Steam Boiler seems to be pulling a vacuum causing it to shut down on low water.

JRM6868

My church had a 2 boilers one to feed the church and another boiler to feed the convent. They sat side by side and were piped so that if one went down the other could feed both in an emergency. About 11 years ago the church boiler went down and it was replaced. 5 years ago the convent boiler went down and was basically shut down and abandoned. The church boiler fed both for about 2 years until they installed an HVAC system in the convent. The church boiler just went down this past year after only lasting 11 years. After researching and finding out fresh make up water all the time can create leaks and eat away the cast iron, we discovered the boiler piping had leaks underneath in the dirt cellar of the old church. The piping has since been fixed. The 2 old boilers from the convent and the church have been removed and a new peerless sectional has been installed. The boiler is using the existing condensate collector/feed unit.
As the boiler fires everything runs fine. When the boiler senses it needs water the boiler the feed pump kicks on and the water drops out of the sight glass, with the boiler feed pump shut off valve fully open it would pull and vacuum and you would hear banging where the feed water goes in. Eventually the makeup water would fill the system and the feed unit would shut down and run normal again. So we throttled the feed unit valve down to being about 1/4 of the way open and it stills pulls the vacuum but the water stays in the bottom of the sight glass holder and will eventually come back up into the sight glass and shut off as normal. It will function fine like that for a few cycles and over night it will lock out again on low water. We came back and reset it and it works fine again and by fine I mean its still pulling the vacuum but its more controlled now with the valve only being 1/4 open. However it keeps locking out on low water because of the vacuum. Also it seems the low water sensing probe and the low water cutoff are approximately the same level.

Why would it keep doing that?

Someone told us to add a F&T on the condensate line going to the receiver coming from the steam supply line above the boiler.

Does that solve our problem?

Being a small town church and low on cash we want to do what is right but not add things just to see if it works.

JUGHNE

Is your condensate tank, with the pump, vented to atmosphere?

Any end of main condensate line needs some steam trap, usually a F&T.

Can you post pictures of the boiler piping and condensate tank piping? Also the end of the steam main connection.

Jamie Hall

As @JUGHNE says, some pictures will help. It's likely not an expensive problem -- it's likely that there is woefully inadequate venting somewhere or somewheres, and quite likely an incorrect equalizing connection. And, possibly, a trap missing.

Steamhead

@JRM6868 , I'm wondering if you even need that condensate tank and pump- BTW, that unit is called a "Boiler-Feed Pump".

Most of these systems were designed to return condensate to the boiler by gravity. Once you introduce the tank and pump, you open up a whole can of worms, as @Jamie Hall and @JUGHNE said. We've returned quite a few systems to gravity-return operation with good results.

I'd bet, however, that the problem is the water being pumped into the boiler too fast. A slug of cold return water will cause the water in the boiler to cool off and shrink, which can result in pulling a bit of vacuum.

Sometimes you can get away with just throttling the pump's output, but I'd look at simply getting rid of the boiler-feed pump altogether. Doing so will eliminate a bunch of moving parts that will need servicing as time goes by.

Where are you located?

Fred

How big is the condensate tank and how far from the boiler? Is the tank and the pipe between the tank and the boiler large enough to hold most of the contents of the boiler, in addition to its normal water level? If so, it is very possible, with a new boiler and new piping the boiler just needs a few good skims and the tank needs a good cleaning. The oils that were introduced into the water from the new piping and boiler, will sit on top of the water and make it difficult for steam bubbles to break through. The result is that the boiler water is pushed back into the return pipes or possibly, in this case, the tank. If it worked before and the change has been the new boiler and some piping, I would try a few boiler skims and draining and cleaning the tank.

dopey27177

Problem of vacuum in the boiler can be resolved with installing a steam vacuum breaker on the boiler.

Assuming the installation of the boiler feed pump is correct and it is vented to the atmosphere and all else is correct the steam vacuum breaker can be the solution without looking for problem that may not exist.

Taken from my book steam the perfect fluid for heating and some of the problems. Available at Amazon, Barnes and Noble and Dorrance Publishing Pittsburg Pa.

JUGHNE

Are you certain that the retired boiler has no piping that could be connected to the operating one?

I agree with Steamhead that to large a flow could cause this.
I have throttled down the feed pump inlet to the boiler to just feed enough water to keep the boiler from going off on LWCO.
Also a bypass was added to take the output water back into the feed tank. Even with a 3/4" line and non full port ball valve which is fully open there was still too much flow and was throttled down again at the boiler.

