Purging: Do You Really Understand It? (Series) – Steam Purging
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This post was last edited by Zhizhi Ming on 2025-1-8 at 12:28. III. Steam Purging: The steam systems in chemical plants typically have multiple pressure level parameters to meet the requirements of different equipment and process conditions. For example, there is low-pressure steam used for mixing and heating, medium-pressure steam used for heating, in direct process applications, and in industrial steam turbines, as well as high-pressure steam used to drive large industrial steam turbines. Especially regarding the steam pipes used to drive industrial steam turbines, during steam purging, it is necessary not only to remove any debris from the pipes but also to eliminate any rust on the metal surfaces, as such substances, if carried along by the high-speed steam flow, can cause serious damage to the rapidly rotating turbine blades. Therefore, it is very important to properly master the steam purging methods and strict quality requirements. Steam purging is usually carried out in two levels – high, medium, and low pressure – based on the operating parameter range of the pipelines (there are also three levels: high, medium, and low pressure), and the requirements for purging vary among these levels. 1) Selection of purge steam source and parameters: To improve purge efficiency and reduce purge costs, pressure-reduction purging is commonly employed for steam system purging. However, the steam consumption remains substantial; generally, it amounts to 50–70% of the steam flow rate inside the pipes at their rated load. High parameters (medium pressure, high temperature), long duration. Therefore, the purging of steam networks is mostly carried out simultaneously with the startup of their feed steam boilers. In the steam generators (waste heat boilers) of chemical plants, such as those for ethylene and synthetic ammonia production, where high-temperature process gases are involved, in order to shorten the commissioning time, external steam is generally used, or steam is supplied by a commissioning boiler built within the plant itself, before chemical feed is introduced into the plant. The requirements for the steam source in steam purging are that its steam parameters (pressure, temperature) and steam volume must be sufficient to meet the purging needs of the steam pipelines at various pressure levels. Steam parameters are the main factors that directly affect the purging effect. Steam purging utilizes the energy (also known as momentum) generated by the flow of steam within the pipe to wash away rust and debris inside it; the greater the energy, the better the effect. The factors that affect the energy of the steam medium during purging include: the steam parameters during purging (pressure, temperature) ； Hydraulic characteristics of steam pipes ； The degree of opening of the main valve during purging, etc. In fact, these three factors are interrelated, and it is necessary to select appropriate purging parameters based on the specific circumstances. The principle for selecting steam purging parameters is that the steam momentum inside the pipe during purging (also known as blowdown) must be greater than the steam momentum under rated load; in other words, the purge coefficient at any point within the system being purged should be greater than 1. That is, the purging coefficient k = (flow rate of purging steam)² * specific heat capacity of steam at the area to be purged / [（flow rate of steam at rated load）² * specific heat capacity of steam under rated conditions]. Typically, when using steam for purging, the steam flow rate inside the pipes is set at 50–70% of the rated value; therefore, the pressure of the steam source and the superheating temperature of the steam during purging can be calculated by using the reference calculation method based on the parameters of the purging steam, in combination with the hydraulic characteristics of the pipe section being purged. Generally, the flow velocity of the purge steam in pipes at different pressure levels is set as follows: high-pressure steam pipes (4–12 MPa) ≥ 60 m/s; medium-pressure steam pipes (1–4 MPa) ≥ 40 m/s; low-pressure steam pipes (
2) Preparations before steam purging: a. Before purging, a comprehensive purging plan should be developed based on the actual conditions of the steam pipeline network. This plan includes the scope of purging, the classification of different levels in the steam pipeline network, the determination of the steam flow rate used for purging, as well as the calculation and determination of the parameters related to the purging steam at each level (pressure, superheat temperature). It also covers the methods of purging, the sequence in which purging is carried out, the locations of discharge outlets, the temporary piping, valves, and supports required for purging, the methods and standards for assessing the quality of the purging process, the organization of personnel involved in the purging process, as well as the safety measures and precautions to be taken during purging. b. Conduct a thorough inspection of steam pipes, fittings, pipe supports, pipe hangers, and spring supports to ensure they are secure and reliable. After removing the spring’s fixing device, verify that the spring can stretch and contract flexibly. c. Check and confirm that there are no obstacles to sliding in the steam guide tube, and that there are no residual weld spots or scars on the sliding surface. d. Insulation of all steam pipes has been completed. e. The high and medium pressure steam pipelines have been acid-washed and passivated. f. In accordance with the purge plan requirements. All temporary piping, valves, vent pipes, target plate supports, etc., used for purging have been installed and meet the strength requirements. g. All instrument components installed on the purged pipeline (such as flow meters, orifice plates, venturi tubes, etc.) have been removed, and the control valves on the pipeline have been taken out or protected by appropriate measures. h. A temporary steam outlet pipe has been connected to the inlet of each steam turbine to prevent steam from entering the main steam valve and the turbine blades during purging, thereby avoiding damage to these components.
