Unwanted vacuum in steam system

Unknown

have the reverse check valves and they may be stuck, I have seen that happen especially on old systems.

jrw

vacuum in steam system

I have 3 low pressure steam water tube boilers operating at 12 psi. They feed a heat exchanger w/ 2 motorized valves and 6 roof top heating units.
The 2 motorized valves on the heat exchanger are rated at 1/3 and 2/3 inlet.
When the lead boiler shuts off the entire system goes into a vacuum 4"w.c. to 5"w.c. and sucks water out of the boiler as the water level drops to the minimum level after the burner goes off. ( The burners are not over fired and the traps are working )
I did isolate the heat exchanger and have narrowed the problem to it.
Does anybody know why the heat exchanger is causing this problem?

Brad White_185

It is not the Heat Exchanger...

Well, actually it is, but it is not it's fault!

You need a vacuum breaker.

Vacuum is a normal occurrence with the collapse of steam- you want that for that is a creation of the steam giving up it's useful heat. You just have to accommodate it.

Nearly every shell and tube heat exchanger I have ever seen has a tapping for a vacuum breaker. Some old-timers used two horizontal check valves in series, aiming into the vessel. (Usually small, about 1/2" or 3/4", it does not take much and this depends on your vessel size and tapping size of course.)

You could do that or install a bona-fide vacuum breaker made for the purpose. Personally, I like to elevate them to above the HEX inlet flange to avoid a flood-line condition.

Now, for that matter, ALL of your steam consumers should have vacuum breakers, especially the air coils on the RTU's. Easy to freeze if you do not.

Brad White_185

A perfectly fair

observation and one reason I specify proper VB's. At the very least, the check valves should be brass or bronze- but you make an excellent point.

Speaking of which, a good pencil may solve it for the moment..