Note: Psig is a measure of pressure that stands for Pounds Per Square Inch Gauge. Learn more about psig on the Energy Education website.


It’s important to note that the term “pressure vessel” can refer to a variety of storage containers used in industrial settings. 

Here are some of the most common types of pressure vessels:

  • Storage tanks/vessels. Often constructed of carbon steel, storage vessels are typically used to store liquids and come in a variety of sizes.

  • Boilers. Usually made of alloyed steel so that it can withstand high pressures and temperatures, a boiler is a vessel whose purpose is the creation of hot water or steam, which is then used as a power source for various operations. 

  • Heat exchangers. Carbon steel is also a common material used for constructing heat exchangers, which are used in a huge range of industrial applications, from industrial processing to food service.

  • Process vessels. Typically made of carbon steel and used to perform some kind of processing of a material (as the name implies) process vessels are used to combine, break down, or remove elements from a material.


Because pressure vessels are potentially very dangerous, they are usually designed with safety consideration in mind.

Here are two of the top safety considerations in the design of a pressure vessel:

  • Safety valves. Also called a relief valve, these valves allow for the quick release of pressure so that the desired pressure is not exceeded to a dangerous extent during use.

  • Leak before burst. This feature refers to a design element in pressure vessels that allow them to crack instead of exploding. Most pressure vessel standards throughout the world, including the ASME Boiler and Pressure Code and the AIAA metallic pressure vessel standard require that pressure vessels have this feature, which allows the vessel to crack and leak fluids rather than suddenly burst, allowing for a safer option for releasing pressure in the event of failure.

Pressure Vessel Inspections

Inspections are a crucial part of the maintenance process for pressure vessels.

This section covers information on the frequency with which inspections should happen, what is done during inspections, the types of testing that can be used in inspections, and ends with a checklist of what is generally covered during a pressure vessel inspection.

pressure-vessel-flyability-3The Elios 2 collects visual data during a pressure vessel inspection


Most pressure vessel regulations provide specific requirements for the frequency of inspections. As a general rule of thumb, pressure vessels should be inspected at least once every five years. An inspection should also be conducted once the vessel is installed, prior to it being put into service.


Pressure vessel inspections can refer to an inspection of the vessel’s condition externally, internally, or both.

In these inspections, inspectors may:

  • Collect visual data regarding the condition of the vessel, including the condition of insulation, welds, joints, or structural connections

  • Collect thickness data to determine whether the vessel has changed due to use

  • Conduct a stress analysis to determine whether the vessel is still OK for use

  • Inspect the vessel’s pressure release valves to make sure they functioning properly

  • Conduct a hydrostatic pressure test.


There are five common types of tests inspectors perform during pressure vessel inspections:


Visual testing is the most common type of non-destructive testing (NDT) an inspector might perform. The goal of a visual inspection is to visually review both the interior and exterior of the vessel to look for any cracks or flaws (see the checklist just below for more details). Learn more about visual inspections


Ultrasonic testing uses sound waves to measure the thickness of a material’s surface in order to detect any defects that may have arisen. This kind of testing is volumetric, meaning it can detect flaws inside the vessel as well as on its surface. Learn more about ultrasonic testing.


Radiographic testing uses radiography to detect defects near the surface or on the surface of a vessel. This testing method is also volumetric. Learn more about radiographic testing.


Magnetic particle testing uses magnetic current run through the pressure vessel to identify deformations or defects on the surface of the vessel, which will interrupt the flow of the magnetic current and appear as a “flux leakage field.”  Learn more about magnetic particle testing.


Dye or liquid penetrant testing uses liquid (i.e., the penetrant) sprayed onto the vessel to identify defects or flaws on its surface. A fluorescent chemical can be added to the penetrant to make flaws visible under U.V. light. Learn more about dye penetrant testing.



Here is a checklist of the things inspectors usually look for during a pressure vessel inspection.


Here's what inspectors look for when inspecting the exterior of a pressure vessel:

  • External coverings, including insulation and corrosion resistant coatings, inspected for defects

  • Entire vessel exterior inspected for any kind of leakage of gas, vapor, or liquid

  • Mountings inspected to see if they allow for appropriate expansion and contraction

  • Vessel and vessel connections inspected for deformations, cuts, cracks, or gouges, including on nozzles, manholes, and reinforcing plates 

  • Nuts, bolts, flange faces, vessel surface inspected for corrosion or other defects)

  • Shell surfaces and heads inspected for blisters, bulges, or other deformations

  • Welded joints and adjacent areas inspected for cracks or defects


Here's what inspectors look for when inspecting the interior of a pressure vessel:

  • Interior of vessel inspected for cracks, blistering, corrosion, deformation, or any other defects

  • Threads inspected to ensure the adequate number of threads are engaged on threaded connections

  • Openings leading to any external fittings or controls inspected to ensure they are free from obstruction

  • Special closures inspected to ensure they are adequate

  • Areas of high stress concentration inspected for cracks or other wear

Looking for a pressure vessel inspection checklist template? We recommend you paste the list above into a document and insert boxes next to each item.


Common Questions about Pressure Vessels

Here are some common questions about pressure vessels, with answers. 

Since some of the answers are fairly long, we’ve included a list of the questions so you can jump to the ones you’re most interested in:

  • 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 throughout the world?



The answer to this question can vary, but typically a vessel is legally considered a pressure vessel when it holds vapors, gases, or liquids at pressures of 15 psig or above.


A fired pressure vessel is any pressure vessel that receives heat.

