Fault symptoms of differential pressure transmitters and analysis and handling methods

Thread Content

This post was last edited by My Chemical Romance on 2018-1-31 at 16:52. Fault phenomenon: During the catalyst production process, it is necessary to measure the pressure and flow rate of the slurry. Although the pressure transduction tubes are short, these instruments often become clogged after being used for some time, which results in a slow change in the instrument’s output, or even no change at all. Analysis and measures: During the production process, it is common for the medium being measured to contain solid suspended particles or powders; over time, some of these may even solidify, leading to blockages in the pressure sensing tubes and preventing proper measurement. In this case, ordinary pressure tap instruments cannot be used; instead, diaphragm pressure gauges or remote flanged transmitters must be employed. Although remote flanged transmitters are more expensive, there is no risk of clogging since pressure guide tubes are not required ； The flange diaphragm has a large area, making it easier to remove the medium being measured, and it can withstand high temperatures. So it is now very widely used. In addition to being used for measuring slurries containing suspended particles, as well as media that are too viscous or prone to freezing, setting, or crystallization, it is also commonly used in situations where condensation is likely to occur in gases within pressure guiding pipes, or where vaporization is likely to take place in liquid media. In such cases, using ordinary instruments for measurement can cause changes in the static pressure within the pipes, leading to unstable instrument readings. Flanged transmitters are also commonly used in food production where hygiene must be maintained, or in processes that require rinsing between two batches. Fault phenomenon: There is a flow measurement system in which the primary element is an orifice plate, and it serves as the measuring instrument. When the system was put into operation, the output of the differential pressure transmitter not only failed to increase but instead dropped below zero. Analysis and judgment: When a flow measurement system consisting of a throttle orifice plate and a differential pressure transmitter is put into operation, the instrument output goes to zero. This may be due to the following reasons: (1) The high-pressure conduit of the transmitter is blocked or leaking ； (2) The high and low pressure tubes of the transmitter are connected in reverse ； (3) The flow direction of the medium in the process pipelines is opposite ； (4) The transmitter is faulty. Upon inspection, the transmitter is in good condition; its output changes in accordance with variations in the differential pressure signal. The pressure transfer tubes are not clogged or leaking either. However, the high-pressure and low-pressure pressure transfer tubes are connected in reverse (the flow direction of the fluid is reversed, which can also be considered as the tubes being connected incorrectly). Remedial measures: Addressing the issue of reversed connection of high and low pressure pressure transmitters is quite difficult for traditional pneumatic transmitters as well as some electric transmitters; it requires reinstallation and the use of welding techniques. This is especially troublesome in the case of operational process units and instrument systems equipped with insulation and heating systems. However, for smart transmitters, there are relatively simple ways to handle the situation where high- and low-pressure conduits are connected incorrectly, and various methods are available. For example, there are two types of the Fuji FCX-A/C series intelligent transmitters. (1) Change the relative position between the detection component and the conveying component, as well as the pressure guide tube interface. The FCX-A/C series transmitters have two high-pressure ports and two low-pressure ports, each of which is interchangeable. If the preceding port is connected to a pressure guide tube, then the following port is connected to a drain/vent plug; therefore, it is sufficient to rotate the relative positions of the detection component and the transmission component by 180° and move the pressure guide port to the other side. When changing the relative position of the two components used for detection and transmission, it is first necessary to remove the connector of the flat cable inside the transmission component; thereafter, the two external hex head screws should be loosened in order to rotate the component. It is absolutely not allowed to try to rotate it without removing the cable, as this will cause it to break ； (2) Change the position of the positive and negative contact pieces. Previous transmitters had only one function: as the differential pressure signal increased, the output of the differential pressure transmitter also increased; this is commonly referred to as direct action. However, for the FCX—A/C transmitter, there is also a reaction effect: as the differential pressure signal increases, the instrument output decreases. The method of selecting forward or reverse operation involves changing the position of the switch; therefore, if the pressure guide tube is connected in the wrong direction, it is sufficient to change the switch inside the gauge from the forward operation \"NOR\" position to the reverse operation \"REV\" position. When changing the position of the positive and negative contact pieces, first remove the cover with the display window, loosen the two screws that hold the display panel in place, and take off the display panel; the contact pieces on the circuit board will then be visible. Gently pull them out with your hand and insert them in the desired position.