Xylem Bell & Gossett Hoffman Speciality Series Engineering Data Manual page 21

22
Example: Assume a steam supply header to
feed tracer lines is 11/4" pipe size operating
at 30 psig and is 800 ft. Iong, insulated 75%
effect, minimum ambient at start-up is 10°F.
Calculate running load and warm-up load.
Step 1
Running load Ib./hr. = L x U x ∆T x E
= 800 x 2.7 x (274 -10) x .25
2.301 x 926
= 66.9 Ib./hr.
Step 2
Calculate warm-up load:
C = W x(T - T ) x.114
1 2
L
= (2.27 x 800) x (274 -10) x .114
926
= 59 Ibs. of steam
A warm-up time must be selected to compute
Ibs./hr. Assuming a warm-up of 15 minutes,
we must multiply 59 Ibs. x 4 = 236 Ibs./hr.
Thus, the trap must be sized for 236 Ibs./hr.
for a 15 minute start-up period, plus a safety
factor.
Step 3
We will size the trap for the large value (run-
ning load vs. warm-up load), which in almost
all instances will be the warm-up load.
Traps should be sized with a safety factor to
handle start-up and abnormal loads. Normal
practice is to size the trap at three times the
running load or two times the warm-up load.
Installation at end of steam main drip trap
STATIC
HEAD
The final sizing for our example would be to
size the trap for 236 x 2 = 472 Ibs./hr. con-
densate based on the differential pressure
between the header supply pressure and
return line pressure. Assuming a gravity return
line, this would be 30 psi.
Step 4
S x H Select the trap.
Based on the previous description of traps, if
the trap from a steam header is not subject
to freezing conditions, the normal selection
would be an F & T or Bucket Trap.
If the trap is located in an area subject to
freezing, to assure complete condensate
drainage during shut-down, we should use
either a Thermostatic or Thermodisc Trap.
Based on the calculated condensate rate plus
the applied safety factor, we would go directly
to the trap manufacturer's catalog and select
the trap.
The drip trap should be installed in a drip con-
nection that is at least equal to the size of
the steam main for pipe size up to 4 inches.
Above 4 inch size the drip connection should
be at least 4 inches minimum or 1/2 the size
of the steam main, whichever is larger. The
height of the drip connections should be the
larger of 5 inches or 1
the pipe.
The static head will provide the differential
pressure across the trap during automatic
start-up until the steam pressure is above 0
psig. A 15 inch static head will provide 1/2
psi differential assuming the trap drains into a
gravity return line. A 2.4 ft. static head will
provide 1 psi differential pressure.
SHUT-OFF
VALVE
"Y" STRAINER
1 ⁄
times the diameter of
2
TRAP
SHUT-OFF
VALVE