Feasibility Analysis of Replacement of Thermocouple Gauge with Pirani Vacuum Gauge in Cryogenic Vessel

The cryogenic adiabatic pressure vessel is a pressure vessel that stores cryogenic liquids such as liquid oxygen, liquid nitrogen, liquid carbon dioxide, and liquefied natural gas. Unlike ordinary pressure vessels, the structure uses a vacuum jacketed structure. According to different insulation requirements, vacuum powder materials (such as pearl sand), fiber materials, metal insulation, etc. are used to make the sandwich meet the vacuum requirements. Once the vacuum level is exceeded, the heat insulation effect will be greatly reduced, and the evaporation rate of the inner cylinder of the cryogenic container will rise straight. Therefore, accurate measurement of the sandwich vacuum is a prerequisite for safe operation of cryogenic vessels.

According to the regular inspection rules for pressure vessels (TSGR7001 - 2004), it can be determined that the vacuum measurement range of cryogenic vessels is generally between 1CT1102Pa. Suitable measurement methods are: Pirani, thermocouple, and capacitive pressure method 10. And these three Among these methods, the thermocouple type has been widely used due to economic cost advantages.

However, with the development of vacuum measurement technology, the manufacturing cost of Pirani-type gauges has dropped a lot. For example, the superior measurement performance of the second-generation Pirani vacuum gauge exceeds the thermocouple vacuum gauge, and the cost increase is not obvious.

1 Pirani vacuum gauge and thermocouple vacuum gauge Introduction The thermal conductivity of gas has a certain relationship with the degree of vacuum. The lower the vacuum degree, the smaller the number of gas molecules, the lower the thermal conductivity of the gas. The vacuum gauges manufactured based on this measurement principle are collectively referred to as heat conduction vacuum gauges, and mainly include Pirani vacuum gauges, resistance vacuum gauges, and thermocouple vacuum gauges.

The Pirani vacuum gauge keeps the temperature of the metal filament constant during the measurement. The measurement range is generally KT113Pa, while the upper limit of the second generation convection-enhanced Pirani vacuum gauge reaches 105Pa (atmospheric pressure), and the lower limit can reach 102Pa. The use of the thermistor to replace the Pirani vacuum gauge hot wire, more resistant to oil and gas molecules and other pollution, the measurement range and the first generation Pirani vacuum gauge is similar, as 1CT1KPa0. Thermocouple vacuum gauge on the metal wire The constant current, the temperature of the hot wire is related to the thermal conductivity of the material and the pressure of the gas medium. The pressure is measured by measuring the voltage change of the thermocouple wafer welded on the metal hot wire. The measurement range is generally 10°12 Pa. Currently, people in China The concept of the Pirani vacuum gauge is rather confusing. The first is to confuse the Pirani vacuum gauge with a resistance vacuum gauge. For example, the newly revised standard vacuum insulting cryogenic equipment performance test method Part 1: Basic requirements (GB/T18443.1-2010) The resistance vacuum gauge called in 4.2.3.4 is actually a Pirani vacuum gauge; The websites of the less domestic regulatory manufacturers also confused the two when they were introduced. The resistance vacuum gauge is called a resistance vacuum gauge because the thermistor replaces the hot wire of the Pirani vacuum gauge.

With the development of the Pirani vacuum gauge measurement technology, the actual resistance vacuum gauge application after 1975 has rarely been 0, and many resistance vacuum gauges are no longer described in many vacuum textbooks abroad. Many domestic resistance vacuum gauges are actually Pirani vacuum gauges. The second is to confuse the Pirani vacuum gauge with a thermocouple vacuum gauge. For example, in the domestic thermocouple regulatory technology standard thermocouple vacuum gauge (B/T 6873), the thermocouple vacuum gauges have been classified into constant-flow thermocouple vacuum gauges and constant-temperature thermocouple vacuum gauges from 1993 to 2005. 4, constant-flow thermocouple vacuum gauge is commonly known as thermocouple vacuum gauge, and constant-temperature thermocouple vacuum gauge is actually Pirani vacuum gauge. The measurement principle of the Pirani vacuum gauge is that the Wheatstone bridge is not a thermocouple effect. In view of this, the author believes that the names of the two should be distinguished in order to facilitate the promotion and application of the Pirani vacuum gauge.

2 Comparison of Pirani vacuum gauge and thermocouple vacuum gauge 2.1 Range of measurement Thermocouple vacuum gauge measurement range is generally 11 ~ 12Pa, beyond the upper limit will not be measured. If a cryogenic vessel is vacuumed in a manufacturing plant, the thermocouple vacuum gauge cannot distinguish between vacuum pump abnormal failures and pipeline leakage failures. The thermocouple vacuum gauge only has a normal reading when the vacuum pump is evacuated to below 102Pa. Manufacturers' operating personnel must wait until the thermocouple vacuum gauge has a normal reading to confirm that the vacuum pumping system is working properly.

The Pirani vacuum gauge (convection enhancement type) measuring range is 1CT1105Pa. Under normal pressure, the vacuum pump operation can be monitored immediately after vacuum is started. There is no pressure blind zone such as a thermocouple vacuum gauge, which improves the production efficiency.

