Inherent Valve Characteristics

Inherent valve characteristics are specific to a valve’s construction. This includes factors such as valve type, valve size, and a valve’s characteristic curve.

A comparison in construction

A valve with a more torturous path will incur more pressure loss as energy is lost to redirecting the flow. Compare the construction of a ball valve and a globe valve in Figure 1 to understand their implications in inherent valve characteristics.

Figure 1: Cross-sectional comparison between a ball valve (left) and a globe valve.

A ball valve is essentially a short connecting segment of pipe with negligible pressure drop when fully open. Compare this to a globe valve, through which the fluid must flow up and around several baffles regardless of valve open percentage. The impact of the flow path is reflected in valve resistance handbooks, with Miller reporting a K resistance of 4 for a fully open Globe valve but a K of only 0.1 for a fully open ball valve. Obviously, the more redirection and constriction in flow a valve has, the more pressure drop it will create.

Characteristic Closure Profiles

While the fluid’s path through a component informs pressure drop, the valve’s pressure drop at different degrees of open percentage is also important to consider during a closure event. How a valve’s changing open percent relates to resistance to flow is known as the valve’s characteristic curve. Since valve types have unique shapes and flow paths, there is a wide range of characteristic curves such as those as found in Figure 2.

 Figure 2: Val-Matic graph demonstrating the different inherent valve characteristic closure profiles – Sourced from Surge Control in Pumping Systems – 2018

These characteristic curves can produce drastically different flow responses for theoretically similar cases. In some cases, flow may be reduced quickly at the start of the closure (as found in an Equal Percentage valve). In other cases, the valve may start reducing flow near the end of the closure (as with the Quick Opening valve). Thus, two valve closures with the same overall closure time and same initial Cv can have drastically different responses based solely on the valve’s characteristic curves. How the closure of these valves either gradually or aggressively reduce flowrate impacts the severity of the surge event.

A closure comparison

It is important to recognize that Figure 2’s dimensionless “Cv % of Max” cannot tell the whole story as different valve types start from drastically different Cv’s. This effect is demonstrated in Figure 3, comparing a 30-second ball valve closure to a similar 30-second globe valve closure.

Figure 3: Comparison of 30-second valve closures with more realistic initial Cv’s but identical initial flow rates.

In Figure 3, it is clear the ball valve begins at a much higher initial Cv, and as such the valve has a more significant reduction in Cv to overcome during its closure compared to the globe valve. Luckily, the ball valve is able to reduce its Cv very quickly due to its characteristic curve. Note that this rapid decrease in Cv does not necessarily indicate when the valve starts to reduce the flowrate, or put another way, gain control of the system’s flow rate.

From the flow rate graph, we can see that despite a smaller change in Cv over the transient closure, the globe valve begins controlling and reducing flow at roughly 12.5 seconds. By controlling the flow rate a full 7.5 seconds faster than the ball valve, the globe valve was able to more gradually slow the fluid, reducing the peak pressure surge in the process.

In both cases, the flow rate begins decrease near a specific Cv value. The Cv at which either valve will begin controlling is based upon the valve’s installed characteristics. How a valve approaches the system’s controlling Cv can inform use cases for different valves and closure techniques when complete closure is necessary.