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Maintenance of Automatic Control Devices in the Frigid Regions of the North Abstract: The winter temperatures in northern China are extremely low; in the Heilongjiang region, the lowest temperatures can reach -40 °C. Such low temperatures severely affect the proper operation of electronic automatic control devices, which is why insulation measures must be taken. The author of this article has drawn conclusions based on years of practical experience in this field. Keywords: heat preservation, heat tracing, maintenance; Chinese Library Classification Number: TP273; Document Code: B; Article Number: 1003-7241 (2002) 03-0067-02. The lowest winter temperature in northern China is -40 ℃, classifying it as a frigid region. This poses significant challenges to the safe and stable operation of automatic control systems, and certain maintenance measures must be taken to ensure their proper functioning. The maintenance experience is discussed as follows: 1. Primary components, primary valves, and control valves need to be reinforced thermally and insulated. The main hazards caused by low temperatures to primary components and primary valves are two-fold: (1) The medium freezes, leading to the expansion and rupture of the equipment. (2) The medium solidifies, causing blockages in the pressure tapping pipes and the primary components. All of the above situations can prevent the detection of medium parameters and disrupt the normal operation of production, so they must be resolved. The solution is to provide insulation and heat tracing for equipment that is prone to freezing, as well as for primary components and control valves. The primary components and control valves are installed directly on the equipment pipelines, and their heat tracing pipes can be connected directly to those of the process heat tracing system. For example, regulators such as the PIC225 and 295 draw steam from the process companion pipe for insulation and heat tracing. 2 Insulation and heating shall be provided for on-site installation panels (outdoor). 2.1 The technical requirements for providing insulation boxes for on-site transmitters are as follows: (1) The panel box must be tightly sealed, with rubber sealing strips installed on the door to improve sealing performance. (2) Most transmitters on-site should operate at temperatures above 5 °C; therefore, heating systems must be installed inside the instrument cabinets in cold regions. (3) The volume of the enclosure should be moderate, to facilitate the maintenance of the transmitter and the heat tracing system. 2.2 Heat tracing for pressure introduction pipelines and transmitters: The design requires dedicated heat tracing pipes that are separate from the process heat tracing pipes; this is to ensure that the shutdown of process heat tracing for maintenance does not result in a shutdown of the instrument heat tracing as well. Low-pressure steam for heating: P=0.6MPa, T=140 ℃. This steam can be drawn separately from the main low-pressure steam pipeline and routed to various equipment through branches. Shut-off valves are installed on these branches to connect them to the various areas that need heating. The steam then reaches the drain valves and the valve for adding water, before finally returning to the return pipe. A discharge valve must be installed at the lowest point of each branch. The entire heating system also requires instrument air, which is used to blow away any condensation accumulated within the heating pipes. 2.3 Centralized heating should be installed for on-site transmitters and pressure guiding pipelines; the advantages are (1) it facilitates construction and saves materials. (2) It facilitates the centralized discharge of return water and condensate. (3) Facilitates insulation construction. Heat tracing pipes are generally made of steel tubes with a diameter of Ф 4.2; when the number of devices that need to be heated increases, the diameter of these steel tubes is increased. For transmitters that cannot be installed together, separate heat tracing pipes must be provided. Heat tracing pipelines over long distances are prone to freezing in extremely low temperatures, so steel tubes with a diameter of Ф 18 are usually used for such applications in order to increase the amount of steam supplied. 2.4 Two problems associated with centrally installed instruments: (1) Direct contact between the pressure transfer tube and the steam tracing pipe can cause the fluid inside the pressure transfer tube to boil, resulting in bubbles formation, which prevents accurate monitoring of the parameters being measured. Solution: ① Isolate the pressure transfer tube from the tracing pipe. ② A isolation tank is added, filled with a high-boiling-point, high-density oil substance, to prevent fluctuations. (2) Freezing prevention issue. To prevent the return pipeline from freezing and causing a freeze in the entire system, it is necessary to thicken the insulation layer. 3 Insulation and heating for single and double flange (silicone oil isolation fluid) transmitters (1) In single and double flange transmitters, the capillaries are filled with silicone oil whose freezing point is below –42 ℃; under normal conditions, no heating is required for these capillaries, but insulation is necessary in cold regions. (2) In high-cold regions, it is necessary to provide insulation and heating for the pressure tapping points (gaskets) with single or double flanges. Single and double flange transmitters are suitable for use in environments with high-viscosity, easily coagulating media, media with high density that tend to settle, as well as media containing solid particles. Therefore, at low temperatures, if the pressure-taking points are not insulated or heated, they can freeze, which affects pressure transmission and makes it impossible to take measurements. (3) When the temperature inside the container is above 30 °C and the distance between the flange and the container is small, insulation can be used without the need for heating. Specific method: Reserve space for the flange and the container; first wrap insulating material from the outside inward to allow convection of air inside, and finally cover it with an aluminum box to keep the temperature of the flange surface above 0 °C. 4 Experience Summary 4.1 Requirements for the heat tracing system (1) The heat regulation of the heat tracing system can be achieved by adjusting the opening degree of the inlet steam shut-off valve and the steam trap, thereby controlling the flow rate and thus the temperature. (2) The discharge valve of the heat tracing system cannot be installed vertically, as this can lead to water accumulation and blockage; it must be installed horizontally, with its length kept as short as possible, to ensure proper heat circulation within the valve and prevent freezing. (3) For the winter maintenance of the heat tracing system, it is necessary to conduct regular inspections of the system, carry out periodic checks, and drain condensate on a regular basis to ensure proper heat circulation. (4) When the heat tracing system is shut down or under maintenance, the condensate must be completely drained; if necessary, instrument air should be used to blow away any remaining condensate. 4.2 Maintenance of instruments for measuring media that solidify easily and cause deposits: (1) Install isolation tanks, filling them with liquids whose specific gravity is higher than that of the measurement medium, in order to prevent condensation and blockages in the pressure transfer pipelines. (2) Water heating and electric heating are used for the pressure guiding pipelines filled with isolation fluid to maintain the temperature above the freezing point, thereby preventing condensation and deposition. (3) The pressure transduction pipelines prone to sedimentation must have their sediment regularly removed. (4) A backwash device is added to remove deposits adhering to the pipe walls, ensuring accurate measurements. The flow meter FT008 for the phenol water spray in the vaporization and cooling system uses regular discharge along with high-pressure water backwashing, which helps to maintain high accuracy in measuring flow rates. (5) In cold regions, temperature fluctuations cause variations in the transmitters. On one hand, these fluctuations can be reduced by adjusting the temperature of the heat tracing; on the other hand, measures such as changing the PID control parameters are taken to enhance the interference resistance of the entire control system, thereby ensuring accurate measurements.