BOILER
During load rejection event
in a power plant, particularly in systems with steam boilers, several significant
changes can occur that affect the boiler’s operation and condition here’s what
typically happens to a boiler during a load rejection:
1.)
Sudden reduction in steam demand:
Load rejection results in an immediate drop in the demand for steam from the
boiler. This sudden decrease in load can lead to an imbalance between steam
generation and steam consumption.
2.)
Rapid increase in Steam Pressure:
With the reduction in steam demand, the pressure inside the boiler can increase
rapidly. The boiler’s steam pressure control system may attempt to maintain
pressure within the desired operating range, but if the load reduction is severe
and sudden, the pressure may exceed safe operating limits. Thus, pressure
relief valve will begin to pop, initially at the secondary superheater to
relieve pressure and if not enough, the pressure relief valve in the steam drum
may pop up as well to further decrease increased steam pressure. (Depending on
the pressure settings of pressure relief valves)
3.)
Potential for steam drum overfilling:
IN boilers with a drum, the sudden reduction in steam demand can lead to a
temporary overfilling of the steam drum. Without sufficient steam leaving the
drum to maintain the water level, the water level may rise above normal operating
levels. This condition, known as carryover, can lead to water entering the
steam lines and causing damage to downstream equipment.
4.)
Steam Temperature fluctuations:
Load rejection can also cause fluctuations in steam temperature within the
boiler. As steam demand decreases, the temperature of the remaining steam may
rise, potentially exceeding design limits.
5.)
Boiler trip and protective actions:
If the sudden load reduction exceeds the boiler’s capacity to adjust,
protective systems may trip the boiler to prevent unsafe operating conditions. This
can include tripping of fuel supply, closing of steam inlet valves, or initiating
emergency shutdown procedures.
6.)
Loss of heat sink:
Load rejection may also result in a loss of the turbine’s condenser as a heat
sink for the steam exiting the turbine. Without adequate cooling, the temperature
and pressure of the steam leaving the turbine may increase, potentially causing
damage to turbine blades and other components.
7.)
Control system response:
Boiler control systems and protective relays monitor operating conditions and
respond to deviations from normal parameters. During a load rejection event,
these systems may initiate corrective actions, such as adjusting fuel flow,
activating auxiliary equipment, or tripping the boiler if necessary.
8.)
Post-event inspection and maintenance:
Following a load rejection event, operators and maintenance personnel conduct
inspections of the boiler and associated equipment to assess any damage or
stress resulting from the event. Repairs and adjustments may be necessary to
ensure the safe and reliable operation of the boiler during subsequent start up
and operation.
Overall, load rejection
events pose significant challenges to the safe and stable operating of the
boilers in power plants. Proper design, control, and maintenance practises are
essential to mitigate the risks associated with load rejection and ensure the
continued reliability of boiler operations.
TURBINE
During a load rejection
event in a steam turbine system, sudden changes in load demand can have
significant impacts on the turbine’s operation. Load rejection occurs when the demand
for power decreases rapidly, causing the turbine to experience sudden changes
in operation conditions. Here’s what typically happens to a steam turbine
during load rejection:
1.)
Pressure surge:
When the load is suddenly rejected, the pressure of the steam within the
turbine increases rapidly. This pressure surge can be damaging to the turbine’s
components if not managed properly. It can lead to mechanical stress on the
turbine blades, casing, and other parts.
2.)
Increased speed:
As the load is rejected, the speed of the turbine may increase due to the
sudden reduction in resistance. This can lead to overspeed conditions if not
controlled promptly, posing a risk of mechanical failure.
3.)
Governor response:
The turbine’s governor system responds to the sudden load rejection by
adjusting the steam flow to maintain turbine speed within safe limits. Governors
are designed to regulate the turbine’s speed and prevent if from exceeding safe
operating limits.
4.)
Valve adjustment:
Turbine often have control valves that modulate steam flow to the turbine. In response
to load rejection, these valves may rapidly adjust to maintain proper steam
flow and prevent overspeed conditions.
5.)
Bypass system:
Some steam turbine systems may have bypass valves or systems that divert excess
system away from the turbine during load rejection events. These systems help
mitigate the effects of sudden load changes on the turbine.
6.)
Emergency shutdown:
In extreme cases, if the turbine’s speed cannot be controlled or if other
safety mechanisms fail, an emergency shutdown may be initiated to prevent catastrophic
damage to the turbine.
Overall, load rejection
events pose significant challenges to the safe operation of steam turbines. Proper
design, control system, and operator training are essential to mitigate risks
and ensure the turbine’s reliability and safety during such events.
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