POWER October 2016 - 44

OPERATIONS & MAINTENANCE
High-Speed Turbine Rotor
Balancing Lowers Costs and
Improves Operation
High-speed turbine rotor balancing was once rare because of the costs and logistical
challenges involved in doing it during an outage. That's begun to
change as economic options emerge, and experience is showing that highspeed
balancing can pay big dividends in reliability and maintenance costs.
Keith Collins
T
o a maintenance engineer or fleet manager,
unwanted vibration in a turbine
or generator is like a mosquito buzzing
around the room on a hot summer night: annoying
and elusive. But unlike a mosquito,
unwanted vibration can be expensive in
terms of extra wear and tear on rotating parts,
and it can lead to added cost in the form of an
unexpected outage.
That's why performing a high-speed balance
on steam and gas turbines and generator
rotors is increasingly accepted as a vital
aspect of a generating unit's maintenance
routine. A high-speed balance helps ensure
reliable turbine operations, an important advantage
in competitive power markets that
demand extreme unit cycling (Figure 1).
In the past, rotor balancing was only performed
at low speeds. That's because steam
turbine rotors were typically repaired either
on-site or in a facility that lacked access to
a high-speed balance cell. In addition, original
equipment manufacturers (OEMs) only
on occasion would recommend a high-speed
balance.
This approach, however, is not necessarily
the best option. That's because relying on
a low-speed balance can cause maintenance
engineers to miss issues that surface only
when the machine is run under full-speed
conditions. Power plant professionals know,
for example, that in some generator rotors,
turn-to-turn shorts are noticeable only at or
near rated speed. As a result, if the machine
is not high-speed balanced before it returns
to full operation, undetected faults can cause
vibrations. In a worst-case scenario, these vibrations
can damage rotating parts and even
force an unplanned outage.
This factor recently came into play for
a regulated utility in Wisconsin. The utility
elected to perform high-speed balancing and
run its generating units with vibration levels
44
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POWER | October 2016
even lower than what was specified by the
OEM. Its maintenance engineers reported
that the high-speed balance meant fewer
problems with bearings and hydrogen seals
in particular, and with all other major parts
in general. They found that lower vibration
reduced unit wear and tear, which in turn reduced
overall maintenance expenses.
Customer-Driven Demand
The increasing demand for high-speed balance
services is largely customer driven
and stems in part from frequent unit cycling.
Consider that liquid-cooled units
rated at 400 MW and larger were neither
designed nor intended to cycle. But market
changes and regulatory mandates have
shaken up unit dispatch orders, in some
cases to favor renewable energy resources
during their peak availability. As a result,
some formerly baseload units cycle as often
as twice a day.
In addition, spark spreads in many markets
may favor natural gas generation one
day and coal the next. This near-price parity
also factors into unit-cycling decisions.
Whether it's to comply with regulatory
mandates or in response to fuel price signals,
unit cycling leads to increased maintenance
issues.
Although high-speed balancing can potentially
help minimize maintenance issues, it
still has not become an integral part of power
generator maintenance outage planning. This
is largely because service centers have not
been available to perform high-speed balanc1.
Balancing act. High-speed balancing has been shown to minimize testing stresses during
repairs, ensuring effective performance when equipment is returned to operation. Courtesy:
Sam Fentress Photography

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POWER October 2016

Table of Contents for the Digital Edition of POWER October 2016