A presentation from this past October’s Emerson Exchange conference on control valve vibration problems was recently published by ControlGlobal.com. The article, Ethanol plant learns why pipe cracks, describes the problem-solving path to address this situation.
Emerson’s Shawn Anderson and Adin Mann joined in an ethanol energy producer’s presentation to share the story of how the teamed with the plant staff to determine the cause and the fix for the piping failures. The ethanol production site experienced:
…repeated fatigue crack failures on a steam let-down system for an evaporator feed… The system let the steam pressure down from 125 psig to 12-14.5 psia prior to a steam diffuser.
The traditional approach to solving vibration problems is to add mass or stiffening reinforcements to change the frequencies of resonance. This approach does not address the root cause of the noised-induced vibration.
Shawn described their approach:
“We put magnetic sensors upstream, downstream, on the actuator body, low on the actuator, and on the valve itself… We also collected the sound pressure level,” Anderson says. A plot of acceleration over time for the sensors… showed that while “some were mild and some were wild,” they were generally below 2-3 G. Then they would suddenly pop up. “We learned the vibration was related to process changes,” Anderson says.
Closely coupled systems, where valves are closer to elbows and equipment, have more problems due to flow-induced turbulence… Noise radiates from the connector, and it wobbles… Some of the bolts were loosening and coming out.
The noise-induced vibration created a resonant frequency at 1800 Hz. The solution to address this resonance was:
…to quiet it down, to use a quieter valve… It was as simple as an accelerometer measurement of the frequency and a calculation by someone who understands the basic physics.
Although at the time of their Emerson Exchange presentation, a solution was not yet in place, Shawn explained:
We’ve looked at the valve sizing, the system, and the diffuser sizing, and we determined the diffuser sizing is too big. The vibration is being caused by different pressure drops due to different operation at different times. The answer may be a smaller valve, or two valves. On the Emerson side, we learned a ton. We want these chances to monitor valves that are having or causing a problem, so we can learn from them with semi-continuous or continuous monitoring.