SOOT IN THE FLUES


| March/April 1995



Soot in the flues

By now, most of you know that Anna Mae Branyan, the author of this column, passed away last September. We want to see the Soot in the Flues continue, and for all of our readers to share information in this column, so please do write to us with you questions, your pictures or whatever you'd like to share with your fellow subscribers.

ROLAND S. BRODBECK, 6540 Clark Road, Ottawa Lake, Michigan 49267, writes: 'I wish to respond to the article of William Lamb and Robert Rhode in the January/February issue. There are a few facts that need to be clarified on butt and lap-joints.

'First, the butt-strap joint in a boiler is twice as strong as a lap-seam joint with the same number of rivets. Why? A lap joint has only a single shear area; this is the cross-sectional area of the rivet multiplied by its shearing, which gives you the force necessary to shear the rivet. The butt-strap has a double shear; it must shear the rivet on the outside strap plus that same rivet must shear in the inside strap. This makes a butt-strap joint twice as strong with its double shear area than the lap-joint. Other factors that affect joint strength are rivet size, spacing, number and installation. All these factors are equal in both joints. The only way to have a lap-joint proportionally the same strength as a butt-strap joint is if the butt-joint has half as many rivets as the lap-joint. I have yet to see a lap-seam boiler on a traction engine have more rivet area than a butt-strap boiler. Remember, two rows of rivets on a butt-joint is equal to four rows on a lap-joint.

'Second, in Audel's Engineers and Mechanics Guide #5, the main problem with the lap-joint is the barrel of the boiler is not a true cylinder in overlapping the sheet and riveting it together. This causes a crease in the boiler sheet. The crease makes it possible to get stress cracks and even possible failure of the sheet at that location, especially with poor quality metal. Most of these problems were because of substandard material and workmanship. That's why in 1915 the American Society of Materials Engineers issued its boiler code to try to standardize the industry with safe boiler rules.

'Third, size of the boiler is a main factor in the choice of a joint. The larger the diameter, the more force that is pushing on the sides and on the joints. Like a small and large hydraulic cylinder, both have the same oil pressure pushing on the cylinder, but the large one has more force because it has more surface area (Area x Pressure = Force). So the more area in a boiler, the more force it has on it. This is why you may see a lap-seam joint on the steam dome of a boiler, yet the barrel has a butt-strap joint. This is because the dome's area is less than a quarter of the barrel's area, so you only need 25 percent of the strength in the dome than in the barrel of the boiler. The same goes for the flue sheet. The flues take up a majority of the surface area so there is very little force on the sheet itself. Thus a lap-joint does fine.

'The butt-joint and lap-joint have their place in boilers, and if they were properly designed for the boiler area they were both adequate. Materially speaking though, the butt-joint is twice as strong.