Retractable and sensitized packing can improve steam turbine efficiency by reducing friction during operation. Retractable packing maintains a wider gap between the packing and rotor during startup to prevent wear, while closing during operation. Sensitized packing provides tighter clearances at all times using springs. Both require proper installation to ensure expected performance, such as correct butt clearances between packing segments. Improper installation of retractable packing at one plant led to excessive wear; correcting procedures ensured proper operation.

1. Retractable and Sensitized Packing
Reduces Friction and Increases Turbine
Efficiency
Issue 8 and Volume 108.
8.1.04

Retractable packing (RP) was first installed in a steam turbine in the 1980s. Today,
the packing is installed in more than 600 turbines worldwide. Over the last 20 years,
research has shown that the use of RP helps improve a plant’s heat rate when
compared to conventional packing seals. A study by the New York State Energy
Research and Development Authority found the RP produced a 1% increase in
turbine operating efficiency and a 2% increase in capacity. Some users have
reported gains in capacity of 2-3%.
To obtain the benefits of RP, specific installation procedures must be followed.
Through observation of errors at existing RP applications, a proven installation
procedure has been developed that ensures RP will operate as designed. These
improved installation procedures have also been applied to a new seal called
sensitized packing (SP). SP has been developed for use in low-pressure
environments where RP cannot be applied.
In operation the SP does not retract and during startup it remains in the closed
position. Lightly loaded springs are used to minimize the clearance between the
packing segments and the rotor. To avoid rubs and wear the packing is able to yield
should it come in contact with the rotating shaft.
Packing Evolution
To properly install retractable packing, it is necessary to understand the operating
principles and how it differs from conventional packing. Packing used in steam
turbines has two primary but competing goals. The first is to limit the heat losses
caused by steam traveling along the gap between the shaft and the packing. This is
achieved by locating the packing as close as possible to the rotating shaft. The
second is to eliminate friction caused by the packing contacting the shaft.
A compromise solution to this problem is to use spring-loaded packing segments.
Conventional packing, retractable packing and sensitized packing all utilize springs.
Because the springs allow the packing segments to be located closer to the rotating
shaft, heat loss is minimized from the turbine. Conversely, should rubbing occur, the
springs increase the clearance between the packing and the steam turbine’s rotor. At
low flows and low speed, the rubbing between the rotor and the packing, and the
heat it generates, can result in rotor bowing and wear of the packing teeth and tip
seal.

2. Because most packing rubs are caused from vibration and thermal distortion of the
rotor during startup, RP is designed to produce a wider gap during steam turbine
startup. Unlike conventional packing, where the flat springs press the segments
toward the shaft, retractable packing springs maintains a 0.15-inch clearance
between the packing and the shaft.
As the turbine steam flow and pressure increases the steam pressure pushes the
packing closer to the shaft’s recommended OEM design clearance of 0.025-inches.
This not only helps to prevent startup problems, it also increases the steam turbine’s
efficiency and lowers the cost of maintenance.
Sensitized Packing
Retractable packing, while superior to standard packing, has limitations. During
startup the packing must be retracted, which results in some heat loss. However, this
is minimal when compared to the savings gained from reduced wear to the packing
and tip seals during startup. Further, since it requires a pressure differential to move
the packing toward the shaft, there are certain environments where it cannot be
used. In general it is not used in low-pressure (LP) stages due to inadequate
pressure differential.
Sensitized packing can be used by itself, or in conjunction with either RP and/or
standard packing. Unlike RP, sensitized packing does not rely on steam pressure to
close. As a result, it can be used in the LP stages of the steam turbine and the end
seals. During operation the SP is always in the closed position as sensitized coil
springs hold the segments in place.
In December 2003, Toshiba Corporation’s Turbine Design Group released the
results of tests it conducted comparing SP’s performance with conventional packing.
Among the findings:
 The rotor disk temperature rise from friction using sensitized packing was
small compared to conventional packing.
 Because of the lower amount of rubbing and the lower temperature rise, the
packing produced less rotor bowing and vibration.
 The packing was effective in preventing hard rubbing and reducing steam
leakage.
SP was initially applied to two turbine boiler feed pumps in China. The pumps were
having problems with excessive steam leakage from the end packing rings, which
resulted in contamination of the bearing’s lubricating oil. Since installing the SP, the
steam leakage has been eliminated. Currently SP is installed on 12 turbines in
China, six in the U.S. and one in Puerto Rico and Guatemala. The units vary in size
from 10 MW to 350 MW units.
U.S. Installation

