Key points in the design of steam pipelines for chemical plants

Thread Content

When designing steam pipes in chemical plants, to ensure the quality and efficiency of the design, it is also necessary to select an appropriate pipe diameter. The piping layout must meet stress requirements as well, and attention should be paid to other detailed aspects to avoid water hammer phenomena. 01 Design of steam pipelines: There are many different types of pipelines in chemical plants. Generally, they are installed outside the buildings or along the plant premises; they are supported by brackets in mid-air, thus forming pipe racks. There are specific requirements for the layout of the pipe trays: process material pipes are generally arranged on the first layer and in the first-level pipe trays, utility pipes are placed on the third layer, and instrument cable trays are located on the fourth layer. Among them, the steam pipes are arranged on the third floor. To facilitate the installation of the π-shaped compensator, steam pipes should generally be arranged on one side of the pipe tray. At high temperatures, steam pipes expand, and at this time, π-shaped compensators can be used to absorb the thermal expansion of the pipes. Due to the relatively high cost and limited service life of bellows expansion joints, they are generally not used to absorb thermal expansion in steam pipelines. When determining the installation location of the compensator, it is first necessary to conduct a thorough analysis of the pipeline to ensure that the compensator can be installed in a centralized position. Those pipes with higher temperatures and greater compensation requirements are usually placed on the outside, while those with lower temperatures and smaller compensation requirements are placed on the inside. The π-shaped compensator is generally installed in the middle, and guide frames must be placed on both sides of the compensator; the distance between these guide frames and the compensator is determined based on the stresses experienced by the pipeline. When calculating the thrust on the supports and the stress in the steam pipe, the stress throughout the entire steam pipe must be calculated. In chemical plants, multi-layered pipe galleries are generally installed, with steam pipes located in the upper layers of these galleries; this arrangement ensures that the low-temperature pipes and the pipes for liquid hydrocarbons are not placed next to each other. On the same layer, steam pipes and electronic instrument cables can be installed simultaneously, provided that the distance between them is not less than 200 mm. The steam pipes can also be arranged below the cable for electronic instruments, but the spacing must also be no less than 500 mm. 2 Design of drainage facilities for steam pipes. Generally, dedicated drainage installations are provided in steam pipes during the pipe warming phase. When driving, a large amount of condensate is generated; therefore, dedicated drainage facilities must also be installed. The installation of drainage facilities should be chosen based on the different levels of steam pressure. Under normal conditions, ultra-high pressure pipelines do not produce condensate, and there are no condensate pipes of appropriate specifications installed in these ultra-high pressure steam pipelines; therefore, hydrophobic devices are generally not installed in such pipelines. Ultra-high pressure pipelines are characterized by thick wall thickness, difficulty in making openings, and high pressure; therefore, a liquid separation vessel is generally not installed either. Under normal conditions, condensate generally does not form in high-pressure, medium-pressure, and low-pressure pipelines. However, to prevent a large amount of condensate from forming in the steam pipelines during the heating-up or startup phase, drainage valves, demisters, and other hydrophobic devices are installed in these steam pipelines. When installing drip collection devices for steam pipes, a drip collector should be placed at the end of the main steam pipe. There are also specific regulations regarding the spacing between these drip collectors along the main steam pipe; when the steam is in a saturated state, this spacing should be 80 mm. If in an overheated state, the spacing between drip pans should be 160 mm. If it is on a slope facing uphill, the spacing between the liquid separation tanks outside the device should be 300 mm; if it is on a slope facing downhill, the spacing between such tanks should be 200 mm. The steam separator is generally installed when the main saturated steam pipe enters the unit; it should be placed near the boundary on the unit side. In addition, measures to prevent water accumulation should also be implemented at the bottom of the diverter. If the superheated steam main enters the device, there is no need to install a water separator. Since the steam vent pipe should lead directly to the atmosphere, a drainage hole should be provided at the lower end of the steam vent pipe; moreover, a DN15 pipe should be connected at a suitable location such as a drain ditch or funnel. Guiding and load-bearing supports should also be installed on the steam vent pipe. Since the Panqi pipelines often discharge or are connected to discharge systems, they should be led to the main operation area or places with few operators. 