MIL-HDBK-1003/6
allows the condensate to be discharged. As condensate is discharged and the
level drops, the mechanism closes the discharge path. Some examples of
mechanical traps include a float trap and an inverted bucket trap.
Thermostatic. A thermostatic trap uses temperature differences to
8.9.1.2
discharge condensate and air. Since steam contains more heat energy than
condensate, its heat controls the actual operation of the trap. As condensate
and air enter the trap, they are discharged through a port. When steam
enters, heat energy expands different materials, which closes off the
condensate flow. Thermostatic traps are very useful during start-up. One
type of trap, a bimetallic trap, uses two types of metals in its design. Some
types of thermostatic traps use bellows filled with liquid, or a thermal
expansion element.
Thermodynamic. A thermodynamic steam trap uses kinetic energy
8.9.1.3
differences between flowing steam and condensate. As steam flows through an
orifice, its velocity will be much greater than that of condensate. There
will also be a pressure drop between the steam and condensate flow. Some
types of thermodynamic steam traps are the piston impulse trap and the disk
trap.
Applications. The condensate level controls the float trap. The
8.9.2
float trap is capable of lifting condensate to a higher discharge level. The
venting of air and non-condensables in thermostatic traps make them useful
with unit heaters, radiators, and convertors where condensate flow is gravity
controlled. Inverted bucket traps are used on low pressure systems. Impulse
and thermodynamic traps are useful for steam tracing of pipe lines where there
will be continuous flow. Additional guidance for the application and
selection of steam traps may be found in MIL-HDBK-1003/8, and Federal
Specification WW-T-696, Steam Traps, and Naval Civil Engineering Laboratory
UG-0005, Steam Trap Users Guide.
Selection and Sizeing. A steam trap must be: the right size, the
8.9.3
correct type, in the best location, and properly installed to serve the system
most efficiently. The wrong trap can reduce equipment efficiency by 35
percent. When selecting a trap consider the following: type of service, rate
and range of load, system temperature and pressure, operational conditions,
and economical conditions. Manufacturers' recommendations are also useful
when selecting steam traps.
Determining the correct size steam trap requires calculating or
estimating the maximum condensate load (lb/hr). Both the minimum and maximum
inlet pressure at the trap, and the outlet back pressure must be determined.
Consult manufacturers' capacity tables when sizing the type of trap. Safety
factors, influenced by the operational characteristics of the trap, are also
selected from manufacturers' literature.
Controls.
MIL-HDBK-1003/12 provides design criteria required for
8.10
plant controls.
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