Maintenance and troubleshooting of ABB SCC-K converter

Practical Guide for Maintenance and Troubleshooting of ABB SCC-K Converter

In modern industrial continuous emission monitoring systems (CEMS), accurate measurement of nitrogen oxides (NOx) is the core link for environmental compliance and process control. However, the nitrogen oxides produced during combustion typically include nitric oxide (NO) and nitrogen dioxide (NO2). Due to the fact that the vast majority of commercial infrared or ultraviolet analyzers can only directly measure NO, it is necessary to rely on a converter to convert NO2 to NO in order to calculate the total NOx concentration.

The ABB SCC-K series converters (including NO2/NO converters based on carbon molybdenum catalysts and thermal converters based on high-temperature quartz glass tubes) are the industry benchmark designed for such demanding applications. When the on-site analyzer experiences faults such as low NOx readings, slow response, or no conversion at all, the problem often points directly to the inside of the converter. This article will start from engineering practice, deeply dismantle the hardware architecture, standard operating procedures, and high-frequency fault troubleshooting strategies of SCC-K, and provide a detailed practical guide for on-site engineers.

Analysis and Application Boundary of Core Conversion Technology

The ABB SCC-K system includes two distinct conversion technologies, with strict physical boundaries for their applicable scenarios and operating parameters.

1.1 NO2/NO Converter: Carbon Molybdenum Catalytic Technology

This converter is mainly used to reduce NO2 in the sample gas to NO, and is widely used in standard flue gas desulfurization and denitrification (DeNOx) monitoring.

Reaction principle: The sample gas passes through a special stainless steel filter filled with carbon molybdenum catalyst. The catalyst significantly reduces the activation energy of the reaction, causing the reaction equilibrium to shift towards the product side.

Linear matching of temperature and flow rate: The catalyst temperature is not necessarily better the higher, but must be accurately matched with the sample gas flow rate. According to official engineering specifications, to ensure a conversion rate of ≥ 95%, the recommended settings are as follows: when the flow rate is 30 l/h or 60 l/h, the temperature is set to 320 ° C; when the flow rate is 90 l/h, it is set to 340 ° C; when the flow rate reaches 150 l/h, it needs to be set to 360 ° C. The preheating time is about 30 minutes, and when the LED of the temperature controller displays "1", it indicates that preheating is complete.

Special engineering limitation: Due to the generation of CO during the reaction of carbon molybdenum catalysts, if simultaneous measurement of NOx and CO (ppm level) is required on site, CO measurement must be carried out in a separate sample gas path, otherwise the CO generated by the converter will seriously interfere with the measurement results. In addition, the ammonia (NH3) in the sample gas will convert some NO2 into N2O and N2, which can significantly decrease the conversion rate depending on the concentration of ammonia.

1.2 Thermal converter: High temperature quartz glass reaction tube

For more complex chemical and papermaking processes, thermal converters utilize high-temperature environments (without the need for catalysts) to achieve extremely high conversion efficiency (≥ 99%).

Reaction principle: Convert unmeasurable gas components into measurable compounds or remove interfering components.

Typical application scenarios:

Paper industry (TRS measurement): Organic and inorganic compounds of sulfur dioxide, analyzer Uras26, working temperature 650 ° C, flow rate 60 l/h.

Chlor alkali plant (H2 conversion): Use Cl2 to convert H2 into HCl, analyzer CaldoS25, working temperature 450 ° C or 500 ° C, flow rate 60-120 l/h.

PVC/chlorinated hydrocarbon plant: oxidizing chlorine component, measuring HCl, working temperature of 650 ° C, flow rate of 60 l/h.

This version requires the user to install the quartz glass reaction tube themselves. The preheating time is relatively long, about 60 minutes. Its maximum operating temperature can reach 680 ° C.

Core consumables replacement and installation specifications

The maintenance of converters highly relies on standardized operations, especially when it comes to disassembling and assembling high-temperature components.

2.1 Installation of Quartz Glass Reaction Tube (Thermal Converter Only)

Quartz tubes are prone to breakage and have high temperatures inside.

Power off and disassembly: Disconnect all power sources and open the chassis cover. Quartz tubes are usually fixed on side wall brackets with separate transport protective covers.

Joint assembly: Place the quartz tube flat and sequentially insert the union nut, pressure ring (with opening), and clamp (with conical seal). It is strictly prohibited to use brute force. When installing elbow joints, use a torque wrench to tighten to 5 Nm (or use an NW17 wrench to tighten approximately 1.5 turns).

Fixing and sealing: Install the quartz tube into the tube furnace, install the front panel cover, and conduct an air tightness test.

2.2 Replacement of Catalyst Filter (NO2/NO Converter Only)

The lifespan of catalysts is usually greater than 6 months (under conditions of 30 l/h, 320 ° C, 10 ppm NO2, and 5% O2). When the conversion rate is significantly lower than 95%, it must be replaced.

Switching the bypass: extremely important! Before starting the replacement, the sample gas path must be switched to "bypass", otherwise the high-temperature corrosive sample gas will directly spray out and injure people.

Disassembly and assembly of components: Unlock the adapter through the rotating handle on the front panel and pull out the high-temperature tube furnace. Gently rotate and pull out the old filter element, remove the two outer O-rings and two inner O-rings. Do not damage the O-ring.

