Valves and Waterhammer – Part 2: Installed Valve Characteristics
This is the second blog in a 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.
This blog serves to continue the discussion of valves’ impact on mitigating waterhammer effects. You can read part one “Inherent Valve Characteristics” here.
This blog will focus on two specific system characteristics that impact a valve’s installed characteristics. These characteristics determine how a valve will behave in a specific system:
- The ratio of the pressure drop due to the valve and the pressure drop across the entire system
- The steepness of the pump and system curves
The ratio of ∆Pvalve/∆Psystem can indicate the amount of control a valve has over the system. Generally, the sooner a valve can control flowrate, the more often a pressure spike will be decreased.
The impact of this ratio is most significant during valve closure transients. Only once the Cv is low enough to cause a significant pressure drop relative to the pressure drop of the system will the valve begin controlling. As seen in the Globe and Reduced Bore Ball valve example in Figures 1, we can see that the flow begins decreasing in each case once each valve reaches a similar Cv regardless of their starting Cv. This is because the starting initial pressure losses in both valves is very small relative to the system.
Plot a valves' inherent characteristic curve against its installed characteristic curve
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 increases, and thus frictional pressure losses increase, the potential for a valve to begin controlling flow early in the closure is diminished.
One way to visualize this is to plot a valve’s inherent characteristic curve against it’s installed characteristic curve, as found in Figure 2.
Remember, the inherent curve exclusively describes losses through the valve itself. For the installed curve, the valve only makes up one component of the system’s pressure loss. Since it is the overall pressure drop which governs whether the valve is controlling, we can see there is less impact of the valve’s closure on flow reduction in an installed system. Thus, the valve must create a much more significant pressure drop to have an impact. This is especially noticable for a valve with a linear inherent characteristic which may only significantly impact the installed flow in the last percentage of closure. Please 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.
While our Globe and Reduced Bore Ball valve example begins at essentially negligible pressure drops, a larger initial pressure drop through a component would also help to control sooner. This explains why generally high pressure drop Globe valves, combined with their characteristic curve, are used for control systems. In a similar fashion, changing valve size can drastically change the control characteristics. For example, an 8 inch ball valve will have a much larger pressure drop than a full bore 12 inch ball valve. Large initial pressure drops through the valve relative to the system will control much sooner.
The next aspect of a system to consider for a valve’s installed characteristics is the steepness of both the pump and system curves. Below in Figure 3 are example curves for steep and flat pump curves for different percentages of a valve closure. As the valve closes, the system curve inherently changes, approaching a vertical line at zero flow once the valve is closed.
Figure 3: Comparison between potential system and pump curves for a valve closure.
The steepness of the system curve can also influence the rate at which a valve controls
A very steep system curve acts like a very steep pump curve in that a valve closure would need to drastically add pressure drop to significantly shift the operating point. By comparison, a very flat initial 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 it is the ratio of the valve pressure drop and the system pressure drop which governs when a valve begins controlling.
In summary, the system a valve is a part of has a significant impact on a valve’s closure behavior. Valves begin controlling based on the ratio of a valve’s pressure drop and the system’s pressure drop. Long piped systems may dampen the valve’s ability to control flow, as was found when comparing inherent and installed characteristic curves. Finally, it is important to consider how the steepness of both pump and system curves can affect a valve’s ability to control flow during closure.
In the next blog, creative valve closure techniques to help mitigate waterhammer effects will be explored. Other facets of valves and are explored in the accompanying blogs in this series (Part 1 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.