“Introduction:

This paper presents some unusual vibration problems experienced at various power stations in Egypt. These cases are unusual because the problems encountered are rarely seen in such units.

The first case is of two 627 MW steam turbines with both their LP rotors operating within their critical response envelope. Moreover, these turbine units exhibit unusual behavior at the generator rotor with power factor variation. This leads to high vibration at lower power factors, with wide 1x phase changes which probably would mean that the unit is hard to balance in the range of power factor 0.85 to 0.95.

The second case is for a tilting pad bearing failure during coast-down. Root cause analysis of this failure at the first bearing of the high pressure turbine (eight bearings away from the generator) is shown to be due to oil film breakage caused by electric charge leakage through the rotor.

These cases illustrate the need to investigate the machine as a unit, with all machine data. The most widely used tool for diagnosis, the vibration spectrum, was not helpful in any of these cases. Rather machine data and special measurements were needed to analyze these problems.

PROBLEMS IN A 627 MW STEAM TURBINES

Two new 627 MW steam turbine units were experiencing difficulties three years after start-up. The Original Equipment Manufacturer (OEM) was experiencing difficulties in fulfilling the contractual obligation of the units operating below 2 mils pp, (4 mils pp at critical), despite repeated balancing efforts. Moreover, the units were experiencing high vibration at varying power factor. The owner asked us to investigate the reasons for these. problems: As a first step to investigate these problems, we requested to review the contract for these turbines. The turbine unit trains are shown schematically in Figure 1.

The striking feature on studying the contract data is that the units operate very near the second critical speed of the LP Rotors. The units operate at 3000 rpm, while according to the Contract Technical Data (table 1), the LP rotors exhibit their second critical speeds at 2850-2950 for one LP rotor and at 3250-3300 for the other LP rotor. This gives a separation margin of about 1.7% for one LP rotor and 8.3% for the other LP rotor.

This is in contrast to a 15% separation margin recommended by most international standards (eg. APT). This means that both LP rotors are operating within the critical response envelope.”