Cold-state operating pressure of the valve

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The cold operating pressure of the valve. I. Overview of valves 1.1 The role of valves in industrial piping systems Valves play a crucial role in industrial piping systems. It can open or close the flow of the medium, determining whether the fluid can flow smoothly through the pipeline. When it is necessary to stop the flow of fluid, the valve closes, creating a solid barrier to prevent the medium from continuing to flow ； When it is necessary to convey a medium, the valve opens to provide a path for the fluid. Valves can also regulate pressure and flow rate; by changing the size of the flow channels, for example, they adjust the pressure of the fluid to meet the requirements of the pipeline system, ensuring its stable and efficient operation. They are essential components in industrial production. 1.2 Emphasizing the importance of safe valve operation: The safe operation of valves is of great significance for industrial production, equipment protection, and personnel safety. In industrial production, if valves fail, such as by leaking or failing to open and close properly, it can lead to production disruptions and significant economic losses. In severe cases, it can also lead to safety accidents, such as explosions resulting from the leakage of flammable and explosive substances when exposed to fire, or harm to human health due to the leakage of toxic and harmful substances. Moreover, the safe operation of valves can effectively protect the equipment, preventing damage to it caused by factors such as abnormal medium pressure and temperature, thereby ensuring the long-term stable operation of the equipment and maintaining the normal production order of the enterprise. II. Explanation of the Concept of Cold-Condition Operating Pressure 2.1 Detailed Definition of Cold-Condition Operating Pressure The cold-condition operating pressure of a valve, as the name implies, is the key pressure value that the valve can withstand at normal temperatures. This temperature range is typically set between -29°C and 38°C, covering various operating conditions such as valve installation, commissioning, hydrotesting, as well as the system being shut down or operating at normal temperatures. At this time, the valve housing, as the core component that bears pressure, must ensure its safety under the highest non-impact operating pressures, without experiencing rupture or permanent deformation. This parameter is crucial for valves; it directly determines whether the valve can operate stably in normal temperature conditions. It represents an important aspect of the valve’s performance and serves as a key factor in selecting valves as well as ensuring the safe operation of systems. 2.2 Clarifying the role of cold operating pressure among valve performance parameters Among the numerous performance parameters of valves, cold operating pressure holds a central position. It is a key factor in selecting valves; only when the valve’s operating pressure at low temperatures is greater than or equal to the highest operating pressure that may occur in the pipeline system at normal temperatures, with an appropriate safety margin, can it ensure safe and reliable operation of the valve within the pipeline system. From the perspective of safe operation, the cold-state operating pressure is crucial as it determines whether the valve can effectively withstand the pressure of the medium under normal temperature conditions. It prevents the valve from being damaged due to excessive pressure, which could lead to production disruptions, equipment damage, or even safety accidents. Therefore, it serves as an important factor in ensuring the safe and stable operation of valves and the entire piping system, and is of great significance for the smooth progress of industrial production. 2.3 Comparison with hot operating pressure and rated operating pressure: There are significant differences between the cold operating pressure, the hot operating pressure, and the rated operating pressure. The hot-service pressure refers to the pressure-bearing capacity of a valve when it is operating at high temperatures; since the strength of materials decreases as temperature rises, the hot-service pressure for the same valve is usually lower than its cold-service pressure. Rated operating pressure is a more general concept, referring to the pressure that a valve can safely withstand at a specified temperature; the cold-state operating pressure is actually a special case of the rated operating pressure at normal temperatures. For example, a carbon steel valve of Class 300 has a CWP of around 50 bar (725 psi) at room temperature, but as the temperature of the fluid increases, its maximum allowable operating pressure decreases accordingly. III. Factors affecting the cold-state working pressure of valves
3.1 Influence of material properties on cold-state working pressure
The material properties of valves have a decisive influence on their cold-state working pressure. Material strength is one of the key factors; materials with high strength can withstand greater pressures, enabling valves to operate at higher pressures in cold conditions. For example, high-strength steel valves can handle higher pressures under the same conditions compared to ordinary carbon steel valves. The toughness of the material is also important to consider, as it determines the valve’s ability to resist deformation and fracture under pressure. Materials with high toughness can maintain stability under complex operating conditions such as pressure fluctuations, thereby preventing damage to the valve due to sudden changes in pressure. Furthermore, the corrosion resistance and fatigue resistance of different materials, among other properties, also affect the pressure-bearing capacity of valves from various perspectives, thereby influencing their operating pressure at low temperatures. 3.2 Effect of temperature changes on the cold-service pressure As the temperature rises, the strength of the valve material generally decreases, resulting in a lower cold-service pressure. Because the lattice structure of the material changes at high temperatures, the bonding forces between atoms weaken, and its ability to resist external forces decreases. For example, in the case of carbon steel valves, when the temperature rises to 200°C, the maximum allowable operating pressure drops from 50 bar (725 psi) at room temperature to around 45 bar. Conversely, as the temperature decreases, the brittleness of the material may increase; although its strength improves, it is more prone to brittle fracture under external forces such as impacts, posing a threat to the safe operation of the valve. Therefore, under different temperature conditions, the cold-state operating pressure of the valve must be adjusted and considered accordingly based on changes in the material properties. 