I am working in a project where FRP line is extensively used & are also stress analysed.
The analysis method is based on the vendor specification & also all the properties are as per vendor data.
The vendor has given different allowable stresses for different size of pipes and also the allowable stress is different for pipe & elbow.

Can anybody help me understanding why the allowable stresses are size dependent & different for pipe & elbow.

Ashish

FRP is not a uniform, homogeneous, isotropic material like steel. The corrosion resistance is provided by the epoxy or polyester resin, while the structural strength is provided by strands or filaments of fiberglass. Wall thickness consists of a resin layer with little strength, and layers of fiberglass bonded with same or different resin to form the structural shell. The proportion of resin versus fiberglass in the wall thickness can be made to fixed ratios, or to fixed dimensions depending on the manufacturing process. Usually the corrosion barrier is a fixed dimension typically selected as 20 mils, 50 mils, 100 mils, etc. The wall's structural fiberglass thickness can be built up to strength requirements. OVERALL ALLOWABLE STRESS (ALSO WEIGHT, MODULUS) BECOMES A PROPERTY OF THE COMPOSITE OF RESIN + FIBERGLASS REINFORCEMENT.
Different manufacturing processes also cause the properties to vary. Pipe could be filament wound with different angles of fibers, or "hand lay up" of matt instead of filaments. Small fittings could be molded with mortar filler or short fiber resin compounds. Large fittings would only be made by "hand lay up" method or "chopped strand spray-up". There are some other manufacturing methods like centrifugally cast pipe, and filament wound fittings. These all have some variation due operator technique as well as size limitations of the tooling. The I.D. or O.D. control of dimensions also has consequences for design. There are also the joints to consider. Smaller diameters might be socket adhesive bonded, while the larger sizes of joints would be butt wrapped resin + fiberglass.
Vendor design and installation directions should be followed. Be cautious of mixing pipe or fittings of different suppliers. Conservative allowable stress must be used. The stress analysis is used more for design of the supports, rather than prediction of safety margin of stresses. MORE OFTEN THAN NOT, IT IS THE VACUUM COLLAPSE RESISTANCE THAT HAS DETERMINED WALL THICKNESS OF FRP PIPE SYSTEMS. Regards, Richard Yee

Addressing the second question first (because it's easier): The pipe is probably manufactured by filament winding, and the fittings are probably manufactured by hand lay-up. The different manufacturing processes yield materials of different strengths. To maintain consistent factors of safety, the allowable stresses must be different.

The explanation concerning size is more complicated, but here goes: The manufacturer probably has a minimum thickness for filament winding, regardless of the pressure rating of the pipe. Even though the structure of a 2" diam. pipe could be 0.04" thick, the manufacturer might not use anything less than 0.08". Now, filament wound pipe is best suited to resisting axial stress when that axial stress is as a result of pressure. In fact, the typical 55° fiber winding angle is used because it is the most efficient to resist pressure. This winding angle will result in the fibers being stressed in pure tension (theoretically at least), when the pipe is exposed to hoop stress that is twice that of the axial stress, i.e. pressure. The axial strength of that same pipe is lower when exposed to purely axial stress, e.g. bending due to pipe weight. To yield consistent factors of safety, the allowable stress must be a function of the two axial strengths. In the smaller pipes, which are thicker than necessary for pressure containment, the bending strength plays a more dominant role. Hence, the allowable stress will be lower in the smaller pipes.