Keep Your Hydraulic Manifolds Working Longer By Reducing Cavitation

Cavitation is a deeply interesting physical phenomenon that occurs when liquids are subjects to sudden massive changes in speed. For example, if fluid moving under 40 megapascals (MPa) of pressure suddenly hits a hydraulic valve and the pressure drops to 20MPa, the sudden increase in speed (described by Bernoulli’s Theroem) would lead us to the conclusion that a small bubble of nearly empty space — a hard vacuum — would open up within the fluid at the point of pressure drop.

This does in fact happen, and the bubble both forms and then collapses at supersonic speeds, creating a tiny sonic boom within the hydraulic fluid itself. The shockwave of the bubble’s collapse generates enough energy that it can literally tear apart nearby molecules of hydraulic fluid — or, if the bubble collapses near the inner walls of the manifold, it can rend the molecules of the manifold itself, causing “erosion”.

Reducing Cavitation
That’s perhaps a misnomer — there’s no realistic way to reduce cavitation within a given system short of reworking it to avoid sudden pressure changes. But the critical phrase above is “if the bubble collapses near the inner walls of the manifold.” It may not be possible to realistically prevent cavitation bubbles, but it is possible to more carefully control where those bubbles collapse.

The simple rule of thumb is this: increase backpressure by 5% of the total drop in pressure across the valve or metering edge (or whatever else is causing the change in pressure.) In the above example, where pressure drops by 20 MPa, an increase in backpressure of just 1 MPa should force the cavitation bubbles toward the middle of the fluid stream, thus causing them to collapse where they will do as little damage as possible to the hydraulic manifold.

If that doesn’t work, switching from an aluminum manifold to a ductile iron manifold will reduce the rate of “erosion” by 90%. Alternately, if possible, increasing backpressure to 10% of the total pressure drop will have a similar effect, though often switching manifold materials is the easier solution.

How do you know if it worked? Listen carefully at the point of pressure change. White noise coming from within the system is the sound of cavitation bubbles collapsing; if the noise is reduced or gone, your attempt was successful.

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