Calcul ate the installation flow and gain of the control valve
When specifying control valves, multiple tasks are involved. First, the valve style must be selected. This is usually based on a combination of process parameters (such as pressure, temperature, medium corrosion and abrasion), plant preferences and economy. Next, you need to select the inherent flow characteristics of the valve (linear or equal percentage). The rule of thumb to choose the inherent characteristics. For example, some authorities suggest that systems with small dynamic pressure losses due to short lines, as well as other systems with low pressure consumption, such as isolation valves, bends, heat exchangers, have little linear characteristics. For long pipe systems and other pressure consumption elements, equal percentage valves are recommended. During normal operation, the specification must also consider how much control range the valve should use. For example, it is usually recommended that the valve open between about 60% and 80% at the maximum required flow rate to provide a sufficient but not excessive safety factor and, where possible, not less than 20% at the minimum required opening. Avoid flow characteristics that can not be predicted when the opening is less than 10. Once you determine which valve style you will use, you can perform the size calculation using one of the many available valve size calculations.
The two tools that can help the specifier select the valve are diagrams of installed flow characteristics 1 and installed gain 2 for all valves under consideration for application. Figure 1 shows the installation flow characteristics and installation gain of two possible valve options (partial ball valves, their inherent flow rate, etc.) for the same application, where the process includes long pipe operation and centrifugal pump. A 6-inch valve will control the required flow range of 80 to 550 gpm in a 10% range, while a 40% open range will control only 30% of its total travel capacity, which makes the flow very sensitive to valve position changes. At the low end there is not much safety factor, more than the maximum flow gpm 550 flow wasted a lot, which is expensive and unnecessary.
A 3-inch valve will control the required full flow range between about 50% of its full stroke capacity (22% to 72% open), reducing the sensitivity of the flow to valve position changes. The minimum and maximum specified flow rate is symmetrically placed on the installed flow characteristics of the valve and has the same number of safety factors at each end of the specified control range.
Figure 1: Comparison of sample control valve installation flow and installation gain characteristics of two valves in the same system. a focus is added to show part of the chart, which belongs to the user-specified traffic range, between the minimum required traffic and the maximum required traffic (80 to 550 gpm).
Typically, installed flow characteristics are represented by a diagram of the relative flow versus relative stroke, as shown in Figure 1. The flow rate is drawn in a scale of 0 to 1, that is, from zero flow to 100% of the fully open flow, and the valve stroke is drawn in a scale of 0 to 1, that is, from completely closed to completely open. The diagram shows the linear relationship between flow rate and valve stroke and the position of the required flow range within the valve stroke range. What is missing in drawing the relative flow chart is the relative slope of the installed flow chart of the valve considered. if the two valves on the installed flow chart are drawn with a vertical scale of gpm (or kg / h), the chart of the 6-inch valve will be much higher at 100% open (1.0% relative stroke) and its slope will be much steeper.
Drawing the figure of installation gain can explain the installation performance of the control valve from different aspects, and put the valves of different sizes and different styles on the same base for checking and comparing. The gain of any device is defined as the rate of change of the output divided by the rate of change of the input. For the control valve, the input is the position of the valve and the output is the flow through the valve. On the installed gain diagram, the flow scale is shown as relative flow, and the reference point is the maximum required flow specified by the user. A user's maximum specified traffic is called Qmax.
