2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
46
Condensate Recovery Systems
have been sized using water volume
only and did not include the
flash steam volume that is present.
The specific volume of water
at 0 psig is .017 cubic feet per
pound, compared to 26.8 cubic
feet per pound for flash steam at
the same pressure. Sizing of condensate
return lines from trap
discharges based totally on water
is a gross error and causes lines
to be drastically undersized for
the flash steam. This causes condensate
lines to become
pressurized, not atmospheric,
which in turn causes a backpressure
to be applied to the trap’s
discharge which can cause
equipment failure and flooding.
This undersizing explains
why the majority of 0 psi atmospheric
condensate return
systems in the United States do
not operate at 0 psig. To take this
thought one step further for those
people who perform temperature
tests on steam traps to determine
if the trap has failed, the instant
we cause a positive pressure to
develop in the condensate return
system by flash steam, the condensate
return line now must
follow the pressure/temperature
relationship of saturated steam.
So, trap testing by temperature
identifies only that we have a
return system at a certain temperature
above 212°F (0 psig)
and we can then determine by
that temperature the system
pressure at which it is operating.
Elevated condensate return temperatures
do not necessarily
mean a trap has failed.
When sizing condensate
return lines, the volume of the
flash steam must be given due
consideration. The chart at Fig.
51 (page 43) allows the lines to
be sized as flash steam lines—
since the volume of the
condensate is so much less than
that of the steam released.
Draining condensate from
traps serving loads at differing
pressures to a common condensate
return line is a concept
which many find difficult. It is
often assumed that the HP “high
pressure” condensate will prevent
the “low pressure” condensate
from passing through the LP
traps and give rise to waterlogging
of the LP system.
However, the terms HP and
LP can only apply to the conditions
on the upstream side of the
seats in the traps. At the downstream
or outlet side of the traps,
the pressure must be the common
pressure in the return line.
This return line pressure will be
the sum of at least three components:
1. The pressure at the end of
the return line, either atmospheric
or of the vessel into
which the line discharges.
2. The hydrostatic head needed
to lift the condensate up any
risers in the line.
3. The pressure drop needed to
carry the condensate and
any flash steam along the
line.
Item 3 is the only one likely to
give rise to any problems if condensate
from sources at different
pressures enters a common line.
The return should be sufficiently
large to carry all the liquid condensate
and the varying amounts
of flash steam associated with it,
without requiring excessive line
velocity and excessive pressure
drop. If this is accepted, the total
return line cross sectional area
will be the same, whether a single
line is used, or if two or more lines
are fitted, with each taking the
condensate from a single pressure
source.
The return could become
undersized, requiring a high pressure
at the trap discharges and
restricting or preventing discharge
from the LP traps, if it is
forgotten that the pipe has to
carry flash steam as well as water
and that flash steam is released
in appreciable quantity from HP
condensate.
While the percentage, by
weight, of flash steam may be
rather low, its overall volume in
comparison to the liquid is very
large. By determining the quantity
of flash steam and sizing the
return line for velocities between
4,000 and 6,000 ft/min, the twophase
flow within the pipe can be
accommodated. The information
required for sizing is the condensate
load in lb/h, inlet pressure to
steam trap(s) in psig and return
line system pressure.
Example:
Size a condensate return line
from a 160 psig steam trap discharging
to 20 psig. flash tank.
Load is 3,000 lb/h.
1. Determine percent flash
steam produced using Table
12 (page 41). With a steam
pressure of 160 psig and a
flash tank pressure of 20 psig
read a value of 12.4%.
2. Next, multiply the condensate
load by the percent flash from
step #1 to determine the
flowrate, of flash steam produced.
3,000 lb/h x .124 = 372 lb/h.
3. Enter Fig. 51 (page 43) at the
flash steam flowrate of 372
lb/h at “A” and move horizontally
to the right to the flash
tank pressure of 20 psig “B”.
Rise vertically to choose a
condensate return line size
which will give a velocity
between 4,000 and 6,000
ft/min, “C”. In this example, an
1-1/2” schedule 40 pipe with
a velocity of approximately
5,000 ft/min. If schedule 80
pipe is to be used, refer to
table within body of chart.
Multiply the velocity by the
factor to determine whether
the velocity is within acceptable
limits.