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.
Original Pipe Schedule Calculation
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).
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
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
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.
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.