Installed Valve Characteristics

Installed characteristics describe a valve’s behavior in the context of a system. This is unique from examining the valve itself, known as the valve’s inherent characteristics. While the two types of valve characteristics are very interrelated, this article focuses on specific installed characteristics which impact how a valve interacts with its system:

1. The ratio of the pressure drop due to the valve and the pressure drop across the entire system
2. The steepness of the pump and system curves

Ratio of pressure drop

The ratio of ∆P_valve/∆P_system can indicate the amount of control a valve has over the system’s flow rate. A large ratio indicates changes in valve position will immediately result in a change in flow rate. Generally, the earlier in a closure a valve can control flow rate, the better a pressure surge will be reduced.

In the context of a valve closure, a valve that approaches this controlling pressure drop quickly can adjust the flow rate more gradually. For example, take two valves that close in 30 seconds. If the first valve begins controlling 5 seconds into the closure, the flow rate is smoothly reduced over the remaining 25 seconds. If instead, the valve begins controlling within the last 5 seconds of closure, there will be a significant pressure surge as flow is rapidly stopped. This concept is discussed further in the article Effective Closure Time.

Understanding the pressure drop where a valve controls is where initial Cv and the “controlling” Cv are important to consider. Below in Figure 1 is a comparison between a globe valve and ball valve closure.

Figure 1: Demonstrates similar transient responses from distinct valve types despite different initial Cv’s.

It is clear in Figure 1 that flow begins decreasing in each case once each valve reaches a similar controlling Cv, regardless of their initial Cv. The initial Cv is almost irrelevant to the closure because the initial pressure losses through either valve is small relative to the system. Thus, in both cases, neither valve is controlling while the valve is completely open.

Since the ratio of pressure drop governs control, it is important to consider the pressure drop across the system as a whole as well. As pipe length and their corresponding frictional losses increase, the potential for a valve to begin controlling flow early in the closure is diminished.

How Inherent Characteristics translate to Installed Characteristics

One way to visualize a valve’s reduced control in a system is to plot a valve’s inherent characteristic curve against its installed characteristic curve, as found in Figure 2.

Figure 2: Inherent and installed characteristic curves, showing the “bowing” out of diminished control.

A valve’s inherent characteristic curve exclusively describes losses through the valve itself. In the installed curve, the valve makes up only one component of the system’s pressure loss. These additional system losses bow out the installed characteristic curve. Thus, a change in valve open % which would have significantly reduced flow in isolation may only slightly reduce flow in the context of a system.

Again, since the ratio of pressure drop governs when the valve can control, the valve must create a significant pressure relative to the system to have an impact. A more drastic beginning reduction in Cv across the valve can help approach that control point sooner.

While the globe valve and ball valve example from Figure 1 begins at essentially negligible pressure drops, a larger initial pressure drop through a component would also help to control sooner. This explains why high-pressure drop globe valves, combined with their linear inherent characteristic curves, are used for control systems. Large initial pressure drops through the valve relative to the system will control earlier in a closure as well.

In a similar fashion, changing valve size can drastically change the control characteristics as well. For example, an 8-inch ball valve will have a much larger pressure drop than a full-bore 12-inch ball valve. This is an example of understanding a valve’s inherent characteristics impacting its installed characteristics.

Note the severity of this shift from inherent to installed is highly dependent on the system itself. It is generally desired to achieve a linear installed characteristic to mitigate surge pressure effects. A linear installed characteristic means any change in valve percentage results in a proportional change in flow rate.

Pump-System interaction during a closure event

The steepness of both the pump and system curves is also important to consider during a valve closure. Below in Figure 3 are steep and flat pump curves for different percentages of a valve closure. The intersection of the curves indicates the operating flow rate as the valve closes.

Figure 3: Comparison between potential system and pump curves for a valve closure.

As the valve closes, the system curve inherently changes, approaching a vertical line at zero flow once the valve is closed. For a flat pump curve, there are large reductions in flow rate for very small changes in system losses. This means the valve begins controlling flow much earlier in the closure, enabling improved water hammer mitigation.

By comparison, a steep pump curve requires a much greater change in the system losses to reduce flow. In Figure 3, it is evident that there is nearly negligible change in flow rate from 100% to 25% open. This means that first 75% of the closure barely contributes to controlling the flow. From 25% to 0%, the valve must also cause a much more significant pressure drop across it to reduce the flow, indicating significant potential for pressure surge.

A transient analysis for these two different pump curves, all else constant, reveals the pressure patterns found in Figure 4. The head rise from the steep pump causes a much greater peak pressure than is found in the shallow pump curve case.

Figure 4: Resulting pressure surges caused by the steepness of pump curve.

The steepness of the system curve can similarly influence the rate at which a valve controls. If the system has significant frictional losses, its system curve will be very steep.

A very steep system curve acts like a very steep pump curve in that a closing valve would need to drastically add pressure drop to shift the operating point. By comparison, a very flat system curve is more susceptible to the drastic changes of a valve closure to shift the system curve. This ties back to the idea that the ratio of the valve pressure drop and the system pressure drop governs when a valve begins controlling. For flat system curves, there is little system pressure drop, allowing the valve to control flow easier. For steep system curves, the valve must provide very significant pressure drop to control, likely in the last few percent of its closure.