Emerson Process Management OXYMITTER 4000 - Manual (Page 116)

Instruction Manual
IB-106-340C Rev. 4.1
July 2004
8-20 Troubleshooting Rosemount Analytical Inc. A Division of Emerson Process Management
Hazardous Area Oxymitter 4000
8-6 HAZARDOUS AREA OXYMITTER 4000
PASSES CALIBRATION, BUT STILL
READS INCORRECTLY
There are a few fault conditions where no alarm
indication is present and the probe passes cali-
bration, but the O
2
reading may still be incorrect:
a. Probe passes calibration, but still ap-
pears to read high.
1. There may be a leak that is permitting
ambient air to mix with the process
gases. Since many combustion proc-
esses are slightly negative in pressure,
ambient air can be sucked into the cell
area, biasing the O
2
reading upward.
(a) Make sure that the calibration gas
line is capped tightly between cali-
brations. If autocal is used, make
sure the check valve is seating
properly.
(b) If an abrasive shield is installed
to protect the entire probe from
particulate erosion, a leak in the
probe flange gasket can allow
ambient air to migrate down the
annular space between the probe
and shield, and then into the cell.
Always install a new probe flange
gasket when re-installing a probe.
2. There may be a leak inside the probe
itself, permitting the reference air
(20.95% O
2
) to mix with the process
gases at the cell. To confirm this leak
condition, instrument air will need to be
connected for reference. Pressurize
the inside (reference side) of the probe
by plugging the reference air exhaust
port with your finger for 1 minute. The
O
2
reading should decrease slightly. If
the O
2
reading increases during this
test, there is a leak inside the probe.
(a) Acid condensation inside the probe
can degrade the red silicon tube
(item 38, Figure 9-3) that carries the
cal gas to the cell. Remove the
housing (11) to inspect this hose.
(See Section 9, MAINTENANCE
AND SERVICE).
BOTTOM VIEW
37260045
PROBE HEAD CORRUGATED
SEAL
REFERENCE AIR
EXHAUST PORT
PROBE
FLANGE
GASKET
Figure 8-18. Probe Leakage Paths
(b) The sensing cell is bolted to the
end of the probe, and uses a cor-
rugated metallicseal (item 25,
Figure 9-3) to separate the process
gases from the ambient reference
air. This seal can be used only one
time, so always replace this seal
when a cell is replaced. Always
apply anti-seize compound on both
sides of the corrugations.
b. Probe passes calibration, but still
appears to read low.
The diffusion element at the end of the
probe is a passive filter. It plugs very slowly,
since there is no active flow being drawn
across it. In applications that have a heavy
particulate loading (coal or wood fired boil-
ers, cement and lime kilns, catalyst regen-
eration, recovery boilers, etc.), this diffusion
element will eventually plug.
It is important not to pressurize the sensing
cell during calibrations by flowing excessive
cal gas against a plugged diffuser. Calibra-
tion flow rates should be set only when a
new diffuser is installed. As the diffuser
plugs, do not adjust the flow rates upward.
How do I detect a plugged diffuser?
1. The O
2
signal's speed of response will
degrade. The O
2
trend in the control
room will become smoother.
2. When calibrating, the calibration gas
flow rate will be noted to be lower.
Never readjust this flow upwards.
Adjust this flow only when a new
diffuser is installed.