Time History Analysis Of Piping Systems

The design of piping systems requires consideration of both sustained stress and expansion stress range to demonstrate adequate margins against failure due to plastic collapse, incremental collapse and cyclic loading or fatigue.

Some piping systems are exposed to dynamic loads such as those due to bursting disc rupture or slug flow which require special consideration.

While the adoption of a pseudo-static approach, the application of a static load corrected using an appropriate dynamic load factor, can provide a solution it is often too simplistic and can be overly conservative. The results of such an analysis also tell us little about the way the piping system responds to loads with very short durations. This is why time history analysis is often considered a better approach and can be performed with commercial software such as CAESAR II.

Time history analysis provides a method of assessing displacements, stress and reactions developed in a piping system over time. In order to carry out such an analysis there is a need to be able to define the loading, be it the pressure wave moving through a vent system following bursting disk rupture or the forces associated with a fluid slug travelling through a piping system.

One such application was the design of a blowdown system for the food industry.

The system considered consists of a series of eight vessels connected via a manifold used to transport the vessel contents as each are blow down in turn to a waste hopper. The blowdown of the waste product from each vessel, which takes six seconds, introduces a slug of liquid into the manifold which travels through the blowdown system to the waste hopper on the roof of the building containing the vessels.

The piping had suffered from excessive vibration during operation prior to FCL’s intervention.

Detailed analysis of the piping was carried out using CAESAR II piping design software. Static analysis enabled FCL to position and size spring hangers to support the manifold. This was supplemented by the use of time history analysis to facilitate the placement of snubbers to keep the spring hangers within their limit of travel during blowdown and ensure that the piping system did not oscillate unduly. The slug flow was defined as a series of forces applied at elbows resulting from the change in momentum for the duration taken for the slug to pass and all selected restraint locations were verified during an FCL site visit.

Using time history analysis FCL were able to gain an insight into the performance of the system not possible using other methods. The analysis provided both the amplitude and range of reactions acting on restraints, particularly important when assessing stress levels in vessel connections.

In this case the inclusion of the vessels and their supports in the piping model allowed FCL to directly define reactions on the vessel support brackets.

The piping system described has now given trouble free operation for in excess of 10 years.