2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
4
Sizing Steam Lines On Velocity
Fig. 1 lists steam capacities of
pipes under various pressure and
velocity conditions.
EXAMPLE: Given a steam heating
system with a 100 psig inlet
pressure ahead of the pressure
reducing valve and a capacity of
1,000 pounds of steam per hour
at 25 psig, find the smallest sizes
of upstream and downstream piping
for reasonable quiet steam
velocities.
Upstream Piping Sizing
Enter the velocity chart at A for
1,000 pounds per hour. Go over
to point B where the 100 psig
diagonal line intersects. Follow up
vertically to C where an intersection
with a diagonal line falls
inside the 4,000-6,000 foot-perminute
velocity band. Actual
velocity at D is about 4,800 feet
per minute for 1-1/2 inch
upstream piping.
Downstream Piping Sizing
Enter the velocity chart at A for
1,000 pounds per hour. Go over
to point E where the 25 psig diagonal
line intersects. Follow up
vertically to F where an intersection
with a diagonal line falls
inside the 4,000-6,000 foot-perminute
velocity band. Actual
velocity at G is 5,500 feet per
minute for 2-1/2 inch downstream
piping.
Pressure Drop in Steam Lines
Always check that pressure drop
is within allowable limits before
selecting pipe size in long steam
mains and whenever it is critical.
Fig. 2 and Fig. 3 provide drops in
Sch. 40 and Sch. 80 pipe. Use of
the charts is illustrated in the two
examples.
EXAMPLE 1
What will be the smallest schedule
40 pipe that can be used if
drop per 100 feet shall not
exceed 3 psi when flow rate is
10,000 pounds per hour, and
steam pressure is 60 psig?
Solution:
1. Find factor for steam pressure
in main, in this case 60
psig. Factor from chart = 1.5.
2. Divide allowable pressure
drop by factor 3 . –. 1.5 = 2 psi.
3. Enter pressure drop chart at
2 psi and proceed horizontally
to flow rate of 10,000
pounds per hour. Select pipe
size on or to the right of this
point. In this case a 4" main.
EXAMPLE 2
What will be the pressure drop
per 100 feet in an 8" schedule 40
steam main when flow is 20,000
pounds per hour, and steam
pressure is 15 psig?
Figure 1: Steam Velocity Chart
Solution:
Enter schedule 40 chart at
20,000 pounds per hour, proceed
vertically upward to 8" pipe curve,
then horizontally to pressure drop
scale, read 0.23 psi per 100 feet.
This would be the drop if the
steam pressure were 100 psig.
Since pressure is 15 psig, a correction
factor must be used.
Correction factor for 15 psig = 3.6
0.23 x 3.6 = 0.828 psi drop per
100 feet for 15 psig