Reason why the pipeline temperature remains at 93°C without rising
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Help: The thermal resistor for measuring pipeline temperature is of the Pt100 type, with a range of -50 to 500°C. Recently, it stops functioning once the measured temperature reaches 93°C. The actual temperature of the gas in the pipeline ranges from 20°C to 180°C; the curve shows normal behavior when the temperature drops below 93°C, but once it reaches 93°C, the reading stays fixed at that level. The problem persists even after replacing the temperature probe. The transmission line has been checked and shows no signs of shorting or disconnection; when the probe is connected to the DCS panel, normal readings are displayed. Could anyone please help me figure out what could be the cause?
This post was last edited by Yann on 2017-5-8 at 17:15. Based on this phenomenon, it is recommended to check the following: 1. Since it is a PT100, the resistance value can also be used for calculation once the value displayed by the DCS stabilizes. Disconnect the circuit and measure to calculate whether the temperature is like this. If so: 1.1. Is there any liquid in the pipeline? Although the temperature of the steam is relatively high. However, once the pipe contains liquid, the bottom end of the PT100 comes into contact with the liquid phase, causing the temperature to remain constant at the boiling point of the liquid phase. 1.2. Generally, temperature sensors are equipped with protective sleeves; it is recommended to check whether water has entered these protective sleeves. After water enters, the gas in the pipeline heats it to its boiling point, after which the temperature no longer changes. 2. If not, 2.1. Use a standard resistance box to send signals into the DCS to see if it works , if it is not recommended to check the safety barriers and clips inside the cabinet, as well as whether the configuration has been altered
I have taken into account the possibility of water being present, as you mentioned. However, since the pipes are too high, it’s difficult to work around them. I tried using a thin stick to check inside, but no water was detected. It seems necessary to remove the casing in order to conduct a proper inspection. It is correct to send signals into the DCS using a standard resistance box ； There are no issues with the thermistor element either. Thank you. I’ll check again to see if there’s water inside the sleeve
If there is water inside the sleeve, it will be around 100°.
It boils when the temperature exceeds 100°; once the temperature drops below 100°, everything returns to normal. It’s more likely that there is a problem on site
Use a multimeter to measure the resistance value. If the resistance is 135.8 ohms, it’s possible that there is water inside the casing, or the temperature of the fluid in the pipeline is simply that high. If the resistance is higher than 135.8 ohms, then check the signal output and the screen configuration as well.