Riello VGD 50 Manuel D'entretien page 18

PROPORTIONING GAS VALVE WITH AIR/GAS RATIO CONTROLLER
Description
The SKP75 controls the gas pressure as a func-
tion of the combustion air pressure so that the gas
to air ratio remains constant over the entireoutput
range of the burner.
Using the SKP75, a separate gas pressure gover-
nor is not required. The gas train is this shorter,
simpler and offers considerable savings.
Design features of the controller (Fig. 11)
The controller is fitted to the housing of the valve
actuator and has two diaphragms (1) which, via a
lever system, act on a ball valve (2) located in a
bypass (3) of the pump (4).
The pressure of the combustion air acts on one
diaphragm, the gas pressure downstream of the
valve on the other.
The gas to air pressure ratio can be adjusted from
0.4 to 9 and the selected value is indicated in a
viewing glass.
The selected characteristic for the pressure ratio
can also be displaced parallel (+/-), either towards
"excess air" or "lack of air", to increase the
amount of air at low-fire levels, for example. The
extent of parallel displacement is indicated in
another viewing glass.
The controller can be adjusted during burner
operation.
Depending on pressure conditions, the control
accuracy is between 2 and 10%.
Safety Shutoff Function
The electro-hydraulic actuator consists of a cylinder filled with oil and a piston containing an electric oscillating pump
and a hydraulic 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 orifice) 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 controller. 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 command.
When the gas valve opens, the pressure downstream of the valve increases immediately and thus the pressure at
the gas diaphragm. As soon as the forces acting on both diaphragms are in balance, the by-pass valve in the actua-
tor 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.
1
2
4
3
Very simplified sectional view
18
GB
Air
Combustion
1
1 Diaphragms
2 Ball valve
3 By-pass
4 Pump
Fig.11
2916016