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发表于 2018-7-20 09:45
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来自： 中国上海
Turbine water induction prevention
Background
Water ingression into a steam turbine can occur from a number of sources, and may have
catastrophic consequences. It follows that any protection against water ingression must be
wholly reliable. The 2462 Hydratect electronic system, designed to be fail operative, provides
the ultimate protection.
Problem
Water ingress into steam turbines may have catastrophic consequences and it is essential that
automatic protection devices are in place to prevent this occurring. Even a small amount of
water can cause enormous damage to the turbine blades, the cylinders, and the housing.
There is very little chance of an operator assessing a deteriorating situation quickly enough to
judge whether or not water, water droplets, or flash steam are present in bled steam lines. This
is complicated by the fact that a manually initiated trip of the turbine may further aggravate the
situation, as the decay of pressure in the turbine stages to vacuum can potentially cause
reverse flow.
Causes of water ingression
Water can reach the turbine from various feedwater plant items and under a number of
operating conditions. For example:
(a) High water level in either HP or LP feedheater, usually caused by tube leaks or the failure
of the drainage arrangement.
(b) High water level in the de-aerator: if there is a mismatch between the inflow and outflow
the vessel can flood. In each of these cases the water may flow, via the bled steam lines
and against the steam flow, towards the turbine.
(c) Un-drained bled steam lines. Wet steam can deposit water on the pipework walls, and
condensation can occur at bends in the pipework and at valves. Condensation is also a
problem during start up when the steam lines are being warmed.
White paper: WP04 Turbine water induction prevention
April 2010 – Page 2
(d) A unit trip or sudden load reduction, resulting in a pressure reversal. During a trip, the HP
turbine pressure decays rapidly and the IP/LP pressure falls to condenser vacuum almost
immediately. In contrast, the pressures in the feed system change relatively slowly. Large
pressure differentials are created which will tend to stimulate flow towards the turbine from
the feed system.
(e) Reverse steam flow in the bled steam lines can potentially carry water from heaters or
un-drained low points to the turbine with consequential damage.
Water ingression is not only a problem when the turbine is at operating speed; water flowing
onto hot cylinders while the turbine is on turning gear can cause severe chilling with distortion
or cracking of the cylinders.
Design philosophy
The Hydratect system detects either water or steam where they are not wanted. An allelectronic
system, it is completely reliable by design. It uses the difference in resistivity
between water and steam across an electrode to inform the user of an unexpected condition.
It continuously checks the measurement electronics, connecting cables, and electrodes for
correct operation. It is fault tolerant, informing the user of a fault condition but continuing to
operate normally.
The electrodes of the 2462 operate equally efficiently over the complete temperature and
pressure range experienced in steam generating plant; pressures from vacuum to 300 bar
(4350 psi) and temperatures from 0 to 560 °C (32 to 1040 °F).
Each 2462 has two independent, separately powered channels. These may be used to provide a
fully validated output, for example in a tripping circuit, or separately to open and close a drain valve.
Typical installation sites in feedwater heating systems for the 2462 include:
(a) Alarm annunciation and drain operation on main steam lines
(b) De-aerator tanks for high water level alarms against turbine water ingression and low level
alarms to prevent the boiler feed pump from being starved.
(c) Drain operation on bled steam lines feeding both high pressure and low pressure heaters,
de-aerator.
(d) Condenser hotwell for high water level alarms and extraction pump operation.
White paper: WP04 Turbine water induction prevention
April 2010 – Page 3
In Figure 1, both high and low alarms are
fitted. The inner electrode provides an alarm
output and the outer electrode gives a trip,
providing that the inner electrode validates it.
If the 2462 has detected either electrode to
be faulty then the trip is inhibited and a fault
output is provided.
Figure 1. Storage tank with both high and low alarms and trips
In Figure 2, the lower electrode in the manifold is
configured for water normal, the upper electrode for
steam normal. When the water level in the drain pot
reaches the upper electrode the status output
changes, opening the drain valve and setting a latch.
As the water level falls below the lower electrode, its
status output changes, resetting the latch and closing
the drain valve. In critical situations, a second 2462
could be fitted to provide high and low level alarms in
the case of the valve failing.
Figure 2. Automatically operated drain valve for bled steam lines |
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发表于 2018-7-19 15:04
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