Steam hammer

The coarse calciae cooler operates at 300°C, while the waste-heat boiler cools the gas to 350°C. The tubes ia the boilers have a chain-shaking arrangement operated by paeumatic hammers. Steam productioa is 0.78 kg/kg of dry coaceatrate. The only trouble with dust reported is ia the connection betweea the reactor and waste-heat boiler. It is necessary to cool the gas stream quickly to avoid sulfation, but even so the carry-over calciae coataias oa the order of four times more sulfate than the coarse overflow. In this plant, the composite calciae is 0.1% sulfide and 2.2% sulfate sulfur. 

Impervious graphite heat exchangers machined from solid blocks are also available (15,16). The solid block constmction is less susceptible to damage by mechanical shock, such as steam and water hammer, than are shell and tube exchangers. Block exchangers are limited in size and cost from 50—100% more than shell and tube units on an equivalent area basis. 

Cavitation Loosely regarded as related to water hammer and hydrauhc transients because it may cause similar vibration and equipment damage, cavitation is the phenomenon of collapse of vapor bubbles in flowing liquid. These bubbles may be formed anywhere the local liquid pressure drops below the vapor pressure, or they may be injected into the hquid, as when steam is sparged into water. Local low-pressure zones may be produced by local velocity increases (in accordance with the Bernouhi equation see the preceding Conservation Equations subsection) as in eddies or vortices, or near bound-aiy contours by rapid vibration of a boundaiy by separation of liquid during water hammer or by an overaU reduction in static pressure, as due to pressure drop in the suction line of a pump. 

Where Water Hammer Occurs. Water hammer can occur in any water supply line, hot or cold. Its effects can be even more pronounced in heterogeneous or biphase systems. Biphase systems carry water in two states, as a liquid and as a gas. Such a condition exists in a steam system where condensate coexists with live or flash steam in heat exchangers, tracer lines, steam mains, condensate return lines and, in some cases, pump discharge lines. 

Anotlier cause of water hammer is lack of proper drainage ahead of a steam control valve. When the valve opens, a slug of condensate will enter the equipment at a high velocity, producing water hammer when it impinges on the walls. In addition to this, the mixing of the steam that follows wifli the relatively cool condensate will... 

Water hammer can also occur in steam mains, condensate return lines, and heat exchange equipment where steam entrapment can take place (Fig. I). A coil constructed and installed as shown here, except with just a steam trap at the outlet, permits steam from the control valve to be directed through the center tube(s) first. Steam then gets into the return header before the top and bottom tubes are filled with steam. Consequently, these top and bottom tubes are fed with steam from both ends. Waves of condensate are moved toward each other from both ends, and steam can be trapped between the waves. 

Water hammer results from the collapse of this trapped steam. The localized sudden reduction in pressure caused by the collapse of the steam bubbles has a tendency to chip out pipe and tube interiors. Oxide layers that otherwise would resist further corrosion are removed, resulting in accelerated corrosion. 

To control differential shock, the condensate seal must be prevented from forming in a biphase system. Steam mains must be properly pitched, condensate lines must be sized and pitched correctly, and long vertical drops to traps must be back-vented. The length of lines to traps should be minimized, and pipes may have to be insulated to prevent water hammer. 

Water hammer (also known as hydraulic shock) occurs in two distinct ways when the flow of liquid in a pipeline is suddenly stopped, for example, by quickly closing a valve [13], and when slugs of liquid in a gas line are set into motion by movement of gas or condensation of vapor. The latter occurs when condensate is allowed to accumulate in a steam main, because the traps are too few or out of order or in the wrong place. High-pressure mains have been ruptured, as in the following incident. 

Figure 9-9. Arrangement of valves on steam main that was broken by water hammer.

Cast iron had not been used. It is brittle and therefore not a suitable material of construction for steam valves, which are always liable to be affected by water hammer. 

The operating team as a whole had been aware of the well-known hazards of water hammer in steam mains. 

