Theory question: Pressure/temperature limits

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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Theory question: Pressure/temperature limits

Post by Steam Captain » Thu Feb 11, 2021 12:23 pm

Hello,

Doing the never-ending researches, I've been wondering how much pressure and temperature a reciprocating steam engine could actually work with. I remember watching the Jay Leno's Garage videos about his White, Doble and Stanley steamers and I was actually surprised how much pressure/temperature the engines work with. Numbers like 1000psi and 700°F were named, which was so much I would have even come up with back then. I mean 1000psi is one big house number. I believe I remember the hydroplanes using flash steam plants also run in these pressure magnitudes.

This brings me to the question after the limitations for an engine. The first thing coming up in my mind is: The lubricant needs to be able to handle the heat. And from a metalurgical pov, the creep issue applies as does to turbines (Although I guess to a much lesser degree due to the much smaller centrifugal forces).

Maybe I should ask for what was the reciprocating engine with the highest steam pressure ever built, but I am also curious about the theoretical explanations behind it all.
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Re: Theory question: Pressure/temperature limits

Post by TriangleTom » Thu Feb 11, 2021 4:10 pm

I'll scan and post the page later, but in one of the Audel's books there's a (purely theoretical) design for a quintuple expansion engine that takes 1500 PSI from the boiler.

Ron is pretty active in the steam car community where they push pressures a LOT higher than we do, so he may be able to give better/more contemporary answers.
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Re: Theory question: Pressure/temperature limits

Post by fredrosse » Thu Feb 11, 2021 5:16 pm

The Spilling reciprocating steam engines as currently manufactured are rated for a maximum steam inlet pressure of 900 PSIG (60 BARg). These engines operate with no lubrication on the steam piston, nor the steam piston valves, which are both made of tungsten carbide.

I assume the 900 PSIG is a convenient designed limit, as it corresponds very nearly to Diesel engine practice. Designing for higher inlet pressures is not a technical limit, however there is little incentive on the thermodynamics to go with higher pressure. As far as temperature goes, this limit is occurring on the boiler end, as the boiler metal must endure temperature higher than the maximum steam temperature.

The highest steam conditions I have ever encountered in steam turbine plants is 5000 PSIG with 1200F steam temperature. I understand these limits are being slightly exceeded in some new Japanese steam-electric plants. Reciprocating IC engines endure working gas temperatures well above this limit, and therefore inlet temperatures for a properly designed reciprocating steam engine could be very high, but then again there is little incentive on practical considerations to go with higher temperature, due to the boiler metal issues mentioned above.
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Re: Theory question: Pressure/temperature limits

Post by barts » Thu Feb 11, 2021 6:44 pm

High temperature boilers have some interesting issues; a British text "Marine Steam Boilers" on my shelf has pictures of a water tube boiler ('of Continental design' :roll: ) that suffered a superheater fire that spread to the rest of the tubes; apparently a superheater tube overheated badly ( 1200F )in port and started grabbing the oxygen out of the water. The reaction continued until all the tubes were slagged. I'd guess this was an older incident before the use of really high temperature boiler tubes.

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Re: Theory question: Pressure/temperature limits

Post by DetroiTug » Thu Feb 11, 2021 7:59 pm

Tom,

Through observation of many different vehicles, 500 - 600 psi seems to be the upper range sweetspot for efficiency and practical operation of a small piston plant. Above that the main issue is lubrication, the hoped for lubrication from saturated steam is way past, and oil begins to vaporize and crack to carbon in the cylinder and valve chest. There are some space age lubricants they use in the Doble's as mentioned run in the 1200 psi range, the last few did anyways. The Doble of which only about 43 were built and no two alike, is really an experimental prototype and none should really be used any sort of standard for normal operation, Yes, they ran at 1200 psi, however with the boiler up front and the engines way out back, there was undoubtedly considerable thermal loss which resulted in lower pressures and temps at the valve chest. The burner in a few of the last models was around 1.2 million Btu, thats about 10 gallons per hour continuous burn, and they weigh around 3 tons. BUT, they will run highway speeds although at great cost. I don't know if any of them were actually used as a normal car. They were mostly a novelty for the very rich, Howard Hughes had one or two? of them just to play with.

My Loco runs at 250, far fewer headaches and the engine is holding up just fine, she'll still pull the car hot on about 30 psi.