I had small undersized boilers and that water would kill the steam.

Pumpguy

When the boiler stops making steam, and the steam condenses in the boiler and header, an induced vacuum occurs due to the change in volume between steam and condensate.

With a vented boiler feed tank and pumps, atmospheric pressure pushes feedwater from the tank into the boiler and header causing the flooded condition you describe.

If this is a vacuum return system, or you're interested in maintaining a vacuum on the returns, a positively closing valve on the discharge of the boiler feed pump(s) should be installed. The actuator on this valve should be wired so the valve only opens when the boiler is calling for feedwater and the boiler feed pump is operating.

JRM6868

Sorry for the delay. I never received any alerts that I had comments and my mother passed away suddenly.
The tank is vented to the atmosphere. No piping from the retired boiler is connected it was all deleted. The feed pump has been throttled down to almost closed to a 1/8 open, 1/4 open to 1/2 open etc. Still pulled a vacuum at each although not as bad as it being full open. The boiler was boiled out and skimmed also.it can be done again. I also wonder if we need the feed tank and pump or not and/or maybe just add the steam vacuum breaker?
Thanks for the comments and help.
Here are some pics.

dopey27177

Firstly; You have a boiler feed pump. Although this unit is over kill for the size of your boiler a 20 gallon unit would have been more than adequate for your application. Keep the boiler feed tank you already paid for it.

Based on what I see you have a system utilizing steam traps, correct me if I am wrong.

Your piping system has large steam piping that is uncovered.
The vacuum condition in my opinion is caused by all the uncovered steam piping, steam when condenses shrinks 1700 times in volume when the boiler goes off line thereby creating the unwanted vacuum.

Covering the steam piping will end the condition. Additionally, over the life of the boiler you will save cash by keeping the spent btu's that may overheat the basement.

At this point a 1/2" High quality soft seat check valve will be the most economical cure for ending the vacuum condition.

The easiest place to do the install is at the manifold where the where the pressure gauge is.

If you install at that location install a drip line to a bucket. Make sure the drip line goes below the gas controls.

The best location for the install is at a tapping at the top of the boiler, Also a drip line should be install at that location.

I recommend, if available to use a real steam vacuum breaker.

Oh: incidentally My initials are also Is JRM.

Jake

Erin Holohan Haskell

@JRM6868, I'm so sorry for your loss.

Here are some tips on setting up forum email notifications:
https://heatinghelp.com/forum-user-manual

Jamie Hall

At some risk of pointing out the obvious here, if your feed tank is vented to the atmosphere -- and I see that it is -- and there is adequate main venting on your steam mains and dry returns -- you simply cannot pull a vacuum: the only modern main vents of which I am aware which will seal against a vacuum are Hoffman 76s, and I doubt very much that that is what you are using.

Now that being said... I'd take a look at the main vents. Are they adequate for the size of the piping and the system? Are they working? If there are dry returns, are they vented? They have to be, condensate tank or no.

Then there is one other place I would look: what is the maximum pressure that the boiler can reach in normal operation? Many types of main vents may fail to open until they cool, if they were subjected to more than 3 psi or so. So... if the pressure control on the boiler allows it to go over 3 psi, it's set way too high (probably should be around 1.6 psi cuout if it's a pressuretrol)..

EBEBRATT-Ed

If it is trapped system there is likely their is no air vents except at the feed tank.

It could be a combination of the un insulated piping and cooler boiler feed water entering the boiler.

I would suggest a water meter on the boiler feed tank make up water line. This will help ensure there are no more leaks in the system and that you are not using excessive make up water.

Throttling the pumped boiler feed water will help. I would check the temperature of the boiler feed water.

Install a steam rated vacuum relief valve or a quality Y pattern check valve on the boiler piping up high.

It would seem that the cooler feed water is chilling the boiler water causing the steam to collapse

That is why I suggest the water meter and checking the feed water temp

Jamie Hall

Have to think about that, @EBEBRATT-Ed . Seems to me, trapped or no, that air has to be allowed to get out of the system somewhere, somehow. After all, an F&T has a thermostatic element which is open cool and closes against steam, no? Am I wrong in thinking that an F&T, cool, will not hold a vacuum?

I quite agree that it's most likely too much old water causing the steam to collapse -- but I'm having trouble seeing how that could cause more than a temporary, very short duration vacuum unless the vents/F&Ts or whatever is letting the air out can hold against a vacuum.

What am I missing?

EBEBRATT-Ed

@Jamie Hall

I am thinking on start up the steam pushes all the air through the traps into the returns and the air is vented at the boiler feed tank.

