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Just Close the Valve Slower, right?  Not Exactly.

Just Close the Valve Slower, right? Not Exactly.

It is well known that fast valve closures cause surge problems in a piping system.  A reasonable thing to do would be to close the valve slower.  Simply closing the valve slower does not always lower the surge pressure because the valve closure surge pressure depends on several factors.  Below is the closing profile for a gate valve where the open percentage is closed linearly over 30 seconds and 60 seconds.

Figure 1 – 30 second & 60 second closure profiles for a gate valve. Open % vs. time on the left demonstrating a linear closure & the resulting Cv vs. time closure profile on the right.

In this case, doubling the valve closure time makes no impact on the surge pressure at the valve.  The pressure surge is the same but shifted 30 seconds later for the 60 second closure compared to the 30 second closure.  Also, there is virtually no change in flow through the valve until the last few open percentage values of the valve closure.

Figure 2 – Pressure & Flow response vs. time for gate valve 30 second & 60 second closures.

The surge pressure and flow through the valve is impacted more by the following factors than by simply closing the valve at a longer rate which can sometimes have no effect.

  • Inherent Characteristic
    • Valve type based on geometric properties establishing its Open % vs. Cv curve
    • Gate, Globe, Butterfly, etc.
  • Installed Characteristic
    • The type of system the valve is installed in as well as its Inherent characteristic determines its behavior
  • Closure Profile & Time Frame
    • How a valve is closed is just as much, if not more important than how long it takes to close the valve
    • i.e., constant or linear closure rates vs. variable or non-linear closure rates

For the same system, a globe valve is compared to the gate valve for a 30-second vs. 60-second closure period.

Figure 3 – Open % (left) & Cv (right) vs. time closure profiles for gate valve vs. globe valve. 30 second & 60 second closure rates compared for linear open percentage closure profiles.

Both valves are closed with a linear change in open percentage vs. time.  However, the gate and globe valves have different open percentage vs. Cv inherent characteristic curves and hence, a different change in the Cv vs. time closure profile.  Note that although the initial Cv values are vastly different, the initial flow rates are kept the same by changing the inlet supply pressure to the pipe.