In which case would a tapered plug valve be recommended?

A tapered plug valve is specifically designed to allow for easy and efficient control of flow. Its unique shape helps it to reduce the friction encountered during operation, making it ideal for applications that involve high lubrication. The tapered design allows the plug to fit snugly within the valve body while still permitting smooth rotation, which can enhance lubrication in the sealing surfaces, thus minimizing wear and tear.

In situations requiring lubrication, a tapered plug valve can provide a better seal compared to other valve types, making it a preferred choice. This is particularly advantageous in applications where the medium being controlled could lead to excessive friction or wear, as the lubrication can help to maintain the valve's operational integrity over time.

While other options could also present valid scenarios for valve types, they would not leverage the specific characteristics and benefits of a tapered plug valve to the same extent as applications focusing on lubrication. For instance, precise flow control may be better suited to different types of valves such as globe or needle valves. Similarly, slurry service may require valves designed to handle abrasive materials, while primary pressure relief applications often utilize valve types that provide rapid and reliable total open- or closed-position performance.