3 Design of steam branch pipes: Steam branch pipes are installed at the top of the main steam pipe. Generally, a shut-off valve is provided on these branch pipes. To prevent liquid accumulation, the shut-off valve should be installed on a horizontal pipe, near the main pipe. Some steam pipelines have strict requirements regarding the steam used, so it is not possible to connect steam branch pipes to such pipelines; furthermore, branch pipes cannot be connected from the π-shaped compensators of these steam pipelines. If the branch pipe is connected to the main pipes at both ends of the π-shaped compensator, it should not be affected by the displacement of the steam main pipes. During thermal expansion, the steam main causes displacement at the connection points of the branch pipes, preventing the branch pipes from experiencing excessive pressure or significant displacement. Under normal circumstances, a two-valve assembly is used when a branch pipe is connected to the main steam pipe. However, in order to detect leaks at any time, a two-valve assembly cannot be used for connections that lead from steam branch pipes or the main steam pipe to other process pipelines; instead, a three-valve assembly should be employed. Depending on the circumstances, hydrophobic devices such as drain valves or steam traps should be installed at the lowest point of the steam branch lines. When installing hydrophobic devices on the main pipelines, it is necessary to take into account the different pressure levels in the pipe racks. 4 Design of steam condensate pipes: Generally, steam pipes and steam condensate pipes are arranged on the same floor in the pipe tray. To prevent water hammer, π-shaped compensators can be installed on the steam condensate pipes. Such a π-row compensator must be arranged horizontally, or the vertical pipe should be designed with an inclined section. Click to view--Summary of Chemical Engineering Skill Training Courses in 2023. Condensate coming out of steam traps at different pressures should be connected to their respective recovery main pipes. When the nominal diameter of the branch pipe is 50 mm or larger, it can be connected directly to the top of the steam and condensate recovery main pipe. For the printing plate, flange connection is used to install the steam trap in the steam condensate recovery system; there should be no bell-shaped sections in the pipes at the inlet of the steam trap. If the steam trap is located below the main steam condensate return pipe, a check valve should also be installed behind the steam trap. When installing a check valve, it should be placed on a horizontal pipe, near the main steam condensate pipe. Check valves should also be equipped with flange connections, which facilitates the purging of steam pipes and the removal of the check valves. 5 Key points to consider in steam pipeline design 1. Proper selection of pipe diameter: When selecting the pipe diameter, it is necessary to take into account the demand for steam. When the pipe diameter is too large, it increases investment costs, raises heat losses, and also leads to more condensate water. When the pipe diameter is too small, it results in insufficient pressure at the steam usage point and inadequate steam flow, which ultimately leads to water hammer and erosion. Therefore, when selecting the pipe diameter, it should not be too large or too small. 2 Stress requirements: When installing pipes, it is essential to meet the stress requirements, and stress calculations must be carried out thoroughly. The installation of π-shaped compensators on pipelines, the thrust at the fixing points of these compensators, as well as the layout of the steam pipes connected to the equipment, all need to meet stress requirements; this helps to improve the efficiency of the design process. 3 Avoid water hammer phenomenon: When water particles moving at high speed collide with pipe fittings, equipment, or valves, it generates certain vibrations and noise; this is what is known as the water hammer phenomenon. To avoid water hammer, attention should be paid to the design of the steam drainage system. Additionally, when taking steam from the branch pipes, it should be done above the main pipeline. Pipes should not have too many branch pipes, reducers, bends, etc. To prevent localized sagging of the pipes, the installation of pipe supports must be proper. Install the filter screen at the appropriate level. All of these details need to be taken into account in order to avoid water hammer phenomena and improve the quality and efficiency of steam pipeline design in chemical plants. In summary, there are many strict requirements regarding the installation of steam pipes in chemical plants; attention must also be paid to numerous detailed aspects. Only by doing so can a scientific and rational design be ensured, the efficiency of the steam pipes improved, and their proper functioning guaranteed.