2.6.3 Boiler explosions
Stress corrosion cracks can also build up in other structures. These were a particular problem in locomotive boilers in the early days of the railways in Britain. All such boilers were made from wrought-iron sheet, riveted together to form a cylinder. In the earliest engines, the boilers were constructed using a single line of rivets, thus forming two corners, one inside and the other outside (Figure 25). Initially they apparently performed well, but a number of catastrophic explosions were experienced through the 1840s.
Some of the earliest explosions were caused by failures of the safety valves fitted to the boilers. For example, some of the first safety valves were simply a stopcock weighed down by a steelyard. The weight and its distance along the arm controlled the pressure at which steam would activate the cock, and so blow off harmlessly. It was tempting for engine drivers to increase boiler pressure by adjusting to the highest possible pressure – and if that didn't give enough driving power to the wheels, they would wedge the valve down further. Such manipulation of a safety device was asking for trouble. Another problem that also caused some explosions was that if the water level dropped too far, the structure would overheat and fail.
Many such boiler explosions were investigated by the Railway Inspectors appointed by the Board of Trade (one of the predecessors of the Department of Trade and Industry). They found that they could not always explain why the explosions had occurred, having excluded both human negligence in the use of the safety valves and the water level. When the inspectors examined the failed remains in some detail, they found a pattern revealing that failure almost always occurred from the horizontal line of rivets in the boiler, and there appeared to be a deep groove running alongside the joint that was filled with rust.
Explain why failure tended to occur along the longitudinal axis of the boiler.
Why would a riveted joint represent a line of weakness? How could such a joint be strengthened?
There are two stress components in the wall of a cylindrical boiler, the hoop stress and the longitudinal stress. The hoop stress is twice the longitudinal stress and is given by the equation:
where p is the internal pressure, r the radius of the cylinder and t the wall thickness.The hoop stress acts such that the cylinder will fail by a lengthways crack (rather than a radial crack).
A riveted joint will always be weaker than continuous material, simply because it is a break in the uniformity of the wall. The rivet holes themselves are stress concentrators, and a line of rivets is a line of such defects, so the line of rivets is the weakest part of the structure.
The problem could be tackled by replacing the lap joint with a butt joint (Figure 26), reinforced by extra layers of riveted metal.
The root cause of the problem lay in corrosive attack of the wall to one side of the joint, owing to the nature of the joint itself. The wall on either side would, when the pressure was being raised first thing in the morning, experience higher stress than the double wall thickness at the joint itself, owing to the existence of a corner where the plates met acting as a stress raiser. Corrosion of the iron by the boiler water tended to start here in the form of a slowly growing crack, a process repeated every morning the locomotive was worked. The cycling of hoop stress caused whenever steam was raised and the boiler pressurised could also have given rise to fatigue cracking, although the phenomenon was not recognised as such at the time.
The purity of the boiler water would be important in such a case, because if any dissolved salts were present they would increase its electrical conductivity and hence the likelihood of corrosion cells being set up near the joint.
Such ‘groove cracking’ was a design fault, which could be corrected only by developing a double-riveted butt joint (as shown in the answer to SAQ 2) in place of the lap joint, rigorous inspection and maintenance of locomotives and, ultimately, use of stronger steel in place of the wrought iron.