Boiler fills when shutdown for weekend

rob203

I have a cleaver-brooks steam boiler. It has a feedwater tank with two feed pumps, one normally in auto, to refill the boiler as needed.
The boiler is operated during the day weekdays and shutdown each night and over the weekend. During the week nothing happens to level, but on Monday morning, after being shutdown Friday afternoon, the boiler is full of water. I'm fairly certain the flowpath is the piping from the feedwater (vented to atmosphere) tank to the boiler and as the boiler cools it draws in feedwater rather than drawing a vacuum in the boiler itself. Level is automatically maintained in the feedwater tank and chemically treated.
So my questions are: is this normal (it doesn't seem like it should be), and if not, how to I prevent it?
I could prevent it by shutting the feedwater valve to the boiler but that would cause the boiler to draw a vacuum, which I've been taught is generally bad.
Thanks

Jamie Hall

No, it's not normal. So... there are at least two possibilities. The first is easy enough to check: what is the water level in the feedwater tank when you get there on Monday morning? If the vent on the feedwater tank is higher than the boiler water level, and water is getting into the feedwater tank and rising higher than the boiler water level, then it will flow from the feedwater tank to the boiler. This would suggest checking the valves on the feed to the feedwater tank for leaking by. Note that this doesn't mean that the opening of the vent is higher -- just that the vent pipe goes up high enough at some point that water in it can stand higher than the boiler water level. If the vent goes up, then over and down -- not unusual -- it's that high point that counts, not where the opening is.

The other way to get water into the boiler from the tank requires that it gets sucked in -- which means that the boiler has to draw a vacuum, which in turn means (assuming that it isn't supposed to do that!) that whatever vent or vents are on the steam side isn't opening on a vacuum. If the boiler is intended to draw a vacuum on cool down, this can only be cured with a valve on the feedwater line which opens when the feedwater pump runs.

rob203

Jamie Hall said:

No, it's not normal. So... there are at least two possibilities...

Thank you for your insight
#1 is basically eliminated. the feed tank and boiler are approx the same height and the feed tank is vented to the roof. the tank is heated by steam when the boiler is operating so this is necessary to not die. I have a water meter on the feed tank supply so as level is automatically maintained it registers the flow.
#2 is what I believe is happening. I took over responsibility for this boiler in the last couple months and was told this happened around the same time I took over via an early morning call that the boiler is full! I was also told they replaced the main steam isolation coming off the boiler around the same time. I'm wondering if the old steam isolation leaked by enough to allow air to enter rather than it drawing a vacuum by condensing the steam blanket. I'm not aware of any vacuum breakers installed on the system. Water analysis has been coming back normal but I'm concerned about long term effects if this isn't normal

SlamDunk

Replace your feed water valve. It is probably leaking by.

Or, header steam condensing and pouring back into boiler.

SlamDunk

rob203 said:

Jamie Hall said:

No, it's not normal. So... there are at least two possibilities...

Thank you for your insight
#1 is basically eliminated. the feed tank and boiler are approx the same height and the feed tank is vented to the roof. the tank is heated by steam when the boiler is operating so this is necessary to not die. I have a water meter on the feed tank supply so as level is automatically maintained it registers the flow.

Sounds like you have a de-arator and the vent has an orifice to help maintain pressure and eliminate scavenged oxygen.

rob203

SlamDunk said:

Replace your feed water valve. It is probably leaking by.

Or, header steam condensing and pouring back into boiler.

the feed water valve is open so it does leak by...by design. I'm concerned that if I shut it the boiler will draw a vacuum, which isn't ideal to the best of my knowledge.

theres only about 6" of vertical pipe between the steam header isolation and boiler so I've ruled that out. I like the way you think though; that was one of my first thoughts until I saw the flow meter over the weekend showed makeup water had been added to the feed tank.

rob203

SlamDunk said:

Sounds like you have a de-arator and the vent has an orifice to help maintain pressure and eliminate scavenged oxygen.

yes. so when theres no steam applied to the feed tank its open to atmosphere via the vent line. i havent located an orifice but wouldnt be surprised if it had one to minimize steam loss.

SlamDunk

you wont draw a vacuum.  especially if your feedwater valve is left open and fed from a vented tank.  any boiler can handle whatever vacuum occurs.  close the feed water valve and see if it fills. 

the orifice is typically drilled thru the valve gate.  you said you feed steam to the tank.  I assumed it is a dearator because that what you do to deaerators. 

rob203

I think we’re almost on the same page. That’s what’s happening when it’s shutdown- rather than drawing a vacuum the boiler is filled with water instead. There’s no air in the system so the space has to be filled with either water or steam. The feed water provides the source. 

