Technical Instructions
Document No. 155-753
July 1, 2005
SKP Series
SKP55…U.. Air/Gas Ratio Controlling
Gas Valve Actuator with Safety Shutoff
Only when assembled to Series VG… Gas valves
SKP55… pressure regulating electro-hydraulic actuators are used in combination with
VG… series gas valve bodies to provide shut-off and air/gas ratio control for industrial
and commercial burner applications.
The SKP55… actuator controls the pressure difference across a restriction in the gas
supply line as a function of the pressure difference across a restriction in the air supply
duct so that the air to gas ratio remains constant irrespective of air volume changes.
There is no need for an upstream constant pressure regulator when using an SKP55…
actuator within the applicable pressure range of the VG… valve body.
Since three functions: safety shut-off, constant pressure regulation, and air/gas ratio
control can be performed by a single valve, fewer gas train components and fittings are
required. This significantly reduces both the size and weight of the gas train. In
addition, smaller diameter gas valves can be used.
The compact SKP55… actuator opens slowly and closes immediately when power is
interrupted. The modular design allows the SKP55… to be used in combination with all
VG… series gas valves bodies from 1/2-inch to 6-inch in size The actuator is easily
mounted on the square flange of any VG… valve with four pre-mounted screws. A
visible position indicator on the front of the actuator displays the entire stroke of the
valve. A light indicates when the actuator is powered.
Siemens Building Technologies, Inc.
Technical Instructions
Document No. 155-753
July 1, 2005
SKP55…U.. Air/Gas Ratio Controlling Actuators
UL listed, FM approved, CSA certified for USA and Canada, IRI approvable, ISO 9001
certified; European, Australian and Japanese approved versions available.
Safety shut-off function, pressure regulating function and air/gas ratio control in one
compact unit.
Proof of Closure with Over Travel (POC) versions are available.
Optional NEMA 4 protection.
Simplifies commissioning and reduces start-up time for:
Recuperative burners with combustion air preheating.
Burners with continuously variable air or gas nozzle openings in the burner
Burners where either the air or gas pressure is not representative of the actual
Burners with negative combustion air pressure levels.
Maintains air/gas ratio when the airflow is disrupted.
Automatic compensation for combustion chamber back pressure fluctuations.
No mechanical wear or play that causes drifting.
Visual position indication.
"Power on" indication light
Quick connect wiring terminals
Optional adjustable auxiliary switch available.
Excellent tracking characteristic.
Modular design with 360o actuator rotation for easy field wiring and installation.
Low, 13.5 VA power consumption.
SKP55… series actuators are combined with 1/2-inch to 6-inch VG… series gas valve
bodies. VG… series gas valves must be ordered separately (See VG…U...Technical
Instructions, P/N 155-512P25).
Table 1.
Product Numbers
Product Number1
110 to 120 Vac
220 to 240 Vac
Proof of Closure
Switch 2
Auxiliary Switch 2
Type of Switch
1. European, CE certified models are available (see data sheet 7643)
2. Proof of closure and auxiliary switches cannot be field installed.
Page 2
Siemens Building Technologies, Inc.
SKP55…U.. Air/Gas Ratio Controlling Actuators
Technical Instructions
Document No. 155-753
July 1, 2005
Sealing gasket to provide NEMA 3, 3R, and 4 protection
(for VGG…/VGD…valves)
As safety shut-off valve
UL/429, FM/7400, ANSI Z21.21/CGA6.5 C/I
Agency marks apply only for SKPxx.xxxU
actuators assembled with VGxxx.xxxU series
gas valve bodies.
Power supply
Operating voltage
110 to 120 Vac + 10% to 15%
220 to 240 Vac + 10% to 15%
Operating frequency
Power consumption
Duty cycle
Ambient operating temperature
Mounting position
Maximum inlet gas pressure
50 to 60 Hz + 6%
13.5 VA
5°F to 140°F (-15°C to 60°C)
Any position except upside down
Same as VG… valve
4.2 lb (1.90 kg)
NEMA 1, 2, 5 and 12 for indoor use
NEMA 3, 3R, and 4 with optional AGA66
See Figure 5
See gas valve Technical Instructions
P/N 155-512P25
Two 1/2-inch NPSM threaded knock-outs
Spring loaded terminals for 14 AWG wires
Agency approvals
Specification for valve bodies
Conduit connection
Electrical connection
Gas/air pressure connections
Output force
Maximum stroke
Opening time for maximum stroke
Control signal
Closing time
Reference input signal
Control characteristic
Operation/installation Differential pressure ratio
Permissible pressure differences
during operation for accurate control
Minimum time required for high to low
fire load changes
Maximum sensing line pressure
Maximum sensing line vacuum
Minimum diameter of sensing lines
Low fire bias range
Minimum distance of sensing lines
Auxiliary features
Proof of closure switch
Capacity of auxiliary switch
Setting range of auxiliary switch
Siemens Building Technologies, Inc.