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Re: Theory question: Pressure/temperature limits

Post by barts » Thu Feb 11, 2021 8:34 pm

I really prefer a steam plant in a boat that is quiet and doesn't require exotic materials. In the end, steamboating largely is a hobby activity, much like sailing. I've ridden in a boat with a steam outboard running at 800F and 800 psi - pretty amazing engineering, but not really the sort of boat one uses to travel 30 miles or from which to fish. High superheat and high pressures are not compatible with relaxation, at least in my opinion.

Give me a quiet seaworthy boat, a wood fire and secluded anchorages, please.
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Re: Theory question: Pressure/temperature limits

Post by Steam Captain » Fri Feb 12, 2021 5:37 am

Bart, your poetic words put pictures of link gears and glistening spring waters in my mind.

I have literally never heard of a superheater ending up being a metal fire. That's some chemical reaction. It reminds me of a german experimental locomotive. As is tradition in Germany :D , it was over-engineered. The water wall of the fire box consisted of tubes. So far so good. BUT those tubes were cooled not by water, but by STEAM. The boiler was working with 1700psi, which is outlandishly high for a steam locomotive.

DetroiTug, yeah, I guess the lubrication already sets a temperaturum non post quem (if my half-latin is readable). I've just been pointed at the Carter steam engine running at 2200psi with a bash valve nonetheless. It's a single-acting design and I guess it thus doesn't feed oil into the steam and I guess the oil in the crankcase is not going to suffer the full brunt of the temperature as it is separated from the displacement space by the piston.

Fredrosse, that's very smart of them to circumvent the limits set by lubrication with new pathways. I've read about Tungsten carbide, carbon rings and ceramic coatings and I wish it would be as cheap as grey cast iron for the hobbyist :D It does seem possible to push it very far with the right materials.

TriangleTom, I remember reading about quadruples. Some british and german steam liners used them, but the development was cut short by the development of the steam turbine. They for sure did extract more work from the steam, but the tone set by analysts seem to see the advantage being very little compared to the added complexity and building costs. It's a very interesting research area.
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Re: Theory question: Pressure/temperature limits

Post by fredrosse » Fri Feb 12, 2021 2:00 pm

From Barts: "apparently a superheater tube overheated badly ( 1200F )in port and started grabbing the oxygen out of the water."

Having trouble understanding this statement, as water is a very stable pure substance, and will never dissociate into its base components (H2O to Hydrogen and Oxygen) due to overheating, as far as I know. Water can be split into its base elements, Hydrogen and Oxygen, with electrolysis, but I would think that process was not in action during the event described above. Are there other processes that could be "grabbing the oxygen out of the water"? Having worked in the steam utility industry for over 45 years I have never heard if anything like this, although I have seen a few superheater tubes burst from overheating, typically from restricted/interrupted steam flow allowing metal temperatures to soar.

Perhaps the issue was an uncontrolled fire of superheater tube external deposits catching fire, similar to common chimney fires burning deposited creosote (or other flammable substances) in a chimney, downstream of a wood stove? I have experienced that condition on more than one occasion, and the stovepipe gets red hot very quickly.
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Re: Theory question: Pressure/temperature limits

Post by DetroiTug » Fri Feb 12, 2021 5:16 pm

"I've just been pointed at the Carter steam engine running at 2200psi with a bash valve nonetheless.''

When I see or hear ''bash valve'' in any sort of testimonial, everything beyond that is discounted as any sort of information I'll ever need.

Yes, they can and do work, but nothing in an engine should be ''bashing'', and with no way to control radial timing or duration, they are not much of an engine. I know a guy working on a high pressure bash valve, he couldn't get his 2000 psi dream pressure sustainable from his monotube boiler so he connected direct to an oxygen cylinder, it was old Economy Hit and Miss engine converted to bash valve, it spun over one time, hit the valve and disintegrated the piston and connecting rod, luckily the cylinder didn't explode like a grenade. Some in the experimental steam crowd throw pressure levels around like there is not much difference. They want that 2000 psi for very high efficiency, in those ranges, very little we are doing with low pressure hobby level steam applies. It's a different world in regards to boiler, operation, piping and engine design.

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Re: Theory question: Pressure/temperature limits

Post by DetroiTug » Fri Feb 12, 2021 5:25 pm

"Perhaps the issue was an uncontrolled fire of superheater tube external deposits catching fire, "

That is an unusual occurrence, my guess is they fired the boiler up empty, melted it down and the BT blamed it on the superheater. :lol:

Don't ask me how I know this can happen. (Forget and leave a blow down open bleeding off some water at fire up)

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