Every startup followed the same pattern on our 27.4 MW steam turbine generator. As the rotor accelerated during run-up, vibration would spike sharply within two distinct speed bands. Alarms would trigger consistently. Operators would watch the trend carefully as the turbine passed through those ranges - on a machine that size, that's not a comfortable position to be in. Then something curious happened. Once the turbine reached operating speed, the vibration would settle down and remain stable. At first glance, it looked like a conventional balancing problem. The rotor was sent for low-speed shop balancing before dispatch to the high-speed facility. The balance certificate looked acceptable. But the startup behavior never changed. The real clue was in the rotor dynamics. This turbine operates at 5914 RPM, and the rotor passes through two critical speeds before reaching operating speed. That immediately tells you something important: the rotor behaves as a flexible rotor, not a rigid one. When flexible rotors pass through critical speeds, the shaft does not simply rotate as a rigid body. Instead, it bends into different mode shapes. The first critical corresponds to the first bending mode, the second to a higher bending mode. Each mode responds to its own distribution of unbalance - what rotor dynamitists call modal unbalance. This is where conventional low-speed balancing reaches its limits. Balancing machines operating below the rotor's critical speeds only see the rotor as a rigid body. They can correct static and couple unbalance, but modal unbalance remains invisible in that environment. It is possible to obtain an acceptable balance certificate while the rotor still exhibits high vibration when passing through critical speeds during every startup. To properly address the issue, the rotor was sent to a high-speed balancing facility. It was spun through its critical speeds and up to rated operating speed while vibration response was measured across multiple planes. Using multi-plane correction methods based on API 684, the balancing team calculated corrections that minimize vibration across the entire operating range. After corrections were applied, the rotor was verified at 5914 RPM for approximately 10 minutes. Vibration levels remained well within API limits throughout. The modal unbalance was gone. Startup behavior confirmed it on the next run. Cases like this are a good reminder for anyone working with high-speed turbomachinery. If the machine consistently alarms through the same speed bands on every startup but runs cleanly at full speed — and low-speed balancing hasn't fixed it - the problem isn't the amount of unbalance. It's the type. Stop rebalancing at low speed and get the rotor into a high-speed facility. Sometimes the real solution isn't balancing more. It's using the right tool for what the rotor is actually doing. Howden Maintenance Partners #SteamTurbine #Reliability #Maintenance #Turbomachinery
