POWER March 2011 - 86

PLANT DESIGN
3. Calculating steam flow through vents. The steam flow rate through a vent for a given
pipe size is determined by either the vent upstream pressure (top) or vent exit pressure (bottom) for a
typical steam system. Nominal pipe sizes are shown in the figures. Source: S. Zaheer Akhtar, PE
Vent header size 8 "
500
450
400
350
300
250
200
150
100
50
225
200
175
150
125
100
75
50
25
50,000
100,000
150,000
Vent inlet flow (pph)
200,000
250,000
50,000
Vent header size 8 "
100,000
150,000
Vent inlet flow (pph)
Vent header size 6 "
200,000
Vent header size 4 "
250,000
1. Set the header exit pressure (P2
) initially at
15 psia (use condenser pressure in case of
a manifold connected to a condenser).
2. Using steam tables, and considering
adiabatic expansion (∆H = 0), establish
the exit temperature (T2
stream steam header enthalpy (h1
exit pressure (P2
).
3. Using P2
check the specific volume (2
steam tables.
4. Now use P2 and v2
velocity (Vs
and exit temperature (T2
) from the
),
to establish the sonic
), according to equation [2]:
) based on up)
and
Vent header size 6 "
Vent header size 4 "
the sum of all incoming drain flows, and the
steam properties in the vent/manifold can
be evaluated as the weighted average of the
incoming streams. The calculation for vent/
manifold pressure should take into account
the most severe situations, such as when one
of the drains is inadvertently opened at full
load or during an STG trip, when certain
drains (usually from the cold reheat steam
header) go to full open position with full operating
pressure in the steam header.
You can calculate the vent header pressure
of a vent discharging to atmosphere/condenser
as follows:
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POWER | March 2011
APRIL 2011
03
16
NETWORKING
DAYS
INDUSTRY
LEADERS
Vent inlet pressure (psia)
Vent inlet pressure (psia)
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POWER March 2011

Table of Contents for the Digital Edition of POWER March 2011

Contents
POWER March 2011 - Cover1
POWER March 2011 - Cover2
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