How to accurately set the pressure difference conditions for control valves/regulating valves
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We know that control valves/regulating valves possess ideal flow characteristics and operating flow characteristics, which mainly reflect the relationship between the medium flow rate and the valve stem displacement. Common types of control valves include linear, equal percentage, and quick-open types. In practical operation, the pressure difference before and after the valve is constantly changing, as control valves are always used in series or parallel with process equipment and pipelines. The pressure difference before and after the control valve changes as the resistance of the pipeline system varies, which results in the flow characteristics of the actual control valve deviating from the ideal characteristics. People in the field of instrument and automatic control often say that the pressure drop across the control valve should be as high as possible, as this makes it easier to select the appropriate valve ； However, in some cases, due to process requirements regarding pressure drop, it is necessary to determine an economic balance when increasing the pressure drop of the control valve. I’d like to discuss with fellow marine enthusiasts: when it comes to control valves, what is the typical value for the pressure division ratio of these valves, or what’s also known as the valve resistance ratio?
The greater the pressure difference, the smaller the valve core of the control valve can be, allowing for a smaller size of the control valve to be chosen. However, the pressures before and after the control valve are determined by the process; the pressures of the equipment before and after the control valve determine the pressures on the primary and secondary sides of the control valve. Rather than having some voltage division ratio.
This needs to be considered comprehensively; once the overall pressure drop across the pipeline is determined, the allowable pressure drop for the valves and the pipeline is also fixed. A larger pressure difference is preferable, as it allows the valves to be more sensitive, but the cost remains the same
In some operating conditions, the pressure drop must be determined based on the pressures at the upstream and downstream ends of the process as well as the backpressure, but this approach is not applicable in all situations. The deviation between the actual operating performance of a control valve and its ideal operating curve depends mainly on the ratio of the pressure drop caused by the control valve in that pipeline to the total pressure drop in the entire pipeline. This ratio is also a crucial parameter for determining the control valve (often referred to as the valve resistance ratio), which is the ratio of the valve’s resistance to the pipeline’s resistance. Generally, this ratio S should be greater than 0.3. However, in pipeline systems used for pressure transmission, the principle that \"the greater the pressure drop, the better\" does not apply, as this would result in wasted energy from the power equipment at the upstream end, increasing energy consumption. In such cases, the selection of the valve’s pressure drop needs to be considered carefully, with the aim of determining an appropriate value for S. As for what this value is, there are different opinions
What is the typical value that can be taken for the valve lift-to-resistance ratio?
Every control valve has an optimal range of adjustment, which is generally between 30% and 80%, representing the best condition