[Safety Management and Standardization of Special Equipment] – Why do the descriptions regarding pressure in the catalog of special equipment vary?

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Ensure proper safety management of special equipment; standardization ensures safety. [Special Equipment Safety Management and Standardization] – Comprehensive summary post! Constantly being updated – feel free to communicate! ! ! https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5718553 (Source: Haichuan Chemical Industry Forum (Hua Haichuan Liu hcbbs)). Everyone is welcome to participate in the discussions. ---------------------------------------------------------In daily work, colleagues often wonder: since they are all special types of pressure-bearing equipment, why is some referred to as “rated steam pressure,” some as “exit water pressure,” and others as “maximum operating pressure” or “nominal operating pressure”? What logic lies behind these different expressions? Today, we will thoroughly clarify the differences between these terms, as well as why the catalog of special equipment uses such a \"complex\" way of expression. I. Three different ways of describing the boiler section In the definition of a boiler, we find three entirely different ways of describing it: 1. Rated steam pressure – applicable to pressure-bearing steam boilers. Definition 1: According to clause 3.2.7 of GB/T 2900.48-2008 \"Electrical Terminology – Boilers\": Rated steam pressure: The outlet steam pressure that must be maintained by a steam boiler when it operates continuously over a specified range of feedwater pressure and load conditions. Definition 2: According to clause 3.1.8 of GB/T 16507.1-2022 \"Water-tube boilers – Part 1: General rules\", rated pressure refers to the pressure of the working fluid at the boiler outlet that must be maintained during long-term, continuous operation within specified feedwater pressure and load conditions; it is also the rated operating pressure or rated outlet pressure indicated on the boiler’s nameplate. Why is it described this way: The core function of a pressure steam boiler is to generate steam. As a working fluid, the pressure and temperature of steam directly determine the quality of thermal energy it can carry. The higher the pressure, the higher the saturation temperature of steam, the higher its enthalpy, and the greater its work-producing capacity. Therefore, the most crucial indicator for describing the capabilities of such boilers is the pressure of steam they can generate. The provision in the catalog stating that “the rated steam pressure must be greater than or equal to 0.1 MPa (gauge pressure)” is intended to distinguish pressure-bearing steam boilers from ordinary atmospheric-pressure hot water boilers and steam generators, thereby bringing them under the scope of special equipment safety supervision. 2. Outlet water pressure — For pressure-rated hot water boilers: Definition: Outlet water pressure refers to the pressure value (gauge pressure) at the boiler’s outlet under normal operating conditions. Why is it described this way: In order to deliver hot water to users at high elevations or distant locations, a certain pressure must be maintained at the boiler outlet to overcome the resistance of the piping system. Secondly, to prevent vaporization, the hot water at high temperatures must be maintained under a certain pressure to avoid flash vaporization inside the boiler; otherwise, severe water hammer accidents can occur. Therefore, the catalog uses “discharge water pressure greater than or equal to 0.1 MPa (gauge pressure)” to define this type of equipment. Unlike steam boilers, where the focus is on “what pressure of steam is generated,” here the emphasis is on at what pressure water is supplied to the outside—pressure being a means to ensure circulation and safety. 3. Rated power – Applicable to organic heat carrier boilers and general standards. Definition: Rated power (thermal power) refers to the amount of heat generated by an organic heat carrier boiler per unit of time under rated operating conditions, measured in megawatts (MW). This is described in such a way because organic heat transfer fluid boilers (commonly known as heat transfer oil boilers) use heat transfer oil for heating; this oil remains in a liquid state throughout the system, without undergoing any phase changes. At this point, pressure alone cannot describe its capability—pressure is primarily used to overcome cyclic resistance. The key indicator for this type of boiler is how much heat it can output; therefore, its capacity is expressed in terms of rated power (thermal power). Furthermore, “a rated power of 0.1 MW or more” is the general regulatory threshold. Even for hot water boilers, if the outlet water pressure meets the standards but the power output is too low (less than 0.1 MW), they are not subject to regulation. This excludes devices such as small domestic heating boilers. II. Two different expressions regarding pressure vessels: In the definition of pressure vessels, we encounter the terms “maximum operating pressure” and “nominal operating pressure”. 1. Maximum operating pressure – for fixed-pressure vessels and mobile vessels. Definition: The maximum operating pressure refers to the highest pressure (gauge pressure) that may occur at the top of the vessel under normal operating conditions. Why is it described this way: Fixed-pressure vessels (such as reactors and storage tanks) are usually designed customized according to specific process conditions. When process parameters such as reaction temperature and material ratios change, the operating pressure also fluctuates accordingly. Therefore, regulation uses the maximum operating pressure as the defining parameter, emphasizing the upper pressure limit that the equipment may reach during actual operation. It is based on this logic that the catalog stipulates that pressure vessels with a maximum operating pressure of 0.1 MPa (gauge pressure) or higher are subject to regulation. For pressure pipelines, the “maximum operating pressure” is also used for description, as the pressure within the pipeline changes depending on the transportation conditions; therefore, the upper limit value during actual operation should be taken as the reference. 2. Nominal working pressure – For gas cylinders: Definition: The nominal working pressure refers to the nominal pressure level (gauge pressure) that a gas cylinder is permitted to reach at a reference temperature (usually 60°C or 65°C) when containing a specified medium. Why is it described this way: Gas cylinders are a special type of pressure vessel with the following characteristics: High mobility: During filling, transportation, and use, gas cylinders are exposed to various changes in ambient temperature. High standardization requirements: To facilitate uniform manufacturing, inspection, filling, and interchangeability, gas cylinders cannot be “custom-made” like fixed containers; instead, they must be produced in accordance with standardized pressure rating series. Common filling pressures include series such as 15 MPa, 20 MPa, and 30 MPa; this is what “nominal working pressure” refers to. Regulations require that cylinders with a nominal working pressure of 0.2 MPa (gauge pressure) or higher be placed under supervision, and this is based on exactly this concept of standardized design. III. Conclusion: Why are there these different formulations? From the above analysis, it can be seen that the reason why different pressure expressions are used in the catalog of special equipment is fundamentally due to the varying functions of these devices: steam boilers focus on steam pressure (the quality of the working medium), hot water boilers focus on outlet pressure (the driving force for circulation), while organic heat carrier boilers focus on thermal power (the heating capacity). Different operating conditions: In fixed pressure vessels and pressure pipelines, the pressure changes depending on the process; emphasis is placed on the \"maximum operating pressure\" as the upper limit for actual operation ； Gas cylinders need to be standardized and serialized, with emphasis on the \"nominal working pressure\" as the design parameter. Different regulatory requirements: It is necessary to clearly define the boundaries of regulation to ensure that only devices with specific technical characteristics (i.e., those that pose a certain level of danger or are of a certain size) are included within the scope of supervision for special equipment safety, thereby avoiding any ambiguity. This approach of meeting each party’s needs precisely reflects the scientific nature and targeted nature of regulatory formulation – by focusing on the most essential and core characteristic parameters of different types of equipment, regulation ensures that nothing is overlooked while also avoiding excessive oversight.