Parameter issues of control valves
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First, it should be noted that in the design of my experimental system, the process pipes before and after the control valve are DN150 pipes; therefore, a control valve with 【equal percentage characteristic】 is required. However, I have some doubts regarding the rated Kv value. I understand the definition of the rated Kv value; one of the conditions is that the valve is fully open, so it makes sense that the rated Kv value is related to the valve’s diameter. But are the Kv values the same for valves with the same diameter? It has nothing to do with the internal design of the valve? I contacted several domestic manufacturers, and all of them specified a rated Kv value of 400 for a DN150 diameter ； Some textbooks state that the rated Kv value for control valves with a DN150 diameter is calculated as 360 ； But in Aspen Plus, the Globe valves in the valve library have a Cv of 390 (Kv=337) or a Cv of 360 (Kv=311). So the first question: Can a valve with a DN150 diameter have a larger Kv value selected? ? ? Like 500, 600, or even 1000? Because the control valve with a Kv value of 400 results in too high pressure loss for my system. I also analyzed in Aspen Plus the impact of different rated Kv values on the system, but when purchasing the valve, I was told that its Kv value would be 400. So is the rated Kv value indeed determined by the valve’s diameter? Second question: It is stated on Zhihu (https://zhuanlan.zhihu.com/p/64576931) that the diameter of the valve seat does not have to match the size of the pipeline. The pipes before and after the valve are DN150, but I have chosen a valve with a valve seat diameter of 200 (with a rated Kv value of 630). Is this in compliance with the technical specifications? I hope an expert can help me understand this. Thank you
Whether it is a cage-type or single-seat type, the KV value of DN150 control valves generally does not reach 400. The KV value of a control valve is related to the flow channel design of the valve body. As for the selection of the valve seat, it can be chosen differently for control valves, but only the size can be reduced; it cannot be increased. You can choose a DN200 valve with elbow connections to meet the process flow requirements.
Change the diameter and then change it back; the curve handles this
1. The Kv value of ball valves can exceed 2000； 2. It is acceptable for the flow cross-section of the valve core to be larger than that of the pipeline; however, in such cases the flow coefficient of the valve depends on its internal flow channel structure. Usually, only valves of the gate type (where the closing element moves along the center line of the valve seat) benefit significantly from having a valve core flow cross-section larger than that of the pipeline, as this helps to increase the Kv value ； 3. When the Kv value does not meet the requirements, it is usually possible to resolve this by using a different valve design, or by increasing the diameter of the valve and its installation location.
Hello, when you say that \"the valve seat is optional and can only be made smaller, not larger\", do you mean that I can choose a valve core with a size of 200mm and pair it with a valve seat of 150mm? Is such a combination available from manufacturers? Because what I’ve asked so far is that the spool must be smaller than the valve seat, not larger; I have too little experience in engineering work, so I’m not sure if such a combination is possible. If it is possible, could you recommend a manufacturer? Thank you
Hello, dear engineer. Due to the requirements of manufacturing processes and various standards, there are limits set for things such as the end face distance and wall thickness of valves with a diameter of DN150, as well as the overall size of their flow channels. Taking design requirements and costs into consideration, the manufacturer states that \"the valve seat can be optional, but it can only be made smaller, not larger.\" Abbreviated as valve necking fabrication. Because when the pipeline dimensions remain unchanged but the process flow rate and pressures before and after change, it is often necessary to use valve reducers.
This post was last edited by lushiru on 2022-4-5 11:02
For DN150, the inner diameter is taken as 152. KV up to 1800 is roughly the limit; I still can’t believe the value of 5100. I have simulated the flow coefficients for many ball valves, and for DN150 they never exceed 1850. Moreover, software simulations are based on ideal conditions – in reality, there is roughness at the interface between the medium and the pipe walls. Only by treating the ball valve as a simple pipe fitting, without considering flow velocity or friction during closing, can a value around 5000 be obtained.
Teacher, may I ask what software you use to simulate the flow coefficient of valves? Ansys or Flowsimulation?