An article authored by Restrict Flow’s Chief Engineer was published in Processing. The article examines how cavitation can develop in industrial liquid piping systems when pressure is reduced across restriction orifice plates and other flow-control components.
Cavitation is commonly associated with pumps, but it can occur anywhere local static pressure falls below the liquid’s vapor pressure. In restriction orifice applications, the fluid accelerates through the reduced flow area and pressure reaches its minimum near the vena contracta.
When that pressure falls below vapor pressure, vapor cavities form. As pressure recovers downstream, the cavities collapse and can produce noise, vibration, localized erosion and mechanical stress.
The article uses a 14-inch cooling-water bypass line as a real-world example. Severe vibration occurred whenever the system was placed into backflush operation. The source was ultimately traced to a restriction orifice plate taking a substantial pressure drop while passing a high liquid flow rate.
Cavitation and water hammer can both produce piping movement and noise, but they are different hydraulic events.
Water hammer is generally associated with a rapid change in fluid velocity, such as a fast-closing valve or sudden pump event. Restriction-induced cavitation can continue under otherwise steady operating conditions as vapor cavities repeatedly form and collapse.
Recognizing the difference is important before determining the appropriate corrective action.
One established approach is to divide the required pressure reduction across a multi-stage restriction assembly. Each stage takes a portion of the total differential pressure.
Multi-stage systems can be effective, but increasing the number of stages can also increase overall assembly length, piping modifications, supports, connections and installation requirements. The article describes one evaluated application where the multi-stage assembly would have been approximately 40 feet long.
Another approach is to engineer the restriction geometry itself so the development and collapse of cavitating flow are managed within a single engineered flow path.
Restrict Flow’s Anti-Cavitate Orifice Plate™ uses this single-stage approach rather than distributing the required pressure reduction across multiple separate restriction stages.
The appropriate method depends on the actual operating conditions. Flow rate, upstream and downstream pressure, differential pressure, liquid vapor pressure, temperature, pipe size, installation space and operating duty all affect how a pressure-reducing application should be evaluated.
Correctly identifying where the pressure reduction occurs and whether local pressure is falling below vapor pressure is the first step toward addressing damaging cavitation in a liquid piping system.