1/4” NPT (see installation notes)
100 lb (450 N)
1 inch (26 mm)
Varies with valve size, 14 seconds for
<0.8 seconds
Combustion air pressure difference
Integral action
Gas: Minimum 0.1" w.c.
Gas: maximum 80" w.c.
Air: minimum 0.1" w.c.
Air: maximum 80" w.c.
5 seconds
20 psi
3 psi
1/4” inside diameter
-0.4 to 0.4" w.c.
3 pipe diameters before and 5 pipe
diameters after any valve, damper, elbow,
coupling, etc.
6 A/250 Vac resistive; 3A/ 20 Vac pilot duty
40% to 100% of stroke
Page 3
Technical Instructions
Document No. 155-753
July 1, 2005
(See Figure 1)
Safety Shut-off
SKP55…U.. Air/Gas Ratio Controlling Actuators
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. SKP55… Operation.
Regulating Function
During the burner pre-purge period, when the gas valve is closed, only the air pressure
difference acts on the regulator causing the air diaphragm to move to the left and closes
the regulating hydraulic bypass valve. When the actuator is powered the gas valve
begins to open. The downstream gas pressure difference immediately begins to increase
until the gas pressure difference is in balance with the air pressure difference. The
bypass valve is now partially open so that the oil flow supplied by the pump is identical to
the return flow.
If, for example, heat demand increases, the air damper opens further increasing the
differential air pressure. The SKP55… air diaphragm moves to the left, causing the
bypass valve to close and the gas valve to open further. The opening of the gas valve
increases the differential 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. Unlike conventional direct acting regulators, the
SKP55… servo operated regulating system displays virtually zero droop (offset) across
the turndown range.
Page 4
Siemens Building Technologies, Inc.
SKP55…U.. Air/Gas Ratio Controlling Actuators
Operation, NOTE:
Technical Instructions
Document No. 155-753
July 1, 2005
The SKP55…/VG… is a 1:1 differential pressure air/gas ratio controller. This means that
the control adjusts the same pressure difference on the gas side as it senses on the air
side. Any other gas to air ratio adjustment will require a modification to one of the
restrictions or the installation of an adjustable orifice (this is normally an adjustable
metering orifice in the gas line). For this purpose the VG… gas valve with manual
operation kit AGA61 may be used. See gas valve Technical Instructions 155-512P25.
Many burner designs, because of reduced mixing energy at the low fire level, require increased
excess air at low fire to maintain optimum combustion parameters. To accommodate this
requirement the SKP55… incorporates a bias adjustment, which allows the characteristic of the
regulator to be displaced either towards excess air or reduced air.
Pressure fluctuations in the combustion chamber do not influence the performance of the SKP55…
air/gas ratio controller. There is no need for any compensation circuit.
1. For optimal performance, both the air damper and SKP55… /VG… gas valve must
be installed upstream of its pressure differential orifice. For other layouts please
consult your authorized Siemens Building Technologies Combustion Products sales
2. To avoid oscillation, do not oversize the VG… valve body. (See Technical
Instructions 155-512P25.)
Personal injury or loss of life may occur if procedures are not followed as specified.
All installations must be performed by qualified personnel only.
Do not pull the actuator shaft.
If minimum gas pressure detection is required, the pressure switch must be
mounted upstream of the regulating gas valve to ensure sufficient gas pressure
before starting the burner. If maximum gas pressure detection is required, the
pressure switch must be mounted downstream of the valve.
Air proving safety devices normally required to guarantee minimum air flow must
also be provided when using the SKP55...
The gas pressure sensing line connections must be flush with the inner wall of the
gas pipe in order to sense turbulence free pressure. The gas sensing line
connections should be located at least three pipe diameters upstream and five pipe
diameters downstream of the valve, elbow, coupling, or other flow disturbing fitting.
Do not use the taps on the valve body for the gas sensing line connections
since these locations typically have strong turbulence. All sensing lines should
be as short as possible to allow the regulator to react to sudden changes.
The AGA66 sealing gasket must be installed between the actuator and the gas
valve to provide NEMA 3, 3R, and 4 protection rating for VGG… and VGD…
The SKP55… actuator is directly coupled to the VG… series valve body by four pre-mounted
4 mm Allen key screws.
The square mounting flange can be rotated in steps of 90o to provide four different mounting
positions. The SKP55… actuator can be mounted in any position with the diaphragms vertical,
except upside down.
The actuator can be mounted or replaced while the valve body is under pressure.
The SKP55… actuator has two knock-outs for the installation of 1/2”-14 NPSM conduit
When conduit routing is connected, flexible conduits must be used.
Siemens Building Technologies, Inc.
