Some issues with hydrotesting

In cases where the design pressure for the shell side of a heat exchanger is lower than that of the tube side, it is necessary to raise the hydrostatic test pressure for the shell side to match the test pressure of the tube side. The hydrostatic test should then be used as the design pressure to verify whether the thickness of the shell side meets the strength requirements. Why isn’t verification used during the hydrostatic test to determine whether the reinforcement through openings meets the requirements when the design pressure is applied?

A hydrostatic test is an overload test that involves conditions higher than those specified in the design requirements; the design requirements meet the conditions for hydraulic testing, and a pass is achieved when the design pressure is 0.7 MPa. It is to ensure that it passes the hydraulic test at 0.9 MPa without any leaks

There are two reasons for this: 1. The classical theory of opening reinforcement is based on the equal-area reinforcement principle, that is, the same amount of material is removed as is replaced; as long as the reinforcement meets the required standards, the strength of the equipment remains equivalent to that of an unopened structure, meaning it can withstand the loads required in pressure tests； 2. If the pressure used for the withstand voltage test is set at the design pressure, and the allowable operating pressure is taken to be a certain percentage lower than the design pressure, this will result in significant waste of material. Moreover, it increases the difficulties associated with processing, testing, transportation, and installation, making it hard to ensure the quality of the equipment. This is even more true for high-pressure and ultra-high-pressure equipment.

The metaphor of overload is very apt.

The design conditions are required to ensure long-term safety over the design life, while hydrostatic tests are only needed to ensure safety under short-term loading; therefore, the allowable stress values for the two are different, or in other words, the safety factors are different. The allowable value for the former is ≤σs/1.5, while that for the latter is 0.9σs. As long as the design conditions meet the strength requirements, and pressure testing is carried out at the pressure specified by the standards, the strength requirements will naturally be satisfied. In your example, if the test pressure of 0.9 MPa already takes into account the temperature correction for the allowable stress, then there is no need to perform a strength check for the hydrostatic testing conditions ； For a device at normal temperature, a test pressure of 0.9 MPa equals 1.286 times the design pressure, which is higher than the required value of 1.25 times. In accordance with Article 4.6.3 of GB150, it is necessary to verify the stress levels of all stressed components. GB150 provides a method for checking the maximum overall film stress; other checks are left to the discretion of the designer. If you don’t know how to verify it, do not increase the hydrostatic test pressure; just follow the standards. A hydrostatic test is a strength test; whether there is leakage is determined by a leakage test (such as a airtightness test).

The allowable stress used for verification during the hydrostatic test also increases, and just as the water level rises, so does the ship’s height:lol:lol

The explanation on the 6th floor is relatively easy to understand; thank you.

It is possible for a person to eat three bowls of rice per meal, but eating three bowls of rice at each meal every day will cause problems over time

The specifications are already qualified when pressure is designed based on experience at the time of formulation. It is to ensure that it passes the hydrostatic test without any leaks