During what was considered routine switching of liquid product lines and tankage at a storage and transfer facility, an incident occurred that resulted in approximately 1000 ft of 20-inch nominal pipeline being displaced laterally and falling from its supports. The event also resulted in the failure of a gasket at a valve flange and loss of containment. This paper discusses hydraulic and structural analysis of this event.

Because the pipe movement was identified during what was believed to be normal pumping, the earlier alignment activities and pump startup were investigated for the potential to generate a hydraulic transient capable of generating sufficient force to cause the movement. A transient hydraulic model of the fluid in the piping as it was aligned at the time of the event was generated to assist in determining the cause and to help prevent a similar occurrence in the future. Several scenarios were modeled and are discussed in this paper: 1) pump shutdown from steady flow, 2) pump start up with the discharge valve open, 3) pump startup with the discharge valve closed, and 4) pump start up with a vapor cavity present near a closed valve at the high point in the line.

The hydraulic simulations showed that cavitation in the pump likely occurred when starting the transfer pump with the discharge valve open. The effects of cavitation in the suction line could not be fully modeled and cannot be ruled out as a potential source of a hydraulic shock. However, the highest forces generated in this scenario are more likely to occur on the suction side of the pump than the location of the pipe displacement.

A section of branch pipe leading to the closed valve at which the gasket failure occurred was elevated above the rest of the pipeline. Due to the high vapor pressure of the product, it is possible that a vapor pocket formed along this section of piping. The collapse of a vapor pocket and the resulting high velocity liquid impact on the closed valve at pump startup can result in high pressure transients. Modeling showed that this behavior could cause forces high enough to overcome the static friction of the pipeline and result in the type of incident observed. Due to the high forces generated and the damage at the closed valve, this scenario was considered to be a likely cause of the incident.

Pipe stress and finite element analysis were used to assess the displaced pipe and to assist with lifting it back onto the supports. The piping stress analysis indicated that the highest stress in the piping was significantly less than the yield stress of the material. Results from a detailed Finite Element Analysis with material nonlinearity showed that there is no global plastic strain in the piping due to the event. This left a potential for local plastic strain due to the impact of the pipe against any hard surfaces, and this was addressed with visual examination. In the end, the pipe was lifted back onto the supports, horizontal supports were added, and procedural changes were implemented to reduce the likelihood of future events.


Pipe Knocked From Supports by Hydraulic Transient Event

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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