Page 282 - Boiler plant and distribution system optimization manual

Steam Traps 267

• Reliability return system. Table 14.7 gives examples of the
• Failure mode most likely to occur reduction in trap capacity caused by this back
pressure which must be taken into account when
The above considerations were used in devel- sizing traps.
oping Table 14.2, the Steam Trap Selection Guide.
This guide was developed by the U.S. Navy for Safety Factor
their many and varied shore bases including hos- The safety factor is a multiplier applied to
pitals, industrial facilities, air bases and training the estimated condensate load since trap ratings
centers. are based on maximum discharge capacities or
Operating characteristics of various types of continuous flow ratings (Figure 14.22). Safety
traps are summarized in Table 14.3, which gives factors vary from 2:1 to 10:1 and are influenced
a comparison of various common trap configura- by the operational characteristics of the trap, ac-
tions for specified service. curacy of estimated condensate load, differential
When installing and evaluating traps, an pressure and expected changes in pressure and
understanding of their limitations is essential to the configuration of the installation design.
long, reliable and trouble free service. The limita- Safety factors are needed to cope with the
tions listed in Table 14.4 should be given careful varying condensate loads experienced in the “real
consideration in the steam trap selection process. world “ of actual use.
During start up conditions the cold piping
Sizing Steam Traps will generate a large amount of condensate un-
Factors that affect the accuracy of trap sizing til it is up to operating temperature. Whether this
are: occurs over several hours or a few minutes the
A. Estimating the maximum condensate load. amount of condensate will be the same, but the
B. Range of operating pressure and differential trap will have to handle it more quickly requiring
pressure. added capacity.
C. Selection of a safety factor. To add to the challenge of condensate remov-
al during warmup, the system pressure is lower
Condensate Load than normal so the trap output will be lower than
The amount of condensate generated by rated capacity. Waterhammer must be avoided so
equipment can generally be obtained from equip- the trap must be designed for this unfavorable
ment manufacturer’s literature or specification condition.
sheets. Steam trap manufacturers, through years Other conditions affect condensate load also,
of experience, have also developed formulas, ta- cold winds and rain can reduce insulation values.
bles and graphs for estimating condensate load Sudden surges in load on heat exchangers can
for most applications. Table 14.5 gives samples also lead to waterlogging.
of simplified estimating aids Table 14.6 gives es- A boiler which has carry-over or is produc-
timates of condensate loads for various sizes of ing low quality steam containing unevaporated
steam mains and different pressures. water places an unusually high load on the whole
system.
Pressure differential Waterlogging cuts production and comfort
Trap capacity is affected by the differential and waterhammer can be dangerous. Insufficient
pressure across a trap. If a trap exhausts to atmo- trap capacity causes these problems. On the other
sphere, the differential pressure will be the sup- hand, wasted energy and large dollar losses can
ply pressure. In some plants, traps are installed occur if the trap is oversized and is leaking.
with the outlet connected to a pressurized return The answer to this situation may be an effec-
system. The trap must operate against this head tive trap maintenance program.
plus any requirement to lift the condensate to the