Purging and cleaning of chemical plants before commissioning

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This post was last edited by yinkuilin6868 on 2016-2-18 at 15:41. I. Purging and cleaning purposes and methods: Before a chemical plant begins operation, it is necessary to purge and clean all the process pipelines and equipment that have passed inspection after installation (hereinafter referred to as purging). The purpose of this is to use fluids such as air, steam, water, and various chemical solutions, along with purification and flushing processes as well as physical and chemical reactions, to remove debris such as sediment, grease, welding slag, and rust that remain inside these pipelines and equipment from the installation process. This helps to prevent blockages in the pipelines and equipment during the commissioning phase of the plant； Damage to machines, valves, and instruments ； Contaminating catalysts and chemical solutions, affecting product quality, and preventing fires and explosions ； It is an important commissioning procedure to ensure the smooth commissioning of equipment and its long-term safe operation. In chemical plants, there are a wide variety of pipes and equipment, each with different operating conditions as well as varying materials and structures; consequently, the cleaning methods suitable for them also differ. However, it usually includes the following methods: water flushing, air blowing, steam blowing, acid pickling and passivation, oil cleaning, degreasing, etc. Their main features and areas of use are summarized as follows. 1. Water flushing: Water flushing is a method that uses water as a medium, pressurized by a pump, to clean pipes and equipment. It is widely used to remove residual dirt and debris from pipelines for transporting liquid media, as well as from equipment such as towers and tanks. Water flushing of pipes should be carried out at the maximum flow rate that can be achieved within the pipes, or at a flow velocity of not less than 1.5 m/s (this does not include high-pressure or ultra-high-pressure water jet cleaning equipment, nor methods for cleaning fouling on the inner and outer surfaces of pipe bundles). For general chemical processing equipment and pipelines, clear water with a turbidity of less than 10*106 and a chloride ion content of less than 100*106 is commonly used for flushing. However, for equipment and pipelines made of austenitic stainless steel, such as those in urea production plants, deionized water is required for flushing in order to prevent stress corrosion cracking (SCC) of the equipment and pipelines due to the accumulation of chloride ions (Cl-). Water flushing features ease of operation and no noise. 2. Air purging: Air purging is a method that uses air as a medium; after being pressurized by a compressor (usually at 0.6–0.8 MPa), this air is used to remove any remaining contaminants from the pipes through which the gas is transported. When using air for purging, there must be an adequate volume of air such that the flow velocity of the purging gas is greater than that of the gas used in normal operation; generally, this velocity should be at least 20 m/s. This ensures that there is sufficient energy (or momentum) to remove any residual deposits in the pipes and equipment, thereby enabling smooth commissioning of the installation and safe operation. During air purging, the amount of air consumed is generally high, and a certain purging time is required. Therefore, air purging is usually provided by using the largest air compressor in the facility, or a large compressor capable of handling compressed air in the facility (such as the pyrolysis gas compressor in ethylene plants). For small and medium-sized chemical plants that lack the capability to supply large amounts of continuous purge air, a segmented purging method can also be employed. This involves dividing the system pipes into multiple sections, each of which is further divided into several sub-sections; purging is then carried out section by section, with each section being isolated from the system after it has been purged. In this way, the purging quality can be ensured even when the gas supply volume is low. For large-diameter pipes or pipes where dirt is difficult to blow out, the blast purging method can also be used for purging. Oil-free air shall be used to purge oil-sensitive pipelines and instrument air pipelines. Due to reasons such as source and cost, nitrogen is generally not used as a purging gas for ordinary pipelines and equipment; instead, it is commonly used for air purging, and for the protective displacement of pipelines and equipment after the system’s air has been dried to an appropriate standard. 3. Steam purging: Steam purging is a type of purging that uses steam with varying parameters as the medium, with the steam source provided by a steam generator. Steam purging has a very high purging speed, and therefore possesses a large amount of energy (or momentum). The intermittent steam purging method, in turn, causes the pipeline to undergo thermal contraction and expansion, which facilitates the detachment and removal of deposits on the inner wall of the pipeline; thus, optimal purging results can be achieved. Steam pipelines should be purged with steam, and this is especially necessary for power steam pipelines. When purging power steam pipes, it is necessary not only to thoroughly remove any dirt and debris attached to the inside of the pipes, but also to eliminate the rust on the metal surface; otherwise, if this rust is carried along by the high-speed steam flow, it can cause severe damage to the rapidly rotating turbine blades, nozzles, and other components. Steam purging involves high temperatures, high pressures, and fast flow rates; as a result, the pipes expand when heated and contract when cooled. Therefore, steam pipes are equipped with compensators, steam traps, pipe supports, hangers, slides, etc., all of which are designed to account for such expansion and contraction. When air purging is not sufficient for cleaning non-steam pipelines, steam purging can also be used; however, it is necessary to consider whether the pipeline’s structure can withstand high temperatures and thermal expansion and contraction, and take appropriate measures to ensure the safety of personnel and equipment during the purging process. 