ASHRAE Journal - June 2023 - 18

TECHNICAL FEATURE
impose significant pressure drop or contain flash steam.
In most cases, the proper drain line size is the larger of
equipment connection size and steam trap inlet size.4
The latter will often govern due to safety factors and
warm-up loads considered when sizing steam traps.
Equipment drain piping should slope a minimum of 1.0
in./10 ft (8.3 mm/m) in the direction of flow to ensure
efficient removal of condensate from equipment.
Excessive drain line lengths from equipment should
be avoided whenever possible. Should the length exceed
30 ft (9 m), the pipe size determined from the criteria
above should be verified not to exceed a velocity of 3.0
fps (0.9 m/s) and a pressure drop of 1.0 feet of water
gauge (ft wg)/100 ft (98 Pa/m).4 Equipment drain piping
should be increased in size as required to comply with
these parameters.
Steam Trap Outlet Piping
Steam trap outlet piping contains two-phase fluid
flow: flash steam produced by pressure drop across the
trap and liquid condensate. Percentage of flash steam is
calculated according to Equation 5:2
% Flash steam =
where
hf1 = enthalpy of liquid at higher pressure,
Btu/lbm (kJ/kg)
hf2 = enthalpy of liquid at lower pressure,
Btu/lbm (kJ/kg)
hf2 = latent heat of vaporization at lower pressure,
Btu/lbm (kJ/kg)
Although flash steam represents only a small portion
of mass flow, it accounts for 96.2% to 99.7% of internal
pipe volume for two-phase piping systems with
upstream operating pressures between 5 and 150 psig
(35 and 1000 kPag). Consequently, flash steam flow rate
is commonly used as the basis of sizing for steam trap
outlet piping. This empirically proven method does
not quantify the nominal volume of liquid occupying
the pipe, or differences in velocity between steam
and condensate; sufficiently low velocity and pressure
drop limits ensure that this simplified approach yields
acceptable results. A maximum flash steam velocity of
4,000 fpm (20 m/s) is suggested for falling lines. Steam
trap outlet piping requiring condensate to be lifted
to a higher elevation is sized for a flash steam velocity
18
ASHRAE JOURNAL ashrae.o rg
J U N E 2023
100()
−
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(5)
not exceeding 3,000 fpm (15 m/s) and a pressure drop
not exceeding 0.125 psi/100 ft (28 Pa/m) to prevent
water hammer. Steam trap outlet piping is sloped at a
minimum of 0.125 in./ft (10.4 mm/m) in the direction
of flow. This piping should never be smaller than the
steam trap outlet connection size.
Dry-Closed Condensate Return Piping
Condensate returns conveying two-phase fluid from
multiple steam trap outlets are used when adequate
back pressure is continuously available. Flash steam at
steam trap outlets serves as the motive force to convey
condensate through the piping network. Alternatively,
this condensate may discharge to a flash steam recovery
vessel, where thermal energy is captured and used
to supplement steam service to low-pressure loads.
Common dry-closed condensate return piping is
nonvented and has a continuous pressure difference
between the point where condensate enters the
line and the point where it exits.1 These piping networks
are commonly referred to as high-, medium-,
and low-pressure condensate lines, in reference to
the steam operating pressure upstream of the trap.
However, these naming conventions are not indicative
of actual condensate line operating pressure. Properly
sized condensate piping will be subjected to normal
operating pressures much lower than that of the associated
steam supply.
Dry-closed return pipe sizing follows the same general
principle as steam trap outlet pipe sizing, in that
flash steam flow rate is typically used as the design
case. Flash steam velocity in falling lines should not
exceed 4,000 fpm (20 m/s), and piping should slope at
a minimum of 0.5 in./10 ft (4 mm/m) in the direction
of flow. Common return lines requiring condensate
to be lifted to a higher elevation are sized for the combined
flash steam flow rate, with a maximum velocity
of 3,000 fpm (15 m/s) and a maximum pressure drop
of 0.125 psi/100 ft (28 Pa/m).
The operating pressure of dry-closed returns is highest
at steam trap outlets, where it is approximately equal to
local supply pressure less the steam trap pressure drop.
Total equivalent length of return piping and the amount
of back pressure at the point of exit must be considered
when determining the design total system pressure
drop. Typical pressure drops per unit length range from
0.125 to 2.0 psi/100 ft (28 to 450 Pa/m), with higher
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ASHRAE Journal - June 2023

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