ASME B31.3 compliance question 1

[knowlittle]

I have a 1/4" OD nylon tube. My question is if I am using this tube in compliance to ASME B31.3.

Manufacturer's product info says 450 psi working pressure and 1350 burst pressure @ 70 deg F.
The tube carries 150 psi nitrogen gas. Pressure relief valve is set at 200 psi.

Nylon is not a listed material.

Is it true that ASME B31.3 has safety factor of 4 for listed plastics? Thanks.

 

[rconner]

Am curious what is the range of ACTUAL service temperatures the pressurized gas tubing will see in the application?

 

[knowlittle]

Actual temperature is actually slightly lower than 70 deg F, in the sixties.

 

[bimr]

The B31.3 Code allowable stresses of are based upon a factor of safety of 3.

http://www.pipingstudy.com/difference-asme-b311-and-b313.html

The pressure/temperature design of the component should provide the same safety margins as the Code. When evaluating these components the manufacturers should be contacted to determine what documentation is available for the components. Suppliers or distributors of the component may not understand the Code and may not be helpful in resolution of the issues associated with qualifications of the unlisted component. Manufacturers are the best source of information.

Plants designed to B31.3 generally have a life of about 20 to 30 years. Plants designed to B31.1, on the other hand, may be expected to have a plant life of about 40 years. The difference between these two codes is the factor of safety in the lower to moderate design temperature range. B31.3 uses a 3 to 1 factor of safety, where B31.1 has a 4 to 1 factor. This factor can reflect differences in plant cost. For example, the same design conditions for a B31.1 piping system may require schedule 80 pipe wall
thickness, while a B31.3 system on the other hand, may require only schedule 40 pipe wall thickness.

 

[knowlittle]

Thanks for the reply. Is the safety factor of 3 same for both metallic and non-metallic materials? Someone told me B31.3 requires higher safety factor for plastics. Thanks.

 

[bimr]

Not really, there is no simple answer.

The code does not use the words "factor of safety". For example, it states this requirement as an allowable stress of 1/3 of the tensile strength.

Safe design rules and guidelines are set forth in the ASME B31.3 Piping Code. In this code the requirements for plastics piping, including those for PE, are placed in a separate Chapter V1, titled “Nonmetallic Piping and Piping Lined with Nonmetals”.

The Table on page 219 does not list the hydrostatic design stress for nylon. Calculations for materials not listed shall be substantiated by extensive, successful service experiences at comparable design conditions as stated in A304.7.2.

 

[rconner]

You may be interested in relative impact test results for some comparative materials, and locations of ductile to brittle transitions etc. at

http://www.doitpoms.ac.uk/tlplib/BD6/printall.php.