The enemy within: critical operating conditions in control valves

摘要

Control valves are, above all, used to control the flow of gases and liquids. In contrast to fluids carrying solids, which quickly cause abrasion in the controlling the flow of gas and liquids does not seem to present an obvious problem. The liquid phase, however, holds the danger of cavitation and flashing. To be able to evaluate such conditions as accurately as possible before choosing the appropriate control valve, it is crucial to know the exact process data. This applies in particular to the conditions downstream of the control valve. Extremely high flow rates in the valve or downstream of it can cause the pressure to drop locally to the extent that it falls below the vapor pressure of the liquid. As a result, small gas bubbles (cavitation bubbles) form in the process fluid. They are carried along by the flow to areas of higher pressure, where they collapse. These bubble implosions result in microjets that damage the surface of the material (cavitation erosion).