FAQ Category: CLD (NOx analyzer) Troubleshooting

If you’ve stumbled on to this FAQ, you must have a Rosemount NGA-2000 CLD analyzer to measure NOX either dry or wet (referring to the WCLD version).   The detector portion of this analyzer is OEM p/n: 659754 (obsolete) and is sold by us under this part number:  35K0104R0.   It is the same OEM detector and fully compatible with your NGA-2000 or uCEMS.
I’d like to point out that this detector assembly is COMPLETELY repairable and should never be discarded.  Here’s why:

1. If the capillary gets clogged:

• Unclog it or solder in a new one

2. If the heaters fail open:

• Replace the heaters

3. If the resettable fuse fails open:

• Replace the fuse

4. If the barrier window gets cracked/busted:

• Replace the window

5. If the temperature sensor goes bad:

• Replace it

6. If the reaction chamber leaks:

• Replace the bad seal (or both seals)

7. If a pin breaks off or a wire opens in the harness:

• Fix the harness

8. If the metal barrel rusts through (this has never happened, but there’s always a first time):

• Replace the housing

9. If the photosensor gets weak or dies completely:

• Replace the sensor

• This is the most challenging aspect as the OEM wants about $8000 for this chip; and, most of the time, it’s the barrier window that gets clouded/stained from the ozone- NO reaction; we don’t change out very many sensor chips.*

10. If the photosensor won’t cool down to 1°C, it might be a bad Peltier section of the chip -or- most likely, it’s a bad Darlington Pair power transistor (mounted on the aluminum divider panel).

Important: *We’ve been fixing about two of these per year for the past 20 years.  Don’t listen to the rumors that you can’t procure a NGA-2000 CLD detector assembly, we can get them or repair them.*

The designer of this analyzer failed on two major points:

1. He specified all sample tubing to be 1/8″ OD.  This is OK for some parts of the internal gas handling system but not for all parts of the sample tubing.

2. He did not include a flow restrictor at the sample inlet.

Discussion (later)

Fix / upgrade / resolution:

1. Change the reaction chamber exhaust tubing to 1/4″ OD Versillon or Tygon or Neoprene tubing.  Also change the BPR exhaust lines to 1/4″ OD tubing.

2. Install a special, modified (with embedded restrictor), adapter at the sample inlet (4-4 TRBZ-SS or 2-4 TRBZ-SS) at the inlet (it attaches directly to the OEM’s SAMPLE INLET port).  RIGAS makes these on demand.

1. Sample pressure: 5 psig*    (* but can be anything that gives you the desired calibration results; the factory calibrates the capillaries for a 5 PSIG sample head pressure.  And because sample flow is controlled by a capillary, there is no danger of over pressurizing the reaction chamber.  However, excess sample pressure will, at some point, exceed the capabilities of the BPR (back pressure regulator) and control of sample pressure will be nonexistent).     NOTE: some customers had a limited sample pressure so the OEM configured many CLDs to run on 2 psig (internal) sample pressure; see below).

2. [opt] Sample pressure: 2 psig*    (* but can be anything that gives you the desired calibration results; the factory calibrates the capillaries for a 2 PSIG sample head pressure.  And because sample flow is controlled by a capillary, there is no danger of over pressurizing the reaction chamber.  However, excess sample pressure will, at some point, exceed the capabilities of the BPR (back pressure regulator) and control of sample pressure will be nonexistent).

3. Sample flow:  controlled by sample pressure (mentioned above).  The capillary that controls flow to the reaction chamber was sized to either 70cc/min (for high NOX concentrations) or 200cc/min (for low NOX concentrations).

4. Converter temperature: 350°F to 450°F  (this is empirically determined by performing a NOX converter efficiency test)

5. Sensor temperature: -1°C to +1°C (check the setpoint value in the Technical menu)

6. Block temperature: 50°C (check the setpoint value in the Technical menu)

7. Bypass flowrate (if the McMillan turbine flowmeter is installed):  400cc/min to 2000 cc/min

See linked PDF for a discussion about ozone starvation (usually caused by diminishing UV output and therefore lessoning O₃ production from the ozone lamp).  CLD_NOx_ozone_starvation_response_graph_discussion.pdf

Possible symptoms of ozone starvation (O₃): poor linearity; bad CGA results; failed RATA; poor raw counts on span gas (and low signal delta).

1. The ozone lamp will die after many years of service

• but along the way, it will begin to decrease its ozone (O3) production and you WILL experience Ozone Starvation (see other FAQ)

• The ozone lamp power supply might live long enough to power two or three lamps

2. The Darlington Pair power transistor that controls current to the sensor chip’s Peltier cooler will fail sometimes age;

• sometimes the Peltier chip pulls too much current;

• sometimes a technician will unplug and re-plug the Darlington cable and blow the transistor (I’ve done this at least once)

3. When reassembling, technicians can cause stress on the ACU02 (the CPU / controller board that sits on top of the BPR (backpressure regulator)) by not routing the ribbon cables neatly; this will break solder connections on some very sensitive chips causing hard failures or the very frustrating intermittent failures.

4. The capillary will foul causing a loss of sensitivity.5. The heaters in the reaction chamber are covered with termite paste; the paste will dry out over time causing poor thermal conduction. This could induce an instability in the temperature control of the “block” (the upper region of the reaction chamber); and this could induce variations in zero, span, and process readings.

6. [later]

7. [later]

 

[later] a discussion of chemiluminescence

See linked PDF for a discussion about ozone (O₃) starvation (usually caused by diminishing UV output from the ozone lamp).  CLD_NOx_ozone_starvation_response_graph_discussion.pdf