3.3 Influence of design standards on cold operating pressure: Valve design standards establish strict requirements and provide guarantees for the cold operating pressure. Standards such as SHT 3501-2021 specify the maximum non-shock operating pressure of valves at normal temperatures, ensuring that during their design and manufacturing, the strength of their housings meets this pressure requirement. The standards also specify the selection of valve materials, manufacturing processes, testing methods, etc., to ensure from the outset that the valve’s operating pressure at low temperatures meets the requirements for safe operation. Through inspections such as hydrostatic testing of the valves, and by referring to relevant standards, it is determined whether the valves can safely withstand the maximum operating pressure at normal temperatures, thereby ensuring their reliable operation within the entire piping system and preventing safety accidents caused by design or manufacturing defects. IV. Selection of Cold-Operating Pressure in Valve Selection 4.1 Determining the Maximum Operating Pressure of the Pipeline System at Normal Temperature To determine the maximum operating pressure of the pipeline system at normal temperature, various factors need to be taken into consideration. It is necessary to analyze the possible pressure changes that may occur during flow, based on the properties of the medium transported through the pipeline, such as flow rate, density, viscosity, etc. Refer to the design standards for pipelines, and take into account factors such as transient conditions during system operation and instrument measurement tolerances, to provide an appropriate margin. For water systems, the formula can be used to calculate that the highest pressure point at rest is the hydrostatic pressure; at the moment of operation it is the sum of the hydrostatic pressure and the total pressure of the pump; and during normal operation it is the sum of the hydrostatic pressure and the pump’s hydrostatic pressure, minus the dynamic pressure. Through these methods, it is possible to accurately determine the maximum operating pressure that the piping system may experience at normal temperatures, providing a crucial basis for valve selection. 4.2 Application of the safety factor in the selection of cold-state operating pressure When selecting valves, it is crucial to apply the safety factor appropriately when determining the cold-state operating pressure. The selection of the safety factor must take into account factors such as variations in the properties of the valve material, manufacturing errors, and medium corrosion. Generally, the cold-operating pressure of the valve should be selected to be greater than the highest operating pressure of the pipeline system at normal temperature, multiplied by a safety factor. The safety factor is usually between 1.5 and 3, with the specific value determined by factors such as the importance of the valve and the severity of the operating environment. By applying safety factors appropriately, it is possible to ensure that the valves operate safely and reliably under various complex operating conditions, preventing damage to the valves due to pressure fluctuations and other factors, thereby maintaining the stability and safety of the entire pipeline system. 4.3 Valves of different pressure ratings are suitable for different operating conditions; the applicable conditions vary for valves of various pressure ratings. Low-pressure valves (PN≤1.6MPa), such as those with PN0.25MPa, are commonly used in systems with low pressure and minimal pressure fluctuations, such as urban water supply and agricultural irrigation. Medium-pressure valves (PN2.5-6.4MPa) are commonly used in petrochemical production for pipelines transporting general-purpose media. High-pressure valves (PN10.0–80.0 MPa) are suitable for pipeline systems in industries such as petroleum and chemicals, where high-temperature, high-pressure media or toxic, flammable, and explosive media need to be transported. Ultra-high pressure valves (PN≥100MPa) are commonly used in special processing processes such as synthetic fibers, nuclear energy, and high-pressure polyethylene. V. Requirements regarding the cold operating pressure in the SHT 3501 – 2021 standard 5.1 Requirements for valve materials The SHT 3501 – 2021 standard sets strict requirements for the materials used in valves in terms of cold operating pressure. The materials are required to have sufficient strength and toughness to withstand the highest non-impact operating pressures at room temperature. The material must also have good corrosion resistance and fatigue resistance, as well as the ability to function in various media and environments. For different operating conditions, such as valves used to transport corrosive media, the materials must exhibit excellent corrosion resistance ； Under complex operating conditions such as high pressure and high temperature, the material is required to have higher strength and stability to ensure the safe and reliable operation of the valve under cold conditions. 5.2 Test method for cold operating pressure in standards: Standard SHT 3501 - 2021 specifies that for the cold operating pressure test, the valve must first be opened, water must be filled in and any air removed, after which the pressure must be increased slowly using a pressure testing pump until the specified test pressure is reached. Maintain the pressure at the test pressure for a certain period of time to observe whether there are any abnormal phenomena such as leakage or deformation in the valve. If there are no abnormalities, it is considered that the valve’s cold operating pressure meets the standard requirements. During testing, it is necessary to ensure that the temperature of the test medium remains within normal ranges, and that the pressure gauge has the required accuracy, in order to accurately measure the pressure-bearing capacity of the valve in a cold state. 5.3 Methods for verifying that the cold operating pressure meets the standards during construction The main methods for verifying that the valve’s cold operating pressure complies with the standards during construction are hydrostatic testing and visual inspection. During the hydrostatic test, before the valve is installed, testing is carried out at the test pressure and holding time specified by the standards to check for any leaks in the valve. Visual inspection involves checking for defects such as cracks and deformation in the valve, whether the flange sealing surfaces are flat, and whether the bolts are loose. For critical valves, non-destructive testing techniques such as ultrasonic testing and magnetic particle testing can also be employed to further verify whether their operating pressure at low temperatures meets the standards.