A control valve's flow characteristic defines the relation between opening and flow rate: linear, equal percentage or quick opening.
Those curves are measured at constant pressure difference. In a real system the pressure difference is not constant: as the valve opens the flow rises, the system loss rises, and the drop across the valve falls.
Valve authority
The result is that the installed characteristic differs from the inherent one. The governing parameter is the ratio of the pressure drop across the fully open valve to the total system drop — the valve authority.
If the authority is low, the flow reaches nearly its full value within the first twenty percent of the opening and the remaining eighty degrees do nothing. Control resolution is lost and the system hunts. The equal percentage characteristic exists precisely to offset this effect.
Cavitation and flashing
During throttling the flow accelerates at the valve throat and the pressure falls. If the throat pressure drops below the vapour pressure, bubbles form.
Cavitation. If the pressure recovers downstream, the bubbles collapse. This produces noise, vibration and erosion; the damage is usually not in the valve but in the pipe wall a few diameters downstream.
Flashing. If the downstream pressure stays below the vapour pressure, the bubbles do not collapse and two-phase flow continues. The erosion has a different character and the flow chokes.
The distinction is practically important: the remedy for cavitation is multi-stage throttling (dropping the pressure in steps), while for flashing it is material and geometry selection.
Modelling
The local minimum pressure at the throat is markedly below the steady average; the question of cavitation inception calls for a transient solution. See the cavitation note.
In gas service, choking can occur at the throat and the flow becomes independent of the downstream pressure. See the compressible flow note.
References
- Singhal, A. K., Athavale, M. M., Li, H. & Jiang, Y. (2002). Mathematical basis and validation of the full cavitation model. Journal of Fluids Engineering, 124(3), 617–624. DOI: 10.1115/1.1486223
- Hydraulic Institute. ANSI/HI 9.6.1 — Rotodynamic Pumps Guideline for NPSH Margin. Hydraulic Institute, Parsippany, NJ.