[2026 Pressure Knowledge] Design pressure, operating pressure, test pressure

2. Design pressure: The design pressure is the maximum pressure specified at the top of the container. The design pressure, together with the design temperature, serves as the design load condition. The design pressure is used to calculate the component thickness, select the flange class, and determine the setting pressure of the safety valve. The design pressure value is indicated on the nameplate. The design pressure shall not be lower than the operating pressure. When the container is equipped with a safety relief device, the design pressure shall not be lower than the opening pressure of the safety valve or the burst pressure of the rupture disc. The design pressure is usually taken as 1.0 to 1.1 times the maximum operating pressure.

3. Calculate pressure: Pressure calculation is used to determine the thickness of the component. It is the design pressure plus additional loads such as the hydrostatic pressure of the liquid column at the design temperature. When the hydrostatic pressure of the liquid column is less than 5% of the design pressure, it can be ignored, and the calculated pressure equals the design pressure. When the hydrostatic pressure of the liquid column is 5% or greater, the calculated pressure equals the design pressure plus the hydrostatic pressure of the liquid column. The calculated pressure is the direct input value for the wall thickness formula.

4. Maximum allowable working pressure The maximum allowable working pressure is the highest pressure permitted at the top of the container at a specified temperature. It is calculated based on the actual effective thickness of each component, taking the minimum value of the calculation results for each component. The effective thickness is the nominal thickness minus the corrosion allowance and the negative deviation of the material thickness. The maximum allowable operating pressure is determined after the equipment is manufactured, reflecting the actual load-bearing capacity of the container. The maximum allowable operating pressure is usually greater than or equal to the design pressure. When not specified in the design documents, the design pressure is considered to be the maximum allowable operating pressure. The maximum allowable operating pressure can be used to determine the overpressure limit, thereby increasing the difference between the operating pressure and the value at which the safety valve activates, resulting in a more stable operation.

5. Test pressure: The test pressure is the pressure at the top of the container during a pressure resistance test or a leak test. It checks the manufacturing quality and structural integrity of the container. The hydrostatic test pressure is taken as 1.25 times the design pressure, multiplied by the ratio of the allowable stress of the material at the test temperature to its allowable stress at the design temperature. The pneumatic test pressure is taken as 1.15 times the design pressure, multiplied by that ratio. Based on the maximum allowable operating pressure, the test pressure is determined anew according to the maximum allowable operating pressure.

II. Their relationships and differences 1. Core relationship These pressures form a sequence from low to high. The operating pressure is the lowest, while the test pressure is the highest. The design pressure shall not be lower than the operating pressure. The calculated pressure is equal to or greater than the design pressure. The maximum allowable operating pressure is equal to or greater than the design pressure. The test pressure is 1.25 or 1.15 times the design pressure or the maximum allowable operating pressure, whichever is higher.

Operating pressure → Design pressure → Calculation pressure ≥ Design pressure → Maximum allowable operating pressure ≥ Design pressure → Test pressure (1.25/1.15 times the base value). These constitute multiple layers of safety protection. Operate with work pressure control. The design pressure provides a basis for strength design. The calculated pressure includes additional loads such as the hydrostatic pressure of the liquid column. The maximum allowable operating pressure indicates the true safety limit after manufacturing. The test pressure verifies structural integrity.

2. Systematic comparison: The differences among the five terms are distinguished by their time of determination, basis, and function. Work pressure is determined during the process design phase and is used for operational monitoring based on the process conditions. The design pressure is determined during the equipment design phase, based on process requirements and safety margins, and is used for strength calculations and marking on the nameplate. The calculated pressure is used in the strength calculation phase; it is based on the design pressure and the hydrostatic pressure of the liquid column, and serves as an input for wall thickness calculation. The maximum allowable operating pressure is calculated after manufacturing, and the true load-bearing limit is determined based on the effective thickness of each component. The test pressure is applied during the manufacturing inspection phase, and by multiplying the design pressure or the maximum allowable operating pressure by a coefficient, it is used to verify the strength and integrity of the container.

3. Key difference: Under working pressure, the question is \"What is the normal operating pressure?\" Design pressure answers the question of \"what pressure should be used for design purposes\". The design pressure is a set value. The calculated pressure is the calculated input value obtained by adding the additional load to the design pressure. The maximum allowable operating pressure is the true limit after the container has been manufactured. The maximum allowable operating pressure is usually higher than the design pressure, as the actual wall thickness, after rounding, is greater than the calculated thickness.

The design pressure is considered the maximum allowable working pressure only when the design documents do not specify it. Using the maximum allowable operating pressure as the starting pressure for the overpressure limit allows full utilization of the rounded wall thickness, thereby improving operational stability. The test pressure is based on the design pressure or the maximum allowable operating pressure; a higher pressure is applied under controlled short-term conditions to verify the safety margin of the container.

III. Precautions in engineering applications: The design pressure should not be too high or too low; it is generally set at 1.0 to 1.1 times the maximum operating pressure. When the hydrostatic pressure of the liquid column at the bottom of a tall vertical container reaches 5% of the design pressure, it must be included in the calculated pressure. In the design of safety relief devices, the maximum allowable operating pressure is preferentially used as the starting pressure for the overpressure limit. Once the maximum allowable operating pressure is adopted, the pressure for the pressure resistance test should also be recalculated based on this maximum allowable operating pressure. The test pressure for high-temperature equipment must be multiplied by a temperature correction factor.