Design pressure is the expected pressure at which a closed vessel, such as a chemical reactor, operates during normal operations. It is determined by calculation or process simulation and used to measure vessel capacities. The maximum allowable working pressure (MAWP) is the pressure a vessel can withstand before failing. Design pressure is important for process safety and equipment selection.
In pressure vessel terminology, the design pressure of a pipe, tank, reactor, or other vessel is the pressure at which the unit is expected to operate during normal operations. This includes excessive pressure that can occur during normal operation; as a general rule, this overpressure can vary from 10% to 25% of the calculated vessel pressure. The design pressure is used to determine the characteristics of the pressure vessel and to measure the vessel capacities.
A pressure vessel is a closed vessel, such as a chemical reactor or process vessel, designed to hold gases or liquids at pressures above that of the atmosphere. Such vessels are commonly found in petroleum and chemical processing plants. Heat exchangers, distillation columns, boilers, process vessels, pipes and reactors can all be considered pressure vessels under the right circumstances.
The design pressure of a given unit operation is typically determined by calculation or process simulation. The design software is used to calculate the unit’s working pressure based on the material and energy put into the system. Once a convergent model is developed, the design pressure is calculated by taking the pressure predicted by the model and adding an estimated excess pressure. Excess pressure typically ranges from 10% to 25% of the expected pressure and will depend on the designer’s estimation of process stability.
Once the design pressure has been determined from a predictive model, the vessel can be fabricated. This pressure is used to determine the size of the vessel. Practical factors, such as material of construction and design factor safety factors often lead to a manufactured vessel that may exceed design factor specifications. Safety factors are used to ensure that pressure vessels can operate safely above their intended operating pressures for short periods of time.
The design pressure of a vessel should not be confused with the maximum allowable working pressure (MAWP). The MAWP of a pressure vessel is calculated using the vessel’s material of construction and dimensions, and is a measure of the pressure the vessel can withstand before failing. The MAWP is used to determine the settings for the pressure relief valves on the unit. Corrosion from chemical sources, mechanical wear and exposure to the elements can reduce a vessel’s MAWP over time.
Design pressure is a useful measure for engineers and operators regarding process safety. Knowledge of the average working pressure for an operating unit may be necessary to select the appropriate equipment to capture process samples. The design pressure of a tank can also be used to estimate process conditions, although direct measurements are usually more accurate.
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