The air at the same time pushes any remaining cooler condensate into the feed tank. If the boiler had been off a while the condensate can be cooler. Either of those thing can cause cooler than normal condensate to enter the boiler killing some of the steam. Slowing the pumped feed water may help

I am also concerned to the possibility of potential leaks causing excessive cold MU water into the feed tank causing the same problem hence the water meter suggestion,

If the system only has issues on start up (it may be ok once it stabelize's-we don't know) a vacuum breaker may help.

I also had a job where the engineer potentially undersized the boiler(s). This job had a lot of piping. On a cold start the boilers would run with a normal water line and as soon as it just started to steam the steam leaving the boiler condensed in the cold mains this caused a vacuum and pulled the water out of the boiler faster than the boiler feed pumps could keep up and they would trip low water.

If you could keep them running as soon as the boilers started to have a + steam pressure they ran fine

AMservices

https://youtu.be/cI_DJ7v70M8
Here's a video of what happens inside a steam boiler when cold feed water hits it.
I looked over that boilers manual and it calls for a 5" header with a 2.5' equalizer.
Is that what you have?
I agree with you that I think those LWCO's are to close to the same level. I don't like that float LWCO (MM67) being used as a pump controller and the placement of it on this boiler with the supply riser coming off 1 side.
What I think is happening, is you have a slanted water line.
When the water is exploding into steam, it starts heading towards its only exit very fast. All these bubbles blowing through the surface of the water at one end section, could be lowering the water on the other side.

In the manuals drawing, tapping "E" on the left end section is the probe LWCO with manual reset. The MM67 lower tapings is In the "E" tapping on the right hand side. The float is set to turn the pump on 1-3/4" above center of "E" tapping.
At some point in the operation (usually as soon as you turn your back on it) the water line could be slanted enough to trip the probe LWCO, but not the MM67.

First thing I would try is clean, clean, clean that water and boiler feed tank, then running the lowest pressure possible. You will want to get a vapor stat to replace the pressuretrol you have now. Try to keep the pressure under 1 PSI.
Or even better, try to find a good steam guy to stage fire the boiler so after it makes 8-10 ounces, the boiler down fires by lowering the gas pressure to the burner. Think of it as, turning down a stove top when the pasta starts to boil over. You can turn the heat down and still maintain a steady boil right? Why not do it with your heating system?

Another thing to consider would be changing the MM67 out for a MM 150 pump controller and maybe putting another float style LWCO on the left side to cut the burn off, but would automatically reset when the water stabilizes. Definitely keep the probe LWCO in place as an emergency shut down. Float LWCO are reliable, but only if they get regular service. Probes don't require weekly blow downs so i feel better walking away because i can't be around every week for the next 20-30 years, hopefully.

If you still are worried about a vacuum forming, what i would recommend there is a vacuum equalizer line.
A vacuum equalizer line is a pipe connecting the supply and return, separated by a swing check and water seal.


Simply put, if a vacuum trys to formon supply side, the swing check opens and the pressure instantly equalizes.
I've done this with 2 boiler feed system and it works great.


Good luck with everything

JRM6868

Trying to answer everyone's questions in this post...
The boiler feed tank is oversized that is the tank that was there when there were 2 boilers installed. (Is that tank even needed?)or would the boiler flood because its making up fresh water because it takes a while to get the condensate back and when it shuts off it would flood the boiler when all the condensate returns?
The boilers lines are insulated except in the boiler room because it was pulling the vacuum we didn't want the boiler lines insulated and then have to remove insulation to add additional parts and waste the insulation. The piping is a 5" header with a 2-1/2" equalizer line. It does make up water quite a bit until the condensate starts coming back to fill the feed tank. The feed tank gets warm once the condensate comes back but it still pulls the vacuum mixing with some of the cooler water.There were leaks in the piping that were repaired before this boiler was installed.
I can also have them clean the boiler a couple more times.

