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There are no hold points during the ramp. The turbine is a stiff shaft design, and therefore there are no rotor critical speeds below the operating speed. As the turbine rotor was accelerating, the turbine vibration levels increased and after the turbine reached full speed, the vibrations continued to climb. The vibration was not stable and the turbine was shut down when the turbine exhaust end “X” vibration approached 5 mils. The gearbox pinion vibration remained stable the entire time.
A second turbine start-up resulted in the same outcome. The Woodward controller was then put in manual mode and the turbine was re-started and run at 500 rpm for an hour. The vibration levels were stable and all below 1.5 mils. During this time, vibration measurements were taken with a TK-81 using the output connection on the vibration transmitter. A TK-81 is a handheld vibration meter with a tunable filter. It was used to verify the DCS overall vibration readings and it confirmed that the vibrations were at running speed frequency.
After this, the turbine speed
The following day, the turbine speed was increased to 4,250 rpm. As shown in Figure 3, all the vibrations slowly started to increase. After 40 minutes the turbine exhaust end “X” vibration went from 1.5 mils to 3.6 mils. The turbine speed was then reduced to 4,000 rpm, but the vibration levels remained high and after about 50 minutes it appeared the vibrations were slowly increasing.
The turbine speed was then reduced to 3,800 rpm and the vibrations came down quickly. After about 40 minutes the turbine speed was increased to 4,200 rpm and the vibrations increased, but at a much slower rate. After about 90 minutes the turbine speed was increased to 4,400 rpm and the vibrations came up very quickly, as shown in Figure 3. At this point the turbine was shut down.
It was thought that the carbon ring shaft seals might be rubbing the shaft and causing the vibration. This was discussed with the turbine manufacturer and they advised that the carbon ring seals did not need to be broken in and should not be contacting the shaft. They suggested the problem might be due to misalignment or excessive pipe strain. The turbine was re-started, heated up and then shut down so that a hot alignment check could be performed. The hot alignment was good and thus poor turbine-to-gearbox shaft alignment did not appear to be the problem. The turbine and gearbox also were checked for soft feet, and it was confirmed that this was not a problem.
The vibrations were all at running speed frequency. This is typically associated with unbalance, but it did not appear to be the cause. If unbalance was the problem, high vibration would be present at all speeds. And turbine-to-gearbox misalignment did not appear to be the problem based on the good hot alignment check.
At this point problems associated with excessive pipe strain were considered. The turbine skid and foundation were inspected and were visually in very good condition. Although the piping was not modified prior to the outage, the inlet trip/throttle valve was removed for overhaul. There is a spring support under the trip/throttle valve, and a loose support could have been a cause of the unstable vibration. The trip/throttle valve actuator also was vibrating excessively at speeds above 4,000 rpm, but the plant operators reported that this had been a problem dating back to the original start-up. The spring support was inspected and found to be in good condition. The spring support preload was increased, but there was no change in the trip/throttle valve actuator vibration or the turbine vibration.