The No. 1 auxiliary engine has activated a high exhaust temperature alarm. In this article we will analyze the actions of electrical engineer to find and fix these problems.
In the exhaust system on the engine there are two thermocouples that measure the temperature at two points (A and B), these points correspond to their group of cylinders. The values at both points should be approximately equal (at 500°C), but the thermocouple at point B showed a temperature of 620°C and it continued to rise on the AMS monitor.
The first thing to do in such situation is to transfer the load to another generator, stop the first engine, let it cool down and remove the thermocouple from the "glass" (the flask where the sensor is installed).
A thermocouple is a type of temperature sensor that measures temperature through the use of two different metal wires joined at one end. The basic principle behind its operation is the Seebeck effect, which occurs when there is a temperature difference between the junctions of two different metals. This temperature difference creates a voltage (thermoelectric EMF) that can be measured and correlated to temperature.
Components of a Thermocouple:
Two Different Metals: Typically, the metals are chosen for their predictable and stable thermoelectric properties. Common metal combinations include:
- Type K (Nickel-Chromium / Nickel-Alumel)
- Type J (Iron / Constantan)
- Type T (Copper / Constantan)
- Type E (Chromel / Constantan)
Junctions:
- Measuring Junction (Hot Junction): The point where the two metals are joined and exposed to the temperature to be measured.
- Reference Junction (Cold Junction): The other end of the thermocouple, which is kept at a known reference temperature (often at 0°C or room temperature).
How It Works:
- Seebeck Effect: When the hot junction experiences a temperature different from the cold junction, a voltage is generated due to the Seebeck effect. This voltage is proportional to the temperature difference between the two junctions.
- Voltage Measurement: The generated voltage is measured and then converted into temperature using standard tables or calibration data specific to the type of thermocouple.
Advantages of Thermocouples:
- Wide Temperature Range: They can measure a broad range of temperatures, from very low to extremely high (e.g., -200°C to +2500°C).
- Durability: Thermocouples are robust and can operate in harsh environments.
- Fast Response Time: They respond quickly to temperature changes.
Disadvantages of Thermocouples:
- Non-linear Output: The voltage-temperature relationship is not linear and requires calibration.
- Reference Junction Compensation: Accurate temperature measurements require compensation for the reference junction temperature.
- Sensitivity: Thermocouples have lower sensitivity compared to some other temperature measurement devices.
Applications:
- Industrial Processes: Monitoring and controlling temperatures in furnaces, kilns, and reactors.
- Scientific Research: Measuring temperatures in experiments and environmental studies.
- Household Appliances: Used in ovens, toasters, and other cooking devices.
- Automotive Industry: Monitoring engine and exhaust temperatures.
Thermocouples are a versatile and reliable choice for temperature measurement in many fields, offering the ability to measure a wide range of temperatures with robustness and relatively simple implementation.
Using a temperature calibrator, we check the thermocouple, gradually heat it up and compare the value on the Kongsberg K-Chief alarm monitoring system monitor.
As a result of the test, it turned out that at a heating temperature of up to 500°C, the thermocouple shows the correct value with a small error, but when increased to 550°C, it begins to increase sharply (first to 580°C, then to 600°C).
Checking the amplifier and tightening the thermocouple contacts did not give positive results, so it should be replaced with a new one.
Another similar situation was described in a previous article with a pressure sensor. But in that case, simple calibration helped; in this case, the thermocouple will have to be changed.
The new thermocouple was tested using a calibrator up to 550°C and showed stable operation. The temperature was calibrated using the Span potentiometer on the amplifier because... there was a small error.
After installing the new thermocouple, it was tested on a working engine and showed normal, stable operation.
In this article I would like to focus on the fact that the old thermocouple worked correctly up to a certain temperature (up to 500 degrees Celsius). After this temperature, the thermocouple began to give false readings. Apparently, after 500°C, the sensor broke through to the housing.
The same problem often occurs with heating elements (heaters in different systems), when at a certain temperature the heater spirals begin to penetrate the housing and low insulation comes out. This will be written about in other articles.
System model: Alarm monitoring & control system Kongsberg K-Chief 500
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