How to Size Piping for
25-Series Regulators
Principle
When steam pressure is lowered through a
reducing valve, the steam expands creating
a higher velocity. The extreme velocities that
must exist across reducing valve seats cannot
be tolerated in pipes supplying the valves and
leading from them. Erosion and noise would be
prohibitive.
It is recommended practice in heating systems
to limit velocities to between 4,000 and 6,000
feet per minute. Higher velocities are often
acceptable outdoors and in plants where the
environment is already noisy.
This chart 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 per hour
at a reduced pressure of 25 psig, find the small-
est sizes of upstream and downstream piping
for reasonable steam velocities.
Upstream Piping
Enter the velocity chart at point A for 1,000
pounds per hour. Proceed horizontally to point
B where the 100 psig diagonal line intersects.
Follow up vertically to point C where an inter-
section with a diagonal line falls inside the 4,000
to 6,000 feet per minute velocity band. Actual
velocity (see point D) is about 4,800 feet per
minute for 1-1/2 inch upstream piping.
Downstream Piping
Enter the velocity chart at point A for 1,000
pounds per hour. Proceed horizontally to point
E where the 25 psig diagonal line intersects.
Follow up vertically to point F where an inter-
section with a diagonal line falls inside the 4,000
to 6,000 feet per minute velocity band. Actual
velocity (see point G) is about 5,500 feet per
minute for 2-1/2 inch downstream piping.
12
SPIRAX SARCO, INC. • 1150 NORTHPOINT BLVD. • BLYTHEWOOD, SC 29016
PHONE 803-714-2000 • SPIRAXSARCO.COM/GLOBAL/US
Spirax Sarco Applications Engineering Department
Toll Free at:
1-800-883-4411
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