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Valves and Waterhammer - Part 1: Inherent Valve Characteristics

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Valve closures, along with pump startups/trips, are one of the most significant sources of waterhammer within a hydraulic system. An emergency valve closure or a routine shut-down are equally susceptible to waterhammer effects depending on a multitude of factors. The valve itself will play a significant role, but its potential for causing waterhammer is also largely dependent on the system itself and the valve closure technique.

This is the first in a blog series detailing the recent AFT webinar "Importance of Valve Inherent Characteristics for Waterhammer Analysis", as presented by Dylan Witte. Witte serves as the Project Manager of Purple Mountain Technology Group, AFT's hydraulic consulting sister-company. You can watch the full webinar here and sign up for future webinars here.

As the first in a series, this blog serves as an introduction to the influence valves have when designing to prevent waterhammer. This blog series will overview:

  • the difference between a valve's inherent and installed characteristics
  • these characteristics' unique effects on waterhammer potential
  • how one can approach valve closures to mitigate waterhammer effects

This first blog will explore the distinction between inherent and installed characteristics, before detailing the effects a valve's inherent characteristics have on waterhammer potential.

A valve's inherent characteristics are specific to the valve itself. These are typically published by the manufacturer, who tests the valve across a constant pressure drop and a range of closures. The results of these tests relate the valve's open percentage to flow capacity while isolated from a system. This isolation avoids any installed characteristic effects, which will be discussed in detail in the next blog.

As an example, let's compare the differences in construction between a ball valve and a globe valve.

A valve with a more tortuous path will incur more pressure loss. This additional pressure loss is often better for control situations as the pressure drop across the valve is a more significant part of the pressure drop in the overall system. This can also be influential during closures as the pressure drop through the valve becomes more significant (and therefore more controlling) sooner.

In our example, the ball valve is essentially a short connecting segment of pipe with negligible pressure drop when fully open. Compare this to the path through the globe valve which must flow up and around several baffles regardless of valve open percentage. This 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. This partially explains why globe valves are typically used in control systems while ball valves are not.

We can see the influence of these paths on inherent closure characteristics for each valve.

Figure 1: Cross-sectional comparison between a Ball valve (left) and a Globe valve.
Figure 1: Cross-sectional comparison between a Ball valve (left) and a Globe valve.
Figure 2: Val-Matic Graph demonstrating the different inherent valve closure profiles – Sourced from Surge Control in Pumping Systems – 2018
Figure 2: Val-Matic Graph demonstrating the different inherent valve closure profiles – Sourced from Surge Control in Pumping Systems – 2018

These resulting plots are largely driven by how these different valve types are inherently constructed. This plot can indicate the range of control for each valve. We can see with the ball valve the immediate drop in Cv from 100% open to 60%. This is from the cross-sectional area decreasing very quickly, though the resulting pressure drop through the valve may not increase significantly. Near full closure, the ball valve has minimal changes in Cv despite large changes in open percentage. Compare this to the much more linear Cv change in the globe valve, indicating most changes in open percentage will generally produce some change in flow. An ideal inherent characteristic is one that results in a linear installed characteristic to directly reduce flow with any change in valve position. This inherent characteristic in unlikely to be linear unless the system is very short, and has very little pressure drop.

It is important to recognize that the characteristic curves using dimensionless "Cv % of Max" cannot tell the whole story as different valves generally start from drastically different Cv's. To gain a more rounded perspective, we can also look at an example closure with identical starting flowrates and more realistic starting Cv's in AFT Impulse.

Figure 3: Demonstration of 30 second valve closure from more realistic initial Cv’s.
Figure 3: Demonstration of 30 second valve closure from more realistic initial Cv’s.

Here we can see that the ball valve has a very significant decrease in Cv to overcome and is able to decrease very quickly. However, this decrease in Cv doesn't necessarily indicate when the valve starts to control the flowrate through the system.

Figure 4: Resulting volumetric flowrates and stagnant pressure spikes for ball valve and globe valve closures.
Figure 4: Resulting volumetric flowrates and stagnant pressure spikes for ball valve and globe valve closures.

From these flowrate and pressure graphs, we can see that despite a smaller change in Cv over the transient closure, the globe valve begins controlling at roughly 12.5 seconds. Thus, the globe valve begins controlling a full 7.5 seconds before the ball valve. By controlling sooner, the globe valve was able to reduce the peak waterhammer pressure spike seen by the ball valve.

The Cv at which the valve will begin controlling is based upon the valve's installed characteristics. Thus, a potential reduction in pressure spike by swapping a ball valve for a globe valve is not guaranteed and will be heavily influenced by the valve's installed characteristics as well. Stick around for the next installment of Valves and Waterhammer to find out what to consider for a valve's installed characteristics.

In summary, many factors influence how a valve closure can result in waterhammer. These factors include inherent characteristics, accounting for the initial Cv of the valve as well as its characteristic closure profile. However, these inherent characteristics do not tell the full story. The valve's installed characteristics unique to a system and the behavior of the valve closure also contribute significantly to mitigating waterhammer. These other facets are explored in the sequels to this blog (Part 2 and Part 3), or they can be explored in the original webinar.

If you want to explore potential waterhammer scenarios within your own hydraulic system, AFT Impulse will help explore an abundance of potential transient events.

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