Five factors to consider when selecting flange transmitters

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Capillaries are used in flange transmitters in place of pressure guiding tubes, so no insulation or heating is required； The temperature of the medium being measured can be very high; the high-temperature pressure/differential pressure transmitters manufactured by Changhui Instruments can operate at temperatures above 600°C, thus meeting the process requirements in many special applications. In practical applications, there are many factors that affect the accuracy of flange transmitters: it has two layers of isolation diaphragms, a long capillary, high damping, and a large amount of filling fluid. Therefore, in practical applications, flange transmitters often respond slowly, provide inaccurate measurements, and suffer from frequent damage. Flange transmitters: yunrun.com.cn/product/list_80.html. The poor performance of flange transmitters is due not only to the difficulty in manufacturing such instruments and to their inferior inherent properties compared to ordinary transmitters, but also to the fact that the operating conditions often exceed the limits allowed by these instruments. For this reason, when selecting flange transmitters, one must be extremely careful and give thorough consideration not only to the process conditions during normal operation but also to those under special conditions or in case of misoperation. Only in this way can the proper operation of the selected flange transmitter be ensured. Some of the selection requirements for flange transmitters are the same as those for ordinary transmitters. In this article, CHANGHUI Instruments focuses on explaining the specific requirements for selecting flange transmitters, and summarizes five factors that need to be taken into account when choosing such transmitters for sharing with instrument technicians. 1. Capillary length of flange transmitters: A longer capillary facilitates installation and maintenance, but it should not be too long; the reasons are as follows: ① If the capillary is long, the amount of filling fluid in the entire sealed system increases. The greater the initial amount of filling fluid, the larger the volume change caused by temperature variations. When these changes are applied to the sensing element of the transmitter, they cause a change in the output. ②As the capillary length increases, the resistance to the flow of the filling fluid rises, which slows down the response speed of the transmitter and deteriorates its dynamic characteristics. This may have little impact on horizontally placed horizontal tanks and measurement systems with slow level changes, but the error increases for vertically placed vertical tanks and measurement systems with rapid level changes. ③If the capillary is too long, it becomes difficult to evacuate the sealed system before filling it with the filling liquid; in other words, it’s not easy to remove all the air from it. Leaving residual air in a closed system can severely impair the performance of the transmitter. According to manufacturers of pressure transmitters, as the capillary length increases, the accuracy drops exponentially. Therefore, the length of the capillary in a standard flange transmitter generally does not exceed 10 meters. 2. Temperature: When selecting a flange transmitter, the temperature of the medium to be measured must remain within the allowable range. If the flange transmitter is used in high-temperature conditions, the following problems may occur: ① When the high-temperature medium comes into contact with the flange diaphragm, the fluid filling the sealing system heats up, causing its volume to expand or even evaporate. If the volume expansion is excessive and the pressure exceeds the allowable limit, the filling fluid will be forced out through the connections and welds. Sometimes, after a flange transmitter has been in use for some time and its isolation diaphragm is gently pressed by hand, it can be clearly felt that the filling liquid inside is different from the original one, which likely indicates that some fluid has leaked out. If there is less filling fluid, pressure transmission within the sealed system cannot proceed properly, which results in a slower response from the transmitter and increased measurement errors. ②The filling fluid is an oily substance that contains more or less air within it. Furthermore, a sealed system before filling cannot be reduced to an absolute vacuum either; a small amount of residual air may not cause many problems when the system is used at moderate temperatures. However, when used under high-temperature conditions, it expands, and the volume expansion coefficient of gases is much larger than that of liquids; as a result, pressure increases rapidly, leading to leakage of the filling fluid outside as mentioned earlier, and inaccurate measurements. 