Assessing Pressure Vessel Integrity: How Important Is It?
A pressure vessel is an important component in many industrial and petrochemical processes. As a result, any failure or malfunction in this component can result to costly downtimes (plus damage to the vessel and other industrial components within the vicinity).
More importantly, the accidental release of contents is hazardous to people. The gases or liquids being released might cause fires and explosions. Exposure to the contents could also be harmful to the people within the vicinity even with the use of personal protective equipment (PPE).
Importance of assessing pressure vessel integrity
Pressure vessels hold gases or liquids at pressures different than what’s found in our normal environment. This pressure differential creates constant stress in the internal system (our pressure vessel).
Remember that physical and chemical systems always seek equilibrium and strive to achieve the lowest energy states. In our pressure vessel scenario, the contents of the container always seek equilibrium by attempting to go out. Once the contents are released, of course the stress will be gone and finally the entire system is now in its lowest energy state. It’s similar to the potential energy possessed by water at an elevated area. It has the “urge” to fall down and once it happens, the water will be in a lower energy state.
The “urge to normalise” the pressure and achieve equilibrium is what makes pressure vessels fail. The contents and vapours are constantly finding a way to get out. It can be on the main body of the vessel, the vessel support and the attachments and nozzles. The weld joints are also often considered as potential failure points.
The material itself may fail against high pressures and abnormal high temperatures. These are the conditions normal in many industrial and petrochemical settings. Engineers also consider the material’s performance at room temperature. Many physical changes and chemical reactions may occur at any time depending on the prevailing temperature level and/or the fluctuations in it.
Failure to account any of these can result to leaks and gradual release of the contents into the surroundings and atmosphere. Although there are safety features in place such as allowing for the contents to be released slowly instead of causing an explosion, it’s still important to prevent leaks and other failures in the first place.
Aside from preventing accidents and health hazards, maximising the asset’s lifetime is also important. After all, this also translates to maintaining the asset’s integrity. This then results to minimal downtimes and lower maintenance, repair and replacement costs.
As mentioned earlier, it’s a constant battle because there’s a stress that needs to be removed when there are contents inside a pressure vessel. As a result, there should also be ongoing action to prevent that from happening.
Considerations on operational requirements
The design of a pressure vessel should be in accordance to its end use. Pressure vessels hold a wide variety of fluids, gases and substances at varying temperatures and pressures. The operational requirements should be in line with the specific application.
For instance, operational requirements often include the operating pressure (defining the maximum maintained pressure), fluid conditions (the fluid’s temperature as well as its physical and chemical properties), external loads (e.g. dead weight of equipment supported from the vessel) and transient conditions (e.g. vibration, structural and temperature fluctuations).
In other words, the pressure vessel should operate properly and safely given both the dominant and fluctuating conditions. When inspecting the pressure vessel, a professional team will review the operational requirements first so that they can better identify the risks and vulnerabilities. This also provides them with useful information when estimating when the next maintenance should be due or how long the asset will last. After all, the integrity of pressure vessels not just depends on the strength of their construction. It’s also about their end use and what are the conditions these vessels consistently face.
Functional requirements and design rules
Before manufacturing the pressure vessels, the functional requirements should be defined first and ensure that the design strictly adhere to codes and standards. These can be dictated by the operational requirements defined earlier by the engineering team.
It can start with defining the size and shape of the vessel. The next steps are about determining the method of vessel support (typical vessels are spherical or cylindrical in shape) and the location and size of nozzles and attachments (e.g. connecting to pipes and previous/succeeding processes).
Then, the proposed design should be checked against the most current codes and standards. For instance, the AS 1210-2010 (source) sets out minimum requirements for the materials, design, manufacture, testing, inspection, certification, documentation and dispatch of fired and unfired pressure vessels constructed in ferrous or non-ferrous metals by welding, brazing, casting, forging, or cladding and lining and includes the application of non-integral fittings required for safe and proper functioning of pressure vessels. This Standard also specifies requirements for non-metallic vessels and metallic vessels with non-metallic linings.
In other words, there are already standards and design parameters set to better ensure safety and performance of pressure vessels. These standards and design parameters may vary from one country to another because these are often controlled or regulated by relevant organisations and legislations.
This applies not just in the design and production of pressure vessels but also in the inspection of these important industrial components. This is to further verify that standards were followed in the production of the vessel. It’s a good way to further ensure the vessel’s safety and performance for the succeeding months and years.
For instance, the standards may include details on the production conditions, welding procedures and key weld details. Specific materials and their thicknesses might also be determined beforehand because of the current codes and standards. Some of the most common materials are carbon steel, low alloy steels, high alloy steels and non-ferrous materials such as copper, nickel and aluminium.
Welding procedures and key weld details should also be specified. That’s because the weld joints can be the initial failure points of pressure vessels. Corrosion and other chemical reactions might initiate on those points. Also, the application of a suitable coating might not penetrate 100%. As a result, there will be microsites directly exposed to the vessel’s contents or the outside surroundings (where moisture and oxygen will give way to rusting).
Inspection and testing
To minimise corrosion and prolong the integrity of the pressure vessel, there should be both an upfront and ongoing inspection of the asset. After all, physical and chemical changes happen all the time on the interior and exterior of a pressure vessel.
With a prompt and ongoing inspection in place, the asset’s lifetime will be maximised while also minimising the downtime and potential repair and maintenance costs down the road. This could make the investment more worthwhile because you get more use out of the purchased asset.
How to accomplish that? Codes and standards are also at play here. These were determined by a team of experts beforehand based on the relevant incidents regarding the operation of pressure vessels. The standards might also have been the result of advanced material and stress testing to identify new risks.
For prompt and rigorous inspections, here at Corrims we provide pressure vessel inspection services in accordance with the API 510 (Pressure Vessel Inspection Code). The inspection includes determining the corrosion rates so the internal and external inspection intervals will be accurately set. The inspection also includes the following:
- Joint efficiencies
- Static head and internal pressure calculations
- Impact testing
- Weld size for attachment welds at openings
There are many other facets in inspecting pressure vessels depending on the risk profiles and end usage of the containers. The overall goal is to ensure the safety and integrity of the key operations of an industrial facility.
Our certified inspectors here at Corrims also assess the conditions of aboveground storage tanks that hold water, crude oil and other chemical products. We do the inspection in accordance with the API 653 standards for tank inspection, repair, alteration and reconstruction.
Contact us today here at Corrims if you require a professional and accurate pressure vessel inspection service. We already have a wealth of experience in working on steam pressure vessels, boilers (gas, coal, and multi fuel), recovery boilers, water heaters, fired heaters, process vessels, storage tanks, process piping, heat exchangers, steam generators, reboilers and much more. We help you identify defects and perform all necessary repairing and maintenance work for pressure vessels.