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Schedule 40 and Scheduel 120

Schedule refers to the thickness of the pipe wall and therefore how much pressure it will hold .
For a given schedule , the OD increases with NPS while the wall thickness stays constant or increases .Using equations and rules in ASME B31.3 process piping , it can be shown that pressure rating decreases with increasing NPS and constant schedule

Schedule 40 is used because :
It works for most applications and is readily available
In small quantities , it is only slightly more expensive then tht thinner wall

Schedule 120
The wall is thick enough so that it will not distort when walked on
From half inch to 2 inch  it is rated no less than 140 psi at 73 degree fahein .
The highest pressure typically found at any facility is 65 psi 

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