May I ask whether the hot water control valve, when used to regulate flow, can maintain a constant pressure difference before and after it?
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Could someone please help explain the use of pneumatic diaphragm control valves for hot water? Is it true that when adjusting the water volume, the pressure is also reduced simultaneously, thereby maintaining a constant pressure difference before and after? The existing project’s temperature control valve adjusts the valve opening based on the outlet temperature of the heated medium. Hot water is used to heat the medium to be heated through a heat exchanger. The specifications state that control valves are selected based on the pressure difference before and after the valve to determine the appropriate flow coefficient. Then, select a suitable control valve ranging from low flow rate to high flow rate. But it is clear to everyone that hot water also needs to return to the hot water circulation system; this means that the pressure of the water at the valve outlet cannot be too high, as this would exceed the design pressure of the downstream piping network, and it also cannot be too low, as otherwise the water cannot return to the circulation system. I'm confused. . For example: if the inlet pressure of the control valve is 50 barg and the outlet pressure is 40 barg, can the control valve reduce the pressure of hot water from 50 barg to 40 barg under normal operating conditions? If the control valve adjusts its opening under normal operating conditions, what will be the pressure at the valve outlet – 40 barg, higher than 40 barg, or lower than 40 barg? What is the range of fluctuations in the valve outlet pressure? Is the back pressure of the control valve ensured by the downstream pipeline network, or by the control valve itself? Does it serve a similar function to a current-limiting control board?
Push on your own. :hug:
This post was last edited by ylb913 on 2016-12-12 at 19:27. It mainly deals with the issue of diameter selection, as well as the relationship between flow rate and pressure drop. The pressure drop caused by control valves is not the main factor, but it is quite common for a control valve to result in a significant reduction in pressure.
There is a pressure difference before and after the control valve, but this difference is controllable and is far not as large as you claim.
Thank you for your reply. The pipe diameters before and after the valve are chosen to ensure reasonable flow rates; however, the pressures upstream and downstream of the valve are not at the same level. Therefore, I would like to confirm whether it is possible to reduce pressure while using the temperature control valve for temperature regulation, and whether there are any relevant standards to support this.
However, the selection of control valves is indeed related to the pressure difference before and after; to find out whether a temperature control valve can actually reduce high pressure to low pressure.
You can explain it more clearly; for example, steam is used to heat the materials inside the reaction vessel. Temperature control generally involves using control valves to adjust the opening degree of the steam supply valve (in other words, the flow rate of the heating steam), thereby achieving temperature control over the materials within the reaction vessel. In this control mode, the pressure difference before and after the control valve is generally not very large. As another example, in order to maintain stable heating of the reaction vessel, or due to limitations imposed by the pressure in the vessel’s jacket, we implement pressure control on the heating steam, and control valves can also be used for this purpose. The pressure before the valve can be 0.6 MPa, or even higher. However, the pressure behind the valve can be adjusted to 0.2 MPa or lower as needed. I’m not sure if what I say can help solve your problem.
Thank you for your reply; it didn’t solve the problem. Control valves are all adjusted using analog signals. But with this control valve, I don’t use a pressure regulator to stabilize the pressure; instead, I use a temperature transmitter to regulate the flow rate. However, when regulating the flow rate, can it ensure a pressure difference upstream and downstream of the valve? For example, with 3 t/h of water and a valve opening of 20%, the pressure upstream of the valve is 40 barg while it is 20 barg downstream; for 10 t/h of water and a valve opening of 70%, the pressure upstream is still 40 barg and the pressure downstream is 20 barg; and for 12 t/h of water with a valve opening of 90%, the pressure upstream remains 40 barg and the pressure downstream is 20 barg. In other words, the control valve is able to regulate flow rates while also reducing pressure, thereby ensuring the desired pressure at the valve outlet. I am particularly concerned about the outlet pressure of the valve – how will this outlet pressure fluctuate? I selected the valve positioner based on a 1% setting, but there are also quite a number of factors involved in this closed-loop control system.
Everyone, please provide technical support :)
The pressure difference upstream and downstream of the control valve, as you mentioned, can be adjusted during the design and selection of the control valve. If a large pressure difference is desired, a valve with a small diameter and high resistance can be selected. The same is true in reverse.