To learn how pressure vessel design is calculated, it’s important to understand what pressure is and what its effects on pressure vessels are. Pressure is defined as force per unit area, i.e.: (Pressure = Force/Area). So, for any given pressure, the larger the area, the larger the force will be.
It’s impossible to imagine the oil and gas industries without pressure vessels such as steam drums, columns, knock out drums, separators, etc. The thermodynamic energy contained in a pressure vessel can be huge. This is why pressure vessel design calculations should be done with extremely high accuracy and precision.
Effects of Pressure
You have probably participated in a tug of war game where two teams pull on the opposite ends of a rope. It’s the same when we apply stress or tension to a metallic vessel by pulling it to different levels, the vessel under stress begins to elongate and deform. Pressure inside also subjects a vessel to stress. If a vessel is designed for a particular pressure and is exposed to higher pressures, it may result in catastrophic failure.
Designing for High Pressure
Ideally, all pressure vessels should be designed to withstand the maximum pressure that can be reached during normal operating conditions. However, in some cases, it may not be economically viable to do this. Vessels are typically designed for a particular pressure, which is calculated by adding a margin to the maximum operating pressure. A relief valve can be used to ensure that maximum operating pressure is not reached.
However, not all pressure vessels have relief valves. Centrifugal pumps, for instance, do not feature relief valves because they are designed to operate at the highest possible pressures whereas the associated vessels and piping are protected by a relief valve.
Pressure Terminology
1). Operating Pressure (OP)
The OP is the gauge pressure that exists inside a vessel during normal operation.
2). Design Pressure (DP)
Design pressure is the pressure at the top of the vessel in its operating position. Design pressure is used to determine the minimum thickness of the vessel at Design Temperature. Design pressure is usually determined by the process engineer in close consultation with the pressure vessel mechanical engineer.
Designing for Low Pressure
Low pressure design takes into account the atmospheric pressure or external pressure that a vessel is subjected to in its operating position. Low pressure design is used to determine the minimum thickness of stiffening rings at the design temperature.
In low pressure vessels, the pressure is normally directed inward and comes from the atmosphere. Some processes that make use of low pressure vessels include:
- Pumping out without using vapour
- Steam side of heat vessels or exchangers with steam
- Components with a boiling point below 0 °C
- Processes where cool-down is expected
After pressure, it is important to understand the different terminologies related to temperature.
Temperature Terminology
1). Operating Temperature (OT)
This is the temperature that exists inside a vessel during normal operation.
2). Maximum Operating Temperature (MOT)
The MOT is the maximum equilibrium temperature of the contents of a vessel at the maximum operating pressure. If operational flexibility is required, the MOT is set higher than the OT, otherwise, they are equal.
3). Upper Design Temperature (UDT) or Maximum Design Temperature (MDT)
The Upper Design Temperature (UDT) is often referred to as the Maximum Design Temperature (MDT). The UDT is the highest temperature a vessel can handle at the lower and/or upper design pressure.
The design pressure and temperature are used as the foundation for pressure vessel design and are used together to calculate minimum wall thickness for pipes and vessels.
AtSherwood 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 inpressure vessel designand pressure vessel design verification, as well as plant engineering where design, plant layout and preparation of arrangement, fabrication and manufacturing drawings are required.
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