BCU 460, BCU 465 · Edition 11.19
89
Parameters
11 .8 Air control
11 .8 .1 Pre-purge time t
PV
Parameter A034
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 connect-
ed areas as well as the flue gas ducts is less than 25%
of the lower flammability limit of the fuel gas. In gen-
eral, a pre-purge is performed by the protective system
(FCU) to ensure compliance with these requirements.
Parameter A034 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 standards (on the basis of EN 676,
EN 746-2, NFPA 85 or NFPA 86, for example), especial-
ly on tube firing burners. This task is not implemented
by the central protective system but by the BCU 465.
For a parameterized pressure switch (e.g. function of
sensor 1, parameter A101 = 1, 3, 5, 7, 9, 11, 13 and 15),
the pre-purge time is counted as soon as all the pres-
sure switches in use have switched. Without a pressure
switch, the pre-purge time starts as soon as the air ac-
tuator has reached the purge position.
11 .8 .2 Pre-ventilation time t
VL
Parameter A036
This parameter is used to define the length of time dur-
ing which the air valve is open before the normal start-
up. This time may be used for pre-purge. Suitable for
burners starting with full air capacity.
Adjustable from 0 to 250 s.
t
SA1
t
Z
t
FS1
A2
A3
00
t
t
VL
A1
A4
A
88
ϑ
V1
V2
V3
PZL
1
1
35
L
51
22
60
61
62
96
82
36
65
4
After the start-up signal (
ϑ
) has been applied and after
the flame simulation check and “no flow” state check
have been conducted successfully, the air valve is
opened. Start-up of the burner commences with no in-
terruption of the air supply after expiry of the program-
mable pre-ventilation time t
VL
.
Parameter setting for this example sequence:
A048 = 1; A036 > 0, see also page 87 (Delayed low air
pressure protection). The gas valve does not open until
the pressure switch has switched.