Could this happen??

JUGHNE

Hospital with two 80 HP steam boilers that are alternated weekly.  After a change over the new on line boiler shut down.  It was the manual reset high limit (set at 15 PSI). These are hi/low burners that run at 5 PSI max.  I reset the limit and watched operation for more than an hour thru numerous cycles.  Always shut off at 5 PSI as usual.  The off line boiler would usually go into a vacuum as it cooled off.  Sometimes the off line valve will not shut off 100% and the idle boiler may gain water.  Not sure of what the change over procedure is and if it is always followed.  The controller pigtails have not been cleaned within anyone's memory.  There are two maint personal there and all I could suggest was to clean all the pigtails on the off line boilers.  My theory is that an accumulation of pressure may have pushed past sludge into the high limit controller.  I have seen this happen to fire sprinkler systems with a check valve on the inlet water line; every time there was a surge in water pressure the closed line would get additive PSI locked on the customer side of check valve.  Or am I grasping at straws????  Thanks for your thoughts.  Also the PRV of 15 PSI never opened; I'm sure some would have noticed that.

Hap_Hazzard

What's more likely is that

The low limit was plugged or otherwise failed to cut out, allowing the pressure to climb until the high limit cut out. Are the high and low limits on separate pigtails? If not, they should be. I'd also want at least a 5 psi margin between the high limit and the prv threshold. If one prv opens the whole system is essentially open to the atmosphere.