Blower Speed Monitoring - Siemens LMV3 series Technical Instructions

Technical Instructions
Document No. LV3-1000

Blower Speed Monitoring

The LMV3 relies on its connected devices to achieve an accurate, repeatable fuel-to-air ratio from low
fire to high fire. These connected devices are typically the SQM33 actuators and a VSD blower. The
accuracy and repeatability of the burner's fuel-to-air ratio is directly dependent on how accurately the
actuators can be positioned, and how accurately the blower's speed can be controlled. Different
burners in different applications have different requirements for the accuracy of the fuel-to-air ratio. To
accommodate these requirements, the LMV3 has adjustable tolerance bands for both the actuator's
angular position and the VSD's blower speed.
To help ensure that the burner is either operated at a safe fuel-to-air ratio or is shut down, the blower
speed is constantly monitored while a flame is present in the boiler. The speed is monitored in a way
that nuisance shutdowns are eliminated, but fast shutdowns will occur if the speed deviation is large.
To do this, the LMV3 evaluates the magnitude of the speed deviation in combination with how long the
speed deviation exists. Three distinct tolerance bands and one limit centered about the standardized
speed line are used. These bands are:
1. Neutral Band - if the speed is within this band, it is considered to be OK and no action is taken. The
width of this band is adjustable via parameter 662 and the range is +/- 0.5 to 3.5% of the
standardized speed. Burner modulation is temporarily paused if blower speed is out of the neutral
band.
2. Near Zone Band - if the speed is within this band, the active speed control (in the LMV3 or VSD) will
be working to bring the speed back into the Neutral Band. If the Neutral Band speed cannot be
achieved in 8 to 16 seconds (adjustable by parameter 664) a lockout will occur. The width of this
band is adjustable via parameter 663 and the range is +/- 2.0 to 5.5% of the standardized speed.
3. Outside Near Zone - if the speed is outside the Near Zone Band but does not exceed the High Risk
Limit, the active speed control (in the LMV3 or VSD) will be working to bring the speed back into the
Near Zone Band and then ultimately into the Neutral Band. The width of this band varies with the
setting of the Near Zone band, parameter 663. If the Low Risk Band speed cannot be achieved in 3
to 7 seconds (adjustable by parameter 665) a lockout will occur.
4. High Risk Limit - if the speed exceeds the High Risk Limit threshold for more than 1 second, a
lockout will occur. The timing and width of the High Risk limit is not adjustable. It is set to +/- 10% of
the standardized speed.
These bands are shown graphically in Figure 5-5 below.
Section 5 Page 16
LMV Series
SCC Inc.