Thermal shock. Is it real or theoretical?

ratio

Guru448 said:

Hey guys I am new to posting but had to comment on this one! When I first started in the mid 80s I added water to a steam boiler that had no water and it smelled a little wierd when I walked in( only a couple of months in), and I will never forget that shrieking cracking sound of the sections. Learned a lot from that moment and I hope any other new guys will never experience that in their careers because the feelings that come along with that kind of mistake really stink(to put mildly)

You're pretty lucky to be alive to feel bad about that. Steam explosions can be a lot more dangerous that just cracking sections.

JohnNY

ChrisJ said:

I could be way off on this but my assumption was John was asking about in normal use e.g, adding water too fast to a hot boiler that's slightly low. Water that's too cold returning to a HW boiler etc.

Adding water to a dry fired empty one is going to be a bad day.

Yeah, I guess I had a narrow focus when asking that question. I was thinking residential (~150 MBH) boiler, ½" makeup water piped into the return, maybe an automatic water feeder, etc. I honestly can't say I recall coming across a boiler cracked by thermal shock in that setting. Though maybe I just didn't know...

JUGHNE

What is the general consensus on how cool an empty boiler should be before refilling? (smaller 250,000 btuh or so)

I believe if one can hold their hand painlessly on the lowest return pipe, where the water would be added, you are good to go.
Though I still add very slowly.

Any thoughts on this?

captainco

Why don't water heater have thermal shock. Do they use a more expensive material that allows them to take on 50 degree water while they are operating under full load, The main cause of boiler shock is ignorance. Firing a boiler without the pumps running is ignorant. Operating a boiler at full temperature and just reseting the loop is ignorant, In fact I think they passed a federal law that states in residential boilers they can only operate when there is a call for heat and can't maintain temperature unless there is a indirect water heater coil.
I am nowhere near as experienced and knowledgeable about boiler systems as most on here and don't mind behind corrected, However I have worked with hundreds of techs and engineers over the years trying to solve their boiler maintenance problems. Two of their problems I just listed. After reading the lab study on boiler shock I get a little confused. According to their chart, the higher the firing rate, the more wear and tear on the boiler. In 100% of all fire-tube boilers I have been involved with, the boilers that leaked the most and needed re-tubing were the ones that modulated or had a low fire hold control.
Had a boiler repair company attend class a few years ago because he wanted to know why boiler tubes and sections fail. Most of it was do to some type of combustion problem, sometimes a control problem or like furnaces it could be water flow but rarely. When a customers calls and says this is the first year his boilers didn't need repair or re-tubing, and everything that was changed was against the norm, one wonders if the norm needs to be updated. It is just like every time I see someone say the efficiency of the equipment is whatever their combustion analyzer shows and is absolutely fictitious. I discovered this in 1980 when the calculated efficiency went down after burner adjustments and yet the customer saved then of thousands of dollars in fuel. Same thing when I measured over 5000 ppm of CO on a smokeless oil burner and read that the only time oil makes CO is when it is making smoke.
Firing a boiler with the pumps off is ignorant. Pumps should be controlled to come on several minutes or degrees before burners fire. Resetting a loop and keeping the boiler hot is ignorant. The exception might be if the boiler is also making domestic hot water, but what a waste of energy that is!

If I am to answer JohnNY question it would be Thermal shock is caused by ignorance and real creativity. Ready for the firing squad!!:)

EBEBRATT-Ed

I don't know about the water heater shock question but WH are not made of cast iron.

Not that running low return water temps are good for a steel boiler (there not, can still cause condensing etc.) but I was told way back you couldn't shock a steel boiler filling it while it was hot.

With CI I agree with @JUGHNE they always say if you can hold your hand on the boiler let the cold water rip. That's the textbook answer

I am sure we all have filled them when hot faster than we would have liked to "get er done"

Labenaqui

Our "Neo-Gravity Hydronic Heating Appliance(TM)" U.S. Patent 10,690,356 seems to negate this risk by combing the embedded LWCO function of the W/M UO 3250+ Aquastat with Taco VT2218 Delta-T ECM Distribution Management.
We are emulating a pure, Natural Gravity Heating Profile by utilizing a very high-mass boiler (W/M UO Series) with Taco Delta-T ECM Circulation. Our systems typically operate between 132 - 165 Deg. F, depending upon setback recovery to stabilized demand operation.
To date we have accrued over 40 operating-years and 125,000 hours of flawless system operation on our 10 Betas without a system-related incident.
Further, we do not recommend Magnetite alleviation, the high-mass cast historically being a natural ferrous oxide scavenger.
Our maintenance intervals average three years with a max. of five, depending upon fuel consumption.
BTW - In my sixty-five years of heating I've never occasioned a cracked boiler. A few "Cherry-topped" Dry Steamers and FHW "pipe-bangers". I feel like I've missed something .....

