Design of steam tracing pipeline for chemical process pipeline

The healthy development of the chemical industry has played a key role in China’s economic construction, which can effectively improve the overall level of the economy and promote sustainable social development. In the process of using steam tracing pipe, attention should be paid to the design process and construction concept of its tracing pipe.

Chemical industry has always been one of the important pillar industries of China’s national economy. In the chemical production process, some media have low freezing point, so steam tracing must be carried out during transportation, which can not only prevent the media from freezing and condensation during transportation, but also ensure that the temperature of the media is always in the best state. This paper briefly analyzes the design method and content of the steam tracing system of chemical process pipeline.

Connotation of chemical process pipeline steam tracing pipeline

There are three common heat tracing methods for process pipes, namely, inner pipe heat tracing, jacket heat tracing and electric heat tracing. Due to the problem of thermal deformation of the inner tube heat tracing, it is not possible to transport corrosive and heat sensitive media in use, and is rarely used in petrochemical plants. The jacket heat tracing is divided into pipe cap jacket and flange jacket. Electric tracing is a form of tracing using electric energy as a heat source, which can effectively control the temperature. However, with the continuous development of the times, in recent years, in the newly built chemical plants, the main heat tracing media are hot water and steam. The external steam tracing pipe can not only improve the heat tracing effect, but also make it easier to adjust the temperature, lower the cost, improve the overall heat tracing effect, and have a wider application range. At present, most petrochemical enterprises in China will choose steam tracing system when they choose heat tracing mode and relevant devices. Therefore, the research on the service quality of steam tracing system has become an important part of the development. With the continuous development of science and technology, steam heat tracing pipes are mainly selected in the heat tracing system of chemical process pipes. Steam heat tracing pipes are very convenient for temperature regulation and become a common heat tracing method in chemical process heat tracing pipes. The heat tracing pipeline is mainly composed of the following parts, namely, the total number of main branch pipe, heat tracing pipe, incubator, steam trap and return branch pipe, main return pipe, shut-off valve, etc.

Design of steam tracing system

In the process of chemical medium transmission, due to its long-term low temperature environment, it is easy to occur condensation, solidification and other problems, resulting in heat loss. As a result, the viscosity of the process medium itself is increased, which also affects the further use of related equipment. The normal use quality of both pipelines and instruments will decline. In order to improve the transportation status of all media without special heating and waste of heat energy, the steam tracing system can be selected. This system can improve the transmission quality of media and prevent a series of negative problems during the transmission of media.

Design of heat tracing gas supply header

During the design of steam tracing system for chemical process pipeline, it is necessary to maximize the role of heat tracing. The heat tracing station shall be designed separately, and the steam powder distribution station shall be set on the first floor in consideration of the heat tracing gas supply header design in the overall design process. Introduce steam from the top of the distribution station, and put condensate and steam main pipe into the outside air, or directly install them on the top of the gallery during the design process, in order to improve the use effect of steam. In the design, it should also be considered that if the position of the main steam pipe is relatively low, or even lower than the inlet pipe of steam, it needs to be changed through a self guided steam powder distribution station to ensure that all steam introduction can be carried out smoothly. At the same time, a heat tracing station should also be set. The thermal tracking station is designed to effectively support and simplify various operation contents. The overall steam pipeline shall be designed close to the handrail, platform, column or wall. The arrangement of heat tracing must be scientific and reasonable to prevent problems in the design of the heat tracing station from affecting the operation of the steam tracing station. Once relevant problems occur, stop them as soon as possible to prevent blockage. During the analysis and management of the steam distribution station, it is necessary to calculate the s value, which should always take into account the size of the pipe diameter and be calculated according to the relevant calculation formula. In the process of calculation, the main parameters to be considered include the number of steam distribution pipes, steam inlet pipes, condensate outlet pipes and other relevant data.
The selected formula is: s=a+2b+2c
When calculating and analyzing the above formula, you need to query the data, where a refers to the number of dn15 tracing pipes in the whole steam tracing system, b refers to the number of dn20 tracing pipes, and c refers to the number of dn25 tracing pipes. Using the calculated s value, it is necessary to analyze the s value. When the s value is greater than 16, the overall number of steam distribution stations and drainage stations should be redesigned to ensure that the number is greater than or equal to 2.
When designing the steam tracing gas supply header, it is also necessary to design the relevant flow chart, and all codes and legends are required to be consistent with the flow chart of the pipeline and instrument. The length of heat tracing pipe is higher than the general length in most cases, so each segment shall be clearly marked when selecting segmented heat tracing. Condensate collection station and steam distribution station, whether horizontal or vertical, should be consistent with the layout in the heat tracing pipeline. All steam tracing pipes shall be clearly planned on the layout drawing, and the steam collection station and tracing station shall be fully shown. Condensate collection station and steam distribution station shall also be marked to effectively identify all tracing stations. For tracing pipes, a very obvious mark must be made. The purpose of the mark is to identify the number and starting point of all tracing pipes, so as to ensure that each tracing pipe can be found at any time and anywhere. Each tracing pipe should have two identical marks, which are respectively set on the condensate collection station and the steam distribution station. This way can improve the working speed and quality of the staff. Once a certain pipeline fails, maintenance can be carried out in the shortest time. The steam distribution station and drainage station also need to meet the following requirements: First, if the steam distribution station is installed horizontally, the steam pipe is connected from the top or side. Second, if there are three heat tracing stations with a diameter of 6m, it is necessary to build relevant distribution stations and drain stations according to the number of heat tracing stations, leaving two left and right joints to prevent the phenomenon of no spare joints in case of failure. The spare joint can ensure the service quality during use. In the same steam distribution station, it is required to strive for the shortest one to keep the length about 70% of the longest tracing pipe.

