Need help understanding disappearing water level

Mike_Sheppard

Happy holidays to all!

This is a bit of an odd situation that I have only encountered several times as a service technician. I do commercial/industrial work and very seldom small steam boilers. This boiler was a 600hp Cleaver Brooks fire tube. The other boilers I have seen this happen on were also Cleaver Brooks fire tubes. It seems that when there is a large load, the boiler will reach a certain PSI and then out of no where within seconds the water level vanishes from the gage glass. The boiler shuts off on low water and after about 30 seconds or so the water level is back up to it's normal level - with or without the feedwater pumps running. I have read through all of Dan's material and am trying to understand why this can happen. I read that if steam sections are too narrow or firetubes too close to eachother, at the end of a cycle the steam bubbles will collapse causing the water level to drop in the gage glass. But this issue I am having while the boiler is running and steaming, usually around 3-4 psi.

An older technician explained this to me as the load being too great for the boiler and the velocity of the leaving steam is pulling the water towards the outlet from the boiler. But I can't make sense of that.

What the engineer claims to have been causing the issue is the feedwater tank being cold and feeding cold water into the boiler. When the cold water fed into the boiler it would cause steaming to stop momentarily which allowed the steam bubbles to collapse and the water level to fall out of the gage glass. Kind of mimicking what can happen at the end of a cycle on a narrow section boiler. This makes more sense to me, but I wanted to ask here to see if anyone could share some insight. He said that after he added a sparge tube to the feedwater tank to heat the water, the problem stopped.

This particular boiler had great water treatment, 7 pH, and an automatic surface blowdown that monitored TDS, and there was no surging or wet steam at all. The water level would just plummet as soon as the pressure reached around 3-4 psi.

Any knowledge is greatly appreciated! Happy holidays!

AMservices

Any zone valves in the system? If so, maybe under the right conditions when a zone valve opens it will cause steam to rush from the boiler, dragging the water out with it.
If you find that to be the problem, I'd try a slow opening valve, or disabling it completely.

EBEBRATT-Ed

I agree with @AMservices about any possible zone valve issue.

Think of it this way, the boiler steams along at say 4-5 psi the water under the steam is hot and at a higher temperature than water boiling in an open pot. Your under pressure and are making saturated steam. Now anything that stops the steaming, shutting off the burner, tripping low water, cold feed water or a sudden large load will drop the boiler pressure.

What happens when the boiler pressure drops? A large portion of that water that is heated above the new saturation pressure (remember the boiler pressure is dropping) flashes into steam........thus the disapering water line

mikeg2015

Could there be bad traps and stem or air in the dry returns is holding condensate back combined its vacuum on the steam side.

So basically a combination of vacuum and pressurized returns.

Total guess here.

Seems like when condensate doesn’t return its either a plugged returns or being air locked or vacuum locked somehow.

Mike_Sheppard

Thank you for the replies. To be honest I am not entirely sure about the zone valves, but there most likely are. The boiler in question provides steam for the entire block of the city which is comprised of several buildings. I know for sure that it serves several large air handlers in the same building that the boiler was in.

Some more information I gained from the site engineer:

He said he found a completely broken 2 inch return line in the building which was not allowing any condensate to return to the feedwater tank. So the tank was running off of only make-up water that was cold. His temporary solution was to add a sparge tube to the feedwater tank to heat the water as well as to pipe the make-up water from his hot water heaters so the make-up water would be hot instead of cold (I know this is not a good long term solution as hot make up water can cause float valves and solenoids to scale up). But he claims that keeping the feedwater tank around 160 degrees solved the problem of the water dropping out of the gage glass.

EBEBRATT-Ed

You need to get that return line fixed asap. If you have several air handlers you have several zone valves.

You really need to get this job in shape, your risking losing that boiler. Loosing water and adding MU water is no good regardless of the sparge tube

mikeg2015

I was thinking residential boiler. This is probably a 500hp+ boiler.

Hey no biggey. Nothing a few patches in the tube sheet and retubinh at the cost of $100k can’t fix.

2” return should already be fixed.

Mike_Sheppard

The one 2 inch return was already fixed. The boiler is a 600hp.

Is was fixed up back to running properly in a day and a half. A couple of weeks ago.

I was trying to get some insight as to what causes the water level to just disappear in seconds. All of the air handler valves were wide open and the problem only occurred when the feedwater tank was cold.

This is not my boiler.. as I said I was just trying to learn something.

Thanks for the replies.

MilanD

@Mike_Sheppard

Cold water added to steaming boiler changes the volume of water. Enough cold water added, and a large load on the boiler, water shrinks in volume plus pressure drops sucks the water out of the boiler. See this:

https://youtu.be/pwvgMiJLwcQ

Mike_Sheppard

@MilanD

Thank you so much for that video! That explains it perfectly. I greatly appreciate it.

nicholas bonham-carter

An excellent video-so much more interesting than many others!—NBC

awthompson

Just wanted to say thanks for the video.  I’m training in a boiler room position and the simple, clear explanations are quite helpful.