Questions about control valves
Thread Content
Why can a control valve regulate flow? Does this flow refer to velocity (m/s) or mass flow rate (kg/s)? For valves simulated using Aspen Plus, it is generally necessary to specify the mass flow rate of the inlet stream and the valve opening degree; these are values set manually. Using these values, parameters such as pressure and temperature downstream of the valve are calculated. Isn’t what a control valve regulates also these state parameters? Why is it said that flow rate is regulated using control valves? What does this statement mean? I hope an expert can help me understand this. Thank you
Flow regulation and pressure regulation: whether they are related to the specified reference point, for reference.
Taking liquids as an example, if you want to increase the flow rate in a pipeline. 1. You can increase the system differential pressure between the starting point and the ending point. 2. If it cannot be increased, then the additional pressure drop caused by the pipes/fittings must be compensated for, and that is done by the control valve. Opening the control valve increases the flow area, which is equivalent to reducing the local resistance coefficient. When the two pressure drops “balance out” at a certain flow rate, that is the flow rate you need. Since the control valve can continuously change the resistance coefficient, it is possible to achieve the pipeline pressure drop you need (although there is a certain discrepancy between the actual value of the pipeline pressure drop and the theoretically calculated value). 3. In fact, the valve flow rate is calculated based on the actual volume, with units that can be KG/S or M3/S. 4. Additionally, the pressure behind the valve is calculated based on the back pressure. Generally, the final pressure remains relatively stable; changes in pipeline resistance do not affect this final pressure. However, an increase in the final pressure will necessarily force the valve to open wider in order to compensate for the pipeline resistance. 5. If you perform dynamic simulation, after selecting the diameter of the control valve (its rated CV value), you can continuously change the opening degree of the valve and observe the changes in the pressure distribution within that pipeline.
There are theoretical formulas available, such as the Fanning formula, which states that, under conditions of constant pressure drops upstream and downstream, flow rate is proportional to the 0.75 power of the cross-sectional area through which the flow passes. Therefore, the flow rate can be changed by altering the cross-sectional area of flow, but there is no linear relationship between them; instead, it is a 0.75-power relationship.
What is adjusted is the flow rate, which can be in linear or percentage terms.