Steam distribution station

The number of connecting pipes of steam distribution station shall be accurately analyzed and determined according to the actual situation. For steam distribution stations with diameter of dn40, no more than six dn15 or dn20 nozzles should be used, and the matching design of nozzles should be done to ensure that the application quality of distribution stations can be improved. If the designed steam distribution station is dn50 type, it is necessary to consider that ten dn15 type or ten dn20 type steam pipe holes should be used for connecting pipes when selecting steam interfaces, and more interfaces should be reserved to ensure timely needs. According to the analysis of the actual production situation at this stage, the layout structure of the steam distribution station is usually vertical or horizontal. The purpose is to facilitate the diffusion of steam to different areas, and achieve uniform diffusion and uniform heat dissipation in steam diffusion. To improve the quality of heat dissipation and the quality of steam use, the main pipe in the heat tracing gas supply pipe needs to lead out the steam in the reserved pipe to prevent steam accumulation in the pipe. In actual production, it is also necessary to choose to install relevant steam pipes at the top or at the water level. This distribution method can install fixed supports or demarcated supports in advance to facilitate pipeline air entrainment and keep the pipeline in a safe state during use.

Condensate station design

When designing the condensate station, the condenser of the condensate station shall be considered, and the main pipe of the condenser shall be connected with the top of the main pipe of the condensate. Ensure that a stop valve is set at the outlet of the main pipe of the condenser, and a group of drain valves are set on the trapezoidal steam pipe, which can directly return to the end of the condensate, and a branch is drawn out at the top of the gas station as the condensate pipe. Condensate needs to be added to the water circulation, and the diluent can be set by condensation method. Condensate can be directly put into the designated recovery pipe to avoid affecting the surrounding equipment and relevant pipes. The drain valve with filter on the body shall be selected as the drain valve, otherwise the use quality of the drain valve will be poor. In general, y-type filter is required to be selected as the filter. Whether the channel area or the use effect of y-type filter can meet the design requirements of the condensate station, the channel area should be determined according to the selection of the condensate pipe. It is required to select twice the sectional area. When the condensate is higher than the drain valve, it is necessary to select a power type drain valve, which has command function and can be set with a check valve behind the drain valve.

Selection of construction materials and specific requirements for construction

Selection of construction materials

The following materials are selected: stainless steel pipe, copper pipe and first grade carbon steel pipe. In many foreign projects, copper pipes and stainless steel pipes will be used as tracing pipes. Copper tube and stainless steel tube are both high-quality and inexpensive metal materials, but the best choice in actual use is still copper tube, because copper tube is more flexible, can be quickly bent and connected, and does not take time and effort. Carbon steel pipe has very strong corrosion resistance and is also widely used in China.

Specific contents of construction

During the construction of steam tracing pipe, the construction quality can be improved by the following ways: 

• ① During the construction of steam tracing pipe, the steam of branch pipe is directly led out from the top of the main pipe, and the steam is led out to the root of the determined position. At the same time, cut off valves need to be installed in the pipe to ensure the stability of the operation and use of the whole pipe. 
• ② If the condensate contained in the steam tracing pipe is not recycled, it will directly pass through the drain valve and be treated uniformly after passing through the drain valve. 
• ③ During the construction of steam tracing pipes, each tracing pipe has different functions, so the staff is required to understand each half pipe and improve their familiarity with each half pipe. Once problems are found in the half pipe, they can be repaired and replaced in a timely manner. The staff can install an independent steam trap on each half pipe. This way is to prevent the problem of one half pipe from causing all the surrounding half pipes to have the same problem, or even the confusion of the heat tracing system. 
• ④ The phenomenon of liquid resistance or air resistance on U-bend should be avoided as much as possible. Certain problems often occur at the head of U-bend during construction, which is a common problem in the design of steam tracing system. When the steam tracing pipe is constructed, the corresponding steam tracing pipe pressure shall be increased in case of U-shaped bend to discharge the air smoothly, so as to prevent the air from being discharged slowly due to the generation of air resistance. Therefore, when a U-bend is generated, the steam pressure should be considered. When the pressure is greater than 300kPa but less than 500kPa, the height of the U-bend can be increased to about 4m. When the steam pressure is greater than 700kPa but less than 1000kPa, the height of the U-bend can be accumulated to about 6m. When the steam pressure is greater than 500kPa but less than 700kPa, the cumulative height of the U-bend is about 5m. 
• ⑤ In order to avoid water vapor entering the condensate pipe network or even back pressure rising when steam tracing pipe is used as much as possible, bypass valves cannot be installed on the water delivery valve group to avoid negative impact on the overall operation of condensate and ensure that the operation of condensate is in a safe and positive state.

Conclusion

The purpose of steam tracing pipe in the chemical production process is to realize steam tracing. Compared with other transportation modes, it consumes less energy and can better meet the economic needs and actual use needs. In the later stage, when designing the tracing system, it needs to consider multiple factors to achieve the overall optimization design and improve the transportation efficiency and economic efficiency.
Authors: Lv Gaofeng, Meng Yuanjia, Li Xinyan