Page 278 - Boiler plant and distribution system optimization manual

Steam Traps 263

Bellows Steam Traps
Bellows traps are often called balanced pres-
sure or thermostatic traps because the bellows
contains a volatile fluid which closely parallels
the temperature-pressure relationship of the
steam saturation curve. The pressure created in-
side the bellows by this fluid as it vaporizes from
the heat of the condensate closes the trap against
Figure 14.15—A double walled disc trap. the pressure of the steam system (Figures 14.16
and 14.17).
The operating principle is that water (con- During start up the bellows is contracted
densate) flowing through the orifice is restricted. away from the seat, allowing the discharge of
This backs up some of the condensate preventing condensate and gasses. Discharge continues until
the flow of steam through the orifice. Steam tries hot condensate enters the trap, heating the bel-
to flow through the orifice at a very high velocity lows and the volatile fluid. The fluid vaporizes
whereas the condensate is very slow by compar- and expands, causing the bellows to expand and
ison, this phenomena serves to choke the flow of close the valve.
steam with condensate.
The orifice is somewhat self regulating. That
is, if the condensate load is less than the orifice
was designed for, the condensate will be hotter,
causing more flashing, which will further restrict
flow. If the condensate load is greater than the ori-
fice was designed for, the condensate will back up
and cool. This cooling will reduce flashing, thus
allowing a greater condensate flow.
Orifice traps have a constant discharge, not Figure 14.16—Thermostatic trap in the open position.
the cyclic discharge of other thermodynamic traps.
Each orifice trap must be very closely en-
gineered to its specific load because they have
the potential to either blow steam (claimed to be
an insignificant amount) or back up condensate
during start up or during high loads when their
capacity may be exceeded.

Thermostatic Steam Traps
Thermostatic steam traps are operated by
changes in temperature and include Bellows,
Diaphragm, Bimetallic and Expansive Element Figure 14.17—Thermostatic trap in the closed posi-
types. Thermostatic traps respond more slowly to tion.
changing operating conditions than some other
types of traps because of the heat stored within The trap will remain closed until the conden-
the trap materials and the condensate which col- sate, the bellows and the volatile fluid cool. This
lects in the trap. cooling condenses some of the fluid, reducing the
Thermostatic steam trap operating princi- pressure inside the bellows. This reduced internal
ples are simple and thermostatic steam traps usu- pressure allows the external pressure, from the
ally have only one moving part. steam system, to contract the bellows away from