Joe_Dunham

so is the boiler flooding or going off on low water? When a steam boiler goes off there's a collapsing action that induces a Vacuum on the system. Especially pork chop boilers. I have seen this suck water out of the BFU, especially if there's a swing check on the discharge line rather than a spring loaded lift check. i HAVE ALSO SEEN A BOILER SIPHON WATER FROM THE bfu THROUGH THE DISCHARGE DEPENDING ON HOW IT IS PIPED AND THE ELEVATION OF THE BFU, BUT i DONT THINK THAT'S YOUR CASE. As Jamie said since the BFU is vented, you should not pull a vacuum UNLESS the return ties into the tank below the water level. It looks like it does and they probably did that to create a water seal rather than add steam traps where they are needed

JRM6868

Joe_Dunham said:

so is the boiler flooding or going off on low water? When a steam boiler goes off there's a collapsing action that induces a Vacuum on the system. Especially pork chop boilers. I have seen this suck water out of the BFU, especially if there's a swing check on the discharge line rather than a spring loaded lift check. i HAVE ALSO SEEN A BOILER SIPHON WATER FROM THE bfu THROUGH THE DISCHARGE DEPENDING ON HOW IT IS PIPED AND THE ELEVATION OF THE BFU, BUT i DONT THINK THAT'S YOUR CASE. As Jamie said since the BFU is vented, you should not pull a vacuum UNLESS the return ties into the tank below the water level. It looks like it does and they probably did that to create a water seal rather than add steam traps where they are needed

It doesn't flood it just pulls the vacuum and eventually goes off on low water

Jamie Hall

OK. I guess the next obvious question is... where does the water go? That will tell you where the vacuum is forming, and by extension where you have no or poor venting.

JRM6868

I added red lines to the pic on what I'm understanding what we need.
So is this what we need to do to create a equalizer line?
Just add the check, drop it down to create a water seal and tie it into the return?

JRM6868

Jamie Hall said:

OK. I guess the next obvious question is... where does the water go? That will tell you where the vacuum is forming, and by extension where you have no or poor venting.

As far as we can tell it stays in the boiler it just collapses the water from the cooler temperature and slowly builds back up. Throttling down the valve after the BFU pump helped keep the water in the very bottom of the sight glass where before it would pull all the water out of the sight glass. when we reset the safety
switch the boiler will refire and run ok still pulling the vacuum but will keep running. there was a guy there and watched the boiler for over an hour going through cycles and still be ok. So he would leave and over night the boiler would lock out on low fire.

Jamie Hall

Ah... um... well... if there was water in the boiler at a certain level on the sight glass, there is a certain volume of water in the boiler. Injecting cooler water -- or any other way of stopping the boiling -- will not reduce that volume. Water is almost incompressible. If the water level in the boiler is falling, that volume of water has gone somewhere else.

Where?

JRM6868

Jamie Hall said:

Ah... um... well... if there was water in the boiler at a certain level on the sight glass, there is a certain volume of water in the boiler. Injecting cooler water -- or any other way of stopping the boiling -- will not reduce that volume. Water is almost incompressible. If the water level in the boiler is falling, that volume of water has gone somewhere else.

Where?

My guess is the water will be backing up into this piping.

AMservices

Its more likely the water is surging out through the supply, not backing out though the equalizer.  
Does the burner shut off when the pump is topping off the boiler?
Is the system completely filled with steam when the vacuum forms? In other words, are all the steam traps closed? 
Is the boiler building pressure before it makes a vacuum? 
The vacuum equalizer line should come from the supply header. 

JRM6868

AMservices said:

Its more likely the water is surging out through the supply, not backing out though the equalizer.  
Does the burner shut off when the pump is topping off the boiler?
Is the system completely filled with steam when the vacuum forms? In other words, are all the steam traps closed? 
Is the boiler building pressure before it makes a vacuum? 
The vacuum equalizer line should come from the supply header. 

No the boiler keeps running the whole time.
I would say no and yes on the traps.
The vacuum starts as soon as the boiler starts making steam and it calls for make up water.
The condensate eventually makes it back to the tank so I'd say some of the traps are closed and some open still.

JRM6868

So are you saying the equalization line should be like this in yellow?

Jamie Hall

I'd agree that it's backing up into the piping... which means the piping needs venting which opens on a vacuum...

AMservices

That will do it

motoguy128

Just looking at the feed tank location and height compared to the boiler. Looks like you could raise the tank about 6” and remove the pump and the system would work fine by gravity. Would need a place to regularly blowdown the return line. Ideally the return would be larger. But if kept clear whats there should work fine.

I’m not really understanding why there is a feed tank. Just needs some storage due to the system size, and it provides that.

JRM6868

motoguy128 said:

Just looking at the feed tank location and height compared to the boiler. Looks like you could raise the tank about 6” and remove the pump and the system would work fine by gravity. Would need a place to regularly blowdown the return line. Ideally the return would be larger. But if kept clear whats there should work fine.

I’m not really understanding why there is a feed tank. Just needs some storage due to the system size, and it provides that.

I think at the time the feed unit was there because there were two boilers and was left in there for the new installation.