Impingement protection

FIGURE 17.7 Impingement protection designs (a) impingement plate, (b) impingement rods, (c) nozzle impingement baffle, (d) vapor belt. 

Floating head cover Impingement protection None ... 

Two other methods are sometimes used to provide impingement protection, One method utilizes the first two rows of tubes adjacent to the nozzle for impingement protection. The tubes are plugged and rods are sometimes inserted through the tubes to prevent excessive tube movement in the event of a tube being cut sufficiently to allow movement. The other method replaces the first two rows of tubes adjacent to the nozzles with rods of the same diameter at the tubes. Both of these methods are effective and inexpensive in the common materials of construction. 

Impingement or erosion attack can occur when Hquids or gases impact metal surfaces at high velocity. The corrosion rate is high under such circumstances because any corrosion product films that can be protective if adherent are swept away as quickly as they are formed to leave exposed fresh surface. 

Impingement Baffle The tube bundle is customarily protected against impingement by the incoming fluid at the shell inlet nozzle wmen the shell-side fluid is at a high velocity, is condensing, or is a two-phase fluid. Minimum entrance area about the nozzle is generally equal to the inlet nozzle area. Exit nozzles also require adequate area between the tubes and the nozzles. A full bundle without any provision for shell inlet nozzle area can increase the velocity of the inlet fluid by as much as 300 percent with a consequent loss in pressure. 

Impingement baffles are generally made of rectangular plate, although circular plates are more desirable. Rods and other devices are sometimes used to protect the tubes from impingement. In order to maintain a maximum tube count the impingement plate is often placed in a conical nozzle opening or in a dome cap above the shell. 

Valves in the inlet, seal water and pilot gas lines should be located according to permissible radiant heat densities for personnel (refer to the last section of this chapter for specific guidelines). Piping to the burning pit should be suitably protected against flame impingement (e.g., by installation below grade). 

An impingement plate was fitted to the reboiler to protect the tubes, but it fell off, probably as a result of repeated blows by slugs of condensate. The condensate then impinged on the tubes and squashed or broke 30 of them. 

When scale drawings are made, the effectiveness of impingement baffles can be evaluated easily. Sometimes it is necessary to relocate or make slight size changes in order to properly protect the tubes and direct the vapor flow. 

This example of aluminium illustrates the importance of the protective him, and hlms that are hard, dense and adherent will provide better protection than those that are loosely adherent or that are brittle and therefore crack and spall when the metal is subjected to stress. The ability of the metal to reform a protective him is highly important and metals like titanium and tantalum that are readily passivated are more resistant to erosion-corrosion than copper, brass, lead and some of the stainless steels. There is some evidence that the hardness of a metal is a signihcant factor in resistance to erosion-corrosion, but since alloying to increase hardness will also affect the chemical properties of the alloy it is difficult to separate these two factors. Thus althou copper is highly susceptible to impingement attack its resistance increases with increase in zinc content, with a corresponding increase in hardness. However, the increase in resistance to attack is due to the formation of a more protective him rather than to an increase in hardness. 

Impingement attack Copper may occasionally suffer this form of attack in systems where the speed of water flow is unusually high and the water is one that does not form a protective scale, e.g. a soft water containing appreciable quantities of free carbon dioxide . Ball valve seatings may also suffer an erosive type of attack. The corrosion of ball valves, including the effect of chlorination of the water, has been studied by several workers... 

Surface preparation is of prime importance, and optimum performance of modern protection coatings can be achieved only if the surface of the steel has been adequately treated. The method of surface preparation depends on the shape and size of the structure or component. Thus it is preferable to blast-clean an openwork steel structure by manual methods, since with this type of structure automatic blast cleaning would lead to excessive impingement of the abrasive on the machine itself. 

Steel, unless adequately protected with a film of oil subsequent to rolling mill operations, has a tendency to form surface rust rather quickly. This rust on the surface of the metal prevents proper conversion coating. A traditional method of removing rust is an acid applied by power spray equipment. The spraying action cleans both by physical impingement and the etching action of the acid. The power spray action is followed by a brush scrub, which further removes soil loosened by the acid. The brush scrub is followed by a strong alkaline spray wash, which removes all traces of the acid and neutralizes the surface.1-5... 

The point is also made [134] that the very high surface areas and the richly interconnected three-dimensional networks of these micron-sized spaces, coupled with periods of desiccation, could together have produced microenvironments rich in cat-alytically produced complex chemicals and possibly membrane-endosed vesides of bacterial size. These processes would provide the proximate concatenation of lipid vesicular precursors with the complex chemicals that would ultimately produce the autocatalytic and self-replicating chiral systems. A 2.5 km2 granite reef is estimated to contain possibly 1018 catalytic microreactors, open by diffusion to the dynamic reservoir of organic molecules. .. but protected from the dispersive effects of flow and convection [134] as well as protected from the high flux of ultraviolet radiation impinging on the early Earth. [123,137]... 

Steel, aluminum, concrete, and other materials that form part of a process or building frame are subject to structural failure when exposed to fire. Bare metal elements are particularly susceptible to damage. A structural member undergoes any combination of three basic types of stress compression, tension, and shear. The time to failure of the structural member will depend on the amount and type of heat flux (i.e., radiation, convection, or conduction), and the nature of the exposure (one-sided flame impingement, flame immersion, etc.). Cooling effects from suppression systems and effects of passive fire protection will reduce the impact. 

Experiments on gas jet fires impinging on steel tubular members by Shell/British Gas (Offshore Research Focus, 1980) evaluated two types of passive fire protection ... 

Jet fire flame impingement on vessels or structural steel that are not protected for 15 minutes... 

The effect of fire exposure is predictable for pressure vessels, such as, spheres, spheroids or horizontal vessels. If no fire protection is provided or is not adequate or inoperative, the vessel will probably fail catastrophically in a prolonged fire. Vessel failure typically results from excessive metal temperature weakening the tank wall above the liquid level of its contents. This weakening can occur within a few minutes if the initial liquid level is significantly belowthe maximum flame height and the flames impinge on the shell. 

Check positioning of nozzles and spray patterns on protected equipment. The entire surface of the protected area should be covered by the direct impingement of the spray pattern. If portions of the protected equipment are dry or minimally wetted, the system must be readjusted as necessary and then retested. 

Intumescent fireproofing— A passive material that undergoes a chemical reaction when exposed to high heat or direct flame impingement that protects against the heat from a fire without additional intervention. 

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