26
£
GAS SAFETY VALVE PROVING SEQUENCE
If the gas valve proving is enabled (refer to section option select 2.0) and a gas profile is selected, the
unit will begin the gas safety valve proving sequence on stage 3 of the start up sequence. If any step
of the gas valve proving sequence fails, the unit will perform a safety shutdown and lockout the
burner.
1.
Open main valve 2.
Main valve 2 (or the vent valve) is opened for the first safety time (t4), or 3
seconds (whichever is the smaller). If main valve 2 (or the vent valve) does not open, the unit
will indicate a fault. A check is made to ensure that the test section pressure is below 15% of
nominal gas pressure.
2.
Pressure rise interval.
All valves are closed and the gas pressure is monitored for T
test
sec-
onds. During this time, the test section pressure may not rise by more than P
test
. If this occurs,
the unit will indicate a fault.
3.
Open main valve 1.
Main valve 1 is opened for the first safety time (t4), or 3 seconds (which-
ever is the smaller). If main valve 1 does not open, the unit will indicate a fault.
4.
Pressure drop interval.
All valves are closed and after a pre-set delay of 10 seconds the gas
pressure is monitored for t
test
seconds. During this time, the test section pressure may not drop
by more than P
test
. If this occurs, the unit will indicate a fault. A check is also made to ensure
that the test section pressure is between the low gas pressure limit and high pressure range of
sensor.
5.
Valve prove finished.
The gas safety valves have been proved satisfactorily and the unit may
proceed with ignition. The unit remains in state 5 until the burner is switched off.
The gas pressure high and low limits are checked for burner status between 11 and 16.
VALVE PROVE TEST SEQUENCE
For the proving sequence, T
test
is given by the following formula:
T
test
= 3600.V.(P
G
- P
test
)
(P
atm
+ P
test
).Q
Where:
V = Volume of test section (l/cu. ft.)
P
G
= Measured gas pressure above atm. (mBar/psi)
P
test
= Max. test pressure drop/rise (mBar/psi)
P
atm
= Atmospheric pressure (1013mBar)
Q = Maximum permitted leakage rate ([l/cu.ft.]hr-
-1
)
The unit automatically calculates P
test
and t
test
based on the entered gas pressure, test volume and
leakage rate.
During the calculation, the unit chooses P
test
to keep t
test
low and avoid lengthy pre-purges.
The calculated values of P
test
and t
test
may be seen on the engineers key (see the engineer’s key
EK37, 38, 39).
Table 2:
A. Check all
valves closed
B. Open main
valve 2
C. Pressure rise
interval
D. Open main
Valve 1
E. Pressure
drop interval
F. Valve prove
test finished
Main valve 1
Main valve 2
Vent valve
-Energized/high
-Energized if vent valve not selected
-Energized if N.O.V.V. valve selected
