Position Check, Dynamic Safety Time - Siemens LMV50 Series Manual

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6.5 Position check, dynamic safety time

Definition of Safety time fuel-air ratio control:
∂
Safety time fuel-air ratio control is the period of time during which deviations from
the required position of 1 or several actuating devices are tolerated before the
valves are shut down
∂
In contrast to the burner control's safety time, this safety time need not be fixed
since the risk potential of a fuel-air ratio control system increases in proportion to
its deviation from the required state
All safety-related supervision functions of the fuel-air ratio control system, especially
checking of the actuator positions with regard to the required positions, are based on
Safety time fuel-air ratio control.
The Safety time fuel-air ratio control of 3 seconds takes the worst case into
consideration. This describes the situation where the deviations from the actuators'
required positions are such that loss of flame does just not yet occur, that is, the
combustion process is as poor as it can possibly be.
In such a case, it is assumed that the amount of unburnt or partly burnt gases produced
within 3 seconds is not sufficient to cause deflagrations or explosions within the Safety
time fuel-air ratio control or just after it (that is, after the valves are shut down).
As mentioned above, it should also be considered that the smaller the deviation from
the required state, the smaller the proportion of unburnt gases. This means that the
closer the required positions, the smaller the risk of dangerous conditions.
For this reason and for reasons of availability, the LMV5 uses a Dynamic safety time
fuel-air ratio control.
This safety time is implemented as follows:
Actual
position
2
+ Value Pos. tolerance
Required position
- Value Pos. tolerance
-2
7550d16e/1115
Figure 40: Successful change of position
In case of a change of position, the actuator must leave its current position to reach the
required position within 2° in no more than 3 seconds.
Otherwise, safety shutdown is triggered.
Also, the neutral band must be reached within 8 seconds after starting the change in
position.
If, due to external effects, an actuator moves from its required position by more than ±
value Pos. tolerance (neutral band) for more than 8 seconds, the supply of fuel will be
shut down.
Basic Documentation LMV5...
6 Fuel-air ratio control (FARC)
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CC1P7550en
22.05.2018