Location control. This principle of flow path design directs flow in the valve away from solid boundaries so bubble implosions will have less impact and therefore less potential for causing damage.
Flat baffle plates. When the pressure drop ratio becomes very high, it's possible that anti-cavitation trim in a valve won't sufficiently lower the levels of noise and cavitation. In this case, using a baffle plate after the valve can create backpressure to reduce the pressure drop across the valve and thereby minimize noise and cavitation.
A flat baffle is a flow restrictor with a custom-made constant flow area and multiple holes for a particular flow condition. The pressure drop across a baffle plate is proportional to the flow squared. Because a baffle plate is a fixed restrictor, it produces the desired pressure drop only at a single flow — the one used for sizing the baffle. A baffle plate typically is sized for the valve maximum flow. As the flow goes down, the pressure drop across the baffle decreases and the pressure drop across the valve increases. Thus, the baffle plate loses its effect at low flow rates.
Orifice plates. An orifice plate is a constant flow resistance. It only has one large hole rather than the multiple small holes in a baffle plate. An orifice plate can be used for producing linear valve installed flow characteristic and, in limited cases, for cavitation abatement. An orifice plate can be installed both upstream and downstream of a control valve. Sizing basically is similar to that of a baffle plate.
Figure 3 depicts how a Q-trim valve uses a combination of approaches, including multistage pressure drop to lower valve trim velocity and multiple-stream flow to control noise.
Cavitation can cause severe damage within control valves and nearby piping due to the formation and high-intensity implosion of vapor bubbles immediately downstream of the vena contracta. This mechanical impact damage, along with associated erosion and corrosion, can substantially impair control valve performance, resulting in lower productivity and product quality along with the added costs of product loss, labor and materials when valves fail prematurely.
Predicted levels of noise (which always accompanies cavitation) correlate closely with the potential for mechanical damage. Expert sizing software may be used to simulate noise/cavitation levels under actual line conditions to arrive at the most-effective feasible abatement measures. While it may be impossible to totally eliminate cavitation in every instance, its harmful effects and associated costs always can be substantially minimized.
STEVEN HOCURSCAK is North American business manager for Neles & Mapag, Shewsbury, Mass. KYLE RAYHILL is global business manager for Neles & Mapag, Shewsbury, Mass. E-mail them at firstname.lastname@example.org and email@example.com.