Why no Porkies ?

[JonRiley56]

Howdy,

I was studying SB&MSL and happened across an article on porcupine boilers. (P 28 thru 28 MA '61) It would seem a realtively simple design to put together.

Why is it that they arent mentioned at all anymore ?

jon

[Lopez Mike]

My only personal experience of one was a small copper unit for a model locomotive, 1.5" to the foot. I never did get it leak free as it was designed to be brazed up all at once and after about the third tank of gas and after singeing myself several times, it was set aside for a calmer day.

I have been told that they will steam well if you include some divider plates in each tube so that the cold water will flow out beneath the plate and the hot water flow back to the drum on top of the plate.

Others on here have more experience with them I'm sure.

Mike

[fredrosse]

One of the keys to reasonable boiler life is maintaining strong circulation throughout the steam generating tubes. As Mike mentions, Porcupine Boilers need to have internal baffles to promote circulation, yet even with this feature the circulation is not as good as with other boilers that have a definite inlet and outlet for each boiler tube.

Another feature of Porcupine Boilers is the increasing flue gas flow area at the outside diameter of the Porcupine Boiler. This results in most of the flue gas traveling at the outer regions of the boiler casing, and the Porcupine tubes near the drum are starved from hot gas flow.

I think the historic reason for some popularity of Porcupine Boilers is the ease of construction using screwed pipe nipples, before arc welding or brazing was generally available. Today with modern welding and brazing equipment very available even to we amature builders, the better arrangement and less boiler tube connections of an Oldfeldt, Bolsolver, or similar boiler makes these types the clear choice over the Porcupine Boiler.

[JonRiley56]

Thanks guys, that makes sense.............

jon

[87gn@tahoe]

Speaking of circulation of water though the tubes, if one were to build a Porcupine or Lune Valley, one needs to make sure the tubes are always submerged, by making the boiler with steam space above the tubes. Having the tubes above the water level makes for a shorter tube life and potential tube failure.

A coupe years ago there was a boat at the B&W Steamboat meet that had a Lune Valley boiler with no additional steam space at the top (like a conventional Lune Valley). He ran pretty regularly with the fire door open and wore shorts. The boiler was about 15 years old and as far as I know he did annual hydro tests, though only ran the boat at a couple times a year. One of his tubes failed on the way back from the picnic, blowing fire and steam all over the inside of the boat and burning the owner's legs quite severely, as well as another passenger not so severely. He spent quite a while in the hospital due to complications.

Operating with the fire door closed should be a given. The main point here is that even "established" designs (i.e. Box boiler!!) are not ideal, and every aspect of the boiler should assessed before committing to a said design. Also, even with proper maintenance, boilers (especially boiler TUBES), do not last forever, and should be considered a consumable. One should not wait for a failure before considering re-tubing or replacing an aging boiler. The FRA requires locomotive boilers to be re-tubed after so many operating hours.

Learning from others mistakes both recent and less recent will help bring our hobby along to a safer, more responsible future.

Now, if there were a shop that built more affordable ASME code boilers that catered to the smaller steam hobbies...

[barts]

I also lost a tube in my Lune Valley copy boiler; since my firebox config is very different, the steam went up the stack and we got to push a rope back to the dock. The problem with that design is that it is sensitive to overheating if the water level gets below the intake of the top row of tubes. Afterwards, I reworked the gage glass to make sure all the tubes were always covered top and bottom; this produces more steam and has improved the boiler generally.

One design boiler design I'm quite fascinated with is the LaMont; this is a forced circulation boiler. This design provides even higher specific outputs than a monotube, with NONE of the control issues... but it does require a reliable boiler circulation pump,
and power (steam or electric when starting) to drive it. There are extensive discussions ongoing on the SACA (steam car) forums
http://steamautomobile.com/phorum5214/list.php?1, where I've been known to lurk.

- Bart

[fredrosse]

Good comments about operating safely. Even an ASME stamp on the boiler does not provide any guarantee of boiler life, if not operated and maintained properly. This includes renewal of tubes when necessary.

A few questions:

Wesley , can you post a picture or drawing or description of a “Box boiler “? I don’t know of this specific type, or maybe I do under another name, but would like to see what you mentioned in your post.

Bart, for the Lamont forced circulation boiler, there is a statement that it has higher specific outputs than a monotube boiler. I see no reason why, from a heat transfer standpoint, this should be so? In the utility industry the Lamont type boilers were abandoned decades ago, while the “once thru” types (basically a monotube type, except there are actually many “monotubes” in parallel because these boilers range up to about 5,000,000 PPH steam flow) are currently very common.

