The original design process of calculating a suitable pipe schedule
number first devised in 1939 has since evolved into selecting an
appropriate pipe schedule for an application. The original intent of the
Pipe Schedule System was that all pipe diameters within a pipe schedule
designation such as Schedule 40 would have the same burst and working
pressure ratings. Since then, other factors, such as materials strength
and temperature effects, have entered the picture such that designers
use validated physical data to select the right pipe schedule for an
application.
Instructions
Original Pipe Schedule Calculation
1
Define the original pipe schedule calculation. The accepted
formula is Schedule Number = 1,000 x (P/S) where P = internal pressure,
pounds-per-square-inch-gauge (psig) and S = allowable fiber stress
(ultimate tensile strength of the steel in psi).
2
Rearrange terms to solve for P, assuming schedule number and S are known. Therefore P = Schedule number x S/1,000
Calculate P based on Schedule 40 steel pipe, and an S value
of 65,300-psi for mild steel pipe. Therefore, P = 40 x 65,300/1,000 =
2,612-psi. This is reasonable, based on a current-day published value
of 2,849-psi for 1-inch Schedule 40 steel pipe.
Current-day (2010) Pipe Schedule Selection Process
4
Define the pipe application. In this example, 500-psig
superheated steam at 600 degrees Fahrenheit needs to flow through a
2-inch nominal diameter steel pipe from a boiler to a turbine. With
this information, you can determine the suitable schedule number for
this application
5
Calculate the maximum allowable pressure rating. According
to the American Society of Mechanical Engineers, the actual pressure
should be about 25 percent of the maximum allowable pressure rating.
Therefore, 500-psig/0.25 = 2,000-psig.
6
Consult the pressure temperature chart to select the
appropriate schedule for 2-inch steel pipe. The chart shows a maximum
allowable pressure rating at 600 degrees F of only 1,783-psi for 2-inch
Schedule 40 pipe, but a rating of 2,575-psig for Schedule 80 pipe.
Since the 2,000-psig pressure lies between the two, the Schedule 80 pipe
should be selected.
