2000 Hook-up Book

Efficient Steam Main Drainage Proper drainage of lines, and some care in start up methods, not only prevent damage by waterhammer, but help improve steam quality, so that equipment output can be maximized and maintenance of control valves reduced. The use of oversized steam traps giving very generous “safe ty factors” does not necessarily ensure safe and effective steam main drainage. A number of points must be kept in mind, for a satisfactory installation. 1) The heat up method employed. 2) Provision of suitable collect ing legs or reservoirs for the condensate. 3) Provision of a minimum pres sure differential across the steam trap. 4) Choice of steam trap type and size. 5) Proper trap installation. Heat Up Method The choice of steam trap depends on the heat up method adopted to bring the steam main up to full pressure and temperature. The two most usual methods are: aa a Separator Trap Set Steam Supply (a) supervised start up and (b) automatic start up. A) Supervised Start Up In this case, at each drain point in the steam system, a manual drain valve is fitted, bypassing the steam trap and discharging to atmosphere. These drain valves are opened fully before any steam is admitted to the system. When the “heat up” condensate has been discharged and as the pressure in the main begins to rise, the valves are closed. The conden sate formed under operating conditions is then discharged through the traps. Clearly, the traps need only be sized to han dle the losses from the lines under operating conditions, given in Table 5 (page 10). This heat up procedure is most often used in large installa tions where start up of the system is an infrequent, perhaps even an annual, occurrence. Large heat ing systems and chemical processing plants are typical examples.

Draining Steam Mains

Steam main drainage is one of the most common applications for steam traps. It is important that water is removed from steam mains as quickly as possible, for reasons of safety and to permit greater plant efficiency. A build-up of water can lead to waterham mer, capable of fracturing pipes and fittings. When carried into the steam spaces of heat exchang ers, it simply adds to the thickness of the condensate film and reduces heat transfer. Inadequate drainage leads to leaking joints, and is a potential cause of wire drawing of control valve seats. Waterhammer Waterhammer occurs when a slug of water, pushed by steam pressure along a pipe instead of draining away at the low points, is suddenly stopped by impact on a valve or fitting such as a pipe bend or tee. The velocities which such slugs of water can achieve are not often appreciated. They can be much higher than the nor mal steam velocity in the pipe, especially when the waterham mer is occurring at startup. When these velocities are destroyed, the kinetic energy in the water is converted into pressure energy and a pressure shock is applied to the obstruction. In mild cases, there is noise and perhaps movement of the pipe. More severe cases lead to fracture of the pipe or fittings with almost explo sive effect, and consequent escape of live steam at the fracture. Waterhammer is avoided com pletely if steps are taken to ensure that water is drained away before it accumulates in sufficient quantity to be picked up by the steam. Careful consideration of steam main drainage can avoid damage to the steam main and possible injury or even loss of life. It offers a better alternative than an acceptance of waterhammer and an attempt to contain it by choice of materials, or pressure rating of equipment.

SYSTEM DESIGN

a

Figure 4 Trap Boiler header or takeoff separator and size for maximum carryover. On heavy demand this could be 10% of generating capacity

8

a