A pressure vessel is one of the most crucial pieces of equipment in petrochemical and industrial processing plants. In general, the term pressure vessel covers a wide variety of vessels including separation vessels, columns, storage vessels, reactors, and heat exchangers. Because of the risk associated with the accidental failure of pressure vessels, many countries have come up with regulations to govern pressure vessel designing and production.
For construction and design purposes, the pressure vessel is typically referred to as the ‘pressure vessel proper’ including welded attachments like welded or screwed connectors, nozzle flanges and adjacent connecting pipes.
Pressure vessels are designed to meet specific requirements set by a team that typically include mechanical engineers, thermodynamicists and process engineers.
The first step in building a pressure vessel is establishing the design criteria or operational requirements. These directly affect the vessel as part of the overall processing plant. Some common design criteria include:
1). Transient Conditions
Some vessels may need an evaluation of cyclic loads that occur due to acoustic, structural, temperature or pressure vibration loading.
2). External Loads
Loads to be considered include local loads, snow and wind. Local loads include things like dead weight or pipe reactions from equipment supported by the vessel.
3). Fluid Conditions
Minimum and maximum fluid temperatures will need to be stipulated and applied to metal design temperatures. Fluid chemical and physical properties will impact material choice.
4). Operating Pressure
On top of normal steady operating pressure, the maximum maintained pressure should be determined. Standards and/or regulations will define how this maximum pressure is applied in the pressure vessel design.
Failure Modes in Pressure Vessels
There are many causes of failures in pressure vessels, such as the following:
1). Change of service routine occasioned by user: Inexperienced maintenance or operations personnel can differ from the normal and expected service routine, which can upset operational conditions. Some types of services that need special attention both for selection of design criteria and choice of material include:
- Vessel contents: Hydrocarbons, Chlorides, Caustics, Compressed air, Ammonia or Hydrogen
- High vibration or shock
- High temperature
- Low temperature
- Fatigue (cyclic)
2). Poor fabrication: poor quality control during fabrication or insufficient or improper adherence to fabrication procedures including forming methods, heat treatment and welding.
3). Incorrect Design data: Incorrect or inaccurate design techniques and inadequate testing can lead to pressure vessel failure.
4). Wrong selection of material: Choosing the wrong material or a defective material is another major cause of pressure vessel failure.
Types of Failures
- Corrosion fatigue: This happens when fatigue and corrosion impacts occur at the same time. Corrosion can reduce fatigue life by pitting the propagating cracks and surface. Fatigue properties and material selection are the main considerations.
- Stress corrosion: It is widely known that caustic service can cause stress corrosion in carbon steels, likewise chlorides cause stress corrosion in stainless steel vessels. Material selection is crucial in reducing rates of pressure vessel failure.
- Low Cycle-High Strain fatigue is strain governed and happens due to high-ductile, low-strength materials.
- Incremental collapse – Plastic instability: Incremental collapse is cumulative cyclic deformation or cyclic strain accumulation. Cumulative damage causes pressure vessel failure through plastic instability or deformation.
- Creep deformation or stress rupture: This occurs due to progressive fatigue, cyclic loading or fatigue. Fatigue is a cycle-based phenomenon, whereas stress rupture is a time-based phenomenon.
- Excessive plastic deformation. The main stress limits stipulated in pressure vessel design standards are intended to prevent incremental collapse and plastic deformation.
- Brittle fracture: This happens at intermediate or low temperatures. Brittle fractures are common in pressure made of low carbon steel.
- Elastic deformation: This is elastic buckling or elastic instability.
Following pressure design standards is the key to successful pressure design operation. At Sherwood Design & Engineering Pty Ltd, we have been providing engineering design and support services to industries such as steelmaking, minerals processing, water treatment, food/pharmaceuticals, materials handling, and many others for more than 40 years.
We specialise in pressure vessel design and pressure vessel design verification, as well as plant engineering where design, plant layout and preparation of arrangement, fabrication and manufacturing drawings are required.
If you are looking for trustworthy and reliable pressure vessel engineering design and support services, please call us today on (02) 9437 3566 or leave an enquiry.