Sulphuric Acid

March 30, 2018 | Author: girishnitw | Category: Pipe (Fluid Conveyance), Leak, Corrosion, Iron, Steel


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Description

IntroductionPiping is used to convey fluid from one location or vessel to another location. Without piping there would be no process. The importance of piping is equal to that of any piece of equipment. Liquid must be conveyed safely, efficiently and economically from one point to another. Materials used for acid piping have evolved with the design of acid plants over the years and with the introduction of new materials. The two most common materials used for the main acid circulating lines are cast/ductile iron and alloy materials. The main differences, advantages and disadvantages are listed in the following table. Grey Cast Iron/Ductile Iron High potential for leaks due to large number of flanges High corrosion rates, up to 2.0 mm/year (80 mpy) or high in turbulent areas High installation cost due to heavy weight and large number of items High maintenance cost Require large inventory of fittings Low velocities (i.e. large pipe diameters to avoid excessive corrosion and erosion rates Low ductility. Risk of brittle failure especially with cast iron Alloy System Piping is welded with minimum number of flanges thus minimizing the chances of leaks Low corrosion rates, typically < 0.1 mm/year (4 mpy) Low weight. Piping can be shop spooled to minimize field installation costs. Minimal maintenance required. Inventory of spare parts is minimal Higher design velocities are permitted thus reducing line sizes. High ductility which minimizes risk of brittle failure Design Velocities Acceptable velocities in acid piping will depend on a number of different factors such as: Pipe material Acid concentration Acid velocity Impurities in the acid Acid temperature Intermittent or continuous flow Solids Flow conditions and turbulence 5 m/s (5.5 ft/s) Sandvik SX® For all practical purposes there is no upper limit to the acid velocity in SX piping but generally velocities in the range of 2 to 3 m/s are used for pressure drop reasons.8 m/s (9. Carbon steel in the presence of strong sulphuric acid will corrode to form a thin film of iron sulphate on the surface of the metal.1 m/s (6. Carbon Steel Carbon steel is by far the most economical material of construction for conveying strong sulphuric acid but it has its limitations in terms of operating conditions which limits its use to but a few areas of the plant.5 ft/s) 2.5 ft/s) 2.0 m/s (10.0 ft/s) >90°C (>194°F) 1.5 m/s (5.The following are general guidelines for maximum design velocities in various pipe materials. The acid velocity in the line should not exceed 0. It is this film of corrosion product that once formed prevents further corrosion of the underlying material.0 ft/s) 3. .6 m/s (8.0 ft/s) 2. It is this reason that the use of carbon steel is limited to handling acid at ambient temperatures and low velocities such as in product acid transfer and loading lines.6 m/s (8.0 ft/s) 1.9 ft/s) 2.5 m/s (1½ ft/s).7 ft/s) 2.6 m/s (8.5 ft/s) 2.8 m/s (6.3 ft/s) 3.3 m/s (7.7 ft/s) 2.0 m/s (10.6 m/s (8.1 m/s (6.6 m/s (8. However.3 m/s (7.8 m/s (6. Line Size 6" 8" 10" 12" 14" 16" 18" >20" Temperature <90°C (<194°F) 1. General The following figure shows maximum recommended acid velocities in various materials and at various temperatures.5 ft/s) 2. The change in velocity at a particular temperature is not exact and should only be used as a guideline. this protective film is very unstable an is easily disturbed and removed. Mondi™ The permissible velocity in Mondi™ piping varies with the line size and temperature.0 ft/s) 2.9 ft/s) 2.0 ft/s) 1. The velocities given in the following table are maximums. however. safety becomes a major concern. When the fluid being carried is not dangerous. when the fluid is hot concentrated sulphuric acid.Flange Guards The weak point in any piping system are the connections between spool pieces. wearing out of the gasket. The spray is contained and is converted into a drip which is less hazardous. in-line instruments. Flushing and Hydrotesting . Screwed connections are not recommended under any circumstance. poor or inproper installation. This hazard can be mitigated by the use of flange guards. If the system operated under high pressure. Flange guards is a safety shield that complete encloses the flange preventing the sprayout of acid. Leak may occur at the flange connections due to corrosion. leaks may result in acid spraying out into the operating area. leaks at these connections is not a critical safety issue. valves. and equipment. In acid piping systems all connections should be flanged connections. The first step is done during the design phase when the piping is laid out. there may be a violent reaction due to the sudden mixing of water and acid. . a drain should be provided. the formation of weak acid in the system may lead to increased corrosion and eventual leaks. After flushing and hydrotesting the lines. This is of particular advantage for ductile iron piping systems which have flanges at each fitting and spool piece. all water must be drained from the system and the line purged with air until all moisture has been removed.There are two opinions as to whether or not sulphuric acid lines should be flushed with water and/or subjected to hydrotesting during construction. As well. For alloy piping systems the number of flanges is greatly reduced so the need to hydrotest the system is less. Flushing and hydrotesting can be done safely provided certain precautions are taken. Low points and pockets should be avoided and if they are present. Alloy piping system are more easily damaged by weak acid than ductile iron systems. The case for flushing and hydrotesting acid lines is that leaks can be detected before acid is introduced thereby reducing hazards during the initial filling and circulation of acid in the lines. The concern is that if water remains in the lines when sulphuric acid is introduced.
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