=+Hillspring Chemicals Ltd is committed to reducing its environmental impact. As a public company, it also has a responsibility to make a profit for its shareholders. As part of its production process, it generates a highly toxic sludge. A decision has been made to build a facility to process this sludge to remove the toxic materials, which also happen to be very expensive ($550 per litre), so they can be reused in the production process. At a cost of $120, 1 litre of sludge can be processed to reclaim 0.4 litres of recovered materials; 5000 litres of sludge will be processed each year.

Engineers are in the process of building the reclamation plant and have to make numerous design choices. One such decision is the choice between two different shut-off valves that will operate on the pipeline carrying the toxic sludge to the reclamation equipment. A choice has to be made between two options—a standard service plug valve (SSPV) and an everlasting rotating disc valve (ERDV). The following data has been obtained for the two valves:

◗ SSPV—is made of 1 kg of high-quality steel.