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 (f)
C o m b i n a t i o n float and thermostatic trap.
q Cannot be used on superheated steam systems.
q Must
be
protected
from
freezing.
q Operation of some models may be affected by water hammer.
3 SIZING TRAPS.
T h e following factors affect the accuracy of trap sizing.
(a) The unavoidably large range in condensate load for many steam
services.
T h e wide variance in operating pressure and differential pressure.
(b)
( c ) T h e uncertainty of trap capacity because of error In estimating
condensate temperature. S i z i n g errors can offset most of the system savings
p r o v i d e d by trapping. Traps that are too small cause condensate to back up.
O v e r s i z e d traps allow live steam through. A l o n g with selection of the proper
t y p e of trap, correct sizing is the Important step in establishing trapping
standards for the system. I n setting up standards, a review of past practices
a g a i n s t current results may avoid repeating errors in sizing. To determine
t h e correct size trap, p e r f o r m the following requirements;
q C a l c u l a t e or estimating the maximum condensate load.
q Determine t h e operating pressure differential and the maximum
allowable pressure.
q Select a safety factor.
q Size
the
type
of
trap
from
manufacturers'
capacity
tables.
3.1 Condensate Load. The amount of condensate generated by items of
e q u i p m e n t can generally be obtained from equipment manufacturer's literature.
F o r most all applications, formulas, tables, and graphs are available in steam
t r a p manufacturers' b r o c h u r e s for calculation of condensate loads. Table 8-4
p r o v i d e s formulas for simplified estimating of condensate loads as shown In
t a b l e 8-5.
3 . 2 P r e s s u r e D i f f e r e n t i a l . O n e element of trap capacity is the difference in
p r e s s u r e between the supply line and condensate return. If the trap
d i s c h a r g e s to the atmosphere, the differential pressure will be the supply
p r e s s u r e . For sizing traps, the maximum steam operating pressure is used.
F r e q u e n t l y , traps are installed with the outlet connected to a return system,
which is under some pressure. T h e trap must operate against this pressure,
p l u s a static head, if the trap is required to lift the condensate to a return
a t a higher level. T a b l e 8-6 gives examples of the reduction in trap capacity
caused by this back pressure. This must be considered when sizing traps.
3 . 3 S a f e t y F a c t o r . T h e safety factor is a multiplier applied to the
estimated condensate load. Trap ratings are based on maximum discharge
c a p a c i t y ; that is, continuous flow ratings. Factors vary from 2:1 to 10:1 and
a r e influenced by the operational characteristics of the trap, and accuracy of
8-22
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