ASHRAE Journal - February 2023 - 37

TECHNICAL FEATURE
steam condensate drainage system.
The " P " numbers indicate points in
the piping system where the fl uid
conditions (temperature, pressure)
change. The " H " numbers indicate
critical height dimensions. Pressures
in each section can be determined as
follows:
* P8 = Atmospheric pressure + Vent
5 ∆P, which is based on friction drop
of steam fl ow + entrance and exit
losses;
* P7 = P8 + ∆PP7 - P8;
* P6 = Backpressure P9 + Vent 4 ∆P
(similar ∆P calculation as Vent 5);
* P9 = Atmospheric pressure when
vented outdoors or the pressure of
the steam line used as the receiver of
the fl ash steam;
* P5 = P6 + ∆PP5 - P6 (∆P for vertical
drops can be estimated using fl ash
steam pressure drops);
* P2 = Steam pressure P1 + head H1
created by any liquid in the drip leg;
* ∆PP2 - P5 = ∆PP2 - P3 + steam trap
FIGURE 3 Steam drainage diagram.
Control Valve
Steam Line @
10 psig, 240°F
P1
Condensate @ 10 psig,
240°F, h = 208.45 Btu/lb
P4
Steam Condensate Line
Backpressure
P5 = P6 + ∆P5-6
P5
P9
Vent 4
Flash Tank/
Receiver
P6 = P9 + Vent 4 ∆P
Vent 5 to
Atmosphere
H4
Overfl ow
with P-Trap
H5
Pump
∆PP3 - P4 + ∆PP4 - P5 + exit loss of high velocity steam/liquid
into a low pressure at P5.
Pressure drop ∆PP2 - P3 should be less than 1 psi
(6.9 kPa) to minimize bubble fl ow. Many trap
manufacturers recommend an additional height H2
to offset the ∆PP2 - P3 to ensure wet fl ow and reduce
potential damage due to possible slug fl ow at the inlet of
the stream trap. Most trap manufacturers recommend
that steam traps be sized for 80% of the pressure drop
from the trap inlet to the receiver for high pressure
systems and 0.5 psid (3.45 kPa) for low pressure systems.
The trap discharge line P4 to P5 starts out at the
trap outlet size and usually remains the same size
until it connects to P5. In section P4 to P5 pipe, fl ow
is transitioning from wet to plug fl ow and ends in a
rapid pressure drop as fl ow enters the low velocity of
the drain line at P5. This line should be kept as short as
possible length (<10 ft [3 m]) to keep forces from plug
fl ow to a minimum. The connection to P5 should be
made at the top of the pipe to minimize turbulence at
this connection point. Increasing the pipe size by one
size at the trap discharge will reduce turbulence at P5.
Condensate Pump
Receiver
Occasionally designers will pipe the trap discharge into a
lower pressure steam line. This works provided that the
steam line pressure is lower than trap discharge at P5.
A fl ash tank is required when the Vent 5 connection
size at the condensate pump receiver is less than the
drain line size at P6. Most drainage systems with high
pressure traps will require a fl ash tank. A fl ash tank for
a low pressure system is dependent on the fl ash steam
fl ow. Flow from the fl ash tank P6 to P7 can be either
" vented-dry " or " dry-closed " depending on the pressure
P9. If P9 is connected to a steam line, P6 to P7 should be
designed as a " dry-closed " system. If P9 is at atmospheric
pressure, P6 to P7 can be designed as a " vented dry "
system. When P6 to P7 is in a wet fl ow regime, the height
H4 needs to be greater than ∆PP6- P7 and Steam Trap 2
can be a simple p-trap with the suffi cient depth greater
than the pressure drop of Vent 5.
For heat exchangers, P10 equals P1 minus the pressure
drop of the control valve minus line losses from the
steam main to the heat exchanger. P10 can be above
or below atmospheric pressure depending on the load
conditions. P11 equals P10 minus the pressure drop of
F E B R U A RY 2 0 2 3 ashrae.o rg ASHRAE JOURNAL
37
P8
Steam
Trap 2
P7
Liquid Condensate
@ Pressure P8, 212°F, h = 180.17 Btu/lb
P8 = Vent 5 ∆P
P6
H3
Steam
Trap 3
P3
Steam Trap 1
H2
Heat
Exchanger
P11
Air Vent/
Vac. Breaker 3
P2
Air Vent/
Vac. Breaker 1
H1, Drip Leg
P10
Air Vent/
Vac. Breaker 2
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ASHRAE Journal - February 2023

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