Siemens LMV5 Quick Start Manual page 175

Section 6 O2 Trim
Introduction and Principle of Operation 6-1
The LMV52 features an integrated O2 trim functionality that enables the LMV52 to monitor the
percentage of O2 in the stack and adjust the positions of actuators when the burner is in operation
(Phase 60). When the system is trimming, the LMV52 will move the actuators that are designated
as "air influencing" independently of the fuel actuator(s) in an effort to achieve an optimal
percentage of O2 in the stack. Typically, only the air actuator is set to be air influencing, but the
VSD (if equipped) and other actuators (Aux 1,2,3) can also be set to air influencing if the dampers
that the other actuators are connected to are also to be adjusted in accordance with the percentage
of O2 in the stack.
As the name implies, the O2 trim functions to regulate the amount of oxygen and therefore the
amount of air entering the combustion process. Since this is the case, the LMV52 O2 trim system
does not and cannot affect the position of the fuel actuator for a given load. In other words, if the
LMV52 senses an O2 level in the stack that is above setpoint, it will react by slowly closing the
actuators that are set to air influencing until the O2 in the stack is at the desired setpoint. The
LMV52 will not close the fuel actuator to reduce the O2 level in the stack. Also, the LMV52 will not
open any of the air influenced actuators further than they were open on the original Fuel-Air ratio
curve to achieve an O2 setpoint.
When setting up the O2 trim curves, a total of three curves are set having to do with the measured
percentage of O2 in the stack. The three curves (in order lean to rich) are:
1) O2 Ratio Control (Fuel Lean Curve) – This is the percentage of O2 read at the stack sensor
when the actuators are at the positions that were defined on the original Fuel-Air Ratio Curves.
In other words, this is the measured O2 corresponding to each point set in Section 4-1, Figure
4-1.11.
2) O2 Control (Trim to Curve) – This is the O2 setpoint that the LMV52 will try to achieve by
backing actuators designated as air influencing down their respective curves.
This may also be referred to as the "O2 setpoint" curve.
3) O2 Monitor (Fuel Rich Curve) – This curve serves as a lower limit or an alarm curve. If the
measured O2 drops below this value at a certain point, the LMV52 will either deactivate the O2
trim and operate on the normal combustion curves, or it will lockout depending on what the
LMV52 is set to do.
This may also be referred to as the "O2 Guard" curve.
Figure 6-1.1 illustrates how these O2 curves might look after they have been set. Notice that for
each of the three curves, a certain percentage of O2 is set and actuator positions (in degrees) or
VSD % are not directly set during the O2 trim commissioning. The actuator positions in degrees or
VSD% were directly set when the Fuel-Air ratio curves were commissioned. See section 4-1 for
details on that procedure.
Figure 6-1.1 also shows that the O2 curves must be set at every point that was defined when the
Fuel-Air Ratio Curves were set. For example : If 12 points were entered during the Fuel-Air Ratio
Curve commissioning, then it will be necessary to enter 12 points for the O2 Monitor, 11 points for
the O2 control and 11 points for the O2 Ratio Control. The LMV52 does not trim on Point 1, and
this is the reason that the O2 Control and O2 Ratio Control curves cannot be set on Point1.
LMV5 QSG Rev 4 Sec 6 Pg 1 www.scccombustion.com