Questions about system pressure drop

On the one hand, I once observed an example. When the pressure upstream of the steam pressure reducing valve is stable, no matter how much the flow rate changes (within the normal adjustment range), the pressure downstream of the valve is basically a fixed value, that is, the pressure drop is constant. On the other hand, there is the pressure regulating circuit that everyone is familiar with. The pressure sent from the upstream is unstable. This pressure regulating circuit changes the pressure drop of the regulating valve to ensure that the downstream pressure meets the user's requirements. Third, I learned from an instrument engineer that various calculations of the regulating valve are based on a constant pressure. Finally, when designing, we usually give the instrument professional the allowable pressure drop of the regulating valve at the beginning. After ordering, usually the supplier information fed back by the instrument professional is basically consistent with the set value. So now I am a little confused. Can it be understood that within the normal regulating range, the temperature, flow regulating loop and the upstream pressure of the regulating loop are stable. The pressure drop of the regulating valve is a fixed value; when the upstream pressure of the pressure regulating circuit changes, the pressure drop of the regulating valve is a variable value. Please give me more opinions! For linear control valves, flow rate = coefficient × valve opening × square root of pressure difference. The coefficient is only related to the valve and is a fixed value. Generally, control valves use equal percentage valves with better control performance. The essence is the same. You can refer to the valve manual. 1. When the pressure upstream of the steam pressure reducing valve is stable, no matter how much the flow rate changes (within the normal adjustment range), the pressure downstream of the valve is basically a fixed value, that is, the pressure drop is constant. This is easy to explain, because for the system, pressure is a relatively important control parameter. To ensure normal production, the pressure must be kept stable. When the flow needs to be adjusted, it is adjusted by changing the valve opening, so that the pressure difference remains basically unchanged. There can be many reasons why the downstream pressure is kept constant, and in many cases this can be achieved by a pressure controller. 2. The pressure sent from the upstream is unstable. This pressure regulating circuit changes the pressure drop of the regulating valve to ensure that the downstream pressure meets the user's requirements. /Since the purpose of control is different, what needs to be controlled at this time is the pressure, not the flow, so you change the above formula, pressure difference = (flow/coefficient/valve opening) squared. When the upstream pressure changes, it is necessary to ensure the downstream pressure. At this time, the pressure difference needs to be adjusted. By adjusting the opening of the valve, the flow rate is changed, thereby achieving the effect of regulating the pressure. 3 Various calculations of the regulating valve are based on a constant pressure situation. / Before designing, a rough estimate of the pressure of the entire system will be made. From the required pressure of the system, the approximate pressure drop of each throttling component, and the pipeline pressure drop, the approximate load of the compressor (pump) can be obtained. Therefore, the pressure drop of the control valve (also the throttling element) is generally evaluated based on experience. Generally, the pressure drop obtained under normal flow conditions and the selected appropriate valve conditions (usually a valve with an opening of about 50% that can meet the flow rate) is considered. So this value is a fixed value. The pressure drop of the control valve is generally about 20% of the pipeline pressure drop. Too large a pressure drop will not save energy, and too small a pressure drop cannot control the flow rate well (you can think about this clearly). Typing is hard. If you are right, please give me a reward.

Account for at least 30% of the total system pressure drop. If the pressure drop of the regulating valve is too low, the regulating quality will be poor.

The friend who asked the question is probably a designer. Generally, it is more appropriate for the resistance drop of the regulating valve to account for 30 to 50% of the variable resistance of the system. A general system can simply be considered as 30 to 50% of the friction resistance. The larger the proportion, the more sensitive the impact of valve adjustment on the system, but the greater the power consumed, so the designer needs to determine an appropriate value within this range.

About 30%. When explaining the function of this, we generally understand it according to the sliding rheostat in physics. That is, the regulating valve is equivalent to a sliding rheostat. To have a good regulating effect, a certain resistance change value is required.

About 30%, the general system can simply be considered as 30~50% of the friction resistance.

It seems that I can take part in a foreign book called Design of Practical Devices for Chemical Engineering, which includes calculation of pressure drop of control valves. However, I don’t quite understand this book. There is a Chinese version.