Mitigating Cavitation and Outgassing Control Valve Damage

Emerson's Jonathan VanceAt the Emerson Exchange conference in Austin, Emerson’s Jonathan Vance presented Outgassing and Cavitation: Not So Good Vibrations. Here’s the abstract:

Outgassing and cavitation are two of the most common issues associated with control valves. Both phenomena often occur in high pressure drop applications and result in excessive vibration; however, the solutions to combat these critical applications vary greatly. This presentation will help control valve users differentiate between the two phenomena and introduce Emerson Process Management’s broad portfolio of products designed to combat these severe service applications. Proper control valve selection allows users to maximize product reliability thereby limiting unplanned downtime.

Jonathan opened with some of the challenges of the misapplication of control valves in outgassing or cavitation services. Cavitation is when the pressure falls below the liquid vapor pressure forming bubbles. As the pressure increases the bubbles implode causing energy to be released inside the valve. Cavitation can cause damage to the valve plug from pitting. Cavitation occurs in a single component fluid.

Outgassing involves multiple components in a fluid. As the pressure drops one of the components can come out of the liquid phase to the gas phase. Think of a can of soda. Once opened, the pressure drop causes some of the carbon dioxide to come out of the solution as a gas which you can hear escaping. Damage is caused by the jets of gas causing erosion to the valve internal surface as well as excessive vibration. The trim can no longer shutoff the flow.

dirty-service-trimSpecification sheet warning signs include when the liquid vapor pressure is close to the valve inlet pressure. Also when the pressure is greater than the critical pressure. Level control valve applications are a common area where outgassing occurs. Multi-component fluids where the valve is having issues a good spot to investigate outgassing issues.

Fisher technologies that can help address cavitation issues are single-stage Cavitrol valve trim which moves the imploding bubbles to the center of the valve away from the internal surface where pitting can occur. Cavitation still occurs but away from the surface area. Multi-stage trim designs drop the pressure in stages where no one pressure drop is enough to cause cavitation.

Jonathan described Whisper Trim technology for outgassing applications. Slotted or drilled hole cage helps breaks up the flow causing the energy to be dissipated during the outgassing event, thereby reducing vibration.

Dirty Service Trim (DST) technology protects against cavitation damage and vibration/noise from high-velocity gas flow.

Jonathan shared a couple of examples where these technologies were applied to mitigate problems caused by cavitation and outgassing.

You can connect and interact with other valve experts in the Valves group in the Emerson Exchange 365 community.

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