The diameter variation issue of control valves
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Gentlemen who work with instruments, how do we determine whether a control valve has a reduced diameter? Let’s talk about a pipe with a diameter of 65, using water as the medium. The pressure before the valve is 0.3, the flow rate is 7.5 m3/h, the dynamic viscosity is 1, and the operating density is 1000 kg/m3. How can we determine whether its diameter changes or not? Experts, please help: lol
Find a manufacturer to do the calculations for you; send them the relevant data. It’s mainly related to the flow rate of the valve – the CV value varies depending on the different valve models offered by the manufacturer, with the size of the valve usually resulting in a corresponding change in that CV value. Many diameter changes are due to oversized pipe sizing!
It is also necessary to know the pressure behind the valve; only after calculation can it be determined whether the diameter needs to be changed. Generally, the design institute makes an initial estimate, and the manufacturer then carries out the final calculations to determine this. The greater the pressure difference before and after the valve, the more the diameter changes.
Isn’t it usually set to 0 on the side after the valve? I’m not quite sure either
With a pipe diameter of 65 and a flow rate of 7.5, the flow velocity is very low, which indicates that the pipe diameter chosen is too large. The KV value of the control valve is much higher than what’s required for such a flow rate; therefore, it’s necessary to use a pipe with a smaller diameter. Choosing the right control valve will help improve control accuracy and reduce the costs associated with purchasing such valves.
There are calculations in the Altar; you need to find them by yourself. I’ve just started to understand a bit too.
Pressure after the valve = Operating pressure of the receiving equipment + Pressure drop in the pipeline from the control valve to the receiving equipment https://bbs.hcbbs.com/thread-1764891-1-1.html https://bbs.hcbbs.com/thread-1764890-1-1.html
Provide the pressure downstream of the valve; based on the flow rate, the pressure difference before and after the valve can be used to calculate Cv. Once Cv is known, it’s possible to estimate what size valve should be used. Control valves usually specify three operating conditions: minimum, normal, and maximum. If, as you say, the operating conditions are such that the pressure downstream of the valve is 0, then what’s the need for a control valve? It’s just like a faucet – it’s sufficient to have something that can be turned on and off to regulate the flow rate. Since you don’t care about what the pressure downstream of the valve becomes, you can simply choose a on/off valve with the appropriate flow capacity.
Without a pressure drop, it is impossible to calculate the required diameter of the valve. A simple estimation method: for liquid non-obstructed flow control valves, if the pipe flow velocity is less than 1 M/S and the pressure drop across the valve is greater than 1 BAR, the valve can be set to one lower setting, with an opening degree of around 70%. By the same logic, if the flow rate is 1.5 M/S and the required pressure drop is greater than 1.5 x 1.5 = 2.3 BAR, it is also possible to downsize one stage. Your flow rate is less than 0.7
How to convert m3/h to m/s, experts?