What exactly is a lap seam boiler? When did they fall out of favor with the builders, and why are they supposedly so hard to keep legal in today's operating evnironment? Thanks for the help.

wilkidm@wku.edu

> What exactly is a lap seam boiler? When did
> they fall out of favor with the builders,
> and why are they supposedly so hard to keep
> legal in today's operating evnironment?
> Thanks for the help.

A lap seam is where the plate is lapped over each other and then rivetted. Think of it as placing one of your hands over the other.

A rivetted lap seam has the lowest joint efficiency of any rivetted seam. The term "joint efficiency" is a way of expressing the strength of the joint as a percentage of the strength of the plate that is joined together. The efficiency of the joint has a direct bearing on the MAWP of the boiler.

For example, in its simplest terms, if a given barrel diameter, plate thickness, and plate strength gave a pressure strength of 500 psi and the joint efficiency of the longitudinal seam is 50%, then the MAWP of that part would be 250 psi. As you can see, a lot of material is wasted, from a strength standpoint, in the plate for the part.

If the joint efficiency is increased to say 90%, then the pressure strength of the plate would only have to be about 278 psi. With this higher efficiency joint, the amount of material needed for the barrel is reduced by about 45%.

The low joint efficiency is one of the reason why lap seams were no longer used as boiler pressures increased.

The other reason has to do with the mechanics of the seam. A rivetted seam has to be caulked in order to provide a pressure tight seal. The caulking is done by peening the edges of the plate. In a lap joint, due to the non colinear action of the forces acting upon the joint, the adjacent plate nearest a free edge of the lap joint tends to rotate away from the other plate. This rotation within the joint causes a tendency of the joint to leak.

With this tendency moisture can become trapped between the plates of the lap joint as the boiler cools. This can then cause the joint to corrode from the inside out.

Current FRA regulations require that a lap seam be inspected inside and out on an annual basis.

Hope this helps.

pkurilecz@yahoo.com