Turbine developments |
Advanced sealing for steam turbines increases efficiency, lowers costs
Guardian® packing rings and Vortex Shedder® tip seals (also called spill strips) are improved steam
turbine seal designs developed by MD&A, drawing on its decades of experience inspecting and repairing steam turbines from all the main OEMs. Retrofit projects to date show that this advanced sealing technology delivers significant efficiency gains relative to conventional sealing systems
Mark Passino MD&A Chief Engineer of Product Development, Latham, NY, USA
Left, Vortex Shedder®
tip
seal; right, Guardian® packing ring; middle, Guardian®
/Vortex Shedder® hybrid
Seals within a steam turbine serve a very important purpose: to prevent steam from leaking between the rotating and stationary components, and to keep air out.
Over time, seals deteriorate, and components become distorted. This could lead to excessive clearance, which reduces turbine efficiency. Conversely, clearances may become too tight, causing turbine rubs and operational problems. To avoid these issues, it’s important to do regular inspections, and for seals to be properly replaced before smaller issues become much larger, costlier ones.
While the temptation may be to “simply” replace a seal with the OEM part, in reality the new sealing system will need to be designed for the specific turbine that is being serviced. Why? No two systems will wear the same way, thus requiring sealing components that are designed and manufactured for the specific conditions of the steam turbine.
In an impulse steam path, leakages associated with conventional seals and shaft-packing
can account for as much as 29% loss of stage efficiency.
There are two common forms of distortion that cause efficiency and operational issues over
time. These are: ● Elliptical bore distortion. This is where components such as diaphragms, blade rings, and gland cases become elliptical from years of service. This distortion alters the seal radial clearances, resulting in seal rubs, wear and increased seal clearances. In turn, sealing efficiency and overall turbine efficiency and output are decreased.
● Dishing distortion of the diaphragm. The axial relationship between seal teeth and the turbine rotor is critical to a smooth-running and properly sealed turbine. As a turbine ages, the interstage diaphragms/nozzles may creep axially due to differential steam pressures. This changes the relationship between the stationary seal teeth and the rotor. If the change becomes too great, the rotor will rub the seal teeth, causing vibration and increased seal clearances.
30 | July/August 2022|
www.modernpowersystems.com
Leveraging experience Leveraging decades of experience inspecting and repairing steam turbines from dozens of manufacturers, MD&A has developed products designed to address these sealing challenges: Guardian® tip seals.
packing rings; and Vortex Shedder®
In summary, Vortex Shedder® tip seals provide improved performance over conventional designs, while Guardian® packing rings prevent seal damage while maintaining seal performance when compared to conventional designs. Guardian packing rings will replace virtually any conventional OEM packing ring without requiring modifications to either the holder or the rotor. Made of a patented, non-galling, low coefficient of friction material, they are set with a radial clearance 0.005” (0.127 mm) less than the conventional teeth. They will perform in any location, at any temperature, pressure or flow condition.
During normal turbine operation, nothing should be touching. However, if a rub condition is
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