Sizing Steam and Condensate Lines - FAQ
Following are answers to many frequently asked questions about correctly sizing steam and condensate lines.
1. Why are drip traps necessary on steam mains and branches?
They remove condensing steam before it can reach heat exchange equipment during normal operation.
2. Should steam always be taken from the bottom of steam mains so there is no impediment to proper steam removal?
Not always.
3. What is the rule of thumb for boiler carryover into the boiler header?
10 percent.
4. Should drip traps always be sized for the startup condensate load?
No.
5. What is the maximum distance between drip traps on a main?
500 feet (150 m).
6. What should be the diameter of the drip leg for a 6-inch (150 mm) main?
4 inches (100 mm).
7. In general, what is the best choice of traps for main drips?
The inverted bucket trap.
8. On mains with diameters greater than 4 inches (100 mm), why are drip legs sized with a minimum 4-inch (100 mm) diameter?
To prevent the condensate from being carried over the drip leg by the higher velocity gas in the main.
9. On branch lines with diameters less than 4 inches (100 mm), is the recommended drip leg diameter half the diameter of the branch line?
No.
10. On branch lines with diameters greater than 4 inches (100 mm), what is the recommended drip leg diameter?
Half the diameter of the branch line, but never less than 4 inches (100 mm).
11. What is the best method of installing a branch line less than 10 feet (3 m)in length?
Pitch the line back toward the main; no trap is necessary.
12. What is the recommended pitch of a 10-foot (3 m) branch line that is pitched back toward the main?
1/2 inch per foot (50 mm/m).
13. Should condensate return lines be sized to carry only the condensate flow?
No.
14. Are steam lines sized for pressure drops and steam velocities?
Yes.
15. When trapping distribution lines, is it best to use a trap that fails closed?
No.
16. Should drip traps be sized for their startup load, and not their operating load?
No.
17. Is drip leg sizing important when trapping steam distribution lines?
Yes.
18. What occurs as steam velocity increases?
Erosion also increases.
19. Is it generally better to oversize steam and condensate lines than to undersize them?
Yes.
20. What is the recommended warmup rate for a steam system?
Do not exceed 100 degrees F (38 degrees C) per 8 minutes.
21. On a steam main, should drip traps be placed at least 1,000 (300 m) feet apart?
No.
22. Do condensate return lines typically contain up to 99 percent flash steam by volume?
Yes.
23. Is 12,000 FPM (60 m/s) a reasonable velocity for condensate in a return line?
No.
24. On boiler headers, should the steam enter at the bottom and be taken off from the top?
No.
25. What is the recommended trap safety factor when trapping boiler headers carrying saturated steam?
1.5:1.
26. With steam mains, on what should steam trap size be based?
Operating load.
27. When steam traps are used, is steam main warmup time important?
Yes.
28. Should steam mains always be installed level?
No. Not always.
29. Should steam main drip legs always be the same size as the drip trap inlet connection?
No. Not always.
30. What safety factor should be used for traps installed ahead of reducing valves?
3:1.
31. Is a steam velocity of 18,000 FPM (90 m/s) considered a good design goal?
No.
32. Should condensate return line sizing be based on the flash steam velocity?
Yes.
33. Of these four terms - drip leg, boiler header, steam main and branch line - which does not fit?
Drip leg.
34. Is automatic warmup recommended for initial heating of large diameter and/or long steam mains?
No.
35. Should drip traps be installed upstream of pressure reducing valves?
Yes.
36. What is the biggest potential safety issue related to undersizing condensate return lines?
Water hammer.
37. What are condensate return lines mostly filled with?
Flash steam.
38. When is the inverted bucket trap most suitable for use in steam distribution applications?
When energy conservation (time in service), performance on very light loads and mechanical failure mode are important.
39. What is the greatest challenge in trapping steam mains?
While operating loads are generally very small, start-up loads can be huge.
40. What guidelines are essential to the proper installation of steam mains?
Pitch the run down, away from the boiler; drip all low points; and install properly sized drip legs.
41. Which trap requirement is used to select the most suitable trap to drip a steam separator?
Vent non-condensables at steam temperature.
42. What is most significant reason to avoid oversizing steam distribution lines?
Cost.
43. Why is it important to avoid undersizing steam distribution lines?
To prevent excessive pressure drops.
44. Can you give me some good reasons to provide condensate return lines sized one or two pipe sizes larger than the minimum?
To allow for increased plant capacity, to allow for unexpected rises in load and to allow for eventual fouling of the line by rust and scale.
45. What categories can steam distribution systems consist of?
Boiler headers, steam mains and branch lines.
46. Why should steam traps be placed along distribution lines?
To remove any condensate that has formed due to radiation.
47. Is a 4:1 safety factor recommended for selecting traps for boiler header applications?
No.
48. Where should steam enter and leave headers?
From the top.
49. Where should drip legs be installed along the length of a steam main?
Every 300 feet (90 m) and never more than every 500 (150 m) feet along the length of a steam main.
50. Why are steam separators used?
To remove entrained condensate from the steam, or "dry" the steam.
51. Where should steam velocities in a process steam application be maintained?
Between 6,000 (30 m/s) and 12,000 FPM (60 m/s), with a maximum of 15,000 FPM (75 m/s).
52. Should all condensate lines be pitched down 1 inch per 10 feet (30 mm per meter) to allow for gravity drainage of condensate?
No.
53. A steam main is calculated to condense 300 pounds of steam per hour (136 kg/hr), using insulation of 75 percent efficiency. With an efficiency of 90 percent, what would be the expected condensation rate?
120 lb/hr (55 kg/hr).
54. What trap capacity (including safety factor) is required for a steam separator handling 10,000 lb/hr (4500 kg/hr) steam of 90 percent quality?
3,000 lb/hr (1350 kg/hr).
55. Why is the carrying capacity of a dry-closed return line reduced when a higher pressure steam supply is used?
More flash steam is formed when a high-pressure trap discharges.
