baltur COMIST 180 DSPNM, Instruction

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LDU 11.. GAS VALVE TIGHTNESS CONTROL EQUIP -
MENT
Use
LDU 11 equipment is used to verify tightness of valves on natural
gas burners.
The LDU 11 combined with a normal pressure switch automatically
verifies tightness of natural gas burners valves, before every start
up and immediately after each stop.
Tightness control is carried out by two-stage verification of gas circuit
pressure in the section between the two burner valves.
Operation
During the first stage of the tightness control (TEST 1), the pipeline
between the valves being checked must be at atmospheric pressure.
In plant without atmospheric pressure setting pipes, this pressure is
achieved by tightness control equipment. The latter opens the valve
on the furnace side for 5 seconds during “ t4 ” time.
When the 5 seconds are up, the furnace side valve is closed.
During the first phase ( TEST 1 ) the control equipment ensures that
atmospheric pressure in the pipes is kept constant.
Surveillance is carried out by the “ DW ” thermostat.
If there is blow-by in the safety valve while closing, pressure incre -
ases and as a result the “ DW ” pressure switch operates. For this
reason, in addition to indicating pressure, the equipment goes into
fault state and the position indicator stops blocked in the “ TEST 1 ”
position (red pilot lamp lit).
Vice-versa, if pressure does not increase because there is no
blow-by in the relief valve as it closes, the equipment immediately
programmes the second stage “ TEST 2 ”.
Under these conditions, the relief valve opens for 5 seconds during
“ t3 ” time and introduces gas pressure into the pipeline (“filling ope -
ration”). During the second verification stage, this pressure must
remain constant.
Should it drop, this means that the burner on the furnace side has
a blow-by (fault) when closing.Therefore the “ DW ” pressure switch
operates and the tightness control equipment prevents burner
start-up and stops in blocked state (red pilot lamp lit). If second
stage verification is positive, the LDU 11 equipment closes the
internal control circuit between terminals 3 and 6 (terminal 3 - con -
tact ar2 - outer cross-connection for terminals 4 and 5 - contact
III - terminal 6 ).
This is the circuit that usually enables the equipment start-up control
circuit. After circuit between terminals 3 and 6 has closed, the LDU
11’s programmer returns to rest position and stops. This means
it enables fresh verification without changing the position of the
programmer’s control contacts.
N.B. Adjust the “ DW ” pressure switch to about half the pressure
of the gas supply network.
Key to symbols :
} Start-up = operating position
In plants without a bleed valve = test circuit put under
atmospheric pressure by opening of valve on the
furnace side of the burner.
TEST 1 “TEST 1” pipeline at atmospheric pressure (blow-by
verification at closure of relief valve)
Putting test circuit gas under pressure by opening of
relief valve
TEST 2 “TEST 2” pipeline at gas pressure (blow-by verification
of valve on furnace side of burner)
III Automatic zero (or inoperative mode) reset of pro -
grammer
} Operation = set for new blow-by verification
If trouble is signalled, there is no voltage in all control equipment
terminals excepting terminals 13 which gives remote, visual indi -
cation of trouble.
When verification is over, the programmer automatically returns
to rest position, and is ready to carry out a further programme for
checking tightness of valves as they close.