Can percent characteristic be selected for all control valves? Then what is the purpose of linear valves?

The flow characteristic of a control valve is the relationship between the relative flow rate of the medium passing through the valve and its opening degree, under the condition that the pressure difference across the valve remains constant. The flow characteristics of control valves include linear, equal percentage, and parabolic characteristics. The meanings of the three flow characteristics are as follows: (1) Equal percentage characteristic (logarithmic) – fast. For the equal percentage characteristic, the relative stroke and relative flow do not follow a linear relationship; at each point along the stroke, the change in flow resulting from a unit change in stroke is proportional to the flow value at that point, with the percentage change in flow being constant. Therefore, its advantage is that the flow rate change is small when the flow rate is low, while it is large when the flow rate is high; in other words, it maintains the same regulation accuracy at different opening degrees. (2) Linear characteristic (linear) – fast at low opening degrees and slow at high opening degrees. The relative stroke and relative flow rate follow a linear relationship under a linear characteristic. The change in flow rate caused by a change in unit stroke remains constant. When the flow rate is high, the change in the relative value of the flow rate is small; when the flow rate is low, the change in the relative value of the flow rate is large. (3) Parabolic characteristic – slow at low openings and fast at high openings. The flow rate varies proportionally to the square of the stroke, exhibiting characteristics that are intermediate between linear and equal percentage characteristics. Linear characteristic valves are generally used for adjusting liquid levels, while equal percentage valves are more commonly used for regulating flow. The flow characteristic of the control valve is selected based on the characteristics of the controlled process. Valves with linear characteristics operate with relatively large changes in flow rate when opened to a small degree; their control effect is too strong, which can lead to overshoot and oscillations ； At large opening angles, the relative change in flow is small, resulting in weak regulation that is not timely enough. To address the aforementioned issues, a logarithmic characteristic was developed to ensure that the relative change in flow rate remains constant at any opening degree. Since the amplification factor K of the logarithmic characteristic increases as the opening degree increases, this facilitates system regulation. At low opening degrees, the flow rate is low and its variation is also small; the amplification factor of the control valve is low, resulting in a smooth and gradual regulation ； At large opening angles, the flow rate is high and its variation is significant; the amplification factor of the control valve is large, resulting in sensitive and effective regulation. As can be seen from the graph, the logarithmic characteristic remains below the linear characteristic; therefore, the flow rate is lower under the same stroke length compared to the linear characteristic. From the analysis of these three characteristics, it can be seen that in terms of regulation performance, the equal percentage characteristic is the best, as it offers stable regulation and excellent performance. Moreover, the parabolic characteristic offers better regulation performance than the linear one, allowing any of these flow characteristics to be chosen depending on the requirements of the application.

It still doesn’t address the core issue; let’s just talk about the disadvantages of percentage valves and the situations in which they are not suitable

It is recommended to post this in the on-site dashboard section, where there are many experts; most people here work on system-related tasks.

The flow characteristics of the valve must be determined according to your on-site requirements; generally, the liquid level is indicated in L, while other controls are mostly expressed as EQ%

Some valves need to open quickly, that is, the flow rate should change rapidly at small opening degrees; in such cases, a linear response can be used.

The flow characteristics of the control valve are selected based on the properties of the controlled object.