Technical Instructions
Document No. 155-516P25
Rev. 2, December, 1998
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Landis Division
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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 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
During the burner pre-purge period, when the gas valve is closed, only the air pressure
acts on the regulator causing the air diaphragm to move to the left and close the
regulating hydraulic bypass valve. When the actuator is powered the gas valve begins to
open. The downstream differential gas pressure immediately begins to increase until the
downstream gas pressure is in balance with the air pressure (in accordance with the
pressure ratio adjusted on the regulator). The bypass valve is now partially open so that
the oil flow supplied by the pump is identical to the return flow. Unlike conventional
direct acting regulators the SKP70… servo operated regulating system displays virtually
zero droop (offset) across the turndown range.
If, for example, heat demand increases, the air damper would open further increasing
the air pressure. The air diaphragm would move to the left, causing the bypass valve to
close and the gas valve to open further. The opening of the gas valve increases the
downstream gas pressure moving the gas diaphragm to the right until balance is
restored and the flow supplied by the pump is once again identical to the return flow
through the regulator bypass.
The gas to air pressure ratio is adjustable from 0.4:1 to 9:1. The setting is visible through
a window in the regulator. Once set, the gas to air pressure ratio remains constant over
the entire output range. The gas to air flow ratio will remain constant (provided that the
cross sections of the air and gas orifices in the burner head are fixed). The SKP70… is
not recommended for use with burners incorporating a sliding/continuously adjustable
head arrangement.
Many burner designs, because of reduced mixing energy at the low fire level, require
somewhat more low fire air in order to maintain optimum combustion parameters. To
accommodate this requirement the SKP70… incorporates a bias spring parallel
displacement adjustment which allows the characteristic of the regulator to be displaced
either towards excess air or reduced air.
