Pre-Control; O 2 Trim Terminology - Siemens LMV5 Series Technical Instructions

Technical Instructions
Document No. LV5‐1000
For many nozzle mixing burners (traditional boiler burners), the O
fire and a lower value at high fire. This is true since most nozzle mixing burners need more excess air
(higher O ) at low fire to achieve complete combustion. For this type of burner an O
2
(wet) at low fire and an O
Pre‐control
As mentioned above, all O
readings due to physical realities of the boiler. This fact, combined with an O
varies from low fire to high fire means that the O
makes a large change in firing rate. This begs the question of what happens to the O
burner transitions from 20% firing rate to, say, 80% firing rate. The answer is the O
from active trim to what is referred to as "pre‐control".
Pre‐control is possible because the LMV52 learns the characteristic of the burner at each point on the
curve during the O trim commissioning. The specific characteristic that the LMV52 learns is how much
2
of a change in airflow (air rate) is necessary to achieve a certain change in the %O
characteristic is known as the Lambda Factor, and is essentially the burner's signature from an O
standpoint. Just like the delay time (Tau time) that was mentioned earlier, it is necessary for the fuel
flow to match the load number so that the Lambda Factor is correct and accurate for each point.
Pre‐control uses the Lambda Factor so that the burner can be modulated while keeping the %O
the O setpoint, even though the setpoint changes with the firing rate and the O
2
readings. Once the burner stops modulating or only modulates a small amount (+/‐ 5%), the O
change modes back to "active" and perform active O
A properly commissioned O
0.1% O when the burner is not modulating a significant amount (+/‐ 5% load change). With this small
2
load change, the O trim will stay in the "active" mode. When significant modulation is encountered (+/‐
2
15% load change), the pre‐control will engage and the deviation from setpoint will increase. Deviations
of +/‐ 0.3% or less are typical during modulation when the pre‐control is engaged.
O Trim Terminology
2
1. %O Wet – In‐situ O sensors, like the QGO20 sensor that is used with the LMV52, read the %O
2 2
wet basis. This is in contrast to the vast majority of portable combustion analyzers that read %O
a dry basis. %O wet should always be a lower value than %O
2
section) gives the approximate relationship between %O
2. Tau Time ‐ This is the delay time between when an adjustment is made at the burner (moving the
air damper, etc.) and when that adjustment is read at the O
time will be shorter at high fire and longer at low fire.
3. O Ratio Control ‐ This is the measured %O
2
Control Curve and are not being trimmed. This is recorded for each point on the curve except Point
1. This is also commonly referred to as the "lean curve".
Section 6 Page 4 SCC Inc.
setpoint of 2% O (wet) at high fire is not uncommon.
2 2
trim systems with an O
2 2
trim system cannot actively trim when the burner
2
trim system will typically maintain the %O
2
wet when the actuators / VSD are on the Fuel‐Air Ratio
2
HOME
setpoint will be a higher value at low
2
sensor in the boiler's exhaust must use "old" O
trim once again.
2
setpoint with a deviation of +/‐
2
dry. Figure 6‐11 (located later in this
2
wet and %O dry.
2 2
sensor. For a given burner / boiler, this
2
LMV Series
setpoint of 5% O
2 2
2
setpoint that typically
2
trim when the
2
trim changes modes
2
. This specific
2
trim
2
close to
2
trim is using "old" O
2 2
trim will
2
on a
2
on
2