Vacuum on start up

SpeyFitter

So I'm a Burner Tech (Gas Fitter) and Hydronic Tech that works for a School district up here in beautiful Western Canada. The district I work for still has several aging low pressure steam plants and many more generally newer or new hydronic systems. I don't often work on the steam distribution but mainly the controls on the boiler/burner mainly. I have bought several of Dan's steam books which I have read which have helped my understanding but I definitely don't feel I have attained the command of steam knowledge like I do with hydronics, gas and burners.

An Elementary School in the district I work for went through a seismic upgrade recently so it sat for a year and a half through a couple winters (was drained down, decommissioned prior, etc.) where we just recommissioned it this past fall. It has a 2 pipe low pressure steam system with a vacuum pump, where the Engineer starts the boiler(s) and vacuum pump at the same time (vacuum pump in float and vacuum setting) in the morning to warm up and heat the building. The question I have is if one boiler is having a hard time developing steam pressure on shoulder days when it previously had no issues prior to the seismic according to the previous school engineer, where a vacuum is present on the boiler for much of the day which seems to me to indicate the vacuum pump may be pulling on the boiler directly, is that indicative of bad steam traps not closing after steam is hitting them where the vacuum pump acts on the boiler directly or something else potentially? The only thing that changed in the steam system is some pipes & rads were offset to accomodate structural changes and pneumatic actuators were replaced with DDC (Belimo) actuators for steam control to the class rooms. I am reluctant to suggest running 2 boilers when 1 boiler perfectly heated the school on all but the coldest of days previously (it does seem to be heating with a vacuum to some degree but there appears to be temperature problems in some rooms apparently) and I think trying to over power the system with 2 boilers would just be a band aid solution to try to cover up potentially faulty steam traps or some other condition (wondering if a steam trap might be missing even?) and would waste A LOT of fuel in the process. Any thoughts or info you can glean would be greatly appreciated! Thanks!

Steamhead

SpeyFitter said:

The only thing that changed in the steam system is some pipes & rads were offset to accomodate structural changes and pneumatic actuators were replaced with DDC (Belimo) actuators for steam control to the class rooms.

Go back to what was changed. If it worked before, it should work now. You may have to consult the original plans, but I'll bet you find the issue there.

nicholas bonham-carter

What Steamhead said. I would think the shoulder days would have the system in vacuum, so maybe an outside temperature sensor has failed.
Remember that temperature sensation is subjective, and that there is no substitute for having accurate thermometers in the rooms. Indoor outdoor wireless thermometers can help you see any fluctuations. Then a teacher can give a more accurate complaint, with an a reading of the thermometer.
It can also be a problem with temperature setbacks, where the system takes a while to balance out. A lower constant setting would be better, and may not use much more fuel.
Check the traps using an IR gun, or the old fashioned way with your hand, while the boiler is firing: outlet pipe warm=normal, outlet pipe hot=failed open. Outlet pipe cold=failed shut.—NBC

Pumpguy

The usual indicator of failed steam traps is very hot condensate.

That said, if all the steam condenses before it gets to the traps, the condensate will not be too hot, even if the trap is failed open.

This would be especially true when operating with very low steam pressures.

It would be normal when starting up from cold for the vacuum pump to pull a vacuum all the way back to the boiler. Under this condition, the traps have no steam to hold, so they would normally be open.

Once the steam gets to the trap, the thermostatic elements expand due to temperature, and close. Then the boiler would go out to a positive pressure.

Can you tell us what are the normal operating steam pressures and returning condensate temperatures?

EBEBRATT-Ed

Check the boiler input firing rate and do a combustion test. perhaps the boiler is not running but not on high fire. If the changed building controls and installed new valves they may have messed up the boiler controls