Issue with the hydrostatic test pressure value

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d) The water pressure is intended to eliminate and reduce welding residual stresses as well as the peak stresses in local discontinuities. This hydrostatic test value must not be too low; it should be set at a level where the material is nearly at its yield point. If the pressure test value is very low, it basically won’t serve any purpose. e) To passivate the microcrack tip, it is necessary to cause that tip to yield. As for a, b, and c, it depends on the perspective from which one looks at it; high standards can be set, or basic standards will suffice. The intention behind specifying that the hydrostatic test pressure should be the lowest value among all components is to prevent the use of an excessively high pressure, which could damage the equipment as a result. But if the design relies solely on the lowest possible hydrostatic test pressure, then the two objectives of d) and e) will not be achieved when testing the equipment under hydrostatic pressure, as I mentioned above ； Even the purpose of item a) has been diminished. Therefore, for equipment with significant hazards, those whose thickness is determined by stiffness, and equipment with a large corrosion margin (some composite plate equipment), it is recommended to increase the hydrostatic test pressure appropriately. 1) Equipment with high hazards: Equipment whose medium poses extremely high levels of hazard. 2) Stiffness determines thickness: The nominal thickness is much larger than the calculated thickness (such as in heat exchanger shells and low-pressure tanks); if the hydrostatic test pressure is calculated based on the design pressure, the stress in the components under actual hydrostatic pressure remains quite low. 3) Equipment with a large corrosion margin or composite plate equipment: In this case as well, the actual nominal thickness is much greater than the calculated thickness. Similar to 2) and 4), there is another scenario in which the temperature is relatively high, but the material used for the heat exchange tubes or the inlet pipes of the process pipelines is of high quality – such as certain nickel-based materials, duplex steel, or nickel-based materials used for expansion joints ； It can also result in the correction factor being controlled by individual components; other values may be around 1.4, while some components have a value of only 1.0–1.1. 5) Additionally, if the design temperature falls within the range of high-temperature creep, the allowable stress corrections specified for the area ahead of the black line are used in the hydrostatic test, which in turn leads to a lower hydrostatic test pressure. None of the aforementioned scenarios achieve the goal of truly testing the quality of the entire equipment under overpressure, let alone eliminating stress or passivating the tips of microcracks. Standard ASME VIII-1 provides for option UG-99 c), which allows the hydrostatic test pressure to be calculated based on MAP (the maximum allowable pressure of the equipment under normal temperature and non-corroded conditions); some clients also require that the hydrostatic testing be carried out at the manufacturer’s facility in accordance with UG-99c). Think for yourself by how much more the hydrostatic test value for most devices will be than the value calculated based on the design pressure! Theoretically, the most appropriate pressure for a pressure test should be such that, as calculated using relevant formulas, the stress level in the shell under water pressure conditions reaches 1.25 times the allowable stress of the material (1.3 times according to ASME VIII-1); to be appropriate, this value should be at least 1 time the allowable stress of the material. Finally, it should be noted that using the minimum value as per the standards is indeed correct; in terms of the hydrostatic test pressure, it is still within the acceptable range. However, it does not fully meet the objectives of the hydrostatic test, so it cannot be considered good or excellent. Those who are interested can take a look at the pressure vessel design engineer training course – Basic Knowledge: Components, Version 2019, Chapter 4 on pressure testing.