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.
Figure 2: Schematic of bellows assemblyA 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)
|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.|