Introduction to the Characteristics of Regulating Valves

A control valve (also known as a regulating valve) is used to adjust the flow, pressure, and liquid level of a medium. Based on signals from the regulating system, the valve’s opening is automatically controlled, allowing for the adjustment of the medium’s flow, pressure, and liquid level. Regulating valves are typically classified into three types: electric regulating valves, pneumatic regulating valves, and hydraulic regulating valves.

A regulating valve consists of two parts: the actuator (either electric or pneumatic) and the regulating valve itself. There are two common types of regulating valves: straight single-seat and straight two-seat valves. The two-seat design, which has larger flow capacity, lower imbalance, and more stable operation, is particularly suitable for large flow rates, high-pressure drops, and reduced leakage.

Flow Capacity and Cv

The flow capacity (Cv) is one of the key parameters when selecting a regulating valve. Cv is defined as the flow rate (in t/h) through the fully opened valve when the pressure difference between the valve ends is 0.1 MPa, and the fluid density is 1 g/cm³. The Cv value is a crucial factor in selecting the nominal diameter (DN) of the regulating valve.

Flow Characteristics of Regulating Valves

The flow characteristic of a regulating valve describes the relationship between the relative flow rate through the valve and its opening under a constant pressure difference. The three main flow characteristics are:

1. Equal Percentage Characteristics (Logarithmic)
   In this characteristic, the relative stroke and the relative flow rate are non-linear. The flow change caused by a unit change in stroke is proportional to the flow rate at that point, and the percentage of flow change remains constant. The advantage is that it provides the same adjustment accuracy across various opening degrees, with more significant flow changes occurring at lower flows and smaller changes at higher flows.
2. Linear Characteristics
   In linear characteristics, there is a direct, linear relationship between relative stroke and relative flow. The change in flow rate due to a change in stroke remains constant. At large flow rates, the relative change in flow rate is small, while at low flow rates, the relative change in flow rate is more significant.
3. Parabolic Characteristics
   The flow rate changes proportionally to the square of the stroke, displaying characteristics that are a mix between linear and equal percentage characteristics.

Selecting the Right Flow Characteristic

From the analysis of these three characteristics, it is clear that the equal percentage characteristic offers the best adjustment performance, ensuring stable and accurate control. The parabolic characteristic is better than the linear characteristic, and the ideal flow characteristic can be selected based on the specific requirements of the application.