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LDU 11.. GAS VALVE
TIGHTNESS CONTROL EQUIPMENT
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 increases 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 operation”). 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 - contact 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 :
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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 programmer
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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 indication 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.
