Humidity measuring instruments are widely used in the process industry-power, chemical, pharmaceutical, Water treatment and other fields, and are an important part of field instruments. The personalized on-site environment is also increasingly demanding for the selection of suitable humidity measuring instruments. This article introduces the application of several types of instruments produced by British Michel Instrumentation Co., Ltd. in the power, chemical, pharmaceutical and water treatment industries.
Detection of moisture content in SF6 gas in high-voltage switch in power industry
In the power distribution network of voltage transmission (100kV ~ 400kV), the power switch or load current interruption is often caused by the arc between two contact points. SF6 has insulation, and SF6 with voltage can effectively control the formation of arc. But with the passage of time, the ingress of moisture causes SF6 to decompose into other products, such as hydrofluoride. This will not only affect the operation performance of a single device, but when the moisture content is high, the by-product will also include highly corrosive hydrogen fluoride, which will accelerate the corrosion of the switch contact, cause physical damage, and will spread to the circuit breaker Other areas within the enclosure increase the degree of damage and bring hidden safety hazards to people and equipment.
Before use, first check the moisture content of the new bottled SF6 provided by the industrial gas supplier. Before using the circuit breaker provided by the circuit breaker manufacturer, it should also be tested in advance. During storage of SF6 bottled gas, moisture can easily enter from the inlet. If well preserved, the moisture content of SF6 should be <10ppmv, which is equivalent to atmospheric dew point <-60 ℃. At the manufacturer of the circuit breaker, at the final stage or before pressurizing and sealing with SF6, the interior of the circuit breaker must be purged with dry gas during internal maintenance work. The most common method is to use bottled high-purity nitrogen or other inert gas, and repeatedly pressurize to sweep the moisture into the atmosphere until the moisture content in the shell drops to 10ppmV, which is equivalent to atmospheric dew point <-60 ℃. During on-site inspection / continuous monitoring, the moisture content in SF6 must be controlled at <100ppmv, which is equivalent to atmospheric dew point <-42 ℃.
For inspection of bottled gas or drying before filling the circuit breaker housing, Cermax portable dew point meter (see Figure 1) can be used. This instrument has a wide measurement range and can provide fast and accurate measurement. Using the secondary menu, you can select a secondary input, that is, manually set or signal input gas pressure, and then display the SF6 ppmv unit, and can pass the built-in data acquisition function. Store 10,000 measurement results, and then download the stored data to the host computer by using the Windows Hyper Terminal or similar data transmission system. The above instruments can also be used for on-site rapid detection of SF6 gas in the circuit breaker housing. The sample can be taken out, or the sample gas can be returned to continue to use, or discharged into the atmosphere. Fast measurement means that less sample gas is consumed. For continuous online measurement, Cermet II dew point meter can be used. This instrument needs to be supplied with main power, or use Transmet dew point transmitter, 24V DC power supply, the output signal is 4 ~ 20mA or RS232 / RS485 signal, so the signal can be input into a computer central processing system, suitable for multiple measurement points installation. Note that the sensor needs to be installed in the upper part of the circuit breaker housing to avoid the accumulation of more and more particulate pollutants in the housing. For the measurement that requires high stability and accuracy, you can choose the cold mirror dew point meter, S4000Integrale (see Figure 2), whose measurement range is -60 ℃, which can display multiple engineering units, including SF6 ppmv. The measurement accuracy is ± 0.1 ℃, and the response speed at -40 ℃ dew point is about 10min. The most accurate measurement can be obtained with the least sample gas consumption.
Detection of water content in hydrogen for cooling of generating units
The thermal conductivity of hydrogen is much higher than other gases, so in the power generation industry, the conventional method is to use hydrogen to directly cool the stator coil of the generator set. Because the casing of the generator set cannot be sealed, the moisture in the surrounding air is likely to invade the unit, and the heat exchanger will cause gaps due to aging, and the moisture will also penetrate into the hydrogen. Therefore, the circulation circuit of hydrogen must be equipped with a dehumidification dryer to continuously remove the absorbed moisture. Otherwise, the moisture in the hydrogen will cause the exposed metal parts to condense and cause flashovers and even lead to heavy losses. If the hydrogen dew point exceeds the upper limit, emergency measures should be taken immediately, or the use and repair of the generator should be stopped, or the pump should be immediately repressed to pump dry hydrogen into the system.
