Heating SS with propane

[WalkingWolf]

Lots of folks run on propane and I don't recall this being discussed before. Not sure how significant an issue but thought I'd put it up anyway.

Caution must be taken when heating stainless steel equipment. I know of one home brewer whose cut-off keg boiler began cracking at the bottom. The cracks appeared at the flame line where the flame of the cajun cooker-style propane heater met the keg. This shows that he was running the flame too hot and that, over time, chromium atom diffusion was taking place.

Diffusion is cumulative.
Once this type of cracking occurs, there is no economical way to correct it.

. . . . for type 304 stainless (nominal carbon content of 0.08%), 5 min at 600 °C (1110 °F) or higher will cause chromium diffusion that will later cause cracking in service. Type 304L stainless - "L" denoting less carbon (nominal 0.03%) - is more weldable and can spend about 6 hours at 600 °C before becoming sensitized. Most kegs (in North America) are made from 304L to facilitate welded construction.


The temps do seem a bit extreme but when you crank up one of the high-output burners the heat goes up in a hurry.

[rtalbigr]

I wonder if this is cumlative?

Big R

[Husker]

With a boiler that has liquid in it, I would think the only part that could reach a high temp, might be the skirting material. If heat were a problem, then all SS pots and pans would have this type problem. I have pots passed down from my mother, they are over 50 years old. The manufacturer is called 'lifetime cookwear' and I am pretty sure it will last a lifetime. It has been on nat gas many times, and on elect for the last couple decades.

H.

[Prairiepiss]

This is kinda like the rings around the bottom exploding. I'd the drain holes get plugged. Nice info to share. Thanks I will keep it in mind the day I can run propane.

[WalkingWolf]

This would never be a problem with household use as these temps stay far below the threshold temps mentioned. The 1110 F temps mentioned can be reached rather quickly with a high-pressure regulator on an outdoor propane burner. I wouldn't think there would much significance or we would have had problems reported before now. I think it should serve as a caution for those on gas that "let-it-rip" on heat up that this may have accumulated affects.

[Torp]

In a boiler filled with liquid I would think that no matter how high the flames go the SS will stay the temp of the liquid inside it. If the liquid can get up to 600 degrees I think the fumes that we distill would be moving at a faster rate than we could catch them.

Edit: Re read this today and I am now convinced I should never post drunk as it makes me sound like an A**. Sorry guys

[Ayay]

Stainless is a slow heat conductor. The skirt of the keg won't be cooled by the contents of the boiler quick enough and I'm sure it can get red hot on a propane furnace.

A holy skirt is no problem, it can be cut off

[WalkingWolf]

In a boiler filled with liquid I would think that no matter how high the flames go the SS will stay the temp of the liquid inside it. If the liquid can get up to 600 degrees I think the fumes that we distill would be moving at a faster rate than we could catch them.

While all topics posted here are subject to debate this info was posted here as an FYI. I came across this information while researching physical properties of stainless steel. The original work is an objective observation not a subjective opinion.

I have a friend that has agreed to teach me to braze SS as he does this everday and his work is tested @ 3000 PSI upon final assembly. I figured if I have this opportunity I need to better understand the metal I will be working with.

[Husker]

I have often thought of brazing to SS. SS is a bitch to weld, unless you really know what you are doing. Brazing is pretty easy. However, it is not welded, but dis-similar metals. I always had concerns about metal problems with the heat, etc.

I could see brazing on a fill fitting on the top of the still. Also, brazing a screwin for a heater fitting or a drain fitting low down on the keg. As long as the metal is very close, or right in contact with the liquid inside (or for a fill, it is out of the heat zone), then things are very likely to be fine.

However, doing something like brazing some legs on the the still, especially if brazed to the skirting, is likely a not very smart thing to do.

H.

[WalkingWolf]

Thanks for that feedback H. I've heard that caution about brazing before. I look forward to my time with this guy as he builds high-pressure atomizers that are used in the oil-field. Under normal working conditions there will be minimal heat but will be under constant pressure. He has built 1000's of these pieces and they have very good service life. There is just the one internal component that is brazed and is subject to the full stresses of operation. In particular, I'll be taking notes on exactly what it is he is using in his processes.

