Xylem Bell & Gossett Hoffman Speciality Series Engineering Data Manual page 8

Disc Trap Operation
Start-Up
The disc is pushed off the seat by the inlet
pressure and is held open by the impact force
of the condensate hitting the disc.
Closing
When all the condensate is discharged, flash
steam enters the seat-disc chamber at high
velocity. This high velocity causes a sudden
pressure drop at the lower side of the disc
and it snaps closed against the seat.
Orifice Traps
Advantages
No moving parts to wear.
Disadvantages
Does not close against steam.
Small hole easily plugs due to dirt.
Backs up condensate on heavy loads and
during start-up.
Does not respond to modulating loads.
Does not vent air when handling conden-
sate—causes slow system start-up and
may cause water hammer.
Not easily recognized as trap during
energy survey.
Operating
As the condensate nears saturation tempera-
ture, greater amounts of flash steam will
appear. Some of the flash steam escapes to
the area above the disc, causing the pressure
above the disc to increase, pushing the disc
closer to the seat.
Closed
At the instant the disc snaps closed on the
seat, the pressure above the disc is approxi-
mately equal to the upstream line pressure.
The disc is held closed because the pressur-
ized area above the disc is much larger than
the inlet area. The pressure above the disc
decreases either by steam condensation or by
non-condensables being removed via the
micro-bleed on the disc. When the pressure is
low enough, the disc is pushed off the seat
and the process is repeated.
Built-in small screen plugs easily.
Discharges condensate at saturation
temperature with some live steam, often
causes excessive condensate tempera-
tures and cavitation at condensate pumps.
Waste energy.
Sizing critical.
Applications
Should be limited to constant load
continuous operation.
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