Thread Content
I. Basic Concepts of Pressure Vessels 1. The housings of all chemical processing equipment are considered vessels; they must withstand the pressures, temperatures, and effects of chemical substances involved in the chemical processes. To ensure long-term safe operation, economic considerations also need to be taken into account. Pressure vessel technology integrates elements from applied mechanics, materials science, metallurgical processes, mechanical manufacturing processes, and technical physics, and is specifically used for vessels involved in pressure operations in chemical production. The structural characteristics of pressure vessels are that they typically consist of components such as a shell, end caps, nozzles, seals, and supports. The materials mainly consist of: at most steel; the manufacturing methods are primarily pressure processing and welding. Management standard: «Regulations on Safety Technical Inspection of Pressure Vessels». 2. Scope of pressure vessels: The maximum operating pressure (pw) is greater than or equal to 0.1 MPa (excluding hydrostatic pressure; the same applies hereafter) ； The inner diameter (for non-circular cross-sections, this refers to the maximum dimension) is greater than or equal to 0.15 m, and the volume (V) is greater than or equal to 0.03 m³ ； The filling medium is a gas, a liquefied gas, or a liquid whose maximum operating temperature is equal to or higher than its standard boiling point. 3. Classification of pressure vessels: Pressure vessels are classified into three categories. Those falling under any of the following conditions are classified as Class 3 pressure vessels: (a) High-pressure vessels ； (b) Medium-pressure vessels (only for media with extremely high and high toxicity) ； (c) Medium-pressure storage vessels (only for flammable or moderately toxic media, where the pV product is greater than or equal to 10 MPa•m³) ； (d) Medium-pressure reaction vessels (only for flammable or moderately hazardous media with a pV product of 0.5 MPa•m3 or greater) ； (e) Low-pressure vessels (only for media with extremely high and high toxicity levels, and a pV product of 0.2 MPa•m3 or greater) ； (f) High-pressure and medium-pressure shell-and-tube waste heat boilers ； (g) Medium-pressure glass-lined pressure vessels ； (h) Pressure vessels manufactured from materials with a high strength level (meaning that the minimum specified tensile strength value in the relevant standards is 540 MPa or higher) ； (i) 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 ； (j) Spherical storage tanks (volume greater than or equal to 50 m3) ； (k) Cryogenic liquid storage containers (with a volume greater than 5 m³). B. A pressure vessel is classified as a Class II pressure vessel in one of the following cases: (a) Medium-pressure vessel ； (b) Low-pressure vessels (only for media with extremely high and high toxicity) ； (c) Low-pressure reaction vessels and low-pressure storage vessels (only for flammable media or media with a moderately hazardous toxicity level) ； (d) Low-pressure shell-type waste heat boiler protection ； (e) Low-pressure glass-lined pressure vessels. C. Low-pressure vessels are Category I pressure vessels (except as provided in paragraphs 1 and 2 of this article). When selecting materials for pressure vessels, in addition to mechanical properties and bending strength, compatibility with the medium must also be taken into account. The phosphorus content (melting analysis, the same hereinafter) of steel specially used for pressure vessels shall not exceed 0.030%, and the sulfur content shall not exceed 0.020%. The design pressure of a pressure vessel shall not be lower than the maximum operating pressure. For pressure vessels equipped with safety relief devices, their design pressure shall not be lower than the opening pressure of the safety valve or the burst pressure of the rupture disc. 4. Pressure rating, type, toxicity level of the medium, and classification of flammable media for pressure vessels: A. Based on the design pressure (p) of the pressure vessel, they are classified into four pressure categories: low pressure, medium pressure, high pressure, and ultra-high pressure. The specific classifications are as follows: (a) Low pressure (code L): 0.1 MPa < p < 1.6 MPa ； (b) Medium pressure (code M): 1.6 MPa < p < 10 MPa ； (c) High pressure (code H): 10 MPa < p < 100 MPa ； (d) Ultra-high pressure (code U) p＞100 MPa. B. Based on the principle of function in the production process, pressure vessels are classified into reaction pressure vessels, heat exchange pressure vessels, separation pressure vessels, and storage pressure vessels. The specific classifications are as follows: (a) Reaction pressure vessels (code R): These