Page 5
Technical Instructions
Document No. 155-753
July 1, 2005
SKP55…U.. Air/Gas Ratio Controlling Actuators
Liquid tight conduit must be used in combination with AGA66 to provide NEMA 3, 3R,
and 4 protection.
The terminal marked GND, located above the wiring terminals, must be connected to
the electrical ground.
NOTE: Wiring must meet all relevant electrical codes.
In cases where it is not possible to install an orifice in the air line, (e.g., lack of available air
pressure) the SKP55… actuator may be connected to the air pressure upstream of the
burner and the combustion chamber pressure, using the pressure differential across the
burner orifice. This arrangement is not applicable to installations utilizing combustion air
preheating systems.
Adjustment of the bias
Connection for the air pressure (+) sensing line
Connection for the air pressure (-) sensing line
Connection for the gas pressure (-) sensing line
Connection for the gas pressure (+) sensing line
Position indication
Figure 2. Connections and Adjustments.
Page 6
Siemens Building Technologies, Inc.
SKP55…U.. Air/Gas Ratio Controlling Actuators
Regulator (See
Figures 1, 2 and 3)
Technical Instructions
Document No. 155-753
July 1, 2005
1. The setting screw 1 on the SKP55… is factory-adjusted so that the gas to air ratio
curve intersects the zero point (no bias). If required, on-site adjustment may be
achieved as follows:
Turn setting screw 1 counterclockwise until spring becomes completely loose.
Shut off gas supply upstream of the SKP55… actuator.
Make sure that there is no air pressure working on the SKP55… actuator.
Apply power to the SKP55… actuator.
Turn the setting screw clockwise until valve starts to open.
2. Set the adjustable orifice to the pre-calculated value so that equal pressure difference
on both air and gas side results in approximate stoichiometric combustion.
3. Start the burner and run it at approximately 90% capacity by opening the air damper.
4. Measure the combustion values and correct the flow using the adjustable orifice until
optimum values are obtained.
5. Return to low fire position by closing the air damper and check the combustion values.
If necessary, correct the working characteristic until optimum values are obtained by
rotating the setting screw 1. To obtain more gas: rotate clockwise. To obtain less gas:
rotate counter-clockwise.
6. Limit the low fire air damper position.
7. If a substantial bias of the working characteristic was needed the adjustment of the
90% position must be checked and corrected, if necessary, by adjusting the orifice.
Repeat the procedure from Step 3.
8. Run the burner to the required high fire position and limit the air damper position.
9. Check flue gas values at several intermediate output levels. If corrections are
necessary, note:
At high fire, correct with the adjustable orifice
At low fire, correct with setting screw 1 on the SKP55… actuator.
Figure 3. Adjustments.
Siemens Building Technologies, Inc.
Page 7
Technical Instructions
Document No. 155-753
July 1, 2005
SKP55…U.. Air/Gas Ratio Controlling Actuators
Start up,
Wiring and
When firing at maximum burner capacity, ensure that the SKP55… /VG… is not in
the fully open position. If this is the case, either the gas valve is sized too small or
the gas supply pressure is too low
The actuator is equipped with spring-loaded wiring terminals for 14 AWG wires.
Insert one wire into the opening of the terminal while pressing the lever downward with
a screwdriver or hard object. Make sure that all strands insert into the opening.
The actuator has two line and two neutral terminals.
Adjust the auxiliary switch (if provided) according to the wiring diagram on the label
below the terminals (see figure 4). The adjustment screw and scale are located on the
right side of the terminal box, and are visible through the transparent portion of the
terminal cover.
NOTES: 1. The auxiliary switch is adjustable between 40% and 100% of the stroke. The
factory setting is at 40%.
2. The auxiliary switch must not be used for proof of closure detection or other
interlock functions.
3. The Proof of Closure Switch is non-adjustable.
There are no serviceable parts on the SKP55… series actuators. If inoperative, replace the
actuator. Tag wires before servicing.
Figure 4. Terminal Designations.
Page 8
Siemens Building Technologies, Inc.
SKP55…U Air/Gas Ratio Controlling Actuators
Technical Instructions
Document No. 155-753
July 1, 2005
(Dimensions in inches;
millimeters in parentheses.)
1/4"-18 NPT
1/4"-18 NPT
0.35" (8.5) O.D.
Figure 5. SKP55…U.. Dimensions.
Information in this publication is based on current specifications. The company reserves the right to make changes in specifications and models as
design improvements are introduced. Other product or company names mentioned herein may be the trademarks of their respective owners.
© 2005 Siemens Building Technologies, Inc.
Siemens Building Technologies, Inc.
1000 Deerfield Parkway
Buffalo Grove, IL 60089-4513
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Document No. 155-753
Country of Origin: US
Page 9
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Richmond, VA 23231
Ph. 804-236-3800
Fx. 804-236-3882
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