Retractable Labyrinth Seals - Dresser-Rand

Retractable Labyrinth Seals
Dresser-Rand has developed and patented a retractable labyrinth seal for
interstage shaft sealing on large steam turbines.
Traditional labyrinth seals, designed to float in close proximity
(approximately .010 to .015 clearance) to the rotating shaft, are springbacked.
This allows them to "give" when the turbine rotors experience upsets,
such as during a bow on startup or when going through a critical speed or
other upsets (Fig. 1). During such upsets, the teeth on the labyrinth seals are
damaged. This results in excessive running clearances and decreased power
output, so instead of going through the proper steam path, the steam leaks
through the opened seal area.
The distinguishing feature of Dresser-Rand’s new
retractable seal that sets it apart from the rest is the
integral spring assembly (Fig. 2). The spring is secured in the labyrinth segment,
eliminating the problem of dropping springs or losing them in the turbine case during
assembly.
The retractable seal’s integral spring assembly separates the seal segments and opens
the seal clearance area when the unit is in startup or shutdown mode (Fig. 3). This
spring tension allows the seals to remain open during startup and to close on the shaft
when the differential pressure across the seal reaches a predetermined level. Just as
the seals close when the differential pressure reaches the
preset level, they open when the pressure falls below that level (Fig. 4 & 5).
When the differential pressure falls below the predetermined level – on either
rising pressure during startup or falling pressure during shutdown – the spring
tension opens the seal and pushes it away from the shaft.
During upsets, particularly during a startup rub caused by a temporary shaft
bow, damage to the labyrinth seals on larger rotors could open the running
clearances to more than .050" on radius at the worst impact stage. If this
damage occurs on initial startup or after a scheduled inspection, the unit could
operate at decreased efficiency until the next scheduled outage – perhaps three
or more years later.

For example, assume you have a 20MW turbine generator that produces electricity with an evaluation of
$.04 per kilowatt. Based on a 16-inch shaft diameter and a 16-stage machine, the efficiency loss due to the
opened clearance could be close to 2.5 percent for a .050 rub – versus normal operating clearance of .010"
on radius. Using the $.04 KW evaluation and 8,000 hours of operation per year, running with the damaged
seals would cost approximately $160,000 per year in lost power. The differential cost of the Dresser-Rand
retractable seals over traditional "floating" seals would be covered in less than one year of operation.
Contact your local Dresser-Rand representative, or call us at 1-800-275-3773 (inside the United States) or
716-593-1234 (outside the United States) to obtain a quote on upgrading your steam turbine with Dresser-
Rand’s retractable seals.
Form 2013
Copyright 2001 Dresser-Rand