【Q&A Question 050】February 19, 2017
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This post was last edited by 955559 on 2017-7-21 at 21:57. [Q&A Question No. 050] Why is it necessary to warm up the pipes before starting a turbine? The reference answer can be seen after *replying*; points will be awarded for identifying the key points. (Unless otherwise specified, all questions and answers are based on hydrogenation units.) ) Since the main steam pipeline, valves, flanges, etc. are in a cold state before startup, the pipeline is first warmed up at a pressure of 0.05–0.5 MPa, allowing it to expand gradually and evenly as it is heated, thereby avoiding excessive thermal stress. The requirement for this warming process is to control the rate of temperature increase in the main steam pipeline and its accessories; under a certain pressure, the steam pipeline, connection flanges, valves, and steam chamber are heated up gradually and evenly, after which the pressure is increased. The purpose of increasing the pressure is to continue warming the pipeline, with the pressure being raised at a rate of 0.1–0.15 MPa per minute until the rated pressure is reached. Petrochemical Area – “Creative Ideas for Energy Saving” event: http://bbs.hcbbs.com/thread-1666758-1-1.html The New Normal Cup – Voting for the top 10 members of HaiChuan in 2016 is now underway; vote for the member you care about the most (please cast your valuable vote for the members in the Petrochemical Area). http://bbs.hcbbs.com/thread-1667649-1-1.html Summary post of Q&A questions for 2017 (updates available now). http://bbs.hcbbs.com/thread-1657793-1-1.html Summary post of daily questions for 2017 (updates available now). http://bbs.hcbbs.com/thread-1657794-1-1.html Summary post of Q&A questions from 2016 (updates completed). http://bbs.hcbbs.com/thread-1597792-1-1.html Third-phase selection of outstanding management members at HaiChuan. http://bbs.hcbbs.com/thread-1655902-1-1.html Petrochemical Area – “Creative Ideas for Energy Saving” event: http://bbs.hcbbs.com/thread-1666758-1-1.html Xiaomi phones are up for grabs. Summary post collecting chemical engineering-related materials from the Petrochemical Section. http://bbs.hcbbs.com/thread-1654603-1-1.html Many prizes are waiting for you (Source: HaiChuan Chemical Forum)
⑴Before startup, since the main steam pipes, various valves, flanges, etc. are at a cold state, a small amount of steam is introduced into the inlet piping to allow the pipes to heat up gradually and expand evenly, thereby reducing thermal stress. ⑵During pipe warming, it is necessary to properly control the rate of temperature increase in the main steam pipes and their accessories; the pressure increase rate during this process should not be too fast, as otherwise the flange connection bolts, pipes, and valves may experience excessive thermal stress due to the rapid temperature rise, leading to cracks or damage.
⑴Before startup, since the main steam pipes, various valves, flanges, etc. are at a cold state, a small amount of steam is introduced into the inlet piping to allow the pipes to heat up gradually and expand evenly, thereby reducing thermal stress. ⑵During pipe warming, it is necessary to properly control the rate of temperature increase in the main steam pipes and their accessories; the pressure increase rate during this process should not be too fast, as otherwise the flange connection bolts, pipes, and valves may experience excessive thermal stress due to the rapid temperature rise, leading to cracks or damage.
⑴Before startup, since the main steam pipes, various valves, flanges, etc. are at a cold state, a small amount of steam is introduced into the inlet piping to allow the pipes to heat up gradually and expand evenly, thereby reducing thermal stress. ⑵During pipe warming, it is necessary to properly control the rate of temperature increase in the main steam pipes and their accessories; the pressure increase rate during this process should not be too fast, as otherwise the flange connection bolts, pipes, and valves may experience excessive thermal stress due to the rapid temperature rise, leading to cracks or damage.
Before starting the turbine, its main steam pipelines and valves are in a cold state; to prevent these components from experiencing excessive thermal stress, pipe warming is necessary. The purpose is to prevent pipes, valves, and flanges from experiencing excessive thermal stress or from developing cracks or damage due to large temperature differences between the inside and outside.
Answer: First, before starting the turbine, since the main steam pipes, various valves, flanges, etc. are in a cold state, a small amount of steam is introduced into the inlet pipelines to allow the pipes to heat up gradually and expand evenly, thereby reducing thermal stress. Secondly, during pipe warming, it is necessary to properly control the rate of temperature increase in the main steam pipes and their accessories. The pressure increase rate during pipe warming should not be too fast, as this can cause significant thermal stress on the flange bolts, pipes, and valves due to the rapid temperature rise, leading to cracks or damage.
This is mainly because the main steam pipes, valves, and flanges are all at a cold state; when the high-temperature steam from the warm pipes comes into contact with the cold metal pipe walls, it condenses and releases heat, which is then transferred to the metal pipe walls. At this time, the condensed hydrophobic substance must be removed promptly. During the pipe warming process, it is essential to strictly control the rate of temperature increase in the metal pipe wall and raise the steam pressure gradually, in order to ensure uniform expansion of the pipe. If the rate of temperature increase is too fast and the steam pressure rises too abruptly, it will lead to an increased temperature difference between the steam and the metal pipe wall as well as a higher heat release coefficient, which may result in cracks and deformation
When starting the turbine, 3.5 Mpa steam is required, and the pipes must be preheated first.
Prevent condensate from entering the turbine and causing damage to the blades
This is because when high-temperature, high-pressure steam comes into contact with the metal wall of a pipe at normal temperature, some of it condenses into water. If the steam flow rate is high, the carried condensed water will cause water hammer within the pipe. The hazards of water impact are significant; in mild cases, it can cause pipe supports to loosen and the pipes to shift ； In severe cases, it can cause cracks in the pipes and their fittings. 2 If the steam preheats the pipes too quickly, large temperature differential stresses will be generated on the pipe walls. If this situation occurs repeatedly, it will cause the pipes and their fittings to experience thermal expansion and deformation that is not acceptable from a safety perspective, and may even lead to serious accidents such as cracks. Therefore, it is necessary to limit the heating rate and the heat transfer temperature difference during the steam preheating of the pipes in order to warm them up.
Before starting an industrial steam turbine when it is cold, the main steam pipes should be warmed up; otherwise, the following consequences will occur: 1 When high-temperature, high-pressure steam comes into contact with the metal pipe walls at room temperature, some of it will condense into water. If the steam flow rate is high, the carried condensed water will cause water hammer within the pipes. The hazards of water impact are significant; in mild cases, it can cause pipe supports to loosen and the pipes to shift ； In severe cases, it can cause cracks in the pipes and their fittings. 2 If the steam preheats the pipes too quickly, large temperature differential stresses will be generated on the pipe walls. If this situation occurs repeatedly, it will cause the pipes and their fittings to experience thermal expansion and deformation that is not acceptable from a safety perspective, and may even lead to serious accidents such as cracks. Therefore, it is necessary to limit the heating rate and the heat transfer temperature difference during the steam preheating of the pipes in order to warm them up.