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Forced Cooling

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Forced Cooling

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Forced Cooling of Steam Turbines
A power plant’s competitiveness is measured not only on performance, but also
on overall plant operating economics.
This includes having the ability to start
up and shut down the plant quickly and
effectively. Through extensive analysis
Siemens is able to provide forced cooling
of steam turbines allowing for the possibility of maintenance to begin significantly
sooner than with traditional cooling methods, potentially getting your power plant
back online faster.
Cool down gradients are determined using
the finite element method, to help protect
against excessive thermal distortion and
differential expansion. Existing temperature measuring points are used to monitor
the cool down gradients. This helps to
maintain the recommended temperature
limits during a forced cool down.
Our Solution
During forced cooling, the existing condenser air removal system is used to draw
air from the turbine hall through the blade
path of the turbines, and into the condenser, where it is ultimately exhausted
from the system. This air flow allows for
a greater amount of heat to be removed
from the components, and for the steam

turbine to cool down, more quickly than
with natural cooling. Subsequent work
on the unit can therefore be started
much earlier, allowing for an increase
in availability and improvement of the
overall plant operating economics.
Implementation
Forced cooling is a reliable way of saving
up to 5 days cool down time compared
to natural cooling. Detailed analyses are
performed for the cool down process taking
into account material strength limits and
usage factors. The forced cooling process
combines a modified software package,
filters and mechanical analyses. Siemens
has developed this process into a patented
product, which can be used to potentially
speed up inspections and reduce plant
downtime. Permanent installation of additional hardware or modification of existing
hardware is not generally required.
In some cases the installation of a nozzle
and filter may be necessary at each turbine
admission control valve. This nozzle is
mounted on the existing flanges for the
dehumidifiers. The process itself is monitored using the temperature sensors
installed as standard in the turbine.

Red arrow indicates direction of air flow

Fig. 1: Longitudinal section through an SSTx-5000 (HP/IP turbine).

Performance Enhancement – Steam Turbine

Answers for energy.

1) Scheduled shutdown
with a target deadline
Targeted shutdowns are divided into
the following steps:






Operational cool down of turbine
Prior to the plant shutdown, main
steam/reheat steam temperatures are
reduced to the minimum allowable
value in accordance with allowable
limits and protection criteria. Reducing
the temperature during operation
shortens the overall cool down time
due to the lower starting temperature
of the forced cooling process.
Natural cooling
Once steady-state temperature conditions have been achieved with reduced
steam temperatures the steam turbine
is shut down. While on the turning gear,
the turbine enters a natural cooling
phase to permit the temperature differentials within the steam turbine to reach
an equilibrium state.
Forced Cooling
At the end of the natural cooling phase
for the steam turbine, the nozzles
provided for connecting dehumidifiers
at the admission control valves are
opened and the vacuum pumps are
switched on. This draws in air via the
control valves, which pass through
the turbine blade path. The air flow
is regulated using the control valves.

After completion of these steps and
sufficient reduction of temperature, the
turning gear can be switched off and
working access to the steam turbine can
be commenced.
2) Unscheduled shutdown (e.g. after
unexpected steam turbine trip)
Unscheduled shutdowns are divided
into the following steps:




Natural cooling:
While on the turning gear, a natural
cooling phase allows the temperature
differentials within the steam turbine
to reach an equilibrium state. Assuming
the turbine was tripped at operating
temperatures, the natural cooling phase
in this case takes longer than it would
after an operational cool down in the
targeted shutdown procedure described
above.
Forced Cooling
The Forced Cooling phase is performed
the same as if the steam turbine shutdown had been scheduled. At the end
of the natural cooling phase for the
steam turbine, the nozzles provided for
connecting dehumidifiers at the admission control valves are opened and the
vacuum pumps are switched on. This
again draws in air via the control valves
which pass through the turbine blade
path. The air flow is regulated using
the control valves.

Siemens Energy, Inc.
4400 Alafaya Trail
Orlando, FL 32826-2399, USA

www.siemens.com/energy

References
Already more than 65 plants have
been equipped with forced cooling
(Status: April 2009).

Turbine shutdown
600
Operational cool down
of turbine to 400 °C
Natural cooling

For more information, please contact
our Customer Support Center.
Phone: +49 180/524 70 00
Fax:
+49 180/524 24 71
(Charges depending on provider)
E-mail: [email protected]

300

100
0

Forced Cooling
0

25

50
Time in %

75

100

Natural cooling
Planned shutdown
Shutdown due to unforeseen event

Fig. 2: Temperature curve for various cool down
procedures.

After completion of these steps and
sufficient reduction of temperature, the
turning gear can then be switched off
and working access to the steam turbine
can be commenced.

Fig. 3: Temperature distribution at the beginning of the forced cooling
procedure after the natural cooling procedure; which started at
rated condition.

Published by and copyright © 2009:
Siemens AG
Energy Sector
Freyeslebenstrasse 1
91058 Erlangen, Germany

Benefit to the customer
Our forced cooling solution offers the
following potential advantages:
■ Power plant specific optimization
■ Reduction of overall outage times
for inspection
■ Increased availability through shorter
cool down times at no risk to the turbine
■ Existing hardware and software can
generally be used without modification.

Temperature in °C

A distinction is drawn between two forced
cooling procedures:

For more information please contact
your local Siemens sales representative.

Fig. 4: Temperature distribution after the forced cooling procedure.

Energy Service Division
Order No. E50001-G520-A170-X-4A00
Printed in Germany
Dispo 34805, c4bs No. 7816
TH 258-090798 460326 DB 09092.0
Printed on elementary chlorine-free
bleached paper.

All rights reserved.
Trademarks mentioned in this document
are the property of Siemens AG, its affiliates,
or their respective owners.
Subject to change without prior notice.
The information in this document contains
general descriptions of the technical options
available, which may not apply in all cases.
The required technical options should therefore
be specified in the contract.

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