What is the "pressure rating" of pipeline fittings?
21-11-12
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In response to the common confusion in the industry, we have compiled identification methods and selection criteria for pressure levels of pipeline fittings in all scenarios, helping practitioners avoid common pitfalls.
According to the latest statistics from the Domestic Fluid Engineering Association, in the first three quarters of this year, the number of leakage and burst accidents caused by incorrect matching of pressure levels of pipeline fittings in the fields of municipal pipeline network renovation, industrial chemical transportation, and HVAC home decoration in China increased by 17% year-on-year. Many new procurement personnel and on-site installation personnel misunderstood the meaning of pressure signs, resulting in project rework costs exceeding hundreds of thousands of yuan, and serious safety accidents such as leakage of flammable and explosive media. In response to the common confusion in the industry, we have compiled identification methods and selection criteria for pressure levels of pipeline fittings in all scenarios, helping practitioners avoid common pitfalls.
Firstly, clarify the core understanding: stress level is not "explosive pressure"
The first misconception among many practitioners is to equate the nominal pressure rating of the fittings with the maximum burst pressure that the fittings can withstand. In fact, the official definition of "pressure level" refers to the maximum working pressure that the corresponding material is allowed to operate stably for a long time under a specified temperature environment. The value itself is bound to the temperature conditions and is not fixed. For example, the same carbon steel PN16 elbow can withstand a water pressure of 1.6MPa at room temperature of 20 ℃, but the allowable operating pressure under superheated steam conditions at 200 ℃ will drop below 1.3MPa. If the nominal 1.6MPa is directly selected, there is a high probability of deformation and leakage.
Identification method for pressure level identification of pipeline fittings in different systems
At present, the pipeline fittings in circulation in China are mainly divided into three identification systems: national standard, American standard, and Japanese standard. The identification logic is completely different and must not be confused and converted
The most common identification in the national standard system (commonly used in most civil and industrial projects in China) is "PN+number", such as PN10, PN16, PN25, with the unit of Bar (1Bar=1kg force=0.1MPa). PN16 represents the rated working pressure of 16kg/1.6MPa at room temperature. PPR and PERT in the field of plastic pipes
The fittings will not be directly labeled with PN, but will be labeled with S series: S5 corresponds to a rated pressure of 1.25MPa, S4 corresponds to 1.6MPa, S3.2 corresponds to 2.0MPa. Many home decoration purchases only monitor the diameter of 20/25mm without considering the S grade. Choosing S5 cold water fittings incorrectly for the hot water circuit can lead to cracking and water leakage after 2-3 years of use.
American standard system (applicable to export projects and high-end petrochemical projects)
Newcomers mistakenly believe that Class150 is a pressure of 150 kilograms, and directly choose ultra-high pressure accessories, causing a lot of cost waste. three Japanese standard system (applicable to Japanese and Korean foreign investment projects and ship pipelines)
Identified as "number+K", for example 10K, 20K, K represents kilogram force, 10K is the rated pressure at room temperature of 1.0MPa. Although the value is close to the national standard PN10, the flange screw hole distance and sealing surface thickness dimensions of the two are incompatible and cannot be directly mixed and docked. In addition, civilian plumbing ball valves and fittings will also be marked with "WOG+number", specifically referring to the rated pressure under three non corrosive working conditions of water, oil, and gas, and should not be confused with the parameters marked with "WP (working pressure)".
The top 3 pressure level selection pitfalls in the industry
Only looking at the identification numbers without temperature correction: The nominal benchmark for all pressure levels is room temperature. For steam, hot oil, and high-temperature process pipelines with medium temperatures exceeding 100 ℃, the corresponding material's temperature and pressure correction table must be checked first to convert the actual allowable pressure under high temperature, in order to ensure safe operation.
Cross system component hard matching: Many projects connect the national standard PN16 flange and the American standard Class150 flange in order to gather goods. The two have similar nominal pressure bearing but completely different sealing surface tolerances, which can easily lead to medium leakage after pressure increase.
Treating blasting pressure as working pressure: Many small manufacturers promote their own pipe fittings with a "blasting pressure of 50 kilograms", while many users directly select based on a working pressure of 25 kilograms, without leaving a safety margin of more than 1.5 times. After long-term aging of the fittings, it is easy to cause pipe bursting.
Industry reminder: The newly implemented "General Technical Requirements for Pressure Pipeline Components" this year clearly stipulates that all compliant pipeline fittings must have permanent pressure rating markings engraved on the body. Products without clear markings are not allowed to enter the site for use. When selecting, the principle of "low not high" must be strictly followed: the pressure rating of the entire pipeline's pipes, valves, and joints should be unified, and the component with the lowest pressure should be used as the system rating standard, which will not cause cost waste and can avoid pressure accidents from the source.
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