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Valves that are utilized as fluid control devices are typically “throttling valves”.

Such valves experience internal velocity and internal pressure gradients (both positive and negative) that conclude with a permanent pressure loss (DP) from the inlet pipe-to-outlet pipe connections. Throttling valve trim (plug-seat) experiences relatively high internal velocities nearly 100% of operating time. In comparison, ON-OFF automated or manual valves experience velocity changes ONLY when being actuated from “open-to-closed”, or vice versa; i.e. a few seconds or minutes.

Bernoulli’s Theorem is the most useful tool in analyzing what is going on physically within the walls of a throttling valve, which includes

  • velocity gradients
  • pressure gradients

The other important tool is the 1st Law of Thermodynamics which allows analyzing

  • fluid state
  • thermal effects

Bernoulli’s principles apply to the following for throttling valves

  • inlet pipe reducer
  • pressure drop to main orifice
  • pressure recovery to outlet
  • outlet pipe reducer
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