| |
A steam trap is a device used to discharge
condensate and non condensable gases while not
permitting the escape of live steam.
Nearly all steam traps are nothing more than
automatic valves. They open, close or modulate
automatically.
The earliest and simplest form of steam trap is the
orifice trap. It consists of simply a disc or short
solid pipe nipple with a small hole drilled through
it installed at the lowest point of the equipment.
Since steam condensate will collect at the lowest
point and this hot liquid is about 1200 times
smaller in volume and denser than live steam,
condensate is effectively removed and steam is
blocked. The problem with orifice traps is the fact
that they do not compensate for varying loads and
pressures. If the condensate load increases, liquid
will back up in the equipment. If the load is light,
then there is little condensate present and live
steam will escape through the orifice. Orifice traps
will not handle or remove non-condensable gases
successfully.
These basic inabilities have spawned a multitude of
steam trap designs and configurations to meet a
whole spectrum of applications.
No single steam trap design is ideal for each and
every application. This makes understanding each
design's abilities and limitations important in
selecting the right trap for the right job.
Steam traps can be split into three major types:
1. Mechanical traps. They have a float that rises
and falls in relation to condensate level and this
usually has a mechanical linkage attached that opens
and closes the valve. Mechanical traps operate in
direct relationship to condensate levels present in
the body of the steam trap. Inverted bucket and
float traps are examples of mechanical traps.
2. Temperature traps. They have a valve that is
driven on / off the seat by either expansion /
contraction caused by temperature change. They
differ from mechanical traps in that their design
requires them to hold back some condensate waiting
for it to cool sufficiently to allow the valve to
open. In most circumstances this is not desirable as
condensate needs to be removed as soon as it is
formed. Thermostatic traps and bimetallic traps are
examples of temperature operated traps.
3. Thermodynamic (TD) traps. Thermodynamic traps
work on the difference in dynamic response to
velocity change in flow of compressible and
incompressible fluids. As steam enters, static
pressure above the disk forces the disk against the
valve seat. The static pressure over a large area
overcomes the high inlet pressure of the steam. As
the steam starts to condense, the pressure against
the disk lessens and the trap cycles. This
essentially makes a TD trap a "time cycle" device:
it will open even if there is only steam present,
this can cause premature wear. If non condensable
gas is trapped on top of the disc, it can cause the
trap to be locked shut. |
