HDPE tanks used in residential systems for processing waste water
We performed finite element analysis (FEA) on a HDPE clarification tank. A manufacturer of water and wastewater treatment products and systems, as well as high density polyethylene (HDPE) tanks that are used in residential systems for processing waste water contacted us to determine that the HDPE tanks satisfied design requirements of the National Sanitation Foundation (NSF).
Requirements from the NSF included:
(1) Certification that the tank is structurally adequate to withstand the stresses of installation, hydraulic, and earth loads.
(2) Certification that the tank can be pumped out without floating or shifting in the ground or collapsing under the earth load.
A few assumptions that were made for modelling included:
1) All models and analyses are linear-elastic using large deflection theory. There are some areas in the model that yield and as such an elastic-plastic analysis was performed to determine the total strain at these local strain concentrations
2) Soil is the only significant loading present on the top of the tank. The soil load will be uniform and independent in each of the three directions. There are no “live” loads acting on the tank.
3) Piping and fittings were not explicitly included in the finite element model.
4) Creep was not considered.
The overall Clarification tank is approximately 30 inches long, 70 inches wide and 70 inches high, and has a minimum wall thickness of 3/8 inches. There were two sources of loading: soil and water. Each is a pressure that acts normal to the surface and is linearly gradated, increasing in magnitude as the depth increases. The water loading is hydrostatic and acts on all surfaces. The soil loading, due to its internal friction acts only on the top and sides of the tank. Soil loading acting from the top is taken at full value, whereas soil loading that acts on side surfaces is factored by a value.
Our team performs design and (thermal, stress, vibration and fatigue) analysis on equipment.
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