There are four different cleaning methods you can consider:
- Cleaning with air
- Cleaning with steam
- Cleaning with water
- Mechanical cleaning.
Cleaning with air
During the cleaning operation, the air velocity should be as follows:
- In the gas process lines, the velocity should be the lowest figure between 60 m/s and the velocity obtained at 1.3 time the maximum process flow rate. The minimum velocity should be 30 m/s.
- In liquid process lines, a velocity between 30 m/s and 60 m/s should be used.
Two main methods can be used:
- Continuous blowing of air: Air is continuously blown from a source (directly from Air Compressor or via a large capacity vessel) at a high velocity through the lines (and vessels) to carry away the loose material. Due to the high velocity of the air, a light erosion of the walls generally occurs during the air blowing, which is beneficial with regards to cleaning.
- Quick decompression: A network is filled with compressed air up to the setting pressure of a rupture disk. When the rupture disk blows out, the source of air is stopped and the pressure inside the network decreases quickly. This operation will preferably be done using a "quick opening valve" after having pressurized the network to the desired level of pressure. The existing valves of the network should not be used for this purpose, but additional valves should be provided specially for these operations. The quick decompression causes a high velocity of air in the pipes, thus removing loose material and performing a light erosion on the walls which eliminate rust and scale.
Cleaning with Steam (for steam networks only)
Whenever steam is available, an excellent cleaning fluid is provided. Steam can be used, like air, for continuous blowing, and preferably, for intermittent blowing. The advantage of the latter is that between two blowings the pipes (and possibly vessels) cool and contract causing millscale and rust deposits to crack. These deposits are then easily removed in subsequent blowings. Steam blowing should be used only for steam networks.
Cleaning with water
Cleaning of piping by circulation of water: Line flushing uses water from the normal cooling or firewater systems. Water supply could conceivably be from other sources, i.e. City water lines, supply boat temporary pumps, etc., depending upon the ongoing situation. This type of flushing normally involves taking a section of piping and flushing it from a "startpoint" to an open end. A high velocity flow, in the order of 10-13 ft/s should be achieved in order to be effective. In some circumstances additional pumping capacity will be required. This method of cleaning is effective in removing loose debris from pipework but not particularly effective in removing rust and millscale from piping (or vessel) walls.
Fill and dump water flushing: Fill and dump flushing is often used for cleaning vessels. The method being quite simple, the adequately vented vessel is filled to the desired level with flush water and then dumped to grade, the flush water dislodges loose debris and rust and carries it out of the equipment. Note that some lines connected to the vessel can be flushed by dumping the water through them. One major problem with this method is the disposal of the large quantities of water involved which may overload the normal drain systems.
Cleaning of vessels or equipment by water spray or high pressure water: If there is only dust on the walls of a large vessel, a water spray from a hose pipe might be sufficient. High pressure water may be used to remove solid particles or foreign matter from inside walls or inside parts of vessel or equipment, in particular where access is a problem. Attention has to be given to the potential corrosiveness of water (chloride ions on stainless steel, seawater on carbon steel) and special water quality may be necessary (treated demineralized water or inhibited water).
Manual cleaning: When a system, or part of a system, can be entered, it may be possible to carry out pipewall or vessel wall cleaning manually using either rotary (i.e. air driven) or hand held steel wire brushes. Rust, scale, and dirt that are dislodged may be subsequently removed by the use of an industrial grade vacuum cleaner, by air blowing, or a combination of these two methods. Brushing should be avoided for vessels or piping fabricated from any grade of stainless steel. Carbon steel brushing may cause subsequent corrosion problems. In any event, stainless steel equipment is not likely to have significant deposits of surface dirt or oxide.
Line pigging: For equipment provided with pigging facilities, e.g. interplatform pipelines, platform-to-shore pipelines, etc., special brush pigs are available. These are pigs equipped with peripheral wire spike type brushes and it is often the case that such pigs are used to dislodge rust and scale from pipeline walls. However, deposits of rust and scale are not necessarily conveyed from a pipeline by the use of such a projectile. Such deposits are normally removed by sending a "scraper" or cupped pig through the line which pushes these loose deposits of it for subsequent removal at an "open end".