3. During a 2003 outage, the Pinetree Power Fitchburg, LP plant in Westminster,
Mass., underwent its first major overhaul. Energy Resources Group (ERG) Dover,
New Hampshire was awarded the maintenance contract.
Although the plant had conventional packing in stock, at ERG’s recommendation,
they opted to modify the conventional rings to act as sensitized packing. According
to plant manager Jonathan Clapp they considered using retractable packing, but
since it could not be used at the vacuum end of the turbine, the plant decided that
SP was a better choice. They ended up installing 18 rows of SP, 11 in the turbine
stages and the other seven in the end seals and between the nozzle box and the
turbine shaft.
Clapp reports that although the SP added minimally to the time and cost of the
outage, it made a huge difference in the unit’s performance. The seal upgrade has
reduced the plant’s heat rate by 3% and the investment was recovered in less than
12 months.
Depending upon the existing pressures, packing ring weight and dimensions, power
plants can use either SP or RP. Since the overall efficiency of a steam turbine is
lower during startup, the effect of RP being in the open position is minimal compared
to the gains from avoiding packing and tip seal rubs during startup. Although SP can
experience minor packing tooth tip rubs during startup, it is a minor problem when
compared to the benefit of tighter clearances during operation.
RP Installation Concerns
After nearly two decades of experience and installations at over 600 plants, RP has
shown itself to be a cost-effective and a reliable method for improving efficiency and
boosting output. Any problems with RP packing operation can generally be traced to
improper installation.
A Westinghouse 330 MW turbine at a California power plant is a typical example of
what can happen if RP packing is not installed correctly. Over a period of seven
years the RP was a constant problem. An inspection of the retractable packing
revealed that 12 of the 15 retractable packing rings installed in the high-pressure
(HP) turbine were so badly worn on the upper halves that the teeth required
replacing. This is common with misalignment.
The HP dummy ring, which houses six rows of packing, was found to have a back
travel of 0.115-0.12 inches. In multiple row packing such as this, the last ring should
have closed at approximately 40% flow, based on the calculations performed using
the design back travel of 0.09 inches. However, with an average increase in travel of
0.025 inches, it was uncertain whether the rings were closing properly.
To hold the six upper half rings in place, a single long retaining pin is installed on
each side of the upper half of the HP dummy holder. The retaining pins are located
in slots cut into each of the upper half packing segments. Regrettably, at the
California plant these pin slots had been incorrectly located, which caused the upper
half packing segments to sag onto the lower half segments. As a result, the lower

4. half packing segments moved away from the shaft and increased the lower radial
clearances.
Verification of the existing packing ring butt clearances revealed several locations
having too little clearance. At two of the locations, excessive butt clearance was
found, causing steam to leak. To correct the problems, and to ensure the proper
operation of the retractable packing, the following actions were taken:
 New packing ring steel teeth were installed and machined to the required
diameters.
 The increase in back travel was eliminated by welding Inco 82 buttons onto
the backside of the packing ring. This limited the travel distance to the
Westinghouse design requirement of 0.09 inches, Figure 1.
 Packing rings that had too little butt clearance were re-measured and the
segments cut to obtain the required clearance.
 New left and right upper segments, with new pin slots, were manufactured for
the HP dummy rings.
The only correction for packing rings that have excessive butt clearance is to make a
longer segment or to manufacture a new ring. However, due to outage schedule
constraints, the customer opted to leave the clearances as is.
Author- Douglas R. Brandon, principal, Brandon Retractable Packing Corporation,
has a BS in engineering from Colorado School of Mines and 20 years’ experience
with steam turbines. He has conducted more than 100 steam path appraisals and
has developed and directed the proper procedures for installation of BRP for more
than 15 years.
Installation Techniques
To achieve maximum operating efficiency it is critical that retractable and sensitized
packing be installed correctly. Packing rings must be installed so that all the ring
segments have at least the necessary butt clearance-the circumferential clearance
between the packing segments. This clearance allows the ring to differentially
expand in relation to the holder at operating temperature. It also allows for the
growth of oxide layers between the butts of the segments.
In addition to correct butt clearances, retractable and sensitized packing must be
installed so that all of the rings have adequate clearance around the upper half joint
retaining pins. The clearances around the pin must ensure that the upper half joint
segments can completely open or retract during operation. Likewise, the clearances
must allow the upper and lower half packing to expand and/or grow oxide layers
without interference from the pin. When insufficient circumferential clearance is
provided for the pins the upper-half packing will grow arch bound against the pins.
The end result is the upper half seal radial clearances will be too large.
Conversely, excessive circumferential clearance around the pins will allow the upper
half segments to sag and contact the lower half segments. In the case of retractable

5. packing the clearance between the upper half segments and the lower half segments
should be no more than 0.0625-inches. With sensitized packing, the upper half
packing segments should be completely isolated from the lower half packing.