Steam Trap Sizing for a Low‑Pressure System

First, calculate the condensate load for the largest radiator. The steam is at 2 psig, which corresponds to a saturation temperature of about 219°F (using steam tables, 2 psig ≈ 16.7 psia, saturation temp ~219°F). The latent heat of vaporization at 2 psig is approximately 965 BTU/lb. The radiator EDR is 100 sq ft. The standard heat output of cast‑iron radiation is 240 BTU/h per sq ft EDR at 215°F steam (or 1 sq ft EDR = 240 BTU/h). So the radiator output is 100 × 240 = 24,000 BTU/h. The condensate load is 24,000 BTU/h ÷ 965 BTU/lb ≈ 24.9 lb/h. Therefore, the trap capacity of 2,000 lb/h is vastly oversized. Oversizing a steam trap can cause short cycling, wire drawing, and poor performance. The trap should be sized with a safety factor of 2:1 to 3:1, so a trap with a capacity of about 50‑75 lb/h would be appropriate. The contractor's selection is incorrect.

Factors affecting trap sizing: The trap must handle the condensate load plus air and non‑condensable gases. In low‑pressure systems, the trap must also be able to pass air during startup. Inverted bucket traps can handle air but may be slow. The trap must be sized based on the pressure differential across the trap, which is the steam pressure minus the return line pressure. The return line pressure is atmospheric (0 psig) because the receiver is vented. So the differential is 2 psi. The trap capacity tables are based on this differential. The trap must also be selected for the maximum condensate load, which may occur during startup when the system is cold and the radiators are filled with air. The trap should be sized to handle the startup load, which can be 2‑3 times the steady‑state load. However, in this case, the steady‑state load is so small that even with a safety factor, the trap is oversized. The correct approach is to calculate the load and select a trap that matches it. Also, the trap must be compatible with the system pressure and temperature. Inverted bucket traps are suitable for low‑pressure steam but require a minimum pressure to operate. At 2 psig, they work but may have a minimum differential. The contractor should verify the trap's minimum operating pressure. Additionally, the trap should be installed with a strainer and a check valve if necessary. The discharge piping must be sized to handle the flash steam. The 1/2‑inch connections are likely adequate for the small load, but the trap itself is too large.

Water hammer in a steam system is typically caused by condensate accumulating in the steam lines and being picked up by high‑velocity steam, or by steam entering a cold radiator and collapsing rapidly. Poor heating can be due to trapped air, undersized or failed traps, or inadequate condensate return. In this system, the traps are being replaced, but the symptoms suggest other issues. The water hammer could be due to the condensate return line being undersized or not properly sloped. The return line is 1‑inch Schedule 40, sloping 1/4 inch per foot. For a system with 30 radiators, the total condensate load is about 30 × 25 lb/h = 750 lb/h (assuming similar radiators). The 1‑inch line may be adequate for the flow, but the slope must be sufficient to drain condensate by gravity. If the return line has insufficient slope or sags, condensate will pool and cause water hammer. Also, the traps may be discharging into a return line that is at atmospheric pressure, but if the return line is undersized, the flash steam from the traps can cause backpressure and impede drainage. The contractor should verify the return line sizing using steam tables and the total condensate load. The boiler feed pump capacity of 5 gpm is about 2,500 lb/h (since 1 gpm ≈ 500 lb/h), which is more than enough for 750 lb/h, so the pump is fine. However, the pump may be cycling if the receiver is small. The water hammer could also be caused by the traps being too large, allowing steam to blow through into the return line. Oversized traps can cause steam loss and water hammer in the return. Therefore, the trap replacement should include properly sized traps. The contractor should also check the steam mains for proper drip legs and steam traps at low points. The radiators should be pitched toward the traps. The contractor should install a strainer before each trap to prevent dirt from damaging the trap. The trap discharge should be piped to a common return with a slope and should not be reduced. The contractor should also consider installing a vacuum breaker on the return line if the system operates with a vacuum. Finally, the contractor should ensure that the boiler is not priming or foaming, which can cause water hammer. The boiler water level should be checked. In summary, the modification should include: correctly sized traps (about 50‑75 lb/h for the largest radiator), installation of strainers, verification of return line slope and sizing, and possibly the addition of a condensate cooler or flash tank. The contractor should also check the steam traps on the steam mains and ensure that the system is properly vented. The boiler feed pump should be checked for proper operation and the receiver vent should be clear.