Free Download of Reference Book on Fluid Flow Basics of Throttling Valves

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