Engineering Notes; Installation And Mounting Notes - Siemens QGO21 Series Basic Documentation

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Building Technologies Division

1.2 Engineering notes


If the burner is shut down for no more than 1 or 2 days, do not switch off the
QGO21 and the associated control unit (LMV52 with PLL52)

To ensure a good response, always use the QGO21 together with the AGO21

Flue gas temperatures at the QGO21 must not exceed 300 °C, since higher
temperatures can destroy the sensor

QGO21 may only be used with the approved fuels, as other fuels can destroy the
sensor with their aggressive components (see Technical data)

Heavy metals in the fuel, such as lead, cadmium etc., destroy the sensor element
and therefore may not be contained

Silicones and silicone vapors in the flue gas can damage the sensor element and
therefore may not be contained

Following element in the flue gas reduces the life cycle of the sensor elements and
should not be contained:
Silicon, sulfur, phosphor, boric, bismuth, copper and halogens (F, CI, Br, I) and
their compounds (e.g. FCKW)

NOx, SOx and reducing atmospheres, such as CO by an incomplete burning,
impair the life time of the sensor element depending on the concentration and the
influence time

1.3 Installation and mounting notes


Ensure that the relevant national safety regulations are complied with

The flue gas flow passing the measuring point should be homogeneous. When
mounted too close to air dampers or pipe bends, faulty measurements can occur

The exchange of fresh air in the connection area of the sensor with the reference
air slots must be ensured and may never be covered up (by insulation or similar)

The sensor should not be exposed to aggressive gases (NOx, etc.). This applies to
both the gas and air side since aggressive gases can drastically shorten the
sensor's life

Disturbances can distort the measurements (this can lead to dangerous conditions in
connection with O2 trim control):
– If flueways are not tight, false air can join the flue gases. In that case, the oxygen
content acquired by the sensor is above the real content
– When flue gas velocities are low, the sensor will respond more slowly since the flue
gases need more time to pass the measuring cell
– The further the sensor is mounted from the flame, the longer the dead time

For expected ambient temperatures between 50 °C and 70 °C, the connecting wires
between the terminals Q4 or Q5 and the Pg screw connection must also be insulated
with the high-temperature bushings provided
Note
For detailed information on mounting, refer to Mounting Instructions M7845
(74 319 0866 0).
Oxygen Sensor QGO21...
1 Safety notes
CC1P7845en
06.12.2017