POWER March 2011 - 85

PLANT DESIGN
2. Calculating steam flow through drains. Steam flow through a 2-inch drain pipe
is limited by the cross-sectional area and length of the pipe for different upstream pressures.
Note that the horizontal line represents choked flow for the given steam pressure. Source: S.
Zaheer Akhtar, PE
P1=2,400 psia
200,000
175,000
150,000
125,000
100,000
75,000
50,000
25,000
0.00
5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00
Drain pipe resistance factor
40% of the inlet pressure, the modified Darcy's
formula for flow/pressure drop in compressible
fluids applies. This formula enables
the estimation of maximum discharge flow of
steam through the drain line by considering
the " choking " phenomena (the velocity of
the steam is sonic) at the discharge line exit.
Recall that when the steam velocity is at
sonic conditions or choked, an increase in upstream
or a decrease in downstream pressure
will not result in an increase in fluid flow. In
this case, the choke point occurs as pressure
falls off and velocity increases to sonic velocity
at the downstream end of the pipe. The
modified Darcy formula can be represented
as follows, in equation [1]:
ule xs) under choked conditions (Figure 2).
Note that for a given steam header pressure
(P1) the rate of steam flow through the pipe
reaches steady-state conditions for long
pipes. In other words, the flow is choked.
P1=1,000psia
P1=637 psia
P1=150 psia
Figure 2 also illustrates that the rate of
steam flow through the drain line decreases
with a decrease in upstream pressure (P1)
and an increase in drain line Resistance Factor
(k). The pressures assumed in the figure
are typical of the range of values experienced
in steam headers during start-up of the STG.
Note that the relatively low pressure of 150
psia is typical for the reheat section during
STG start, and this low pressure could limit
steam flow through the drain line. Restricted
steam flow through the drain line could, in
turn, limit the heat-up rate of the steam header,
thus increasing plant start-up time.
Computing Vent Pressures
Atmospheric vents or drain manifolds are
useful for collecting the discharge from
several drain lines for disposal to the atmosphere
and/or condenser. When discharging
to the condenser, the manifold serves as a
single connection to the condenser. Similarly,
if the drain line is discharging to the
atmosphere, it is usually connected to a
large-diameter manifold joined to the vent
stack or directly connected to the vent stack.
The bottom of the vent stack is fitted with a
drain line to discharge condensate collected
at the bottom of the vent.
The flow through the vent or manifold is
Where
W = steam flow rate (lb/hr)
Y = Net expansion factor (dimensionless) for
compressible flow
d = Internal diameter of drain line (inches)
∆P = Pressure drop under choked conditions
(psi)
1
(ft3
= Specific volume at upstream conditions
/lb)
k = Resistance factor (dimensionless) of the
drain line, from the familiar Crane's Flow of
Fluids Through Valves, Fittings and Pipe
To illustrate the usefulness of this formula
to the plant designer, equation [1] is plotted
for steam flow from a typical steam header
at four assumed upstream pressures (P1)
through a nominal 2-inch drain line (schedCIRCLE
25 ON READER SERVICE CARD
March 2011 | POWER
www.powermag.com
85
Steam flow (pph)

http://www.powermag.com

POWER March 2011

Table of Contents for the Digital Edition of POWER March 2011