STEAM PROPERTIES & FLOW CHARACTERISTICS
SIZING OF CONDENSATE RETURN LINE, FLASH TANK DIAMETER & VENT LINE • Velocity of Flash Steam in Condensate Return Lines should be between 4000 and 6000 ft/min.
• Velocity in Flash Tank should be less than 600 ft/min. • Velocity in a Vent Pipe should be less than 4000 ft/min.
Example: A steam trap with a 150 PSIG steam inlet pressure is being discharged into a flash tank operating at 20 PSIG. The condensate load on the trap is 3200 lbs/hr. Problem: (1) Determine the size of the condensate return line from the trap to the flash tank based on velocities of 4,000 - 6,000 ft/min. (2) Determine the diameter of the flash tank based on velocities less than 600 ft/min. (3) Determine the size of the vent line on the flash tank based on velocities less than 4000 ft/min. Solution: The accepted practice of determining condensate return pipe sizing is to base the size of the return pipe on the amount of flash steam in the return line. This is due to the fact that the volume of flash steam is over 1,000 times greater than the equivalent volume of liquid condensate. Therefore, the flash steam is the dominant factor affecting flow in the return line. We must first calculate the amount of flash steam produced. From the Percent Flash Steam Table we find that 11.8% of the condensate will flash into steam. Therefore .118 X 3200 = 377 lbs/hr of flash steam will be produced in the condensate return line and flash tank.
Enter Condensate Line, Flash Tank & Vent Line Sizing chart at A 377 lbs/hr . Move horizontally to point B 20 PSIG Flash Tank Pressure.
Move vertically upwards to point D to determine a 5” Flash Tank Diameter is needed to keep velocities less than 600 ft/min. Continue to move vertically to point E to determine that the Vent Line on the Flash Tank should be 2” Diameter in order to keep velocities less than 4,000 ft/min.
Continue to move vertically to point C to determine that the Condensate Line should be 1 1 / 2 ” Diameter in order to maintain condensate return line velocities between 4000 and 6000 ft/min.
PERCENT (%) FLASH STEAM TABLE Percent Flash Steam produced when condensate is discharged to atmosphere (0 PSIG) or into a flash tank controlled at various pressures Condensate Flash Tank Pressure (PSIG) Pressure (PSIG) 0 5 10 20 30 40 60 80 100 5 1.6 0.0 10 2.9 1.3 0.0 15 3.9 2.4 1.1 20 4.9 3.3 2.1 0.0 30 6.5 5.0 3.7 1.7 0.0 40 7.8 6.3 5.1 3.0 1.4 0.0 60 10.0 8.5 7.3 5.3 3.7 2.3 0.0 80 11.8 10.3 9.1 7.1 5.5 4.2 1.9 0.0 100 13.3 11.8 10.6 8.7 7.1 5.8 3.5 1.6 0.0 125 14.9 13.5 12.3 10.4 8.8 7.5 5.3 3.4 1.8 150 16.3 14.9 13.7 11.8 10.3 9.0 6.8 4.9 3.3 200 18.7 17.3 16.2 14.3 12.8 11.5 9.4 7.6 6.0 250 20.8 19.4 18.2 16.4 14.9 13.7 11.5 9.8 8.2 300 22.5 21.2 20.0 18.2 16.8 15.5 13.4 11.7 10.2 350 24.1 22.8 21.7 19.9 18.4 17.2 15.1 13.4 11.9 400 25.6 24.2 23.1 21.4 19.9 18.7 16.7 15.0 13.5
Flash Steam
Vent Line
Diameter
Condensate Return Line
Flash Tank
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