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How to Identify Fatigue Points While Inspecting Pressure Vessels

July 3, 2019
 

Pressure vessels are used in a variety of industries for storing liquids and gases in various pressure and temperature conditions. Thus, these vessels are subjected to stringent inspections during and after manufacturing. Even though various advanced inspection equipment is employed at the service, they are not enough to ensure the safety of the vessel. A thorough inspection becomes possible only if you have an understanding of the probable fatigue points and failure points of the vessel. If these points are overlooked, there may be severe catastrophic effects on the vessel. This post will lead you through important fatigue and failure points to keep in mind when using pressure vessels at your facility.

Understand the Important Types of Fatigue That Might Affect a Pressure Vessel

Fatigue is one of the important reasons for pressure vessel failure, and it is distinguished into three types— thermal fatigue, corrosion fatigue, and mechanical fatigue. These types are described in detail below:

1. Corrosion Fatigue: Any cracking produced due to corrosion and fluctuating stress is known as corrosion fatigue. This type of cracking occurs more in boilers and can be extremely dangerous, as they may endanger the lives of users. It is seen that corrosion cracking mostly occurs in areas subjected to high stress. Pressure vessels are manufactured using durable and corrosion-resistant materials. Also, they are covered in a thick protective oxide layer, which helps avoid the formation of cracks. Initiation of corrosion begins with the cracks in the oxide layer through the cyclic loading. These cracks make it easier for the corrosive medium to access the bare metal layer and produce several micro-cracks. These micro-cracks are aggravated in the later stages due to regular vessel usage and it may lead to catastrophic failure. Sometimes, the vessels also develop corrosion cracking due to caustic embrittlement. This type of cracking is referred to as stress corrosion cracking (SCC).

2. Thermal Fatigue: This type of fatigue is induced by rapid cooling and heating cycles. These alternating cycles may impact thermal expansion and induce failure in less than ten cycles. At times, symptoms of thermal fatigue are confused with corrosion fatigue. Proper management of thermal cycling helps reduce the effect of thermal fatigue.

3. Mechanical Fatigue: This fatigue is induced by mechanical stresses caused by vibrations or other factors.

What are the Common Symptoms of Fatigue?

The following are some common symptoms of fatigue:

  • Loosened components including bolts and nuts
  • Oxidation or rusting in various areas of the vessel
  • Pitting in the bare metal
  • Misaligned or bowed piping
  • Frozen or stuck latches
  • Valves that are not easy to turn
  • Material discoloration
  • Micro or mini cracks that are only visible through magnifying glasses

All the aforementioned symptoms are indicative of the fatigue that your pressure vessel may have endured. They represent severe potential hazards and must be corrected with utmost importance. If the above signs or other signs are noticed, then the pressure vessel must be removed from service, and subjected to thorough inspection. The inspection must be performed by a certified professional and in accordance with the rules and regulations described in ASME Section VIII. Once you notice these symptoms, it is important to get in touch with your manufacturer. An experienced manufacturer can guide you with the right inspection procedures and also help you with the inspection. BEPeterson is one of the leading manufacturers of industrial pressure vessels and specializes in the design and fabrication of ASME vessels, ASME tanks and vacuum furnaces,vacuum chambers, and more.



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