Figure 3-6.A two-pipe vapor system with a return trap.
mechanism, which permits equalization of the boiler
pressure and the pressure within the return trap.
Vapor-steam systems with return traps are similar
in design. However, it is seldom that two installations
are alike. Since the details differ with the type of heating
equipment, it is recommended that the manufacturers
installation instructions be followed.
However, the mechanical return trap should be
installed on a vertical pipe in the return system that is
adjacent to the boiler. The top of the trap should be level
with, or below, the bottom of the dry return main. The
bottom of the trap should be approximately 18 inches
above the boiler waterline to provide a sufficient
hydrostatic head to overcome friction in the return
piping to the boiler.
The two-pipe vapor system with a return trap
alternately fills and dumps. It returns condensate to the
boiler by a mechanical alternating-return trap instead of
by gravity. The alternating-return trap consists of a
vessel with a float that, by linkage, controls two valves
simultaneously so that one is closed when the other is
open. One valve opens to the atmosphere; the other is
connected to the steam header. The bottom of the vessel
is connected to the wet return.
In operation, when the float is down, the valve
connected to the steam header is closed and the other is
open. As the condensate returns, it goes through the first
check valve and rises into the return trap, which is
normally located 18 inches above the boiler waterline.
The float starts to rise when the water reaches a certain
level in the trap, the air vent closes, and the steam valve
opens. This action equalizes the trap and boiler
pressures and permits the water to flow by gravity from
the trap, move through the boiler check valve, and go
into the boiler. The float then returns the trap to its
normal vented condition, ready for the next flow of
The problems you are likely to encounter in main-
taining the two-pipe vapor system with a return trap will
differ with each system. Some of the more common
troubles are discussed here. For specific instructions,
you should refer to the manufacturer's manual or
pamphlet pertinent to each piece of equipment.
When a radiator fails to heat, the air vent being
plugged or the radiator being waterlogged because of a
plugged or defective trap can cause the condition. In
case there is a plugged air vent, all you need to do is
clean it. When there is a waterlogged radiator, the trap
should be checked to determine if it is plugged; also you
should check to see if the bellows is serviceable. If the
trap is plugged, then cleaning it should solve your
problem. However, if the trap is damaged, the damaged
part, or the whole trap, must be replaced.
When the entire steam distribution system fails, the
trouble can be caused by inoperative return traps or