A stainless steel expansion bellows was utilised in an air conditioning system at a shopping centre. It was used to compensate for linear movement of the pipes associated with inevitable thermal expansion. The bellows failed, allowing massive leakage from the system. The failed bellows is shown in Figure 1, with the burst part visible on the right hand side.
Figure 1: Failed expansion bellows
A failure investigation was required to determine the cause of failure of the bellows.
Visual inspection of the bellows was undertaken. Microstructural analysis was performed on the strip making up the convolution at the failure site. The bellows is classed as an unrestrained type of bellows. It consists of a 145 mm length of convoluted 0.5 mm 304 stainless steel welded to a carbon steel flange assembly. An inner pipe is welded to the flange on the one side and extends longitudinally into the bellows (Figure 2). This is a design feature meant to prevent turbulent effects in the system as a result of fluid flow in contact with the convolutions. The fracture was found to occur circumferentially on the crown of the second convolution from the flange assembly.
Figure 2: Schematic of bellows assembly
A schematic of the relevant section of the piping system layout is shown in Figure 3. The bellows is situated upstream of an overhead 900 elbow which extends to a fixed point upstream. The bellows inline pipe is unrestrained at both ends and thus any loading as a result of linear thermal expansion is considered to extend the bellows in a lateral fashion. The failure occurred on the elbow side of the bellows facing in the direction of the perpendicular pipe. The bellows and pipe is covered with lagging to insulate the system (Figure 4).
Figure 3: Schematic of piping arrangement
Figure 4: Photograph of bellows installation (restricted section)
A microstructural investigation was completed on a section through the bellows and the flange at the weld. No evidence of a heat affected zone could be found in the region of the bellows failure. Further, no evidence of inclusions was found in the material. The microstructure is characteristic of a cold worked austenitic stainless steel. Spectrographic analysis on the bellows material indicates that the material is within the specification for an AISI 304 stainless steel.
Scanning electron microscopy (SEM) conducted on the fracture surface (Figure 5) indicates that the crack probably propagated slowly, and the surface was progressively corroded as the crack propagated. This suggests that a crack propagated slowly over time, probably by corrosion assisted fatigue.
Figure 4 – Scanning electron microscope image of the fracture surface
The results of this investigation indicate that failure was not a consequence of fatigue or stress corrosion cracking. Failure due to corrosion was ruled out as no evidence of corrosion product within the bellows was found. The localised failure of the bellows and the morphology of the fracture surface indicate a point load at this surface.
In terms of the extraneous loading on the bellows, several possibilities were considered, the likelihood of which is discussed in Table 1.
Table 1: Likelihood of loading sources
|Potential Loading Source||Likely/Unlikely||Explanation|
|Vibration||Unlikely||The fluid flow speed was within the design parameters and insufficient to set up any vibrational mode on the bellows material.|
|Water hammer||Unlikely||Pump shut off is ramped down, preventing the formation of a pressure pulse sufficient to be a cause of failure.|
|Linear thermal expansion||Likely||The pipe installation was assembled at ambient conditions, resulting in a maximum linear thermal expansion over the temperature range of 7 – 8 mm in either direction. An 8 mm lateral movement will cause an impingement of the inner collar on the convolution, between which there is a clearance of 3 – 5 mm. In an unrestrained situation the bellows is designed for up to 35 mm of lateral movement. The installation of the inner pipe causes the bellows to become stiff. Since it is not welded to the flange nearest the T-piece, the bellows bends about the free end of the pipe. This causes the load associated with the normal expansion and contraction of the system to be concentrated on one convolution of the bellows.|
The root cause for failure is likely to be linear thermal expansion.