Install new filter element: Gently screw in the new filter element, making sure it is inserted into the bottom limit position, otherwise it may cause air leakage. Before installing into the tube furnace, the external O-ring can be slightly wetted to facilitate insertion (lubricating grease is strictly prohibited as it can poison the catalyst). Finally, tighten the handle.

2.3 Airtightness Test (Core Required Steps)

Whether replacing the quartz tube or catalyst, the converter must be cooled to room temperature for testing after assembly.

Switch the sample gas path to catalyst mode (green LED lights up).

Seal the sample gas outlet and connect the U-tube pressure gauge.

Use a shut-off valve to apply pressure, so that the pressure gauge reading is approximately 50 hPa.

Absolute red line: The maximum operating absolute pressure of NO2/NO converter shall not exceed 200 kPa; The thermal converter shall not exceed 120 kPa.

Observe for a few minutes. If the pressure drops significantly, it indicates a leak and the O-ring or joint needs to be rechecked.

On site high-frequency fault diagnosis and troubleshooting

When dealing with feedback from engineers searching for solutions, SCC-K's faults often focus on electrical heating circuits, air path blockages, and signal transmission.

3.1 The converter does not heat up at all

Phenomenon: The temperature controller displays abnormal or unable to raise temperature, and the conversion rate drops to zero.

Troubleshooting steps:

Measure heater voltage: Measure the voltage between terminals X4/2 and X4/3. If there is no voltage, it indicates that the heater is disconnected and needs to be replaced.

Measure controller output: Measure the voltage between terminals X4/6 and X4/7. If the voltage is less than 8V DC, it indicates that the temperature controller is damaged and needs to be checked or replaced according to the operation manual.

Testing Solid State Relays: If the controller output is greater than 8V DC but the heater has no voltage, it is usually due to the breakdown and damage of the Solid State Relay, which needs to be replaced directly.

Temperature control reset technique: If the set temperature is lowered by more than 10 ° C due to misoperation, the control logic of the sensor will release and cut off the heating circuit. At this point, it is necessary to wait for the actual temperature to drop below the new set value, then completely cut off the main power supply and reconnect it before resetting the temperature control system.

3.2 Non switching solenoid valve (optional dual solenoid valve version)

Phenomenon: Unable to switch between "bypass" and "catalytic" modes, or external control failure.

Troubleshooting steps:

Check the power supply and plug: Confirm if the 9-pin Sub-D plug (X2) is inserted correctly. SCC-K can only ensure normal operation when the plug is fully inserted.

Internal/External Control Conflict:

If internal switching (front panel switch) is used, there must be a short bridge between pins 1 and 6 of the Sub-D plug, otherwise the solenoid valve will not operate.

If external switching (customer DCS control) is used, the short-circuit bridge between No. 1 and No. 6 must be removed. At this point, the front panel switch will fail. The switching status is confirmed by the two green LED indicators on the panel.

3.3 Low NOx reading or no conversion rate

Phenomenon: The analyzer shows a much lower than expected NOx concentration or no response at all.

Troubleshooting steps:

Air path blockage/leakage: Check if the sample gas pipeline is blocked by dust or aerosols. Observe whether the flow meter has a flow rate. If the flow rate is zero, first check whether the front-end SCC-C or SCC-F feeding unit is normal, and then check the internal pipelines.

Catalyst failure: Confirm that the temperature is normal, but the conversion rate is below 95%. At this point, the catalyst filter element must be replaced.

Cross sensitive interference: Analyze the composition of the sample gas. If the concentration of ammonia (NH3) is too high, it will consume some NO2; If the sample gas contains high concentrations of SO2, and if an acid filter is not used to intercept SO3 aerosols at the front end, it can also cause the filter element to quickly become poisoned and fail.

Oxidation problem of NO2/NO: Due to the easy re oxidation of NO to NO2 in long pipelines, SCC-K converters should be installed as closely as possible to the analyzer, shorten the sample gas pipeline, and the temperature of the heat tracing pipeline must be stable above the dew point (recommended 120-180 ° C).

Installation environment and long-term shutdown strategy

In order to extend the lifespan of the converter, on-site environmental control cannot be ignored.

Installation location: SCC-K is designed for 19 inch rack or wall mounted installation. Regardless of the method, at least 1 height unit (1U) of heat dissipation space must be reserved directly above it. The equipment should be kept away from direct sunlight, ovens, or high dust and corrosive environments. The ambient temperature should be controlled between+10 ° C and+50 ° C, and the annual average relative humidity should be ≤ 75%.

Short term shutdown (Stand by): In case of a brief pause in production, in order to avoid unnecessary catalyst consumption and ensure that it can be restored to use at any time, it is recommended to lower the catalyst temperature to about 100 ° C during the short-term shutdown (standby mode).

Long term shutdown: If the equipment is not in use for a long time, it is recommended to use inert gas or air to blow the inside of the converter at room temperature to remove residual corrosive gases and prevent condensation and acid corrosion of internal components.

Packaging and transportation: If it is necessary to return to the factory for maintenance or relocation, the thermal converter must first remove the quartz glass reaction tube, install it in a dedicated transportation protective cover, and fix it on the chassis bracket. Shock proof foam shall be used for the outer package, and it shall be marked with "Fragile goods, upright transportation".