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Valve Characteristics – Inherent vs. Installed

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Valve closures 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. The valve itself will play a significant role, but its potential for causing waterhammer is also largely dependent on the system and the valve closure technique.

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

  • the difference between a valve’s inherent and installed characteristics
  • these characteristics’ unique effects on waterhammer potential
  • how one can approach a valve closure to mitigate waterhammer effects rather than make adjustments to the system

 

Inherent vs. Installed Valve Characteristics

A valve’s inherent characteristics are specific to the valve itself. This includes parameters like valve type, valve size, and Cv vs. Open Percentage. These characteristics are typically published by the manufacturer, who tests the valve across a constant pressure drop and a range of closure percentages. 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.

Installed characteristics are how the valve behaves in the context of a system. This generally involves the pressure drop across the valve relative to the surrounding system, the ratio of which is what ultimately defines a valve’s controllability. Drastically different results can be found by placing the same valve with the same closure behavior in a different system. The root of those different results is due to a valve’s installed characteristics.

 

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