The pipeline described in question 7.1 was used to carry wastewater to a hold-up tank.
The effluent is not hazardous. A decision has to be made on what material to use to replace
the pipe. Three suggestions have been made:
1. Replace with the same schedule carbon steel pipe and accept renewal at 3-year intervals.
2. Replace with a thicker pipe, schedule 80, outside diameter 114.3 mm (4.5 in), inside
diameter 97.2 mm (3.826 in).
3. Use stainless steel pipe, which will not corrode. The estimated cost of the pipes, per unit length is:

 schedule 40 carbon steel $5, schedule 80 carbon steel $8.3, stainless steel (304) schedule 40 $24.8. Installation and fittings for all the materials adds $16.5 per unit length. The downtime required to replace the pipe does not result in a loss of production. If the expected future life of the plant is 7 years, recommend which pipe to use.

Data from question 7.1

A pipeline constructed of carbon steel failed after 3 years operation. On examination it was found that the wall thickness had been reduced by corrosion to about half the original value. The pipeline was constructed of nominal 100 mm (4 in) schedule 40, pipe, inside diameter 102.3 mm (4.026 in), outside diameter 114.3 mm (4.5 in). Estimate the rate of corrosion in ipy and mm per year.