4. Oil cleaning: For the lubrication oil, seal oil, and control oil pipeline systems of high-speed, high-load machinery such as steam turbines and centrifugal compressors, oil cleaning should be carried out after the equipment and pipelines have been successfully purged or pickled. Since such oil pipeline systems require an extremely high level of cleanliness, the presence of even tiny impurities in them can lead to damage to the bearing shells and sealing rings of the machines in operation, as well as malfunctions in the control systems, resulting in serious accidents involving the machinery. The oil cleaning method involves circulating oil within the pipeline system; during this cyclic cleaning process, the oil temperature must be raised and lowered 2–3 times every 8 hours within the range of 35–75°C, so that the particles attached to the pipe walls can fall off and be removed later by the oil as it circulates through the filter elements. The cleaning oil should be high-quality oil suitable for that machine. Effective protective measures should be taken for pipeline systems that have passed the cleaning process. Qualified lubricating oil should be replaced before the machine is tested. The criteria for determining whether oil cleaning is satisfactory: when the design or manufacturing manufacturer does not specify any particular requirements for the cleaning of the oil piping system, there are two methods to determine whether the oil cleaning meets the requirements. The first method involves using the American MOOG Grade 4 standard for testing: the criterion is based on the size of particulate impurities in 100 ml of oil sample, and observation is carried out under a 20x magnifying glass. If two consecutive samples meet the specified standards, then it can be considered that the oil system has been properly cleaned. The second method involves using filters for inspection, with the acceptance criteria specified in Table 1-1-4-2. Table 1-1-4-1: Counting of particles using a JC10 type 20x reading microscope; particle size: μm > 150, 100–150, 50–100. Moisture: Standard range; number of particles: 0 ≤ 2, 1 ≤ 2, 250. Table 1-1-4-2: Inspection for residual contaminants using a filter screen; equipment rotation speed (r/min), filter screen specification (mesh count). Acceptance criteria: ≥6000 at 200-mesh filter screens, with no more than 3 particles of residue per cm2; <6000 at 100-mesh filter screens, with no more than 3 particles of residue per cm2. 5. Degreasing: The degreasing process for equipment and pipelines is intended for those media that may experience combustion or explosion if exposed to small amounts of organic substances such as oils during production, transportation, storage, or use (such as oxygen), as well as for those cases where exposure to oils could affect product quality; it is thus another type of cleaning procedure. To ensure the safety of testing and production, such equipment, pipelines (including fittings, valves, instruments, and sealing materials), as well as the tools and measuring devices used for installation, must all undergo strict degreasing before being installed and put into use. Degreasing takes advantage of the fact that fats are soluble in certain chemical solvents; therefore, the process of degreasing is essentially a form of chemical cleaning. For the degreasing of pipes and equipment, it is first necessary to select an appropriate degreasing agent; for this purpose, Table 1-1-4-3 can be referred to. Name of degreaser, applicable scope, and remarks:
Industrial dichloroethane: For degreasing metal parts; toxic, flammable, and explosive.
Industrial carbon tetrachloride: For degreasing black metal chips, copper, and non-metallic materials; toxic.
Industrial trichloroethylene: For degreasing metal parts; must contain stabilizers; toxic.
Industrial alcohol (≥95.6%): Suitable for equipment, components, or surfaces where only basic degreasing is required; flammable and explosive.
Concentrated sulfuric acid (≥98%): Used for degreasing acid-resistant pipes, ceramic rings, etc., in sulfuric acid processing units.
Additionally, acetone, benzene, and alkaline solutions can also be used as degreasers. The most commonly used degreaser is carbon tetrachloride, as it has high degreasing efficiency, low toxicity and minimal corrosiveness to metals, thus making it suitable for a wide range of applications. The degreaser should be able to dissolve grease quickly. The oil content of the degreaser used must meet quality standards; if necessary, it should be determined through laboratory testing. The usage regulations for the degreaser are shown in Table 1-1-4-3. Degreasing agents have a limited capacity to dissolve grease; they become contaminated after use, so they can generally be used only once. If reuse is necessary, they must be distilled and regenerated, and their oil content must be checked to ensure it is within acceptable limits before they can be used for degreasing. Regulations regarding oil content in ml/l: >500 is not permissible; 50–500 allows for rough degreasing