The TransmetIS transmitter of the British Michel company can install the ceramic humidity sensing part on the sampling device, and perform continuous online measurement on the outlet of the generator set and the outlet of the hydrogen adsorption dryer. The measurement should be carried out at the full range of the system pressure to accurately provide information on whether condensation will occur under the conditions of the generator set. TransmetIS transmitter has CENELEC approval certificate, which can measure dew point of combustible gas in hazardous area. Due to the danger of hydrogen explosion, the digital display should be far away from the generator set and installed in a designated safe location, and the sensor probe cable assembly should be equipped with a current isolation barrier. The monitor has a full-featured display and analog output, and is equipped with two alarms set by the user. The meter can be set in the "warning" and "stop immediately" state. TransmetIS transmitters can provide 4-20mA or 0-20mA analog signals to the control room. Michell Intrinsically Safe Portable Dew Point Humidity Meter CermaxIS can be used as an on-site testing tool. Quick response and convenient operation (with data recording function) can allow users to quickly perform gas analysis.
Water treatment system
Now the water treatment industry has increasingly used ozone (O3) instead of chlorine to treat pure water. There are four main application occasions: drinking water, food production water and pharmaceutical production water, swimming pool water, disinfection after sewage treatment and before being put into circulation. The chlorination process is not only costly, but also poses health and safety risks in water transportation and chlorination system applications. In view of this, the water treatment industry has consistently used ozone for disinfection in recent years. Ozone and chlorination have the same water treatment effect, and are both powerful oxidizing media, which can attack impurities in the water (especially for microbial media), and split them into harmless residues and filter out the water system. The advantage of ozone is that after acting on microorganisms, it will split into completely harmless oxygen atoms.
The working principle of the ozone generator is very simple. Pure air or oxygen passes through the high-voltage discharge (generally 12000V) between the two electrodes of the reaction unit (ceramic or stainless steel). The high voltage causes a reaction between oxygen atoms to form stable ozone molecules. A typical ozone generator must use compressed air from a standard regenerative dryer and enter the ozone generator from a set of reaction vessels like a heat exchanger. The following three necessary reasons determine the need to measure the moisture in the air or oxygen of the process: prevent generator electrodes from arcing, prevent system components from being corroded by acid formation, and increase ozonation efficiency. If there is a lot of moisture in the gas entering the generator, it will cause an arc in the electrode, and in extreme cases, the electrode will contact and break down the cavity wall. The arc will damage the electrode and the outer shell of the system, eventually leading to system paralysis and expensive maintenance costs. The side effects of the arc in the presence of ozone can cause nitrogen and water to generate acids and other nitrogen compounds, causing corrosion of electrodes and system components, and can also cause the ozone generator to be paralyzed. The efficiency of the ozone generator is inversely proportional to the moisture content of the input gas. If the moisture content is reduced from 50 ppm to 5 ppm (-50 to -70 ° C dew point), the efficiency of the generator can be increased by 20%. The general technical specifications stipulate that the minimum dew point of the input air or oxygen is -50 ℃, but if the drying can be accurately controlled to reduce it to -70 ℃ dew point, then the generator's working efficiency and life can be significantly Improve and improve.
Michell's impedance hygrometer can be used to monitor the moisture content of the input gas. The online measuring instruments include Easidew transmitter, Easidew online, Transmet or Cermet II, and the on-site fixed-point detection is Easidew or Cermax portable. The combination of Michell instrument online humidity meter and drying control system can ensure that the input gas is as dry as possible, and maintain the high efficiency and working life of the generator.