are pressure vessels used to carry out physical or chemical reactions of substances, such as reactors, kettles for reactions, decomposition vessels, sulfidation tanks, decomposition towers, polymerization kettles, high-pressure reactors, ultra-high-pressure reactors, synthesis towers, shift reactors, cooking kettles, steam generators, autoclaves, and gas generators ； (b) Heat exchange pressure vessels (code E): These are pressure vessels primarily used for heat exchange between media. Examples include shell-and-tube waste heat boilers, heat exchangers, coolers, condensers, evaporators, heaters, sterilizers, dyeing vessels, drying cylinders, steaming and frying pots, preheaters, solvent preheaters, steam kettles, steam strippers, electrothermal steam generators, and water jackets for gas generators ； (c) Separation pressure vessels (code S): These are pressure vessels primarily used for achieving fluid pressure balance and buffering, as well as for gas purification and separation; examples include separators, filters, oil separators, buffers, washers, absorption towers, steel washing towers, drying towers, stripping towers, steam generators, and deaerators ； (d) Storage pressure vessels (code C, with spherical tanks coded as B): These are primarily pressure vessels used for storing gases, liquids, liquefied gases, and other such substances, such as tanks of various types. In a pressure vessel where two or more process functions are performed simultaneously, the types shall be classified according to the main function within the process. C. The classification of the physical properties of media and the categorization of flammable media are as follows: (a) The classification of the physical properties of chemical media in pressure vessels and the categorization of flammable media are in accordance with the provisions of HG20660 \"Classification of Toxicity Hazards and Explosion Risks of Chemical Media in Pressure Vessels\". When no specific provisions are given, the degree of toxicity is determined according to the following principles: Extremely hazardous (Grade I) – the most tolerable concentration < 0.1 mg/m3 ； Highly hazardous (Class II): Maximum allowable concentration of 0.1–<1.0 mg/m3 ； Moderate hazard (Level III): The maximum allowable concentration is 1.0–pw. 3. When a rupture disc device is installed on a fixed-pressure vessel, the designed burst pressure of the rupture disc shall not be greater than the designed pressure of the vessel; moreover, the minimum designed burst pressure of the rupture disc shall not be less than 1.05 times the highest operating pressure of the vessel, that is: pB ≤ p, and pBmin ≥ 1.05pw. 4. The requirements for installing safety valves are as follows: (1) Safety valves should be installed vertically, and they must be placed in the gas phase space above the liquid level of the vessel, or on a pipe connected to the vessel’s gas phase space. (2) The cross-sectional area of the connecting pipes and fittings between the pressure vessel and the safety valve must not be smaller than that of the safety valve’s inlet; such pipes should be as short and straight as possible. (3) When two or more safety valves are installed at one connection port of a pressure vessel, the cross-sectional area at the inlet of that connection port shall be at least equal to the total cross-sectional area of the inlets of these safety valves. (4) It is generally not advisable to install stop valves between the safety valve and the pressure vessel. To enable online calibration of safety valves, a rupture disc device can be installed between the safety valve and the pressure vessel. For pressure vessels containing media with extremely high, high, or moderate toxicity, flammable media, corrosive or viscous media, or valuable media, in order to facilitate the cleaning and replacement of safety valves, a shut-off valve may be installed between the safety valve (rupture disc device) and the pressure vessel, provided that it is approved by the technical person in charge of pressure vessel safety at the operating unit and that reliable preventive measures are put in place. During the normal operation of pressure vessels, the stop valve must be kept fully open (sealed with lead or locked). The structure and bore diameter of the stop valve should not impede the safe relief of the safety valve. (5) The safety valve should be installed in a location that facilitates inspection and maintenance. Safety valves should generally be calibrated at least once a year; when it is difficult to disassemble them for calibration, on-site calibration (in-line calibration) should be performed. The rupture disc assemblies should be replaced regularly; those that have exceeded the maximum design rupture