【Haichuan Chemical Valve Knowledge】Pressure difference and total pressure difference of valves: stop confusing them when specifying technical requirements!

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Operating pressure difference: The actual resistance that is constantly changing. The working pressure difference refers to the pressure difference measured before and after the valve, and it depends on the valve opening degree, fluid flow rate, and physical properties of the medium. When the valve is closed, the flow channel contracts, resistance increases, and the operating pressure difference grows larger ； When the valve is fully open, the resistance is minimal, and the operating pressure difference is reduced to the lowest level. Simply put, it is the degree to which the valve restricts the flow of the medium in its current state. The adjustment characteristics of the control system and the calculation of the flow coefficient both depend on this dynamically changing operating pressure difference. Total pressure difference: The “base value” when the valve is fully open. The total pressure drop is a specific value of the operating pressure drop—it refers only to the pressure loss measured at the design flow rate when the valve is opened to 100% position. It represents the “minimum resistance” caused by the structure of the valve itself. The lower the pressure drop when fully open, the smoother the valve’s flow path design. But note that the total pressure difference is not equal to the maximum pressure difference the valve can withstand. The maximum allowable pressure difference is usually determined by the sealing material, actuator thrust, and cavitation resistance, and can be significantly higher than the total pressure difference. Why is it so important to distinguish between them? Because using it in the wrong place will cause problems. For example, when selecting a control valve, some people use the total pressure difference to calculate the flow coefficient; as a result, they choose a valve that is too large. After it is put into use, the valve keeps oscillating at an opening degree of less than 20%, resulting in extremely poor control quality. Conversely, if the energy consumption at full open is measured using the working pressure difference, people may panic and assume that the system resistance is too high, leading to the unnecessary use of larger pipes and resulting in waste. In gas or steam systems, when the operating pressure difference exceeds the critical pressure ratio, \"plugged flow\" occurs – the pressure difference keeps increasing while the flow rate no longer rises. At this point, if only the total pressure difference (which is a very small value) is considered, the actual maximum flow rate may be underestimated, resulting in insufficient sizing of the safety valve or discharge pipeline. For liquid systems, it is important to note that an excessively high operating pressure difference can cause cavitation; since the total pressure difference is usually below the threshold at which cavitation begins, it cannot be used as a reference for protection. When specifying technical requirements, it is necessary to separately indicate the \"maximum shut-off pressure difference\", the \"pressure difference for normal flow operation\", and the \"full-open pressure difference\"; do not use a general term such as \"pressure difference requirement\"” ； When selecting a control valve, use the operating pressure difference corresponding to an opening degree of 60%–80%; do not use the full pressure difference for calculation ； The on-off valve and bypass valve are primarily tested for the full pressure difference in the fully open position to ensure that the flow capacity is sufficient. Remember this in one sentence: the operating pressure difference is a process variable, while the total pressure difference is an inherent characteristic of the valve.