To prevent leakage at the point where the shaft protrudes through the case, which components may be used to seal the space between the shaft and the case?

The correct answer highlights the use of packing rings or mechanical seals as effective components for sealing the space between the shaft and the case. This is crucial in various water distribution applications where preventing leakage is essential for maintaining system integrity and efficiency.

Packing rings are typically made from materials that can compress around the shaft, creating a tight seal that prevents fluid from escaping. They are often used in pumps and valves to handle various fluids, ensuring that the equipment operates without leaks. Mechanical seals function similarly, but they rely on a more complex design that includes a set of stationary and rotating elements to create a robust barrier against leakage. Their ability to withstand high pressures and temperatures makes them suitable for challenging environments in water distribution systems.

In contrast, other options do not apply to the sealing function at the shaft. Butterfly valves are control devices used to regulate flow, strainers are designed to filter out debris from the water, and flow control valves manage the flow rate within the system. These components serve different purposes and do not function as seals between a shaft and its housing, illustrating why packing rings and mechanical seals are the best answer for preventing leakage in this context.