WHAT IS valve CAVITATION?

An intriguing phenomenon can occur when fluid passes through a restriction; for example, when fluid flows through a control valve. The valve’s restricted flow path causes the fluid velocity to accelerate. When velocity increases, it corresponds to a decrease in the local pressure (Bernoulli’s Principle for incompressible liquid flow). If the drop in pressure is great enough, it may fall below the fluid’s vapor pressure (the pressure at which a fluid becomes a gas), resulting in the formation of vapor bubbles.
Upon exiting the control valve, the fluid velocity decreases and the pressure recovers downstream, allowing the bubbles to implode. This phenomenon is called ‘cavitation’. Cavitation may seem harmless; however, it can be a destructive process. When a vapor bubble implodes, the bubble’s center pulls in a jet of liquid at supersonic speed. When these implosions occur close to the valve or pipe wall, the jet will erode material from these surfaces. If left unchecked, cavitation can lead to significant noise, vibration and mechanical failure.
What is valve cavitation? Diagram depicts cavitation bubble imploding near control valves and piping.

what about cavitation in slurry?

With cavitation being a common issue, various valve manufactures offer unique ‘anti-cavitation’ trim designs that reduce or prevent cavitation in clean fluids. These products generally have multiple small orifices, and torturous flow paths that resemble labyrinths to keep velocities in check. There are several valve options for clean fluid applications. However, if the fluid contains solids (i.e. slurry), the scenario is much different. These ‘clean fluid’ valves are no longer a viable option for slurry duty, as their complex flow paths can erode or become plugged very quickly.
Therefore, end users needing to modulate slurry flow must select alternate valves with simple ‘line of sight’ designs, such as ball valves, butterfly valves and pinch valves. These designs provide relatively little flow re-direction and minimal interference with the abrasive slurry. The downside is that fluid velocities through these types of valves are much greater, making them susceptible to cavitation. The flow is also directed towards the valve’s body and pipe wall.
Ball Valve TrimComputer Flow Model of abrasive flow in a ball valve.Ball Valve
BALL VALVE: A ball valve directs flow to one side of the valve and piping.
Butterfly Valve DiscComputer Flow Model of abrasive flow in a butterfly valve.Butterfly Valve
BUTTERFLY VALVE: The disc accelerates media onto both sides of the valve body and pipe walls.
Computer Flow Model of abrasive flow in a pinch valve.Pinch Valve
PINCH VALVE: High velocity flow impinges both sides of the valve’s sleeve and downstream pipe.

solutions for cavitation in slurry

A SlurryFlo control valve can reduce cavitation in slurry applications by centering high velocity flow.

Control Valves with Centered Flow

To reduce the effects of cavitation in slurry lines, end users can opt for a SlurryFlo control valve. SlurryFlo’s patented trim design acts as a variable orifice, centering the high velocity flow and resulting cavitation. Directing vapor bubbles away from the valve’s body and pipe wall significantly increases the system’s longevity.
VIEW SLURRYFLO CONTROL VALVE TECHNOLOGY

Multiple Control Valves

It’s also possible to reduce or even eliminate cavitation in slurry lines by introducing back pressure downstream of a control valve. This can be achieved by installing two or more control valves in series, with each valve sharing a portion of the total pressure drop, keeping the fluid pressure above its vapor pressure.
VIEW SLURRYFLO SPECIFICATIONS
Installing two or more control valves in a series can reduce or eliminate cavitation in slurry lines.
Rocketplates are orifice plates positioned downstream of control valves to help reduce cavitation.

Downstream Orifice Plates

Alternatively, back pressure may be provided by installing one or more orifice plates downstream of the control valve, such as Rocketplates, which are specifically designed for slurry service. The goal is to force the control valve to operate above cavitating conditions, resulting in prolonged valve service life. If cavitation can be reduced, but not eliminated, the set-up will be designed to have the cavitation occur at the RocketPlate, which is extremely robust, yet lower in cost compared to the valve.
READ MORE ABOUT ROCKETPLATES

can we predict cavitation?

Yes! Design engineers use Computational Fluid Dynamics (CFD) and cavitation prediction software to size all SlurryFlo control valves and RocketPlate orifice plates. The following case study exemplifies cavitation using CFD and physical flow tests.

Incipient Cavitation

SlurryFlo Control Valve: 4”
Trim: C1
Media: Water
Valve % open: 60%
Flow: 609.6 USGPM
Inlet pressure: 48.49 PSIG
Outlet pressure: 16.43 PSIG
Delta P: 32.06 PSIG
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images Below: ISO surface (blue) on CFD model indicates where vapor bubbles will form. Physical flow test showing the vapor bubbles being carried downstream.
CFD model of incipient cavitation in a control valve.
Physical flow test of incipient cavitation in a control valve.

moderate Cavitation

SlurryFlo Control Valve: 4”
Trim: C1
Media: Water
Valve % open: 60%
Flow: 665.6 USGPM
Inlet pressure: 48.20 PSIG
Outlet pressure: 9.05 PSIG
Delta P: 39.15 PSIG
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images Below: ISO surface (blue) on CFD model indicates where vapor bubbles will form. Physical flow test showing the vapor bubbles being carried downstream.
CFD model of moderate cavitation in a control valve.
Physical flow test of moderate cavitation in a control valve.

severe Cavitation

SlurryFlo Control Valve: 4”
Trim: C1
Media: Water
Valve % open: 60%
Flow: 701.8 USGPM
Inlet pressure: 49.76 PSIG
Outlet pressure: 2.69 PSIG
Delta P: 47.07 PSIG
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images Below: ISO surface (blue) on CFD model indicates where vapor bubbles will form. Physical flow test showing the vapor bubbles being carried downstream.
CFD model of severe cavitation in a control valve.
Physical flow test of severe cavitation in a control valve.