Different packing ring configurations have been developed which provide adequate flow restriction when in good condition. Most seals, or packing rings, are designed to provide a difficult flow path which essentially results in a low flow coefficient. The performance of large steam turbines over time is nearly always compromised by excessive internal leakage that allows steam to bypass one or more stages. There are both design and operating constraints, which influence the type of seal used and the clearances that can be maintained between the rotating and stationary parts to decrease this leakage.
Proper Installation is Key
Proper installation is the key to obtaining the benefits of any type of steam turbine packing. Since the consequences of heavy seal rubs are serious, careful consideration as to how new replacement packing rings will be installed during an outage can result in less rubbing during start-up and higher unit efficiency.
Upper Half Pins and Retaining Keys
It is important during installation that the retaining pin or key slots on the upper half packing segments are machined properly; allowances for required axial and radial clearance must be taken into consideration for the packing rings to operate correctly. Restriction of movement can result in excessive radial clearances due to heavy rubbing.