Pharmaceutical Industry
The industry requires very low dew point temperature for dry air. When the fluid air bed system is used to dry the compound, the dew point is generally -40 ° C. In addition, the cooling stage before the material packaging should be strictly prevented from generating moisture again. All these occasions require precise control, repeatable environmental conditions and traceability. In most cases, the user will choose a relative humidity sensor, but if it is to be accurately controlled within 2% or 1.5%, it is at the critical point of the measurement accuracy of this type of instrument or exceeds the measurement capability of the instrument. Therefore, the best solution is to use Optidev (see Figure 3) with an accuracy of ± 0.1 ～ 0.2 ℃ dew point or S4000Integrale integrated type (depending on the required accuracy requirements), CermetII or Transmet with ± 1 ℃ dew point, and ± 2 ℃ EasidewOnline (see Figure 4) or Easidew transmitters, these instruments can provide accurate humidity measurement to achieve the purpose of precise control of the dehumidifier.
Catalytic reforming of petroleum products in the petrochemical industry
In the process of catalytic reforming of refined petroleum products, in order to ensure the operation efficiency of the plant, it is essential to control the moisture in the recycled hydrogen. Excessive moisture content, such as greater than 50ppmv, will cause catalyst poisoning, which means increased costs. In addition, if the moisture in the recycled gas is not kept within the optimal range of 20 to 30 ppmv, the chemical reaction efficiency will be reduced, which will also affect the product output. At this stage, in order to periodically regenerate and adjust the catalyst, hydrogen chloride must be injected into the gas stream during the refining process to supplement the hydrogen content. Humidity transmitters should be isolated from the process sampling gas to avoid the erosion of hydrogen chloride and the simultaneous increase in moisture content. At this time, the moisture content will be as high as 2000ppmv, or even higher.
TransmetIS is installed in a simple sampling system made of high-quality stainless steel components using the British Michell ceramic humidity transmitter. The sampling system should be equipped with inlet and outlet valves to isolate the transmitter during the regeneration phase so that the transmitter will only contact traces of hydrogen chloride during the pressurized drying process. If the application site requires remote or local display, CermetIIIS can be used. The transmitter can work under process pressure, and can indicate the dew point temperature or moisture content (ppmv) under actual pipeline pressure conditions. If you need to display and output humidity units, and the transmitter still works under pipeline pressure (you can manually enter a fixed pressure value into the hygrometer, or use additional pressure transmitter options for automatic pressure compensation), The standard configuration of the Cermet II hygrometer allows users to choose from multiple ppmv ranges. You can also choose other humidity units ℃, ° F, gm-3, Lb / mmscf, etc.
PET industry
Before injecting PET into the mold pressing machine, it must go through processes such as drying and extrusion. By reducing the free moisture content, the purpose of improving the appearance and quality of the finished product is achieved. Precisely controlling the drying process is the primary process that can not be ignored to ensure product quality (strength and beauty). In the solid state, PET will absorb moisture in the air until it is in equilibrium with the moisture in the atmosphere. This hygroscopic behavior means that 0.6% of its weight is moisture. In order to ensure the performance of polyester products, the moisture content must be reduced to 0.003% (30ppm) before heating. Because in the heating stage, the moisture contained will quickly hydrolyze this raw material, thereby reducing the molecular weight and impairing the physical properties of the polyester product. The processing is carried out in the funnel filling base, which is treated by the hot dry air flow from the dehumidification dryer. The general process is to expose these materials to air with a dew point temperature below -40 ° C for a period of time. It is then transferred to another funnel and passed to the mold press. A typical production plant will have 10 to 50 such dryers and mold presses.
Cermet II of British Michel Company can be used to monitor a single dryer, making the supplied air better than the specified dew point limit temperature of -40 ℃. An alarm can also be used to warn technical maintenance personnel when the dryer condition deteriorates. The sensor probe should be installed at the air inlet of the dryer for measurement and above the debris material. Transmet can also be used, which can output 4-20mA or 0-20mA analog signal for PLC controlled air drying system. Cermax portable dew point meter can detect the efficiency of the dryer under operating load. It can also perform on-site testing at normal intervals, providing a high-efficiency guarantee for improving product quality and reducing costs. It is easy to operate and responds quickly to air sources with a dew point below -40 ° C. Maintenance personnel can monitor multiple dryers at low cost within the required time period.
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