POWER February 2020 - 18

FOCUS ON O&M
3. Recessed
hydropads
technology,
designed
for double-pressurized O-ring pusher
seals, reduces the risk of uneven contact
across the seal interface, and provides better
and stiffer lubrication. It offers the advantage
of a non-contacting seal, but with less leakage.
Courtesy: John Crane
rosive media, and increased shaft speed.
To compete for market share in this
increasingly competitive space, a European-based
company knew it needed to
focus on a cost-effective turbine design
for power outputs up to 40 MW per single
generator. As part of the new design,
the ORC company introduced new silicon-based
fluids into the process. Even
so, the new fluids not only presented
temperature concerns but also corrosive
condition challenges to the turbines' existing
double-cartridge mechanical seals.
To accommodate the silicon-based
fluids, the company worked with John
Crane to provide reliable solutions to
the increased heat and corrosive conditions.
The designs included traditional
heavy-duty, double O-ring pusher seals,
with primary rings made of plain face
abradable inserts in metal carriers, incorporating
different elastomers, including
polytetrafluoroethylene (PTFE), hydrogenated
acrylonitrile butadiene rubber
(HNBR), and silicone rubber.
Based on experience with double Oring
pusher seals, John Crane engineers
tried a number of solutions that would
stand up to the high heat, or 230F to
644F (110C to 340C). Different seal primary
ring designs were tested as well.
However, none met John Crane's heat
transfer, leakage, and other performance
criteria. In addition, concerns were
raised about maintaining proper levels
of fluid film needed to lubricate the seal
faces and the amount of leaking of the
process fluid.
A Recessed Solution
As a result of the testing, the metal carrier
press-fitted carbon inserts were discarded
in favor of graphite-loaded silicon
carbide (SiC) primary rings, providing deformation
resistance with a significantly
better heat exchange coefficient.
The requirements for fluid film formation
were addressed by incorporating
hydropads into the seal design. As the
name suggests, hydropads are slots on
the sealing surface that generate hydrodynamic
separation force between
sealing rings. This technology has been
successfully used for years in the presence
of fluids which tend to evaporate,
The requirements for fluid film formation
were addressed by incorporating hydropads
into the seal design.
the new, demanding operating challenges.
John Crane had worked with
ORC companies since 2009, supplying
engineered products and services
including mechanical seals, couplings,
hydro-dynamic bearings, and seal support
systems.
Reliability and Performance
Working closely with the ORC turbine
manufacturer, John Crane engineers
determined that the original turbine included
traditional heavy-duty cartridge
seals that were not suitable for the ORC
environment. The engineers then evaluated
numerous seal designs to address
18
including propylene, butane, and others.
Because oil has low vapor pressure,
leakage does not vaporize and disappear.
Users often request the oil leakage
rate to be much lower than the typically
acceptable leakage rate. As a result,
traditional hydropads technology, while
effective in many high-pressure applications,
did not meet John Crane's criteria
for the new ORC turbine conditions.
John Crane called upon the experience
of company engineers based in Germany
for a new and effective recessed hydropads
technology (Figure 3) that had proven
itself in other turbine applications. To
accommodate the leakage, an innovative
www.powermag.com
recessed hydropads technology-designed
for the double-pressurized O-ring
pusher seal-was introduced.
Typically working with low vapor barrier
fluid like mineral oil, the recessed hydropads
technology optimizes the liquid
film thickness formation. Fluid film thickness
impacts every aspect of seal performance.
Too low thickness causes contact
between seal rings, resulting in high wear
and absorbed power. On the other side,
leakage is correlated to film thickness
with a cubic factor. Traditional hydropads
technology operates with a full, non-contacting
working mode, but with an unacceptable
amount of oil leakage.
The recessed variant reduces the risk
of uneven contact across the seal interface
while providing better and stiffer lubrication,
making the seal interface less
sensitive to seal ring distortion. It offers
the advantage of a non-contacting seal
but with much less leakage.
The ORC turbine manufacturer was
impressed that the suggested upgrade
to double O-ring pusher seals with recessed
hydropads would provide all the
benefits of the conventional seals. Nonetheless,
before implementing the recommendation,
verification was required.
At constant intervals of 1,500 hours,
the seals were dismantled and examined
at a John Crane service facility. At each
inspection, the sealing rings were found
in perfect condition without signs of
wear or damage. The testing confirmed
the capability of the recessed hydropads
technology to prevent the risk of uneven
contact, even with low film thickness.
The absence of thermal damage (heat
checking) was also the clear result of effective
heat removal.
Results
In part due to the seal upgrade, the
ORC turbine manufacturer can confidently
offer its turbine technology as a
reliable income generator to the geothermal
energy market. Seal leakage
and over-heating issues have been resolved,
and reliability is now a reality.
In direct comparison with the previous
design, leakage was reduced
by 65%, and power requirement decreased
by 32% (the average calculated
at three different pressure values).
Finally, the turbines consistently meet
performance goals, including boasting
a five-year, seal meantime between
maintenance (MTBM) track record. ■
-Mario Severino is a
drawings and application team
coordinator with John Crane.
POWER | February 2020
http://www.powermag.com
POWER February 2020

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