POWER September 2014 - 62

PLANT DESIGN
How to Design the Collector Pipe
for Condensate Return Lines
While several methods exist for sizing collector pipes on steam lines, the results
obtained from the various approaches can be quite different. This article will
show how design parameters can be used to calculate the dimensions much
more accurately.
Emilio Casado
I
n the steam lines of thermal and nuclear
power plants, condensed steam is usually
discharged from components such as
steam traps, control valves, or restriction orifices
through draining lines that join to a collector
pipe that connects to the condenser.
The size of this collector is typically calculated
by applying rules of thumb that can
vary significantly depending on the reference
used. One source may suggest using a cross
section equal to the sum of the sections of the
connecting lines, another source may suggest
several times this sum, while yet another
suggests limiting the fluid velocity to small
values, such as 50 ft/sec or less.
The result is that there are very different
designs. For example, a 10-inch-diameter
collector may have two lines of 4 in. plus
two lines of 6 in. connected to them, while
another collector of 16-in.-diameter only has
two 2-in. lines connected.
In order to optimize the design of these
collectors, the following guide takes into
account the capacity of each draining line,
the pressure in the connecting point, and the
equivalent length of the collector.
System Parameters Are Key
The condensate collected in the drip pots of
the steam lines is saturated or sub-cooled
water that flashes when passing through
the component, which continues flashing
along the drain line. If the pressure of the
condensate at the component entrance is the
saturation pressure (PS
) and the resistance coefficient
of the component plus the line is K,
the critical pressure at the collector connection
(PCR
line is calculated applying the following
equation for saturated water:
W / d2
= 280 CE [(PS - PCR) / (ve K)]0.5 ln PS
where:
62
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) may be calculated using the graphs
in Figures 1 or 2.
The maximum draining capacity of the
W is the flow rate of each drain line (lb/hr),
d is the internal drain piping diameter of the
selected line with the minimum PCR
value (in.),
CE is the expansion coefficient of the saturated
water,
PS
is the saturation pressure at the entrance
of the component (psia),
PCR
is the critical pressure at the end of the
pipes (psia),
ve
is the specific volume to PS
(ft3
/lb), and
K is the resistance coefficient of the pipe.
This equation has been deduced by the author
from a 1961 research paper titled " Adiabatic
Flow of Flashing Liquids in Pipes "
written by M. Sajben and published by the
American Society of Mechanical Engineers
in its Journal of Fluids Engineering.
The CE
tersection of K with the critical conditions line.
Figure 3 is based on PS
= 1,000 psia as a reference
pressure and the equation allows using
the CE
pressure drop (PS
value for whichever other pressure. The
- PCR) is divided between the
component and the downstream piping.
45
40
35
30
25
20
15
10
5
10
20
30
PS (psia)
POWER | September 2014
40
This equivalent line will have the same resistance
coefficient (K) as the selected line,
and it should be designated Ko
.
Third. Calculate the resistance coefficient
1. Low-pressure scenario. Relationship of saturation pressure (PS), critical pressure
(PCR), and resistance coefficient (K) for saturated water flow through pipes. Source: E. Casado
K=2
6
10
20
40
60
80
100
150
300
500
1,000
1,500
50
60
Design of the Collector
When each line that connects to the collector
is defined, knowing its flow capacity (W),
the connection pressure (PCR
pressure at the component entrance (PS
), the saturation
), and
the resistance coefficient (K), the collector
may be designated following these steps.
First. Select the connecting line with the
minimum value of PCR
tal flow (WT
and calculate the to)
that must be drained for the
collector to the condenser as the sum of the
flows of all connecting lines. If there are two
or more connecting lines that have the same
minimum value of PCR
, select the line that has
the greatest flow. For the structural design of
the collector, use the maximum value of PCR
.
Second. Calculate an equivalent diameter
of the selected line (Do
) as:
is obtained from Figure 3 for the inDo
= d (WT
/ W)0.5
PCR (psia)

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POWER September 2014

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