Potential excitation mechanism

Velocity or Kinetic Energy (ρv²) at some extend may cause excitation on valve internal component(s), hence may jeopardize valve and piping integrity depending on its stiffness and toughness.

Predominant potential excitation mechanism are:

  • flow induced turbulence (vortex shading)
  • high frequency acoustic
  • cavitation and flashing
  • surge and momentum change

Unless otherwise specified or engineered accordingly, it is recommended to apply best practices (derived from various calculation and empirical evidence) as follow:

  • Mitigate cavitation and momentum change;  On/Off valve shut-off travel (from fully Open to fully Close) should be minimum 4 seconds per 1 inch valve size. For example 3 in. ball valve travel time should be set to fully close (from fully Open) within 12 seconds or more.
  • General valve sizing criteria should follow below  limit
Service Valve Inlet / Outlet Velocity Limit
Non-Critical (Outlet velocity) <8 m/s (26 ft/s)
Non-Critical, corrosive (Outlet velocity) <6 m/s (20 ft/s)
Partial Cavitation Xfz < Xf(min) (Outlet velocity) <5 m/s (17 ft/s)
Maximum and Severe Cavitation Xfz < Xf(min) (Outlet velocity) <3 m/s (10 ft/s)
Flashing (Inlet velocity)  <5 m/s (17 ft/s)
Flashing (Outlet velocity) <60 m/s (197 ft/s)
Gas service (continuous) Mach.  <0.3
Gas service (intermittent) Mach. <0.7

Xf(min) is pressure ratio;

Xfz is valve-specific cavitation coefficient

Check this link to help you understand correlation between outlet valve velocity and Cv.

  • Kinetic energy
Aspect Fluid phase Susceptibility of failure
Negligible Medium High
 

 

Kinetic energy as result of throttling valve

Liquid and multi-phase ρv² < 10,000 kg/ms² 10,000 kg/ms² < ρv² < 20,000 kg/ms² ρv² ≥ 20,000 kg/ms²
Gas ρv² < 10,000 kg/ms²   ρv² ≥ 10,000 kg/ms²

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