I understand the control issues with monotube boilers, but as far as transferring energy from the fire to the steam/water of the power plant, (be it a 150 PPH steamboat power plant or a 5,000,000 PPH utility plant) what could be the advantage giving higher specific output to the Lamont type boiler?

[87gn@tahoe]

Fred;

A little history on the "Box" boiler:

http://www.cityofart.net/bship/boiler_rect.html

I would think that the higher specific output of the Lamont would mean a lighter package that would be easy to steam and control. Have your cake and eat it too.

[barts]

Bart, for the Lamont forced circulation boiler, there is a statement that it has higher specific outputs than a monotube boiler. I see no reason why, from a heat transfer standpoint, this should be so? In the utility industry the Lamont type boilers were abandoned decades ago, while the “once thru” types (basically a monotube type, except there are actually many “monotubes” in parallel because these boilers range up to about 5,000,000 PPH steam flow) are currently very common.

I understand the control issues with monotube boilers, but as far as transferring energy from the fire to the steam/water of the power plant, (be it a 150 PPH steamboat power plant or a 5,000,000 PPH utility plant) what could be the advantage giving higher specific output to the Lamont type boiler?

The Lamont design uses recirculation rates of 5 - 10 x boiler steam production or more. This means the mixture leaving the generating tube is still quite wet (10-20% quality) and thus much less likely to lead to tube overheating problems. The heat transfer rates as steam dries go much lower so tube sections with dry steam (or superheated steam) must have a lower heat flux. Practical experiments in steam cars bear this out; Lamont designs achieve more BTU/sq. ft. than even the very fine Doble boilers. For very large power plants, control systems are not problematic and not having large very high pressure steam drums are big advantages; these plants often run at constant load as well, so monotubes are a better design.

For our steamboats, some advantages of a Lamont boiler:

• Very high BTU/sq. ft. heat transfer rates reduce size and weight of boilers. Some designs are seeing 40K btu/sq ft - imagine running a traditional steam plant with 1 or 1.5 sq. ft of heating surface per hp, instead of the 5 to 10 we usually suggest
• quick steaming; since circulation is forced, uneven heating is not an issue.
• low sensitivity to contaminated boiler water. Since the tubes don't dry out and circulation is forced, deposition of oil residues or scale is not seen - unlike monotubes.
• steam drum is not fired, and can be replaced with tubes if needed to deal with ASME code issues.

The real disadvantage of these boiler is the need for a reliable circulation pump that will deal with low NPSH conditions. One neat trick is to arrange the coil discharge and suction lines into the steam drum to promote rapid rotation in the drum using the high speed discharge of the water/steam from the heating coil; the pump suction pickup is then situated so as to take advantage of the velocity head of that rotation.

[fredrosse]

Yesterday I had a conversation with Henry Wong, our URS corporate boiler specialist, and the former Director of Engineering for Foster Wheeler Boilers. The conclusion of our conversation is as follows:

If comparing a monotube to a Lamont boiler, with wet steam circulated through the generating tubes, there is virtually no difference in the specific steam generating capacity of the two boiler types.

From a flow & heat transfer standpoint, the main difference between the Lamont and the monotube is that the Lamont forces a rather large flow of water through the tubes, and issues a steam-water mixture into the boiler separator which is, say 15% saturated steam, and 85% saturated water. The saturated water has a very very high heat transfer coefficient (on the order of 500x to 1000x the heat transfer coefficient of hot flue gas to the generating tube surface) so the tube metal temperature is held at virtually the saturated steam temperature.

The monotube (see previous thread on monotube boiler control on this forum, “Perfect control of monotube boilers in our lifetimes!”) forces a smaller flow through the tubes, and issues a steam-water mixture into the boiler separator which is, say 65% saturated steam, and 35% saturated water. Even though the amount of saturated water is a smaller fraction of the total flow in the monotube design, there is still plenty of liquid water such that the system enjoys the very very high heat transfer coefficient, so the tube metal temperature are again held at virtually at the saturated steam temperature.

Since both boiler types hold the tube metal temperatures close to the saturated steam temperature, both types will have about the same specific heat flux capability.

All of this is with regard to the steam generating section of the boiler, regardless of whether there is an economizer section or a superheater section. Of course, if improper operation that leads to loss of water circulation in the tubes (be it 85% moisture fraction in the Lamont or 35% moisture fraction in the monotube), then tube overheating will occur in either type of boiler.