POWER May 2023 - 38

OPERATIONS & MAINTENANCE
5. Downward flowing valves with horizontal actuators (left) were the main option a few decades
ago for turbine bypass valves. New flow up valve designs (right) may be installed without
piping modifications in most instances, providing extended service life and improved performance.
Courtesy: Emerson
6. Downward flowing valves subject the
seat to erosive high-velocity steam. Upward
flowing steam velocities are much lower because
they pass over the seat and accelerate
later as the steam moves through the small
trim holes, greatly extending trim service life.
Courtesy: Emerson
7. These pictures of flow up trim clearly show the erosive impact of high-velocity steam, but
unlike the flow down trim damage shown previously, this valve has no damage to the seating
surface and can still shut off tightly. Courtesy: Emerson
nology and practices have improved, and
upgrading to the latest sealing technology
could inject new life into the valve.
Another example of a technology upgrade
is when there have been repeated
failures with a welded diffuser. In this instance,
a removable two-in-one seat diffuser
will reduce required maintenance.
If a turbine bypass valve must be replaced,
plant personnel are strongly
encouraged to look beyond a direct replacement
and evaluate potential improvements.
A
particularly important
item to consider when looking at replacement
and improvement opportunities
is valve orientation.
Downward Flowing Valves
with
Horizontal Actuators. Historically, most
turbine bypass valves employed a flow
down trim design paired with a horizontal
actuator (Figure 5 left). This arrangement
is ideal for locations with low overhead
clearance, and it places the actuator
closer to the deck or ground for easy access,
but it creates a number of long-term
operational problems. Gravity tends to
result in increased and uneven wear on
trim components, and the side-mounted
actuator is prone to response problems.
Downward Flowing Valves
with
Vertical Actuators. A superior option
for flow down is vertical mounting of the
38
actuator. This arrangement results in reduced
and more even wear, longer runs
between maintenance outages, and improved
actuator response. The challenge
with this orientation is that replacement
opportunities are typically for horizontally
mounted actuators, and the two designs
are not readily interchangeable without
significant piping modifications.
Upward Flowing Turbine Bypass
Valves. When sufficient overhead space
is available with existing horizontal actuator
flow down designs, a flow up design
can easily meet the existing face-to-face
requirements of the old valve, while reducing
some of the wear common to
flow down designs with a horizontal actuator,
such as gear toothing. Because
the trim just switches from flow down to
flow up, this change can be made without
modifications to existing piping.
Flow up valves (Figure 5 right) significantly
extend seat and trim service life
because the steam is moving relatively
slowly as it passes over the plug and seat
(Figure 6). As the steam moves through
the small holes of the cage, it accelerates
significantly, but that energy dissipates
into the large body cavity of the valve instead
of striking the seating surface.
Notice how the flow up trim (Figure 7)
is certainly showing erosion, but when
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compared to the flow down trim shown
above (Figure 4 right), the seating surface
on the seat ring and plug remain relatively
undamaged, allowing the valve to
shut off completely, the required mode
in normal operation.
The flow up valve style design still
provides the same control responsiveness
and flow capacity, but the new
flow arrangement dramatically extends
the operating life of the seals, seat, and
valve internals.
Consult an Expert
Turbine bypass valves are highly specialized
pieces of engineered equipment that
must be carefully specified, installed, and
maintained. It is therefore wise to be proactive
by reaching out to trusted experts
prior to purchase as they can assist with
valve sizing and specifications based on
specific operating conditions.
For existing installations where unplanned
turbine bypass valve failures are
bedeviling your facility, or if it has been
some time since the plant turbine bypass
valves have been fully inspected, it
would be wise to consult your turbine bypass
valve vendor for help and support.
Many of these vendors offer full inspection
and health check services to
identify areas where metal fatigue and
stress cracking are developing. Ideally
the valve can be repaired, but if a valve
replacement is warranted, the vendor can
also provide guidance in evaluating bypass
valve design styles and choosing the
best option for the specific application. ■
-Chris Roling is a product engineer in
the Steam Conditioning Group at Emerson.
POWER | May 2023

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POWER May 2023

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