Steam boiler pressure relief

ecolbeck

I was test firing my steam boiler today after removing a few radiators due to some remodeling. The radiators began to heat as normal but then the pressure relief valve opened on the boiler and blew steam all over the basement. I killed the burner and waited for thing to cool down.

Then a really strange happened (strange to me at least). After about ten minutes of cooling the radiators suddenly began to hiss air and got really hot. Why did this happen? Shouldn't the pressuretrol have cut the burner well before the relief valve kicked in? Why did the radiators suddenly vent and heat well AFTER the burner had been switched off?

Fred

Did you have gate valves on the mains that you forgot to open? Yes, the Presuretrol should have cut the burners off at around a pound and a half of pressure. Of course this assumes the Pressuretrol was working (clearly it's not). When was the last time maintenance was done? the Pressuretrol tested? the LWCO tested/cleaned and the pigtail the Pressuretrol is mounted on taken off and cleaned. If it is clogged, the Pressuretrol can't see the system pressure.

JUGHNE

You must have some pressuretrol issues. Either adjustment or plugged pigtail. You should be able to shut off all radiators and the control would shut down the boiler...preferably below 2 PSI.
That relief valve should not open until 15 PSI.

MBG is that after your relief valve closed, the system went into a vacuum and continued to provide steam, thus heating your rads. This was the method with vacuum systems back in the day of the coal fire dying down overnight. Make steam without fire and deliver it to rads that were in a vacuum.

Now all your vents have been subject to at least 15 PSI.....maybe a problem.... maybe not.

ecolbeck

Thank you for responses. No gate valves on the system. Presssuretrol pigtail issue makes sense although it did seem to cut out properly on a recent test fire.
Could there be a blockage on the main line somehow?

JUGHNE

How many rads did you remove?
Were they still connected for your test fire.
Did the tstat shut off the boiler for the test fire or the pressure control?
Also the LWCO could have shut it down in the past.

Fred

ecolbeck said:

Thank you for responses. No gate valves on the system. Presssuretrol pigtail issue makes sense although it did seem to cut out properly on a recent test fire.

Could there be a blockage on the main line somehow?

If there was no blockage before removing the rads, probably not likely there is one now unless someone stuffed a rag into it. Did you also make changes to the Main as part of your rehab? If not, the most likely reason is a clogged pigtail under the Pressuretrol or a completely failed Pressuretrol.

ecolbeck

Rads were connected. Quite a few have been removed. System is certainly overfired. Only needed for backup heat once per year. Burned less than 1 oil tank in last 5 years. No work done on mainline. Tstat did not cause shutdown I believe it was pressuretrol the first time. The second time pressure valve blew.

EBEBRATT-Ed

Something is wrong with the pressure control, pressure control wiring or the pigtail and connected piping

Jamie Hall

As has been said, something is wrong with the pressure control -- and since it is a safety control, best find out what the problem is before trying again.

Pressure relief valves are not meant to blow under any ordinary circumstances.

I would also be very concerned that having been subjected to pressures well beyond their rated, never mind controlling, pressures, all your vents and traps will need checking, if not replacing. Sorry about that.

mikeg2015

The hissing was probably vacuum being drawn back through the vents. Mine do this as well since it was designed as a vapor vacuum system and is oversized.

Steamhead

I think that with the pressure being so high, you exceeded the "drop-away" pressure of your air vents. The high pressure held them closed. Then, when the pressure dropped enough, they could open, vent the rest of the air, and the rads got hot.

You either have a plugged pigtail or a bad Pressuretrol. If the former, and the pigtail is steel, replace it with a brass one.