RTP tank wall thickness for underground installations

[potvincm]

Hello everyone,

I'm designing an open top rtp tank for an underground application. The liquid to be stored is dirty water for a water treatment plant. There is nothing dangerous or caustic in the fluid. The calculation of all dimensions are clear to me EXCEPT the thickness of the wall.

I've done my homework, and studied the standards ASME RTP-1-1995, UL-S615-1977, ASTM D 4097-95a, and ASTM 582-95. I can't find anything in any of these standards that gives me a clear answer. In RTP-1-1995, section 3A-300, they talk about using external pressure to calculate the thickness, but I have no idea how to find the minimum hoop tensile modulus or axial tensile modulus of the resin. Another thing that bugs me is that they give the required moment of stiffening rings, but don't say when to use them.

For example, I've got an rtp tank, 20 feet deep, installed with the opening flange 2 feet above ground, and the rest in the ground. The internal pressure is atmospheric. I assume that the worst case scenario for this application is when the tank is empty. How do I calculate the wall thickness in this case? Is it going to change with the depth of the tank? Are there any rules of thumb? If not, how would I get the hoop tensile modulus and the axial tensile modulus of a Type 1 resin?

Thank you in advance for your consideration,

 

[patprimmer]

I'm not a structural or civil engineer, nor a geologist, but I am sure the external forces will tend increase with depth, but may also be influenced by point pressures.

Factors I imagine will be significant are:-

1) Height of water table (eg buoyancy of tank and crushing force).
2) Fluidity of soil (eg sand).
3) Activity of soil (eg clay).
4) Variations in soil (eg rocks floating in clay or stable bedrock and moving soil acting on different points).


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[stanier]

The maximum external pressure is going to come from the water table. If this at the surface then 20'(say 6m) will occur. The external pressure will thus be about 200 kPa.

BS 4994 is a good standard to use for GRP structures. this will give your the design information.

Stres & Strain by Roark or Theory of plates and Shells by Timishenko.

What you will be looking at is the buckling criteria rather than hoop or axial stress. Buckling will dominate in this environment.

ISO 14692 is also a very good document for this type of appication. it is for pipelines in GRp but the equations hold for tanks. Check out Roark for end effects that dont really apply to pipelines.