3. Pressure: Flange transmitters operate under positive pressure, and generally no problems arise. However, when operating under negative pressure (vacuum), issues such as slow response and inaccurate measurements can occur. The reason for this is that, when a flange transmitter operates in a vacuum environment, the diaphragm is subjected to an outward pulling force, causing it to bulge outward. This increases the volume of the sealed system of the flange transmitter, resulting in a decrease in the pressure within the filling fluid. The boiling point of the filling liquid inside the transmitter diaphragm chamber, like that of other liquids, decreases as the pressure applied to it decreases. When a flanged transmitter operates under negative pressure conditions, creating a vacuum within the sealed system, the vaporization pressure becomes high. This not only affects the sensor elements of the transmitter and causes measurement errors, but it also causes the isolation diaphragm to bulge outward, leading to damage in remote-flanged differential pressure transmitters and remote pressure transmitters. In response to this issue, Changhui Instruments provides a detailed explanation in the article titled \"Reasons Why Double-flange level transmitters are not suitable for measuring negative pressure levels\". ④High temperature and high vacuum: The effects of the operating temperature (high temperature) and operating pressure (vacuum) on flanged transmitters were analyzed above. If both of these conditions exist simultaneously, that is, when the transmitter operates under high temperature and high vacuum conditions, it can be easily damaged if no measures are taken. Therefore, when dealing with flanged transmitters in high-temperature and high-vacuum environments, pressure transmitter manufacturers must take special measures and precisely calculate the amount of filling fluid based on the application conditions and the properties of the filling fluid. For transmitters used at high temperatures, reduce the amount of filling fluid appropriately, while for those used at low temperatures, increase the amount of filling fluid appropriately, so that they are in optimal condition under their respective operating conditions. The gas present in the filling fluid also needs to be removed through complex and time-consuming processes; as a result, these specially treated high-temperature, high-vacuum flanged transmitters are several times more expensive, or even more, than ordinary flanged transmitters of the same specifications. Flange transmitters are generally not used in negative pressure conditions; they are employed only in a few special process applications, such as measuring the liquid level in vacuum towers in oil refining. However, during the switching process of the container, a vacuum can easily occur due to the evacuation of air from the area in question. Although this situation may last only for a short time, even that brief period can cause damage to the flange transmitter. The operating temperature and operating pressure of flange transmitters influence each other; do not assume that these two parameters are independent! ! Therefore, it should be considered together when making a selection. If the pressure of the medium being measured is high, the operating temperature of the flange transmitter can be higher; if the medium is in a vacuum state, the operating temperature of the flange transmitter must be kept within the allowable range. Transmitters from different manufacturers have varying temperature and pressure operating ranges, which are determined both by the properties of the filling fluid and by the manufacturing techniques of the pressure transmitter manufacturer. 5. Intelligent flange transmitters: Calibrating and changing the range of flange transmitters is very troublesome. It’s not only because it is relatively bulky; special care must be taken with the capillaries – they must not be damaged or bent, and they cannot be lifted by holding them at the capillary end. Moreover, it is difficult to introduce signal pressure, as is not the case with ordinary transmitters where a pressure connection fitting is sufficient – here, matched flanges are required. To this end, where conditions permit, intelligent flange transmitters should be used; this way, range modification and setting as well as zero-point adjustment can be carried out on the handheld operator without having to remove the gauge, which is very convenient. It should be noted, however, that the operator unit only changes the amplification factor of the electronic circuit; if the sensing element is defective, the operator unit cannot rectify this issue. We have discussed this in detail in the article \"Adjusting smart pressure transmitters using only an operator unit is not true calibration.\" Common types of flanged transmitters: The more common flanged transmitters include double-flange differential pressure transmitters, sanitary remote differential pressure transmitters, remote pressure transmitters, remote gauge pressure transmitters, single-flange level transmitters, and ultra-high temperature remote pressure/differential pressure transmitters for 600°C applications. http://yunrun.com.cn/upload/201908/25/201908250122324687.png http://yunrun.com.cn/upload/201908/25/201908250125511030.png http://yunrun.com.cn/upload/201908/25/201908250132388102.png http://yunrun.com.cn/upload/201908/25/201908250133061514.png http://yunrun.com.cn/upload/201908/25/201908250135244581.png Double-flange differential pressure transmitter, Sanitary remote differential pressure transmitter, Remote pressure transmitter, Remote gauge pressure transmitter, Single-flange level transmitter