1423 Steam Traps8

A steam trap is a mechanical device used to remove air, carbon dioxide, and condensed steam and to prevent steam from flowing freely into the condensate return system. Steam traps are necessary for several reasons. If air is not removed from steam, the oxygen can dissolve in low-temperature steam condensate and help cause corrosion of the valves, pipes, and coils in the steam distribution system. If carbon dioxide is not removed, it can combine with steam condensate to form carbonic acid, another major source of corrosion in the steam distribution system. Air and carbon dioxide also act as insulators to impede heat transfer from the steam; their presence creates a partial pressure that lowers the steam temperature and the heat-transfer rate.

Perhaps the main function of steam traps, however, is to permit the removal of steam condensate from a system while simultaneously preventing the free escape of steam. In this last function, the energy of the steam is kept within the system, and the amount of the live steam within the facility is controlled.

Steam traps occur as parts of nearly every steam distribution system. They are often not maintained, and this lack of maintenance can create a hidden cost that is significant. The cost of a steam trap failure is dependent upon the failure mode, with a strong dependence upon the original design. The two main failure modes are failing open and failing shut, and the design consideration of most maintenance interest is proper drainage. Consider these problems in turn.

Problems if Trap Fails Open

If a trap fails open, live steam flows directly from the steam system through the trap, a pressure buildup is caused in the condensate return system and the condensate return lines are heated unnecessarily, or the steam is released directly to the air. In the first case, the back pressure in the condensate lines may cause other steam traps to fail, and the condensate may not be removed from the steam distribution system. In the second case, steam discharging to air costs as much as any other kind of steam leak and creates pressure losses. These costs can be estimated using Table 14.17 and the formula

1100 Btu/lb energy cost/million Btu

boiler eff. 1,000,000

waste =

boiler eff. = energy cost/million Btu =

energy cost per month due to steam loss number of pounds of steam wasted per month, from Table 14.17

approximate value for total heat in saturated steam at 100 psi boiler efficiency, usually 60 to 80%

cost that can be obtained from your fuel supplier

These figures are close estimates for steam at 100-psi pressure; for other pressures between 50 and 200 psi, the cost is nearly proportional to the pressure, using 100 psi and Table 14.17.

Table 14.17 Cost of Steam Leaks at 100 psi Assuming a Boiler Efficiency of 80% and Input Energy Cost of $2 per Million Btu

Size of

Steam Wasted

Total Cost

Total Cost

Orifice (in.)

per Month (lb)

per Month

per Year

1/2

835,000

$2,480

$29,760

7/16

637,000

1,892

22,704

3/8

470,000

1,396

16,752

5/16

325,000

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