Woody_S

I had a call in the 80's that was a no heat call. The job was 120 miles away. They said that the boiler room was very hot, and all of the buildings were cold, but one of 2 boilers were running. They are BIG Kewanee Fire Tube steam boilers. (96 Fire tubes, 2 3/4 diameter. Boilers were 20' long, installed in 1946, 3-16 GPH oil nozzles in a mod burner). Any way; I told them to go back down in the boiler room, Under the bus garage, and check the water level, and shut off the boilers. They told me they couldn't see any water in either gage glass, but had turned both off. Told them to go down and turn off the water feed valves. They said, why? Told them, if you don't, you will have a big smoking hole in the ground if one of those make up water valves decided to open. Then they were scared, and no one wanted to do it. Told them to turn off the building supply water. I arrived on site about 3 hours later. Both boilers out of water. Stuck a temp probe through the insulation and read 325 degrees of Boiler shell temp. Both low water cutoffs had stuck on one boiler, the other one had worked as it was supposed to.
Long story short, the maintenance man had not been blowing down either the boiler or the low water cutoffs for over a month. Isolated the down (HOT) Boiler, filled the second boiler, and got it running to get heat, (winter time).
The end result was some leaky flues, which were rerolled, and the ones which weren't able to make stop leaking were pugged. Some of the flues were sagging. The State Boiler inspector and the insurance inspector signed off on the temporary repairs, as there was only a month and a half that the second boiler might be needed. It cost the customer a bunch of dollars to get them up and running temporally, and a bunch more the following summer for a extensive internal inspection and a reflue. A loss of life could have easily occurred, and a lot of property damage could have occurred because of poor on site maintenance. Kewanee made some real good big fire tube boilers!

DanHolohan

@Woody_S, I hope they appreciated all that you did with this one. Where was it that you had to travel so far?

Woody_S

Minot, ND to Belcourt, ND, Dan.

DanHolohan

I’m imagining what you thought about along the way. What the road was like. The weather. What was happening at home. Bravery. Thanks for caring about those people. Thanks. 

egansen

I can add some comments on the locomotive style boilers although not directly related to the topic of discussion and brought up by someone else.  On the locomotives, steam tractors, and steam shovels that I have been around make up water is fed either by a feed water pump/heater or an injector and two means in any combination are required are required for each each boiler. 

On either feed system the water is always pumped in near the front of the boiler because it is the coolest part of the boiler and is furthest from the fire box.  By the time it circulates from the front to the back it is somewhat heated and tempered.

In terms of thermal stress what I have read is when a boiler is thermally shocked what will usually happen is the tubes will shrink in length and leak in the tube sheets.  They are the thinnest part of the entire boiler so they are more succeptable to damage by rapid temperature changes.

It is usually not a feed water issue that will knock tubes loose but a large amount of cold air admitted through the fire door that cause the tubes to shrink and come loose.   If you are lucky you can reroll them to seal them up otherwise its a tube replacement job.

Most locomotive boilers have one or more fusible plugs screwed into the crown sheet (the top of the firebox).  It is a bushing that is filled with an alloy metal with a low melting point.  As long as they stay covered with water the metal won't melt.  If the water level drops low enough, the alloy melts and the steam/water released puts out the fire.  Since these are 100% make up boilers the plugs need to be removed and any scale cleaned off on a regular basis so they will work as intended. 

retiredguy

@Woody S, said that Kewanee made some real good fire tube boilers, yes they did but their burners were crap.

A melted steel fire tube boiler or a cast iron boiler is a terrible and scary thing to see. Imagine coming to a school job in the morning and seeing one of the 2 H B Smith cast iron boiler's sections being melted almost all the way to the insulated sheet metal jacket and the burner is still firing or a steel fire tube boiler having most of the 2nd pass tubes and fire box stay bolts being melted, and yes, the burner is still firing.