[Husker]

When working with dis-similar metals (304L and brazing material), then go to someone knowledgeable at the welding shop, and get the right brazing stock. Try to get something that sticks well to SS, and has similar expansion characteristics at the temp we work with. If you are running this on the outside of the boiler, but the seam is right against the boiler, then expect the temp to be in the 200-300 C range at max. We have a heat sync inside the boiler, that can only get to 100C. SS is a somewhat shitty thermal xfer material, but if that braze joint is close, it should not be 'too' high above that 100C range, even if it is pretty close to the flame.

However, warnings about issues in the skirting (especially since it IS welded on), are certainly on topic for this thread. How most of us use kegs, this skirting is a non-critical part. As long as it can support a couple hundred pounds of liquid weight, it should be fine. The weld that is critical should be tight against the boiler.

Topics like this ARE very welcome. It gets people talking, and thinking. Who would have thought the gas expansion in the skirting would be dangerous (to almost deadly), under certain extra-ordinary circumstances. The same may go for this issue in general. It may well be that there are a certain style of keg that we find should not be used with super high external heat. Who knows. However, getting the dialog going allows any of these extra-ordinary situations to be brought out in the open, so people can make informed, safe(er) choices.

H.

[spumco]

It's been forever since I've posted anything, but the metallurgy discussion caught my eye. Assuming everyone is interested in debate and discussion, I figured I'd add my two cents in what will be an unreasonably long post.

Seems there are two distinct issues raised in the thread. First is stainless cracking, second is SS brazing.

1. Cracking - 304 and 304L will crack if they're overheated. However, the root cause is not time and temperature at which a keg is typically heated. As the mechanical integrity specialist & boiler inspector at my plant, I regularly specify that stainless steels and nickel alloys be solution heat treated at temperatures well above what was quoted by the OP. These heat cycles do not cause cracking by themselves.

Rather, high temps can cause sensitization if the cooling rate is too slow. In this case, when the metal is really hot the molecules are fairly mobile, and instead of freezing in place, the chromium molecules combine with the carbon atoms and form chromium carbide (hard, brittle) at the grain boundaries. This is one form of sensitization. 304L(ow carbon) is less susceptible as there is less carbon availabe to form carbides, but it is just as likely to suffer from environmental cracking as regular 304. The 304L is not 'more weldable,' but it is less likely to become extremely hard and brittle - prone to weld cooling cracks - than plain 304.

Induced or residual stresses (from welding, forming, heating) will cause/expedite cracking along the grain boundaries as the chromium carbides separate from the iron, carbon and, in the case of 316SS, nickel molecules/atoms. The crystals that make up the metallic structure simply come apart at the seams. Imagine the difference between duct tape stuck to a clean surface vs. stuck to a dirty/oily one.

This is know as Stress Corrosion Cracking, and is a serious problem in the nuclear and petrochem industries.

This is the reason for 321 & 347 SS, which has a bit of titanium & niobium in the mix to help prevent carbide formation.

Sensitization is cumulative, but mostly reversible. The fix is to heat it up to 1080C and quench it,otherwise known as solution heat treating. The temperature unzips the chromium and carbon carbides, and the quench freezes the molecules in place before the carbides can re-form. Unfortunately, the quench usually warps or otherwise wrecks some components.

Another, more important factor in 300-series SS cracking is the chemical environment in which it is operating. Any chlorine or chloride molecules will also cause cracking if the stresses and temperature are 'right.' The chlorides interfere with the formation of chromium oxides - that's what makes it 'stainless' and allow inter-granular oxidation.

Chloride Stress Corrosion Cracking (CSCC) is the reason you shouldn't put bleach or clorox in a stainless pot. It'll crack - and if any welds are present they'll crack even faster. Similar chemical-metal cracking combination exists elsewhere: carbon steel & caustic, ammonia & brass/copper, for example.

CSCC is NOT reversible. If it starts, the part is junk. Welding up the cracks results in a component that has all the reliability of a candy hammer.