2.2 Measurement accuracy The uncertainty is less demanding, as long as less than 50% is s, but it fails.

The measurement accuracy of the thermocouple vacuum gauge is generally ± (20% 50%), and only a part of the inlet regulation can achieve an accuracy of ±20%; while the measurement accuracy of the Pirani vacuum gauge generally reaches 2.3 operating temperatures. Many common heats The operating temperature of the even vacuum gauge hot wire is as high as 250°C. At such a high temperature, the oil molecules entrained in the vacuum pump are likely to burst and carbon traces remain on the hot wire, affecting the measurement accuracy.

The operating temperature of the Pirani vacuum gauge hot wire is about 120C. This temperature guarantees the measurement accuracy. At the same time, it will not allow foreign substances (such as oil molecules) entrained from the outside to contaminate the hot wire and affect the measurement accuracy.

2.4 Remote Monitoring Pirani Vacuum Gauge As the output voltage is as high as 0.36V, the data line can be extended more than 150 meters without affecting the measurement accuracy. The thermocouple vacuum gauge output voltage is only 115mV, the data line is too long directly affect the measurement accuracy H. Vacuum degree is an important indicator to measure the thermal insulation effect of cryogenic vessels.

If the vacuum data can be extended from the site to the management room, it will be more intuitive and convenient to record, and safety management personnel can also timely grasp the vacuum maintenance of the sandwich.

2.5 Regulatory and Instrument Interchangeability Thermocouple vacuum gauges come in a variety of forms, such as filament thermocouple gauges and short-filament thermocouple gauges. Regulators cannot interchange with each other, even if the same kind regulates the heating current value. Different. For example, the domestically produced ZDO4 vacuum gauge is required to adjust the heating current according to each different regulation when measuring the degree of vacuum. The heating current is usually attached to the regulatory product warranty book, and the manufacturer usually puts it in the welding regulation. The value of this heating current is marked on the protective cover. When measuring, the corresponding heating current must be set according to this value before measuring the vacuum. The import regulation such as TELEDYNEHastingsDV4R type regulation does not need to set the heating current. However, differently regulated measuring instruments cannot be used in common with each other. Inspection agencies must be equipped with all common thermocouple gauges, which will incur additional costs for regular inspections.

Pirani vacuum gauges were factory-calibrated, each calibrated with nitrogen, and temperature compensated for the 1040°C temperature range. Measurements do not require manual adjustment and are plug-and-play regulatory.

2.6 Cost-effective thermocouple regulation is cheap, and is suitable for those applications where the measurement accuracy is not high and the measurement environment is harsh (if corrosive). Once the hot wire is contaminated, it can be replaced immediately.

Pirani’s prices are generally twice that of thermocouple control, and thermocouple regulatory prices are inherently low. Substantial performance gains can be achieved if Pillar’s ​​regulation is slightly increased. .

Regulations used on cryogenic vessels are generally welded during manufacture and require direct use and cannot be easily replaced. Therefore, it is advisable to weld more reliable Pirani regulations.

2.7 Leak Detection Capabilities A feature of the Pirani vacuum gauge is its ability to detect leaks.

Cryogenic vessels need to be leak tested when they are manufactured or when they are partially leaking. The helium mass spectrometer is now used for leak detection. Although the accuracy of the Pirani vacuum gauge cannot reach the accuracy of the helium mass spectrometer, it can be used for the coarse detection before the helium mass spectrometer accurately detects leaks. When detecting leaks, as long as the leaking gas (such as hydrogen, argon, or Freon) is blown at a suspicious place, if the leaked gas enters the system, the Pirani vacuum gauge will suddenly change its reading.

3 Example Comparison The Pirani vacuum gauges are often confused with resistance vacuum gauges and thermocouple vacuum gauges to limit the application of Pirani vacuum gauges. The typical foreign thermocouple vacuum gauge and Pirani vacuum gauge parameters are compared in Table 1. Table 1 Typical foreign thermocouple vacuum gauge, Pirani vacuum gauge parameter comparison Model type Measurement range/Pa Leakage detection capability Maximum cable/m Pirani's regulation is that Pirani's regulation is Pirani's regulation is thermocouple regulation. China's special equipment safety, 2012,28 (2) :23~24. GB/T18443.1―2010. Test methods for performance of vacuum insulation cryogenic equipment Part 1: Basic requirements S.2010. 4 Conclusion Currently applicable to cryogenic insulated vessels There are two types of sandwich vacuum measurement instruments: Pirani vacuum gauge and thermocouple vacuum gauge. By comparing the measurement range, measurement accuracy, measurement temperature, interchangeability, remote monitoring, leak detection capability, and cost-effectiveness of the two, you can It can be seen that the Pirani vacuum gauge has the characteristics of large measurement range, high measurement accuracy, low operating temperature, and leak detection capability. The Pirani vacuum gauge is equipped with a cryogenic insulated container, which facilitates equipment monitoring, user management and periodic inspections in the manufacturing process.

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