The vessel may be partially or fully subjected to the heat source and the heat may be received either directly or indirectly. Fired pressure vessels require special precautions since they can overheat, leading to safety concerns.

Here are some of the most common types of fired pressure vessels:

  • Thermal oil heaters used for organic liquid piping systems.

  • Boilers that generate hot water, steam, or electricity.

Industries that use fired pressure vessels include Power Generation (Electricity), Oil & Gas, and Petrochemicals.

An unfired pressure vessel can be used to cool or heat a fluid when combined with another fluid, essentially acting as a heat exchanger. Unfired pressure vessels typically contain several tube bundles and chambers for the heating or cooling of fluids.

Here are some of the most common types of unfired pressure vessels:

  • Steam generators associated with piping systems.

  • Any closed vessel that is not a boiler and that is made to hold hot water, gas, or air.

Industries that use unfired pressure vessels include Power Generation (Electricity), Fire protection, and Automation.


The answers vary by country, but here are some of the most common:


Here is a list of all the some of the most common pressure vessel standards in the world:

  • AD Merkblätter. This is the German standard, which has been harmonized with the Pressure Equipment Directive (97/23/EC).

  • ASME Boiler and Pressure Vessel Code Section VIII: Rules for Construction of Pressure Vessels.

  • API 510. This is the North American standard for a pressure vessel inspection. (This standard is also used in other places.)  

  • B51-09. Canadian Boiler, pressure vessel, and pressure piping code.

  • BS 5500. This standard has been replaced in the UK by BS EN 13445 but is retained under the name PD 5500 for the design and construction of export equipment.

  • CODAP. This is the French Code for Construction of Unfired Pressure Vessels.

  • EN 13445. This current European Standard, which has been harmonized with the Pressure Equipment Directive (97/23/EC). 

  • EN 286 (Parts 1 to 4). This is the European standard for simple pressure vessels (air tanks), which has been harmonized with Council Directive 87/404/EEC.

Five Ways Drones Can Help with Pressure Vessel Inspections

Drones can be a powerful tool for inspectors when it comes to collecting visual data inside a pressure vessel.

Using a drone, inspectors can now enter vessels and visually scan their interior without ever having to step foot inside. 


This means inspectors are kept out of potentially hazardous, uncomfortable environments, while collecting high quality data. It also means that inspections can be conducted more quickly and efficiently than they would be using a manual approach.

Flyabilty’s Elios 2 (pictured above) sits within a cage, which allows it to collide and continue flying, making it uniquely suited for inspections inside pressure vessels.

The Elios 2 also comes with features like flight stabilization, powerful lighting, and high quality imaging to help support inspectors in their work.

Here are four of the major benefits to using drones in pressure vessel inspections: 


Using a drone to enter a pressure vessel for the collection of visual data means that inspectors don’t have to enter that confined space, which helps improve the overall safety of those involved in the inspection process.


Using a drone instead of a person to collect visual data inside a pressure vessel can significantly reduce the cost of the inspection by removing the need for companies to build and take down expensive scaffolding.

Because of their speed and efficiency, drones can also shorten the downtimes needed for pressure vessel inspections, which can help companies realize significant savings as well.


Drones can allow inspectors to get a close up view of welds, burners, and other parts of the pressure vessel that can be hard to access in a manual inspection.

pressure-vessel-flyability-8The Elios 2 inspecting a gas tank


Drones can typically gather higher quality visual data than might be gathered manually. They also allow inspectors to create a detailed historic record of an inspection, which can be referred to in the future as needed.


Decreasing the cost of inspections by using a drone can allow companies to increase the frequency of inspections. A study conducted by Boiler Room Consulting found that an increase in the frequency of inspections supported by the Elios 2 could potentially reduce CO2 emissions by as much as 649 metric tons a year.


The Future of Pressure Vessel Inspections

What does the future hold for the inspection of pressure vessels? 

Emerging trends include the increased adoption of RVI equipment like drones for inspections and advances in sensor technology that enables more diverse data to be gathered by remote inspection tools. 

The Growing Adoption of Drones

Drone use for visual inspections is increasing rapidly in this sector. 

More companies are incorporating flying devices in their operations due to safety benefits and time and cost savings. The quantity and quality of data gathered by drones —and their ability to quickly access otherwise challenging places—make them an excellent inspection tool.

Because inspectors can conduct drone inspections quickly, it's possible to conduct them more frequently. It's even possible to perform a relatively spontaneous drone inspection, including accessing areas that would require significant planning for human inspection or that are typically inaccessible.

Leading energy firms use environmental mitigation techniques to allow drone access to spaces that would otherwise be restricted to intrinsically safe equipment. 

This scenario is ideal for drones, as even when not affected by potentially explosive atmospheres, these locations present significant risks to humans entering the confined spaces while wearing protective equipment.

Recent Technological Advances May Diversify Drone Use Cases

New locational and mapping functionalities are expanding the usefulness of drone inspections. 

Thanks to SLAM technology, for instance, the latest-generation drones, including the Elios 3, can find themselves on a 3D map in real-time as they fly. This way, users can know exactly where a drone is in an asset while looking at the live visual feed and can identify the exact location of defects.

Drones can also be outfitted with sensors commonly used in NDT for ultrasonic thickness testing, thermal imaging, optical gas imaging, magnetic field detection, and more.

Applications for drone-mounted sensors are virtually limitless, and drone engineers are exploring how to make remote inspections even more versatile and effective. 

Similarly, software advances provide inspectors with faster and more streamlined ways to manage and interpret the large quantities of data that inspectors can now receive. 

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