Emerson OXYMITTER 4000 - Manual (Page 124)

Instruction Manual
IB-106-340 Rev. 3.0
December 2003
8-20 Troubleshooting Rosemount Analytical Inc. A Division of Emerson Process Management
Oxymitter 4000
8-6 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 appears
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 par-
ticulate 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 con-
dition, instrument air will need to be
connected for reference. Pressurize
the inside (reference side) of the probe
by plugging off the reference air ex-
haust 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 28, Figure 9-3) that carries
the cal gas to the cell. The blue
electronics housing or probe head
needs to be removed to inspect
this hose (See Section 9, MAIN-
TENANCE 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 metallic seal (item 5,
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. This diffusion element will eventu-
ally plug in applications that have a heavy
particulate loading (coal or wood fired boil-
ers, cement and lime kilns, catalyst regen-
eration, recovery boilers, etc.) It is important
not to pressurize the sensing cell during
calibrations by flowing excessive cal gas
against a plugged diffuser. Calibration flow
rates should be set only when a new dif-
fuser 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. Ad-
just this flow only when a new diffuser
is installed.