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.
Bernoullis 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
Bernoullis principles apply to the following for throttling valves
- inlet pipe reducer
- pressure drop to main orifice
- pressure recovery to outlet
- outlet pipe reducer