3) Methods and key points for purging steam pipelines: a. Steam purging is usually carried out in the order of the pipeline layout; generally, medium-pressure pipelines are purged first, followed by low-pressure steam pipelines. For each level of piping, the main pipe should be purged first, with the exhaust coming from the end of that pipe section; thereafter, the branch pipes should be purged, starting from those that are closer and moving towards those that are further away. It is advisable to temporarily remove or seal the valves on both the main and branch pipes before purging, and to reinstall the valves once the pipe section ahead of them has been properly purified, so that purging can continue on the subsequent sections. For welded valves on high-pressure pipelines, the valve core can be removed for sealed purging. The steam traps for each pipe section should be installed after the pipes have been purged. b. The purging method for steam pipelines involves repeating the process of heating the pipeline–purging–cooling down–heating the pipeline–purging–cooling down. Until the purging is satisfactory. By repeating this process over and over, the pipeline will inevitably undergo thermal deformation, which makes it easier for deposits such as rust on the inner wall of the pipe to fall off, thus achieving a good purging effect. c. Steam purging must first fully warm the pipes, and attention must be paid to drainage to prevent water hammer. During the steam heating of the pipes in the first cycle of purging, special attention should be paid to checking whether the thermal expansion of the pipelines, the sliding of the pipes, and the deformation of spring supports and hangers are all within normal limits. Warming the pipe should be done slowly. That is, a small amount of steam is first introduced slowly into the pipe to preheat it; only when the temperatures at the beginning and end of the pipe section to be purged are similar can the steam flow rate be gradually increased to the required level for purging. d. When warming up pipes using medium-pressure steam, the initial warming-up period should be relatively long—approximately 4 hours. During this time, the temperature should rise by about 100°C per hour. For subsequent warming-up cycles, the duration can be shorter, ranging from 1 to 4 hours. Each purging session lasts approximately 20–30 minutes. Since the cooling is done naturally, the time required for cooling depends on the temperature; generally, it is sufficient to cool the pipelines to below 100°C. The number of times purging needs to be repeated varies: for the initial purging of the main pipelines, more repetitions are necessary due to their length. Purging can be stopped and the quality of the purging checked only when the steam flowing out of the vent appears clean to the eye. Typically, the main pipes are purged 20–30 times, while the number of purges for the branch pipes can be less. For pipes that have undergone pickling and passivation, the number of purging cycles can be significantly reduced. e. The purging effect of medium-pressure steam pipelines must be checked using a target plate to assess the quality of the purging. Its target plate can be a polished copper sheet, about 2-3 mm thick, 5%-8% of the inner diameter of the exhaust pipe in width, and with a length equal to the inner diameter of the pipe. It can also be made from polished aluminum sheets with a thickness of about 8-10 mm. Conduct the target plate inspection twice in a row, with a purging time of 1–3 minutes. If no visible traces resulting from purging can be seen on the target plate, the purging is considered successful (should the design party have other requirements, those requirements shall be followed). For low-pressure steam pipes, a polished wooden board can be placed over the steam outlet for inspection; if there is no rust or dirt on the board and the steam condensate is clear and transparent, then it is considered satisfactory.
4) Safety precautions for steam purging: Purging of medium-pressure steam pipelines is a rather challenging task. Therefore, in terms of scheduling the purging process, determining the purging duration, implementing temporary measures, and ensuring safety precautions, it is necessary to make thorough arrangements based on the actual conditions of the pipeline network and ensure proper coordination throughout the purging process. During medium-pressure steam purging, the temperature is high, the flow velocity is fast, and there is significant noise; moreover, the steam is colorless and transparent. Therefore, safety must be strictly observed during purging. Noise reduction equipment should be installed at the discharge outlet, which must be directed outdoors and upward. A barrier should be set up around the discharge outlet, and no one is allowed to enter this area during purging. To prevent personnel from accidentally entering the purging port area and suffering injuries.
Purging: Do You Really Understand It? (Series) https://bbs.hcbbs.com/thread-5677878-1-1.html (Source: Haichuan Chemical Industry Forum [Hua Hai Chuan Liu])