pressure without bursting must be replaced immediately ； Blowout valve devices used under harsh conditions should be replaced annually ； Generally, rupture disc devices should be replaced within 2–3 years (except where the manufacturer specifies that their service life can be extended). Pressure gauges and temperature measuring instruments shall be calibrated within the period specified by the user unit. 5. The safety valve should be taken out of service and replaced if it exhibits any of the following conditions: (1) The valve core and seat of the safety valve do not seal properly and cannot be repaired. (2) The valve core of the safety valve is stuck to the valve seat, or the spring is severely corroded or rusted. (3) Incorrect selection of the safety valve. 6. The requirements for selecting a pressure gauge are as follows: (1) The selected pressure gauge must be compatible with the medium inside the pressure vessel. (2) The accuracy of pressure gauges used in low-pressure vessels shall not be lower than grade 2.5 ； The accuracy of pressure gauges used in medium-pressure and high-pressure vessels should not be lower than grade 1.5. (3) The scale limit value of the pressure gauge dial should be 1.5~3.0 times the maximum operating pressure, and the diameter of the dial should not be less than 100 mm. 7. The calibration and maintenance of pressure gauges shall comply with the relevant regulations of the **metrology authorities. The pressure gauge should be calibrated before installation; a red line indicating the maximum operating pressure should be marked on the dial, along with the date for the next calibration. After calibration, the pressure gauge shall be sealed with lead. 8. The installation requirements for pressure gauges are as follows: (1) The installation location should allow operators to easily observe and clean the gauge, and it should be protected from adverse effects such as radiant heat, freezing, or vibration. (2) Between the pressure gauge and the pressure vessel, a three-way cock or needle valve shall be installed ； The three-way stopcock or needle valve should be equipped with an opening mark and a locking mechanism ； No accessories or pipes for other purposes shall be connected between the pressure gauge and the pressure vessel. (3) For pressure gauges used in steam media, a trap is installed between the pressure gauge and the pressure vessel. (4) Used for pressure gauges in corrosive or highly viscous media. A buffer device capable of isolating the medium should be installed between the pressure gauge and the pressure vessel. 9. The pressure gauge should be stopped from use and replaced if it exhibits any of the following conditions: (1) In a gauge with a limit pin, the pointer cannot return to that limit pin when there is no pressure ； Gauge with an unlimited scale: when there is no pressure, the pointer’s position relative to zero exceeds the gauge’s allowable error. (2) The glass covering the dial is cracked, or the dial markings are blurred. (3) The seal is damaged or the verification validity period has expired. (4) Leakage in the gauge spring tube or a loose gauge pointer. (5) The pointer is broken or the casing is severely corroded. (6) Other defects that affect the accurate indication of the pressure gauge. 10. Level gauges for pressure vessels shall comply with the provisions of relevant standards and meet the following requirements: (1) They shall be selected appropriately based on the medium contained in the pressure vessel, its maximum operating pressure, and temperature. (2) Before installation and use, level gauges for low- and medium-pressure vessels shall undergo a hydraulic test at a pressure of 1.5 times the nominal pressure of the gauge ； The level gauge of high-pressure vessels shall undergo a hydraulic test at a pressure of 1.25 times the nominal pressure of the level gauge. (3) For pressure vessels containing media at temperatures below 0°C, frost-proof level gauges should be used. (4) For level gauges used outdoors in cold regions, those with a jacketed or insulated structure should be selected. (5) Pressure vessels for liquefied gases that are flammable and pose an extremely high level of toxicity must be equipped with protection devices to prevent leaks. (6) When a stable liquid level indication is required, float-type level gauges should not be used. (7) Glass plate level gauges shall not be used in mobile pressure vessels. 11. The level gauge should be stopped from use and replaced if it is in one of the following conditions: (1) The maintenance cycle has been exceeded. (2) The glass plate (tube) is cracked or broken. (3) The valve component is stuck. (4) False liquid level appears. (5) The liquid level gauge gives an unclear reading