In American Lifelines Alliance July 2002 Report of "seismic Design & retrofit of piping systems", clause 3.4.1 & Fig 3.4.1-2 it is shown that unanchored flat bottom tank slides & rocks leading to loss of contents of tank due to rupture of short pipe connections at the base.

From this what can we infer - not only piping system, but adequate equipment anchorage is also necessary to ensure seismic adequacy of the whole system or can we remain unperturbed and depend on friction to resist sliding/rocking of unanchored tank ?


regards,

sam

Yes, adequate equipment anchorage is always important. But, the problem of how to anchor a flat-bottom storage tank that may have a capacity of 1.5 million gallons at 6-8 pounds per gallon against a lateral seismic load of 0.3g or more awaits a final solution.

Once that problem is solved, the next problem becomes how to design the joint at the intersection of the side walls of the tank with the anchored tank bottom to withstand the shear loads imposed. Presumably, that is achievable, but your employer / client may not be thrilled with the cost of the joint you design. This may have a negative impact on your immediate and future compensation.

It's not a perfect world. Stuff happens. Our job is to analyze problems as best we can, prevent the preventable, and try to let those responsible for disaster planning know what fell through the cracks.

Here, adequate diked areas can minimize the problems caused by a tank nozzle failure, provided that the dikes withstand the earthquake. Max flow valves on selected (or, better, all) tank connections can also help with this issue. Leaving the tanks unanchored helps to prevent tank failures, which are in general far more expensive to repair than piping failures.

As noted elsewhere on this forum, friction is temporarily and randomly suspended during seismic events due to the vertical excitation modes of the sliding supports. A similar effect is used in a positive manner with, for example, vibratory coal feeders. Friction is a most undependable phenomenon - it's unpredictable, goes away just when you need it most, operates in random directions, and in general fights everything you try to do. You should try to account for its effects in analysis where possible, but don't depend on it for anything or you will most likely be disappointed.

One of the important things we have to learn is how piping interacts with the rest of the plant. In some cases, the piping is more important than the equipment. Most of the time, though, it is far more important to design the piping to fail before the equipment does.

As noted in many threads on this forum, it is up to the judgment of the engineer to decide what the weak link in the system should be. You should consult with others in your organization on issues like this; each facility has a different perspective. If, for example, your tanks contain liquid chlorine or other toxic material, it makes sense to go to a lot of trouble to ensure continuing mechanical integrity of the tanks AND the piping during a seismic event. In the far more likely case of stored petroleum products, local environmental rules and other considerations are likely to favor failure of the piping over failure of other system components.

Dear Craig,

Thanks for your response. Thus,as you said, if dikes are designed for seismic event, tank nozzle failure can be tackled. Leaving the tanks unanchored helps.

regards,

sam