Finite Element Analysis of an Ammonia Tank

Objective
To build a FE model of an ammonia tank and walkway structure and to investigate the structure's response to pressure, gravity, thermal and wind loads.

Summary
MSC.Software Australia were commissioned to investigate the structural behaviour of a 12,000 tonne liquid ammonia tank and to assess the response of the tank and the Ammonia Vapour Return Line (ammonia pipe) to in-service loads. The tank and associated structural members were therefore modelled and analysed using the Finite Element Method (FEM) of stress analysis.

The focus of the analysis was to assess:

• the response of the structure to pressure, gravity, thermal and wind loads with the ammonia pipe made from (a) stainless steel and (b) carbon steel;
• the effect that adding supports from the (stainless steel) ammonia pipe to the walkway had on the nozzle/ammonia pipe stresses.

The FE model included the inner and outer tanks, the ammonia pipe/nozzle outlet flange, the ammonia pipe and the outer tank's walkway structure. Comparative analyses were carried out with the ammonia pipe made of carbon steel and stainless steel. The tank structure with a stainless steel ammonia pipe was also analysed with the ammonia pipe supported by two "ties" to the walkway structure. Wind loads were applied in accordance with AS 1170.2 using the Patran Command Language.

The results of the FE analyses showed that there was negligible difference between the stresses for the stainless steel and carbon steel ammonia pipe and that for the gravity, pressure and thermal loads, the stresses were relatively low. It was also shown that restraining the ammonia pipe greatly reduced the stresses produced by wind loads without increasing the stresses produced by pressure, thermal and gravity loads.