Honeywell SV2 Series User Manual page 106

SV2 Series Safety Shut-off Valves
Fig. 61: Control startup delay with Leaner mixture (decreased amplification factor).
Ramped Startup with Control Startup Delay and Enriched or Leaner Mixture (Ramp Offset + Ramp Period + Hold
On Period + Ignition Period + Ignition Setpoint parameters)
In addition to the Control Startup Delay + Enriched/Leaner Mixture concepts previously discussed, ramp-up of the
amplification factor during the ignition period can be adjusted. This helps to smooth out the start-up behavior of the
burner.
By entering a Ramp Offset value, a Ramp Period value and a Hold On Period value in the appropriate fields as shown
in Figs 58-59 (Ignition Setup page), the SV2 Series control controls the gas volume to a lower value at the start of the
Ignition Period and drives it within the Ramp Period timing to the gas volume required by the Ignition Setpoint
Following the Ramp Period, the Hold On Period begins, in which the gas volume is kept at a fixed level while feedback
ignition back-pulse is ignored. This occurs before the Ignition Period timing expires.
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.
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.
For the Ramp Period + Hold On Period timing, 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.
The Ramp Offset has a minimum value of 35.0 steps such that the fuel/air mixture at the moment the two safety shut
off valves open will always be lean. Directly after opening the SSOVs, the stepper motor will ramp from the offset
position to the Learnt Pos. 2 within the Ramp Period timing. After the Hold On Period time expires, the mixture moves
from a lean mixture to a mixture setting specified by the Ignition Setpoint %.
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
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