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Safety Shutoff Function
The electro-hydraulic actuator consists of a cylinder filled with oil
and a piston containing an electric oscillating pump and a hydrau-
lic relief system.
When power is supplied to the actuator, the pump located at the
base of the piston moves oil from the reservoir (through an ori-
fice) into the pressure chamber.
The oil flow from the pump through the orifice creates a pressure
difference moving the diaphragm and the attached spring loaded
plug to the left, closing the return flow path from the pressure
chamber to the reservoir.
This action causes the piston to move downward in the cylinder
opening the gas valve.
When power to the pump is interrupted, the pressure difference
across the orifice instantly reduces to zero.
The plug is immediately pushed to the right allowing the oil to flow
back from the pressure chamber into the reservoir, closing the
valve in less than 0.8 seconds.
This unique hydraulic relief system provides definite closure of
the associated gas valve guaranteeing fail safe operation.
A visible disk fitted to the pump shaft indicates the entire stroke
range of the actuator.
The disk also operates optional potential free switches via a set
of levers.
The switching position of the auxiliary switch is adjustable over
the entire stroke. The proof of closure switch is non-adjustable.
Regulating function
When the gas valve is closed, i.e. during the pre-purge and pre-
ignition time, only the air supplied by the fan acts on the control-
ler. It cause the air diaphragm to move to the left and, via the lever
system, the gas diaphragm to move to the left, thus closing the
by-pass valve in the actuator of the gas valve.
The actuator can therefore open the gas valve if, at the beginning
of the safety time, the burner control gives the appropriate com-
mand.
When the gas valve opens, the pressure downstream of the valve
increases immediately and thus the pressure at the gas dia-
phragm.
As soon as the forces acting on both diaphragms are in balance,
the by-pass valve in the actuator is opened to such an extent that
the return flow through the by-pass valve and the flow supplied
by the pump are identical.
This means that the piston of the actuator and thus the disc of the
valve remain in the position reached.
If heat demand increases and the air damper open further, or the
speed of the fan increases, the controller closes again the by-
pass valve, due to the greater pressure on the air diaphragm, so
that the actuator opens the gas valve further until the forces act-
ing on the diaphragms are in balance again.
The gas to air pressure ratio and thus the gas to air volume ratio
remain constant over the entire output range, provided the orific-
es in the burner head do not change during output variations ei-
ther for the combustion air or for the gas.
Because of the small mixing energy at the low-fire level it is often
necessary to provide somewhat more air in order to achieve op-
timum combustion.
The characteristic of the controller can therefore be displaced
parallel.
Start-up, calibration and operation of the burner
Very simplified sectional view
1
2
4
S8331
Key (Fig. 28)
1
Diaphragms
2
Ball valve
3
By-pass
4
Pump
Controller's working characteristic (Fig. 29)
A
Gas to air ratio for stoichiometric combustion.
B
Adjusted gas to air ratio for burner operation with excess air.
The excess air in percent is constant over the entire range.
C
When the working characteristic is displaced parallel, the
amount of excess air in percent in low-fire operation is
greater.
The controller permits a parallel displayement either
towards "excess air" or "lack of air".
Risulting effects on pressure
PG
29
GB
Air
Combustion
1
3
A
B
C
PA
Fig. 28
Fig. 29
20036762
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