Анализ на общите повреди на устройствата за измерване на термодвойките в индустрията

The simple structure of the thermocouple determines its extremely plastic application shape. It can be processed into different thicknesses, lengths, single or double (usually double thermocouples are used, one as a spare) according to needs. Some thermocouples, such as armored cable thermocouples, can be bent and twisted at the head to meet the different installation shape requirements of the on-site equipment when използва се .
*The measurement principle of the thermocouple is the thermoelectric effect. The biggest difference between it and other sensors is that it can generate signals without adding any excitation power supply. However, it generates extremely weak electrical signals, usually only tens of millivolts. In order to prevent signal distortion, it has high requirements for the transmission of thermoelectric potential: first, a dedicated cable (called compensation cable) must be used, and the compensation cables of different types of thermocouples are also different (E type with nickel-chromium-copper wire, K type with copper-constantan wire). Secondly, the shielding requirements for interference signals are high, and the cables usually use double-layer shielding.
Компенсация на студен край
The cold end compensation function of thermocouples has been widely moved into DCS{{0}} Taking the INFI-90 control system as an example, all I/O cabinets are placed in an electronic room with a constant ambient temperature (generally 18℃~21℃). In this way, when performing cold end temperature compensation in the I/O terminal cabinet, a high-precision measurement value can be obtained. When the cold end temperature of the thermocouple is zero degrees Celsius (referred to as t0), the compensation bridge R1= R2= R{{10}} RF(t0), the bridge is balanced, the compensation voltage Uba=0, and the total potential of the temperature measurement circuit is EAB(t,t0). When the cold end temperature of the thermocouple changes, such as rising to t1, although the thermoelectric potential of the thermocouple decreases, due to the increase in the RF value of the thermistor, the compensation bridge will have a Uba voltage output, and the total potential of the temperature measurement circuit is EAB(t,t1)+Uba. Only by choosing the working current value of the compensation bridge, the compensation voltage Uba can be exactly the part of the thermoelectric potential EAB (t, t1) that is reduced due to the rise in the cold end temperature, so that the total potential EAB (t, t0) of the temperature measurement circuit остава стабилен .
The thermoelectric potential EAB (t, t0) after the above compensation enters the signal isolator, where it is isolated and transmitted to convert the millivolt signal linearly into a 1~5 volt signal (calculated based on 900℃as the full scale) and then input into the I/O card. The analog/digital conversion is performed in the I/O card and finally sent to the Карта на контролера за изчисление и управление .
Обработка и контрол на софтуера
The temperature signal entering the DCS controller card is a digital signal (when monitored from the engineer's workstation, it still displays the millivolt value), which is linearly related to the input millivolt voltage value, so to convert it into a temperature signal, it is also necessary to perform interpolation calculation in a list.
At the same time, in order to ensure the reliability of the entire measurement system, multiple measuring points are installed on-site for each main and important temperature measurement parameter in the control logic. For example, for the outlet temperature of the superheater, dual measuring point input is redundant measuring point input or three measuring point input, and the corresponding control logic adopts two-choice or three-choice.
According to the law of heat transfer, the speed of heat transfer depends on the thermal resistance of the heat transfer body. Therefore, the response of any temperature measurement device to the temperature field measurement is much slower than that of the flow, liquid level, and pressure signal. Besides, there is usually a protective sleeve outside the thermocouple element. Therefore, its response to temperature changes is not a step characteristic but close Към инерционната характеристика . По тази причина някои водещи компоненти трябва да се добавят към основния контролен PID на температурния контрол на сайта, за да се предотврати прекомерното коригиране на температурата .
Общи грешки и анализ
Общите неизправности на термодвойките се проявяват главно, както следва:
A . Компенсационният проводник на термодвойката е свързан в обратна . Това е главно проблем, който възниква по време на конструкцията на инфраструктурата ., той е причинен от временната небрежност на лицата, отговаряща за окабеляването и е трудов фактор ., когато се появява термоклама
b. The insulation layer of the thermocouple's compensation wire is worn out, causing the signal loop to be grounded. This is mainly because the compensation wire is hard and is not installed flat in the junction box. When troubleshooting, the junction box cover is repeatedly screwed and the compensation wire is worn out. Този вид повреда се отразява в показанията за температурата на контролната станция на оператора, която обикновено е малка .
C . Терминалът за окабеляване в кръстовището има лош контакт . Тъй Контролната станция на оператора като никаква индикация или стойността на индикацията надвишава диапазона .
D . отказ на компенсация на компенсация . Този вид повреда се проявява като бавно покачване или падане на стойността на показанията за температурата, след като термодвойката е свързана .
e. The failure rate of the thermocouple measuring the flue at the tail of the boiler is high. During the shutdown inspection, the thermocouple was removed and it was found that the head of the thermocouple, including the sheath tube, was severely worn after being washed by flue gas. After the sheath tube was changed to wear-resistant steel material, this kind of fault Скритата опасност беше елиминирана .
f. The signal protection system DCS cabinet is poorly grounded. This kind of fault can easily cause electric charge to accumulate on the signal line, causing signal drift or fluctuation. Because the fault point of this problem is difficult to find out, the usual treatment method is to disconnect the signal line and discharge it to the ground.
G . Сигналът за вход на температурата се проваля след преминаване през изключването, който се отразява в анормалния сигнал за температура на контролната станция на оператора . е нормално след подмяна на прекъсването .}
The above are common problems encountered during inspections, but when looking for these faults, it is recommended to first use a multimeter to measure the input voltage value on the I/O terminal cabinet of the DCS, so that you can quickly determine whether it is a problem at the on-site measurement end or a problem in the DCS department. For multi-point inputs, you can also compare it with several other thermocouple Сигнали . Ако не можете да го определите, развържете проводниците и измервайте стойността му на съпротивление . обикновено стойността на съпротивлението на термодвойката е около 100Ω .
Устройство за измерване на термичен резистор
The application rate of temperature measurement devices using thermal resistors as temperature measuring elements in thermal power plants is second only to thermocouples. Ordinary domestic thermal resistors are about 1,000 yuan each, which is more expensive than thermocouples, but have better characteristics at high temperatures and higher measurement accuracy than thermocouples. They are mainly used to detect Точки за измерване с температури под 200 градуса . В момента се използва за измерване на температурата на генераторните бобини и вентилаторите и въглищните мелници .