vacuum steam systems

dfaulk

Can anyone tell me why there is a second check valve on the equalizing line on the vacuum tank to the boiler, I understand why its by the boiler but don't understand why there's another one and a vent by the vacuum tank ?

Pumpguy

Reading from left to right, the first check valve is a pressure vent.

If the vacuum pump is running on float only, or if the vented air volume exceeds the air removal capacity of the vacuum pump, steam pressure will push out air through this check valve to atmosphere.

The check valve on the right is for equalizing the pressure between the steam side and return side of the system. When steam stops flowing in a properly tight system, an induced vacuum will occur due to the volume change between steam and water. When this happens, there is a high vacuum on the steam side. If this were not equalized with the return line pressure (vacuum), condensate would not gravity flow back to the vacuum pump.

If you look closely, you will see these 2 check valves are installed to allow flow in opposite directions. The pressure vent check valve allows flow to atmosphere. The equalizer check valve allows flow toward the boiler.

dfaulk

In what application would you have float only vacuum pump, I would think you would want it on float and vacuum ?

Pumpguy

True, except when condensate is too hot for producing a vacuum due to leaking steam traps or other causes.

Other times are when there are air leaks in the system that exceed the vacuum pump's air removal capacity, and in summer when steam is not used for building heat.

Perhaps also when vacuum producing pump parts are too worn, or if vacuum pump is undersized for the system. This last scenario, not enough air removal capacity in terms of CFM, is a far too common situation.