Whether it is caused by a “poor” design, extreme operating conditions or even too much deterioration, turbine failures can occur. In order to help prevent these it is necessary to perform regular maintenance on all parts of the machine and control the conditions at which the turbine is operating at any moment in time as well as performing repairs and retrofits to keep the pieces in good shape.
One way to improve steam turbine efficiency is through better seals. However, when designed incorrectly they can create significant damages and performance losses in the turbine. Sealing steam turbine rotors presents several challenges. Any gap between the rotor and the packing lets the steam escape, dropping the pressure and wasting energy. If the packing ring is too tight, however, the rotor will rub, which creates localized hot spots.
This, in turn, causes the rotor to bow. Such a bow in the rotor deflects the blade cover into the tip seal causing it to rub as well. Therefore, preventing the initiation of the packing rub will also prevent the blade tip seal rub.
An early solution to this problem was the conventional labyrinth seal packing ring. It is a segmented ring that is held at close clearance to the shaft by flat springs that are positioned behind each segment. The force exerted by the flat spring is designed such that freedom of movement of the segment is not possible during rubbing, contact with the shaft. At higher flows, the steam force that is naturally behind the segments adds to the pressure of the spring. These two combined forces then hold the ring segment against the shaft causing the labyrinth teeth to rub, resulting in excessive clearance.
The image below is used to show that as the turbine ages its clearances also change. It can be seen from this figure that an entire percent of efficiency is lost when the considered turbine has its stator seal clearance go from 0.4 mm to 1.025mm. Bigger values for the clearances induce more leakage losses which can significantly reduce the machine’s performance since a more important mass flow is not creating any useful power.
Improving the performance of steam turbines and the efficiency of seals is still a common concern, but the industry is continuously finding more ways to improve upon it.