FRP Duct Thickness

A couple of points:

1. That is a really low vacuum - and the minimum thickness will be dependant on other factors such as resistance to buckling during handling, storage, installation and for any realistic strength for spans (see below)
   
2. It depends on the design and fabrication of the pipe, eg. winding angle for filament wound, resin, .........
   

You will probably still find a lot of US manufacturers referencing fabrication to the old NBS PS15-69; and within that standard the minimum thickness of any ducting is 1/8" for laminates with an minimum ultimate tensile strength of 9,000 psi; and a minimum flexural modulus of elasticity of 700,000 psi.

And consequently:

• 2" - 0.125" thk: allowable vacuum = 405 inches of water
• 6" - 0.125" thk: allowable vacuum = 64 inches of water

You may also find reference to ASTM D2924 for empirical results of external pressure.

Or you may refer to SMACNA 1546