Final answer:
Backflow prevention devices are essential in plumbing to prevent contaminated water from reversing flow into the clean water supply. Examples include air gaps in faucets and various entrainment devices that use fluid dynamics to prevent contamination. Therefore, the correct answer is option 3. A drain is submerged in the drainpipe
Step-by-step explanation:
To keep the water supply safe, a backflow prevention device is required, particularly to prevent scenarios where contaminants might reverse flow and pollute the water system. An example of this need occurs in a situation where a drain is submerged in the drainpipe, which is often referred to as a cross-connection. This can cause a back siphonage or back pressure, thus the need for backflow prevention.
Similarly, plumbing usually includes air-filled tubes near water faucets. These are often referred to as air gaps and serve as a simple but effective backflow prevention method. The air gap works by ensuring there is a physical separation between the end of the water supply and the flood level of a fixture, preventing contaminated water from siphoning back into the potable water supply.
In Figure 14.31, different entrainment devices are described that use increased fluid speed to create low pressures, which then entrain one fluid into another. This principle is applied in various devices, from Bunsen burners to aspirators, to ensure efficient operation and also, in some cases, to prevent backflow.
Therefore, the correct answer is option 3. A drain is submerged in the drainpipe