Problems with heat exchangers
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Sometimes, one encounters heat exchangers in which the tube side contains condensate water at a pressure of around 0.3 MPa; the shell side has a pressure of about 1.5 MPa, and the medium flowing through it is highly hazardous (chlorine). How should such pressure vessels be classified? Should non-destructive testing be considered separately, or should all be designed as Category III containers? How is a pressure test conducted? I hope experts can offer some guidance!
The heat exchanger you mentioned belongs to category 3 containers, and non-destructive testing should be conducted in accordance with the requirements for category 3. After the hydrostatic test, a airtightness test must be conducted!
To facilitate the safe management and use of pressure vessels, **the Ministry of Labor has issued the ‘Regulations on Safety Supervision of Pressure Vessels’. In this regulation, pressure vessels are classified into four categories based on their role in the production process: reactors, heat exchangers, separation vessels, and storage and transport vessels ； Each type of container is classified into five pressure grades—low, medium, high, and ultra-high—based on its operating pressure. Less than 0.1 MPa refers to pressure vessels at normal pressure ； 0.1~1.6MPa is classified as low-pressure vessels ； 1.6~10MPa refers to medium-pressure vessels ； 10–100 MPa corresponds to high-pressure vessels ； Containers with a pressure greater than 100 MPa are ultra-high pressure containers.
Article 6 of the Regulations: Pressure vessels falling within the scope of application of Article 2 of these Regulations are classified into three categories (the pressure grades, types, toxicity levels of the media used in pressure vessels, and classification of flammable media are listed in Annex 1): 1. Pressure vessels fall under Category 3 in any of the following situations: (1) High-pressure vessels； (2) Medium-pressure vessels (only for media with extremely high and high toxicity) ； (3) Medium-pressure storage vessels (only for flammable or moderately hazardous media, with a PV product greater than 10 Mpa·m3); (4) Medium-pressure reaction vessels (only for flammable or moderately hazardous media, with a PV product greater than or equal to 0). 5 Mpa·m3); (5) Low-pressure vessels (for media with extremely high or high toxicity levels, and a PV product of 0.2 Mpa·m3 or greater) ； (6) High-pressure and medium-pressure shell-and-tube waste heat boilers ； (Note 4) (7) Medium-pressure glass-lined pressure vessels ； (8) Pressure vessels manufactured from materials with a high strength grade (meaning that the minimum specified value for tensile strength in the relevant standards is 540 Mpa or higher) ； (9) Mobile pressure vessels, including railway tank cars (for liquefied gases and cryogenic liquids), tank trucks, and tank containers (for liquefied gases and cryogenic liquids), etc ； (10) Spherical storage tanks (volume greater than or equal to 50 m3) ； (11) Storage vessels for cryogenic liquids (volume greater than 5 m3).
1. The shell-side pressure is 1.5 MPa, classifying it as a low-pressure vessel. 2. The medium inside is highly hazardous. 3. In accordance with Article 5, this heat exchanger can be classified as a Class III vessel only when the V-product is greater than or equal to 0.2 MPa·m3; otherwise, it is classified as a Class II vessel
The tube side and shell side are considered separately, designed and manufactured according to their respective categories, with overall management based on the higher category.
Understood, thank you! So, during the hydrostatic test, is it necessary to increase the pressure in the shell side? Should the test pressure still be determined based on the design pressure of the shell side? Should the external pressure applied to the shell side during testing be taken into account or not?
Isn’t the pressure in the shell side higher than that in the tube side right now? If the pressure in the shell side is lower than that in the tube side, it’s possible to increase the pressure in the shell side, but it is necessary to conduct strength checks on the pressure-bearing components in the shell side. If operated at their respective test pressures, an additional inspection of the heat exchange tube joints is required.
1. Regarding the issue of applying external pressure to the shell side during testing of the tube side: The tube side consists of the space within the tubes and the tube sheet; it can be said that the external pressure applied to the tube side acts only on the tube sheet, which is taken into account in the calculations for the tube sheet. And it has no impact on the calculation of the shell-side cylinder. 2. To test the strength of the tube sheet, the pressure for the shell-side hydrostatic test is determined based on the maximum value of the following two options: (1) the maximum combination of shell-side pressure and tube-side pressure (in the case where the shell side is under positive pressure while the tube side is under negative pressure); (2) the shell-side pressure (when both the shell side and the tube side are under positive pressure, the shell-side testing cannot be conducted by considering the difference between shell and tube side pressures). 3. When the shell-side pressure is lower than the tube-side pressure, in order to effectively assess the strength of the tube sheet as well as the connections between the tubes and the sheet, if it can be assumed that the pressure difference across the tube sheet remains greater than the shell-side pressure, then the shell-side pressure can be increased to the maximum pressure level on both sides of the tube sheet. This situation should be taken into account during design. Everyone is welcome to share their opinions :) This post was last edited by ruihuich on 2007-12-3 03:57.]