Old K says every week – talking about things in the petrochemical design industry. Post 51: Selection of control valve diameter

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Old K says every week – Talks about things in the petrochemical design industry, Post 51: Selection of the diameter of control valves. First and foremost, the control valve must be able to handle the maximum flow rate required by the process, with a certain margin of safety. Depending on the variations in the load during operation, at least 15% is generally taken into account. Secondly, depending on the flow characteristics of the selected control valve, the valve is set to operate at an appropriate opening degree at normal flow rates, at which point its operating characteristics are optimal. But in the end, only one valve is selected; it is not practical to require that the valve maintain an appropriate opening degree at minimum/normal/maximum flow rates. When a large range of flow rates needs to be controlled, cascade control is the best option; for flow control valves used for relief, it is sufficient to consider the maximum flow rate that can pass through, without needing to take into account specific opening requirements. Therefore, when selecting the diameter, it is necessary to understand the process characteristics; it is ideal to specify the operating parameters and requirements at minimum/normal/maximum flow rates. It is worth noting that in some pressure control circuits, the pressure drop across the valve differs significantly between normal flow rates and maximum flow rates, sometimes by more than a factor of 2. If it is assumed erroneously that the pressure drop remains constant, the valve selected might be too small. From a static perspective, a control valve can function adequately as long as it can handle the maximum flow rate of the process; the larger the valve diameter, the more reliable it is. However, valves with excessively large diameters have a smaller actual opening degree, resulting in poor dynamic characteristics and inferior system regulation quality. The flow rate that can pass through a control valve is determined by the pipeline resistance distribution. As the flow rate increases, the pressure loss in the piping also rises sharply, resulting in a decrease in the pressure difference allocated to the valve. Therefore, when the valve is fully open, the actual flow rate through it differs significantly from the theoretical value. Tip: You can enter “Lao K’s Weekly Thoughts – Talks about things in the petrochemical design industry” into the website’s search engine to view previous posts