A steam trap is a device used to discharge condensate and non condensable
gases with a negligible consumption or loss of live steam. Most steam traps are
nothing more than automatic valves. They open, close or modulate automatically.
The three important functions of steam traps are:
Discharge condensate as soon as it is formed.(Unless it is desirable to use
the sensible heat of the liquid condensate)
Have a negligible steam consumption.(i.e. being energy
efficient) Have the capability of discharging air
and other non-condensable gases. Basic operation
The simplest form of steam trap is 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 live steam is about
1200 times greater in volume than this hot liquid, condensate is effectively
removed and steam is blocked. However, the vast majority of steam traps in
current operation are of the mechanical or thermostatically operated design.
Mechanical and thermostatic steam traps basically open when condensate and inert
gases need to be removed, and close when all the condensate is removed. The
process repeats when new steam is condensed again and ready to be drained.
Steam traps work best when sized specifically for the application they
are used on. Generally it is better to oversize, as they will still discharge
condensate when present and close or obstruct for live steam. However an over
sized steam trap may wear quickly, waste energy, and if drastically over sized
can cause process issues. Types
Steam traps can be split into four major types:
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. Mechanical steam traps
have a typical service of life of 3 years. Inverted bucket and float traps
are examples of mechanical traps. Temperature traps. They have a valve
that is driven on / off the seat by either expansion / contraction caused by
temperature 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, Bi-Thermostatic
traps and bimetallic traps are examples of temperature operated traps.
Thermodynamic 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. Venturi Nozzle traps. Venturi Nozzle
steam traps work by utilizing the basic physics of two-phase flow for the
effective removal of condensate from steam systems. Steam and condensate pass
through the Venturi nozzle steam traps by a series of staged steps. "The denser
liquid throttles the venturi nozzle, which keeps the steam from escaping.
Erosive eddy currents are eliminated and any contamination is carried away with
the condensate. Because condensate is created continuously in a working steam
system, the condensate continuously chokes the nozzle from losing steam as
condensate is removed continuously". Venturi Nozzle traps require expert
engineering and precise sizing in steam systems.
External links Understanding Steam Traps - American
Institute of Chemical Engineers Steam Trapping, an overview - Spirax
Sarco Limited. How Steam Traps Work - CLAM Engineering.
Steam Basics - Armstrong International, Inc.
Steam Trap Types - US Department of Energy Federal Energy Management Program
- broken link! What Types of Steam Traps Can You
Insulate? - Thermaxx Jackets The History of Steam Traps - TLV
International, Inc. What you need to know about
venturi-orifice steam traps - Thermal Energy International
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