Position Check; Dynamic Safety Time Ratio Control - Siemens LMV5 Series Basic Documentation

Dynamic safety time
ratio control
Siemens Building Technologies
HVAC Products

5.1.2 Position check

Definition of «Safety time ratio control»:
«Safety time ratio control» is the period of time during which deviations from the
−
required position of one 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 ratio control system increases in proportion to its deviation from
the required state
All safety-related supervision functions of the ratio control system, especially checking
of the actuator positions with regard to the required positions, are based on this «Safety
time ratio control».
The «Safety time ratio control» of 3 seconds considers the worst case. 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 unburned or partly burnt gases
produced within 3 seconds is not sufficient to cause deflagrations or explosions within
the «Safety time ratio control» or just after that time (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 unburned 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
ratio control».
This safety time is accomplished as follows:
Pos. [°]
Actual
position
2
0,3
Required
position
-0,3
-2
7550d16E/0502
< 3 s
Successful change of position
In the case of a change of position, the current actuator position must have approached
the required position to within 2° in no more than 3 seconds.
Otherwise, safety shutdown occurs.
Also, the neutral band must be reached within 8 seconds after starting the change in
position.
Basic Documentation LMV51...
5 Fuel / air ratio control (FARC)
< 8 s
t
61/278
CC1P7550en
01.10.2003