My remote (thus, possibly way off) analysis of the keg in the first post is that the keg was:
a)overheated by running dry, and/or
b) cleaned with a bleach solution (or one containing bleach or some other chloride molecule - i.e. salt water). The areas heated by the flame were sensitized by localized heating. The cleaning fluid cause micro-cracking of the sensitized flame impingement areas. Then, when a temperature change happened the residual stresses from forming caused it to make an unhappy PING! sound.

And no, this is not like exploding rings on a keg. Those are just boring old steam explosions. The way to solve that one is to open a bunch more vents on the ring so any liquid trapped can vaporize gracefully rather than building up pressure.

2. Brazing. Assuming we're discussing high temperature (>850F) brazing and not soldering, then the comment about brazing being really strong is quite correct. 3000psi hydraulic fittings are regularly brazed.

My concern is that the brazed fittings be kept away from heat. My advice is to follow the US Navy's requirements: no brazed pipe joints will be used in services higher than 450F. The only way I would braze something - a bung or legs - to the flame end of my keg/pot/bucket/pressure vessel is to monitor the operating temperature with an infrared gun or thermocouple. Does the location I'm going to braze get anywhere near 450F? If it's full of mash, of course not - but what happens if you run it dry?

Just weld it. Welding stainless isn't difficult. The only significant difference is that the puddle is a little sluggish compared to mild steel. And for the price of Grade V silver braze filler, you can pay someone to GTAW weld it.

Sorry for the stupid long post. I've learned so much about the hobby from everyone here, and the only area of expertise I can bring to the party is metallurgy, welding, and industrial failure analysis. You guys keep teaching me how to make good stuff, and I'll try to help you not break your gear.

Regards,
Spumco

[Prairiepiss]

Now that was a well informed thought out response. Very informative. Thank you spumco.

[Braz]

Excellent post, Spumco. Thanks.

[Bushman]

+2 Spumco, very informitive sounds like it is time you start contributing again!

[doctee]

An empirical data point. Since 1993, my propane fired S.S. beer keg boiler has made 83 batches (10 gal ea.) of beer. No cracking problems here.

[Husker]

An empirical data point. Since 1993, my propane fired S.S. beer keg boiler has made 83 batches (10 gal ea.) of beer. No cracking problems here.

I have done propane also, many batches. No problems here.

However, I do not run one of those 150k BTU monster type burners (banjo ??). I run about 65k BTU, and normally do not run it full out. So, I can not speak from experience of running balls to the wall, SUPER hot flame. However, in all of my runnings, I have seen no issue at all with the way I use propane.

H.

[short bus]

Unfortunately, I think my have an idea what this post is about. I have run my keg still twice, using propane. Originally, I planned on installing a MK-5500. After reading this could get a keg up to temp in about 30 mins I did not see a big deal in cranking up the huge burner(big, banjo type?) way up. While heating up I looked down inside the bottom keg ring and it was RED HOT! I cranked the heat way down and have not run it that high since and have added a 1/8" steel difuser plate that leave about 1/2 gap around the ring. I do not let the flame get bigger than the plate. Now it takes 2 hours to heat up. Anyway it seems like I have a slow leak that seems to close with heating. Just a heads up ! I might have a new keg in my future.

SB

[moonshine guy]

Yep, anytime you turn any metal red hot, you are asking for trouble! When the metal starts turning straw brown, it is time to back the burner back as this is drawing the metal temper back and starting to weaking the metal.

[Stoner]

I
Chloride Stress Corrosion Cracking (CSCC) is the reason you shouldn't put bleach or clorox in a stainless pot. It'll crack - and if any welds are present they'll crack even faster. Similar chemical-metal cracking combination exists elsewhere: carbon steel & caustic, ammonia & brass/copper, for example.

CSCC is NOT reversible. If it starts, the part is junk. Welding up the cracks results in a component that has all the reliability of a candy hammer.

This is what I deal with on my unit. We've got some huge (think 18 wheeler trailer) stainless steel heat exchangers. Whenever we bring the unit down and clear it, we've got to put these guys under an inert blanket (nitrogen) to keep them from cracking. Well, that or wash them with a special chemical solution. We call it polythionic acid stress corrosion cracking. It's a consequence of the process, not the pressure or heat. So the only thing I can think of is it was a crap pot, or got into something it shouldn't have (chlorides?)