Buildup of condensate in a heat exchanger can cause operating problems as well as water hammer. If the steam supply is controlled by a motor valve and the valve is not fully open, the steam pressure may be too low to expel the condensate, and its level will rise. This will reduce heat transfer, and ultimately the steam supply valve will open fully and expel the condensate. The cycle will then start again. This temperature cycling is bad for the heat exchanger and the plant and may be accompa-... 

A fire, followed by an explosion at Huddersfield in 1900 was also caused by detonation of iron picrate (presumably Fe++). The iron picrate had been formed on the surface of steam pipes located in the Picric Acid drier shop. It ignited when a plumber, unaware of the fact, struck one of the pipes with a hammer. The flame spread along the pipe and set the drying Picric Acid on fire... 

Pile Driving Device. Said to have originated in America, this device uses a cartridge of expl placed between the head of a wooden pile and a driving ram. The expin of the charge drives the pile into the ground and simultaneously raises the ram which, upon falling, drives the pile further. This idea was utilized in drop hammers which were explosively, not steam, operated... 

Steam traps are automatic mechanisms that remove low heat-content air and condensate from the steam delivery system. The lack of steam traps or use of traps that fail to function properly leads to a gradual decline in heat-transfer efficiency, waterlogged heat exchangers, and water hammer (which may in turn result in ruptured pipes). When adequate maintenance of steam traps is neglected, this ultimately leads to a serious overall loss of operating efficiency. There are various types of steam traps, each designed for a specific function. Some common variations are discussed in the following sections. 

Where condensate forms from the wet steam and drips back through the system, water hammer may develop and cause severe damage. 

NOTE Water hammer is caused by sudden interruptions in flow as when steam meets draining condensate. The steam produces instantaneous surges of pressurized water that hits valves, elbows, and tees at high velocity. This produces a hammering sound and leads to metal stress and possible failure. 

Instantaneous surges of water under pressure caused by sudden interruptions in water flow in a pipe or water system, producing a hammering sound and leading to metal stress and possible eventual failure. Water hammer can develop where a steam main is incorrectly pitched, has un-drained pockets or where steam flows up and meets draining condensate flowing down causing a temporary interruption in both flows. 

Figure 8.7 confirms that this is correct A single nickel catalyst used for steam reforming of n-butane deactivates steadily and gains weight due to the accumulation of carbon, but a Ni-Au catalyst maintains its reforming activity at a constant level [F. Besenbacher, I. ChorkendorfF, B.S. Clausen, B. Hammer, A.M. Molenbroek, J.K. Norskov and I. Stensgaard, Science 279 (1998) 1913]. 

Besenbacher F, Chorkendorff I, Clausen BS, Hammer B, Molenbroek AM, Nprskov JK, Stensgaard I. 1998. Design of a surface alloy catalyst for steam reforming. Science 279 1913-1915. 

Figure 7.9 Schematic illustration of a single frame steam forging hammer. Reprinted, by permission, from Doyle, L.E., Manufacturing Processes and Materials for Engineers, 2nd ed., p. 254. Copyright 1969 by Prentice-Hall, Inc.

The ability of a hammer to deform metal depends on the energy it is able to deliver on impact. Consider a steam hammer (see Fignre 7.9) that has a falling weight of 200 lb and a steam bore, d, equal to 12 in. Assume that the mean effective steam pressure, P, is 80 psi and that the stroke is 30 in. If the hammer travels 1/8 in. into the metal after striking it, determine the average force exerted on the workpiece. You should be able to do this without an equation, through the application of some dimensional analysis. 

Total downward force = Steam force + Hammer weight = 9050 + 2000 = 11,050 Ibf... 

The actual falling weight of a steam hammer is 1200 pounds, the cylinder is 10 inches in diameter, and the stroke is 27 in. The mean average steam pressure is 80 psi. (a) What is the energy of the hammer blow (b) What is the average force exerted by the hammer if it travels 1/8 in. after striking the workpiece ... 

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