Steam Turbine Valves

STEAM TURBINE VALVES O&M
Torus Risk Engineering
BEST PRACTICE ADVICE
Best Practice Centre
Steam Turbine Catastrophe
A steam turbine catastrophe occurred in an
incident when the steam turbine tripped due to an
unknown condition within the turbine hydraulic
system. The throttle and governor valves closed
but not completely. The pilot valves could not seat
completely and the governor valve didn’t seat due
to a jamming valve stem. The generator breaker
opened and consequent to this load throw-off the
machine over sped within seconds of unit trip
resulting in a catastrophe.
The centrifugal forces caused failure of generator
rotor retaining rings, rotor wedges and turbine
blade shrouding – exciter and bearings came
off their mountings resulting in breaking of
generator casing from thrown elements. This in
turn released hydrogen from generator causing
explosion and creating a pressure transient in the
turbine room blowing out exterior block wall.
Over speed events are predominantly a consequence of
valves (stop valve, control valve and/or extraction valves)
failing to close wherein the steam flow doesn’t cease
even after turbine rejects load following a turbine trip.
Over speed can break shaft, damage turbine blades and/
or burst the generator rotor retaining rings.
The best practices to prevent such steam turbine catastrophe
include but not limit to the following:
• Daily Visual inspection of stop and control valves.
• Weekly Cycle/exercise of turbine main-steam stop
& throttle/control valves, combined reheat stop and
intercept valve ensuring their proper closing.
• Internal inspection of main stop, throttle, control,
extraction and non-return valves (wear, seat leakage,
indications of damage) bi-annually.
• Annual visual inspection and functional testing of stop,
throttle, control, extraction non-return, & other critical
valves ensuring their proper closing.
• Test over speed and vacuum trip device annually
(mechanical trips can be tested bi-annually if primary
system is electronic).
Background
The majority of large-scale power plants use steam
turbines which are high hazard to the plants. They,
therefore, become the obvious focus of best preventive
maintenance practices. One of the most critical and
paramount essentials amongst all the best practices
are the steam turbine valves operation and maintenance
which forms the basis of this bulletin.
STEAM TURBINE VALVES O&M
Some plants have seen a tendency of the throttle valves
to stick, particularly during start-up. They also experience
steam flow and speed increase of turbine after a full-load
unit trip. This clogging/sticking of main steam stop valves
and control valves due to system corrosion and excessive
magnetite layer formation results owing to many
factors in conjunction such as poor filtration in steam
and condensate systems, HSRSG start up chemistry
parameters not being controlld before latching/connecting
it to steam header (especially in frequent start/stop
scenarios), frequent switching of de-aerators (D/A) from
pressurized to vacuum operation (in plants having such
operation for D/As), HRSGs lying depressurized without
preservation for extended time, improper chemistry of
condensate returning back from cogeneration process (such
as Desalination units), incorrect location of chemical injection
points in the water circuits and un-reliable online analyzers.
• A fully fledged Root Cause Analysis (RCA) needs to be
carried out in the plants facing sticking turbine valve
problems and recommendations need to be completed
on top priority basis.
• Start-up procedures should include verification of
Throttle Valve closure at 600 RPM prior to placing unit
on-line.
• Include ‘Zero Speed’ check before turning gear
operation into the steam turbine shut down procedure in
order to ensure no leakage of the Throttle Valves prior to
closing the boiler header valve.
Summary
The healthiness of steam turbines depends essentially on
how well the turbine valves are operated and maintained.
This aspect is primarily important not only because
Insurance Companies are interested in it to limit risk
exposures but also because plant owners can curtail
costly downtime and turbine failures in addition to reduce
the insurance premiums.
References
America Society of Mechanical Engineer (ASME)
Performance Test Code 6
America Society of Mechanical Engineer (ASME)
Performance Test Code 6 Report
America Society of Mechanical Engineer (ASME)
Performance Test Code 6A
America Society of Mechanical Engineer (ASME)
Performance Test Code 6S Report, IND. STD;
Insuring Best Practices – Power Engineering
Contact us
For further information please contact our
Engineering Practice Leader for Power & Utilities
Calum Wood
+44 (0)v203 206 8139
[email protected]
This publication is intended as a general overview and discussion of topics that might be relevant to insureds and it is not
intended to be, and should not be used as, advice in relation to any specific situation nor a representation of Torus’ view on
any particular risk, circumstance or claim. Torus Insurance Company (either Torus Insurance (UK) Limited, Torus Specialty
Insurance Company Torus National Insurance Company, Torus Insurance (Europe) AG or Torus Insurance (Bermuda)
Limited, collectively “Torus”) will accept no responsibility for any actions taken or not taken on the basis of this publication.
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