POWER May 2023 - 36

OPERATIONS & MAINTENANCE
Turbine Bypass Valve Challenges
Turbine bypass valves are one of the most difficult applications in a power plant.
Keeping these valves operating efficiently and avoiding unexpected failures is critical
for plant operation.
Chris Roling
W
hen engineers are asked to list
the most difficult control valve
applications, turbine bypass
valves are invariably mentioned. Frequent
thermal cycles, high pressure drops, and
the need for tight shutoff push these
valves to the limit. Unfortunately, many
plants tend to ignore these valves until
they fail, creating unplanned outages,
lost production, and high costs for reactive
maintenance. This article provides
suggested methods of inspection to anticipate
and mitigate issues beforehand,
and it offers upgrade alternatives should
a valve need to be repaired or replaced.
Strictly Severe Service
Severe service control valves are used in
the most difficult installations within process
plants. These installations commonly
include cavitating, erosive, corrosive,
noisy, high pressure, high temperature,
high pressure drop, or high velocity media.
Turbine bypass valves are exposed
to many of these process conditions;
yet, they must respond flawlessly and
remain leak free when closed.
As their name suggests, turbine bypass
valves are used to bypass steam
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will
fatigue the metal and create cracks in both the valve itself and the surrounding piping. Courtesy:
Emerson
turbines during plant startup and shutdown,
as well as when a turbine trips
off-line (Figure 1). In normal operation
the valves are completely closed, forcing
all the steam through a turbine. During
startup, bypass valves divert steam
away from the turbine until the properties
and conditions of the steam are appropriate
for sending it to the turbine.
This process happens in reverse at shutdown.
Using bypass valves at startup
and shutdown helps to protect the turbine
by diverting potentially wet steam,
and by ensuring only appropriate steam
conditions and flows make their way to
the turbine itself.
Should a turbine trip, the steam must
continue flowing to avoid equipment
damage due to overpressure and high
temperature, so the turbine bypass valve
immediately opens to maintain flow
through the system.
As it operates, a turbine uses steam
to perform work, reducing outlet steam
temperature and pressure. When a turbine
bypass valve opens, it will drop the
pressure, but the exit steam will remain
quite superheated, potentially destroying
downstream equipment. To avoid that
situation, turbine bypass valves either
incorporate a water injection system in
the valve body, or employ a separate water
injection desuperheater just downstream,
in either case to lower the exit
steam temperature.
As a result, turbine bypass valves face
a perfect storm of severe service conditions.
While the plant is in operation,
these valves must remain tightly closed
to avoid wasting energy. When a turbine
trip occurs, the bypass valves must respond
immediately, exposing them to
rapid temperature changes and requiring
them to pass very high flows at high
pressure drops, creating high noise and
potentially extreme vibration.
1. Depending on the power plant design, several turbine bypass valves may be employed to
instantly shunt steam around a turbine should it trip offline. Courtesy: Emerson
36
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