Selecting and Sizing Steam Traps - FAQ
Following are answers to many frequently asked questions on selecting and sizing steam traps.
1. Should steam traps be sized based on their piping connections?
No.
2. What determines which type of trap should be used?
Different application requirements involving constant or variable pressure.
3. Does back pressure increase the capacity of a trap by producing a low differential?
No.
4. What do I need to know to figure trap size?
Condensate load, the safety factor and the pressure differential.
5. What determines the safety factor?
The type of equipment being drained and the pressure.
6. In a modulated steam supply system, should the pressure stay the same?
No.
7. Is it possible that a trap may not have to operate at a given design pressure?
Yes.
8. How do I calculate condensate load (lb/hr (kg/hr))?
Divide BTU/hr (kcal/hr) by latent heat.
9. Is it necessary for a trap to operate at maximum operating differential pressure if it operates at the normal operating differential pressure?
Yes.
10. Does atmospheric back pressure mean 0 psig (0 bar)?
Yes.
11. What is condensate load?
The actual amount of liquid created from condensing steam as heat is given up to the application.
12. Are safety factors applied to the load when sizing steam traps to compensate for inaccuracies in catalog data?
No.
13. Is the back pressure important when sizing a steam trap?
Yes.
14. Should steam traps be sized based on the maximum design pressure and temperature of the pressure vessels?
No.
15. When selecting steam traps, can the maximum differential pressure be used as long as the operating differential is within 50 percent of the maximum differential?
No.
16. When sizing steam traps, is it OK to exceed the capacity by a small amount (as long as the correct safety factor has been applied)?
Yes. It is OK.
17. In trap sizing, what does "load" refer to?
Actual condensate flow rate times a safety factor.
18. When sizing steam traps, is it OK to exceed the operating pressure by a small amount?
No.
19. If the maximum condensing rate is achieved with the maximum steam supply pressure, why be concerned about a modulating steam supply?
Reduced steam flow can result in very low pressures at the trap.
20. In determining the required maximum operating pressure of a trap, may I safely ignore positive back pressure?
Yes.
21. What does "design pressure" refer to?
The pressure that must be withstood by the pressure-retaining parts.
22. What must be considered when sizing steam traps?
Load, operating pressure and back pressure.
23. When trapping a unit heater supplied with 15 psig (1 bar) modulated pressure, what sizing rule do I follow?
2:1 safety factor at 1/2 PSI (0.1 bar) differential.
24. Often steam traps that are supplied modulating steam are sized for 1/2 PSI (0.1 bar) differential. Where does the 1/2 PSI (0.1 bar) come from?
It is the static head that is most often supplied by the drip leg.
25. When trapping a specific application, will one trap work as well as any other, providing they are correctly sized?
No.
26. As long as a trap is properly sized for the condensate load and the minimum operating pressure (based on the back pressure in the condensate system), does maximum operating pressure need further consideration?
Yes.
27. What is operating differential?
The difference between the supply pressure and back pressure at the stated condensate load.
28. In a constant supply system, what is maximum differential?
The supply pressure with no reduction for back pressure.
29. In a modulating supply system, how is the minimum differential determined?
Subtract the back pressure from the least amount of supply pressure encountered.
30. Are safety factors applied to steam trap applications to prevent over-pressuring of the vessels?
No.
31. If the operating differential is greater than 80 percent of the maximum differential pressure, can I use the maximum differential when sizing the trap?
Yes.
32. How many factors are required to size a steam trap?
5.
33. What does the L in LAMBS refer to?
The actual load to the trap, times the safety factor.
34. Is dirt in the system a reason for not using safety factors?
No.
