BCU 560, BCU 565 · Edition 02.16
76
Parameters
11 .6 Air control
11 .6 .1 Pre-purge time t
PV
Parameter 34
A burner start may only occur if it has been ensured that the
concentration of inflammable components in all sections of the
combustion chamber and the connected areas and the flue
gas ducts is less than 25% of the lower flammability limit of the
fuel gas. In general, a pre-purge is performed by the protective
system (FCU) to ensure compliance with these requirements.
Parameter 34 determines the time for which pre-purge is to
occur after a safety shut-down (0 to 6000 s).
With this function, the combustion chamber of the burner can
be purged after a safety shut-down in compliance with stand-
ards (on the basis of EN 676, EN 746-2, NFPA 85 or NFPA 86,
for example), especially on radiant tube burners. This task is
not implemented by the central protective system but by the
BCU 565.
If air monitoring has been activated in parameter 15 or 35, the
pre-purge time t
PV
starts as soon as the air monitor detects
an adequate flow for purging, see page 74 (Low air pres-
11 .6 .2 Air flow monitoring during pre-purge
Parameter 35
Function of the air flow input (terminal 48)
The air flow is monitored during pre-purge by the differential
pressure switch connected to terminal 48. If the air volume
and therefore the differential pressure on the air pressure
switch falls below the set value, the BCU will perform a safety
shut-down or fault lock-out.
BCU 565
µC
P
49
15
13
14
50
45
48
p
e
/2
TC
PZL
PZH
PZ
PDZ
ϑ
1
2
3
48
PDZ
When the air actuator is switched off, the “no flow” state (default
position) of the differential pressure switch is also checked if
air flow monitoring has been activated. Activation of air flow
monitoring and the shut-down properties can be set using
parameter 35.
Parameter 35 = 0: Off; the air flow monitoring function is de-
activated.
Parameter 35 = 1: with safety shut-down. If there is no signal at
the input (terminal 48), a safety shut-down will be performed.
Parameter 35 = 2: with fault lock-out. If there is no signal at
the input (terminal 48), a safety shut-down with subsequent
fault lock-out will be performed.
Air flow monitoring is to be set on the basis of the relevant ap-
plication standard (e.g. EN 676, EN 746-2, NFPA 85 or NFPA 86).
