Surging commercial low pressure steam boilers

churchlady

I am running a pair of LES HF3-60 low pressure steam boilers to heat my 42,000 sq ft church. They were installed in 2007, they run as a lead-lag pair cycling everyday. we have vacuum pumps at 3 condensate collection stations, where it is collected and sent to a fill tank from which the boilers get make up water. The chemical pump feeds the chemical mixture into this take and is triggered when either boiler calls for water.
Lately we have gotten into a cycle where the lead boiler fires, and just as it gets to temperature and starts to make steam, the water level drops quickly, and triggers the low water cut off. the pump kicks on, the chemical pump kicks on the boiler fills, tries to fire again, drops out from low water. over the course of a couple of days it gets worse and worse until it can't make steam at all, the system is full of water and too much chemical. and the whole thing has to be skimmed and drained and started all over again. I ran it for two weeks with no chemical at all and it worked fine, no low water cut-off, made steam easily etc. as soon as I add any chemical it gets squirrely and starts to surge again. At first I thought it was the steam line additive because it is designed to flash off in steam, not float around in the boiler, so i took that out of the equation and just added the oxygen scavenger but i get the same result. I'm sure I am missing something.
I am a building manager not a boiler operator, and my longtime boiler guy and the boiler water guy both seem to be confused by how sensitive this system is. Does anyone have any ideas for me? At this point I will try just about anything.

Jamie Hall

What chemical are you adding, and why? Most low pressure steam boilers get severe indigestion whenever chemicals are added. Unless you are adding a great deal of fresh water, you shouldn't even need an oxygen scavenger never mind anything else -- and if you feeding a good deal of water, you need to find out where it's going and fix that.

churchlady

Hi Jamie, we have lost tubes inside the boilers to pitting so we have been adding and oxygen scavenger, powdered sodium sulphite. when we are running properly not very much fresh water is added.

Jamie Hall

I would suggest that you may need to review the water quality in some depth. You may need to add an oxygen scavenget, but not every time a boiler calls for water -- only when there is a call for new, fresh water. What you are doing now will result -- inevitably -- in chemical concentrations which are way too high.

churchlady

It has been my understanding that the condensate needed treatment as well? I agree the chemical pump adding when the boiler calls for water doesn't make sense but it seems to be how its done as an industry standard. My thinking is that the chemicals should be added when water that needs to be treated goes into the tank, not when feed water comes out

Jamie Hall

The condensate does not need treatment -- it's distilled water, after all, with whatever small amount of crud it picks up on the way back to the tank.

Chemicals should only be added when new water is brought in from outside the system, and the amount should be carefully determined by analysis of that new water. More is not better.

Perhaps you are confusing process steam system practice, where a good deal of steam is lost and new water has to be added regularly -- sometimes in large quantities -- with a closed system, which doesn't do that. The process steam does require more or less continuous chemical feed, because of all the new water.

Mike_Sheppard

My bet is the pH is well above 9. I see this all the time with commercial boilers. They have a chemical pump attached to them set to pump at a constant interval or to pump when the feedwater pump comes on. Inevitably your pH level will rise and you will get surging.

Another possible cause is cold feedwater from your pumps. If your feedwater is cold, or if it is feeding too quickly, it can cool the boiler enough to make it stop steaming and that layer of steam bubbles will collapse and it will look like your water level is plummeting. Foaming caused by high pH will make it look even more dramatic.

I had a customer who had a steam boiler used for heating an acid bath to clean the plates from plate and flame heat exchangers. He was adding so much chemical that when I got there, there wasn’t even water left in the boiler. It was 100% chemical. Using a 55 gallon drum every week in a closed steam system that returns 100% of its condensate.

churchlady

I have seen that the pH causes the surging as well as continuity.
Ok. so since I have spent thousands replacing tubes inside the boiler I'm a little gun shy when it comes to making sure the water isn't full of oxygen to drive little holes through the tubes. what is the best way to measure. of course my water chem guy says its measuring the amount of SO3 (the scavenger) in the return water. is there a better way? I am all for not using as much chemical.

Mike_Sheppard

My opinion is, if you’re adding enough make-up water to oxygen-pit your tubes, no amount of chemical is going to cover the problem. It would be much more economical to find and fix the leaks.

churchlady

I was concerned about too much makeup water as well and what I found when I put a meter on it is that what things are running smoothly we only add maybe 20 gallons per week. I'm not sure if that's a lot or a little.

Jamie Hall

That's a little more than I would expect, but not outrageous. You will need an oxygen scavenger in there, and possibly pH adjustment, but only for the new water -- not for the condensate return at all.

Might go looking for leaking vents and the like, though, to see if you can reduce that.

nicholas bonham-carter

“Dude, who stole my water?”
I think that amount of makeup water is quite excessive. Your boilers are 2 million btu each (lead-lag), so 10 gallons a week for a million btu. I have a Peerless 211A, (1million btu), using maybe a gallon a week, and I could probably reduce that.
I would drain and refill at least once, and monitor the Ph weekly. You may have had an acidic problem, rather than an oxygen problem, pitting your tubes. When the water boils, all the O2 is driven out, however excessive pressure can cause CO2 to make weak carbonic acid inside the system, which can cause bad things to happen to good tubes.
What is your pressure?—NBC

Mike_Sheppard

Low condensate temps can increase carbonic acid as well. Make sure everything is insulated.

churchlady

3 psi
the feeder tank has a steam inlet to preheat, typically its around 140.

churchlady

what level of residual SO3 would you consider sufficient? taking measurements of it right out from the condensate tanks,

Henry

I love the chemical sales people. They are my best steam heating boiler sales. I you do one blowdown a week, why spend $3K on chemicals?