Honeywell SV2 Series User Manual page 96

SV2 Series Safety Shut-off Valves
Ramp Offset and Ramp Period Parameters
Refer to Fig. 53 and
back-pulse, the SV2 Series valve allows ramp-up adjustment of the gas volume (# steps and timing) during the ignition
period.
The Ramp Offset (in steps) determines how lean the mixture is when the two safety shut-off valve seats open.
This sets the number of steps that the Learnt Pos. 2 is reduced or offset during ignition. Once the valve seats
are powered, the stepper motor moves from this position to the Learnt Pos. 2 position. Note that Learnt Pos. 2
is re-learned with every burner start.
The Ramp Period (in seconds) governs the speed with which the stepper motor opens from the (offset) idle /
start position to the Learnt Pos. 2 during the ignition sequence. While the burner igniter is on, the air and gas
mixture enriches until a mix occurs for light-off.
NOTES:
₀ The default values for Ramp Offset and Ramp Period are 35.0 steps and 1 seconds, respectively.
₀ If the FARMOD mass flow sensor measured values reach their saved/Learnt Pos. 2 ignition settings before the end of
the programmed Ramp Period, the stepper motor will not open further.
During the Ramp Period, the motor position will change without the fuel/air control being active to prevent the fuel/air
control from reacting on the pressure pulls in the air signals caused by the ignition of the burner.
The slowest allowed Ramp Period is limited by the setting of the Ignition Period; the Ignition period needs to be at least
1 second longer than the Ramp Period.
Hold On Period
In addition to the startup delay of the fuel/air control as a result of the Ramp Period, the startup can be further delayed
up to 1 second before the end of the Ignition Period by entering a value greater than 0 for the Hold On Period
parameter.
The Hold On Period sets the time that the control ignores the fuel/air control feedback following the Ramp
Period. The Fuel Control Valve is held at the controlled valve position for the specified time before it starts to
modulate.
As with the Ramp Offset and Ramp Period, this control startup delay helps to make the system less sensitive to pressure
pulses caused by ignition of the burner. During the Hold On Period, the control loop is not active so that fluctuations
in the measured air signal are not translated into position variation of the Fuel Control Valve. Gas flow to the burner
during this period is determined by the pressure drop over the stepper motor-controlled Fuel Control Valve and the gas
restriction in the mixing unit.
Ignition Period and Ignition Setpoint Parameters
The SV2 Series valve allows the user to make the fuel/air mixture rich or lean during the Ignition Period relative to the
amplification setting for the appliance RUN mode or the start load. This occurs AFTER the Ramp Period and Hold On
Period timers have expired, when the valve control loop starts to modulate.
The Ignition Period parameter (in seconds) sets the time that the SV2 Series valve control uses the ignition
settings. It encompasses the Ramp Period, the Hold On Period and the remaining period in which the
controlled amplification setting can be changed relative to the normal amplification setting (based on the
control curves) at the start load / RUN (as governed by the Ignition Setpoint % parameter).
The Ignition Setpoint % defines whether the mixture during the ignition phase is the same, leaner or richer
relative to normal operation at ignition load.
If the Ignition Setpoint % is at 100%, there is no change compared to normal operation at
ignition load.
If the Ignition Setpoint % is above 100%, it causes an enriched fuel/air mixture during ignition with an
increased amplification factor. This occurs after the Ramp and Hold On Periods have expired. Refer to
Fig. 58.
If the Ignition Setpoint % is below 100%, it causes a leaner fuel-air mixture during ignition with a
decreased amplification factor. This occurs after the Ramp and Hold On Periods have expired. Refer
to Fig. 59.
Note that ± 20% gives approximately ± 10% change in excess air level.
Fig. 62-Fig. 63. In order to smooth out the start-up behavior of the burner and avoid ignition
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