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A pressure vessel is a container used to hold gases and liquids at a different pressure either low or high, but quite different from the ambient pressure.
Examples of pressure vessels are glassware, autoclaves, compressed gas cylinders, vacuum, refrigeration, or custom-designed laboratory vessels.
Pressure Vessel Materials Used
The reliability of Pressure Vessels depends on the materials used.
Six different types of materials are used in manufacturing pressure vessels. These are:
1. Titanium Metal
- It has the capability to retain its structure.
- Highly resistant to corrosion.
- Low maintenance is needed.
- It has a good tensile strength.
- The melting point of this material is higher than that of steel and aluminum.
- Highly compatible.
- It is non-toxic in nature.
2. Nickel Alloys
- Nickel alloys give the best corrosion resistance.
- Protects from a thermal explosion.
- Nickel alloys are best to use in harsh environments.
- This material is reliable and can last long.
- Nickel alloys offer the best oxidation and carburization resistance.
3. Stainless Steel
- Pressure vessels are required to be robust and steady. stainless steel is the best option for that purpose.
- Stainless steel is extremely resistant to chemicals.
- Stainless steel shows good corrosion resistance.
- Stainless steel 304L has an excellent weldability.
- This material has the capacity to withstand all humid conditions, sunlight exposure, or even high temperatures.
4. Aluminum Material
- Aluminum can maintain high tensile strength.
- This material is a lot easier and more cost-effective for machines as than for stainless steel ones.
- Aluminum is also a good material as it has a large coefficient of expansion as compared to other metals.
5. Carbon Steel
- Carbon steel has a high capacity to retain its strength.
- The tensile strength of carbon steel is exceptional.
- Carbon steel can be recycled easily.
- It has the capacity to resist vibrations and shocks.
6. Hastelloy Material
- Hastelloy is a well-suited material to use in gas, oil, petrochemical, and chemical applications.
- Good corrosion resistance
- This alloy is long-lasting and can work for several years.
- It is crack-resistant.
- It is available in many types such as C276, S, C, B2, and many more.
NDT Techniques for Pressure Vessels
The pressure vessels have toxic and hazardous gases in them, therefore it is very likely that some accidents may happen.
To minimize this risk Non-destructive Testing (NDT) Techniques are there.
Under the observation of the best operators and technicians, NDT techniques can be implemented.
For the detection of surface defects following NDT Techniques are used.
1. Visual Testing
A common NDT Method is the visual testing of the vessel. It is used to look out for defects such as corrosion or any physical damage that is visible.
Visual Testing requires good light and perfect eyesight of the inspector. The visual testing method has easy access to the surface however, some sites can be challenging.
If we want to enhance the visual method, the use of a magnifying glass is suggested. Visual testing methods can also be done using computerized video systems.
2. Magnetic Particle Testing
Another NDT technique that is used to detect surface defects is Magnetic Particle Testing.
This technique is only for materials that are ferromagnetic in nature, such as steel. This NDT Technique is easy to carry, portable and inexpensive.
Magnetic Particle Testing is used for detecting cracks, which are responsible for surface breakage of the material.
This NDT technique is a two-stage process. First, a magnetic current is passed through the component. If there is any crack or defect present, it will interfere with the lines of Magnetic Flux.
After that magnetic particles are sprayed, which collects all the surface defects. The use of fluorescent magnetic inks can also be used for more visibility of particles.
3. Eddy Current Testing
A coil is placed near the pressure vessel surface and an electrical current is passed through it. This electric current will induce an Eddy current Testing. If there is a defect on the vessel surface it will hinder current flow. However, if the material of the vessel is non-magnetic, eddy current testing will give a measurement that will show the depth of the defect.
4. Dye Penetrant Testing
A Dye Penetrant Testing i.e a liquid is sprayed on the vessel surface to detect its surface flaws. To make defects more visible under UV light a fluorescent chemical is also added. For internal defects of pressure vessels following NDT techniques are used.
5. Ultrasonic Testing
Ultrasonic Testing requires an excellent surface finish because it uses high-frequency sound waves to detect cracks.
The ultrasonic waves will be reflected on the cracked parts of the surface and give data on wall thickness.
For its working a coupling medium such as water or gel is required, an ultrasonic probe transfers sound waves through the component to detect defects.
The automated technique of this testing is the best to use as the manual application has some speed limitations.
6. Radiographic Testing
This NDT Technique works by detecting material loss. Radiations are applied on the component and any alteration in the material can be detected and recorded on the film.
Radiography is the most popular NDT technique as it can capture defects in the film and also provides a hard copy of it.
Digital Radiography is even better as it can give results on computer screens in seconds.
The best NDT technique for Pressure Vessels is Digital Radiography.
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