So is it normal or detrimental for the boiler to be completely filled every weekend?

Jamie Hall

rob203 said:

I think we’re almost on the same page. That’s what’s happening when it’s shutdown- rather than drawing a vacuum the boiler is filled with water instead. There’s no air in the system so the space has to be filled with either water or steam. The feed water provides the source. 

So is it normal or detrimental for the boiler to be completely filled every weekend?

no.

And if the feedwater tank is vented through the roof, and there is any leakage into the feedwater tank through the fill valve, that water will happily go from the tank to the boiler and fill it. Eventually to roof level...

Or you need to have a vacuum relief on the system.

Look at it this way: if the level in the feedwater tank is normal on Monday morning, but the boiler is full, that extra water had to come from somewhere. The only place it can come from (assuming all is well all week long) is from the feedwater tank. The water can't get from the feedwater tank to the boiler unless the pressure in the boiler is less than the water pressure in the tank -- which either means the boiler and system are dropping into a vacuum and sucking the water in, or the water is standing in the vent above the water level in the boiler and flowing in by gravity.

SlamDunk

it is not normal.  i dont think it is detrimental either.   just a waste of treated water. 

rob203

Jamie Hall said:

I think we’re almost on the same page. That’s what’s happening when it’s shutdown- rather than drawing a vacuum the boiler is filled with water instead. There’s no air in the system so the space has to be filled with either water or steam. The feed water provides the source. 

So is it normal or detrimental for the boiler to be completely filled every weekend?

no. And if the feedwater tank is vented through the roof, and there is any leakage into the feedwater tank through the fill valve, that water will happily go from the tank to the boiler and fill it. Eventually to roof level... Or you need to have a vacuum relief on the system. Look at it this way: if the level in the feedwater tank is normal on Monday morning, but the boiler is full, that extra water had to come from somewhere. The only place it can come from (assuming all is well all week long) is from the feedwater tank. The water can't get from the feedwater tank to the boiler unless the pressure in the boiler is less than the water pressure in the tank -- which either means the boiler and system are dropping into a vacuum and sucking the water in, or the water is standing in the vent above the water level in the boiler and flowing in by gravity.

Sorry for the long quote above. I’m on the mobile site at the moment. 

So if there were a vacuum breaker on the boiler wouldn’t that cause air to be introduced into the system? It’s basically a toss up at the moment which is worse for it: full of water or have air introduced. So far I’ve leaned toward full of treated water being better than air. I’m not too concerned about the cost of the treated water because it’s within budget limits. I just can’t have a boiler explode on me!

SlamDunk

sounds like you’re gonna do what you’re gonna do.  

if I were you, I wouldnt put a vacuum breaker on a boiler.   I would stop the unwanted migration of water.  best of luck!

rob203

SlamDunk said:

sounds like you’re gonna do what you’re gonna do.  

if I were you, I wouldnt put a vacuum breaker on a boiler.   I would stop the unwanted migration of water.  best of luck!

Thanks.
I got ahold of our boiler inspector just now. he confirmed that its due to the water vacuum dragging from the feed tank to the boiler as the steam condenses.
solutions include vacuum breaker which introduces minimal air and are widely used without any known issues, resistor check valve so that a small differential pressure wouldn't allow flow like a conventional check valve but would allow a small vacuum, or just let it fill up like it has been. I'm leaning toward the latter as the cost of a hundred or so gallons every week is minimal, at least until our next annual inspection.
i appreciate everyone's thoughts here.

Jamie Hall

As @SlamDunk said, looks like you're going to do whatever you're going to do.

That said: take into account that a steam boiler -- and the associated heating, process, power, whatever equipment -- is designed and intended to operate correctly and efficiently at a remarkably narrow range of water levels in the boiler. Therefore, if you are content to let the boiler flood during the weekend, you must -- absolutely must -- drain the water level down (to waste) to the correct level on the gauge glass before you fire the boiler on Monday. This is not optional -- for you, or whoever does come Monday morning to get things going. There is a substantial risk of damage if the boiler is fired when it is overfilled -- ranging from minimal (damaged steam vents) to spectacular (if the pressure relief valve is flooded and the boiler starts making steam, you'd best hope that the pressure relief valve is rated to handle that rate of flow of solid -- boiling -- water).

rob203

thanks @Jamie Hall
We will be sure to blow down to the operating range prior to firing