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Technical Instructions SKP25U.. Pressure Regulating Gas Valve Actuators
Document Number 155-752
July 9, 2014
Page 4 Siemens Building Technologies, Inc.
Operation
(See Figure 1)
Safety Shut-off Function
The electro-hydraulic actuator consists of a cylinder filled with oil, a piston containing an
electric oscillating pump, and a relief system. When power is supplied to the actuator,
the relief system closes, and the pump moves oil from the reservoir into the pressure
chamber. This action causes the piston to move downward in the cylinder, opening the
gas valve. When power to the actuator is interrupted, the relief system opens and the
gas valve closes in less than 0.8 seconds.
A position indicator, visible through the transparent portion of the terminal cover, shows
the entire stroke range of the actuator. A light, which is visible through the lower left
transparent portion of the terminal cover, indicates when the actuator receives power.
An optional, non-adjustable SPDT proof of closure over travel switch signals the closed
position after the gas valve has closed. An optional SPDT auxiliary switch is adjustable
between 40% and 100% of the stroke. The adjustment screw and scale are located on
the right side in the terminal box, and are visible through the transparent portion of the
terminal cover.
Figure 1. SKP25 Operation.
Regulating Function
The outlet gas pressure sensing line is connected to the pressure regulator housing.
The outlet pressure acts on the regulator diaphragm. The diaphragm is opposed by an
adjustable setpoint spring force, which represents the desired gas pressure value. The
movement of the diaphragm modulates a hydraulic bypass valve connecting the
pressure chamber to the hydraulic fluid reservoir. When the outlet gas pressure is lower
than the desired value, the bypass valve is closed by the movement of the diaphragm,
causing the actuator piston to open the gas valve. The opposite occurs when the outlet
gas pressure is higher than the desired value; the diaphragm moves to open the bypass
causing the actuator piston to close the gas valve. The movement of the actuator piston
stops once the outlet gas pressure is equal to the setpoint. In this position the bypass
valve is partially open so that the oil flow supplied by the pump is identical to the return
flow. Unlike the conventional direct acting regulators, the SKP25 servo operated
regulating system displays virtually zero droop (offset) across the turndown range.
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