Answer to Question 1
Some fuel oil systems are installed so that the oil tank is at a higher elevation than the furnace. This makes getting the fuel to the burner easier. Unfortunately, it also makes it possible for any leaks in between the tank and the pump to leak oil continuously, especially when the furnace is off. In a worst case scenario, an entire tank of oil can leak out. The purpose of the oil safety valve is to prevent line leaks from draining the tank and causing environmental damage.
Answer to Question 2
Double pipe, open vertical shell and tube, horizontal shell and tube, and shell and coil.