Exam Question

The correct material listed for use as the dynamic seal on mechanical seals in centrifugal pumps for water service is **(c) Carbon**.
### Justification for Material Selection
A mechanical seal prevents leakage along a rotating shaft. It primarily consists of a stationary face and a rotating face, which are pressed together. The interface between these two components forms the 'dynamic seal'. The material selection for these faces is critical for the seal's performance, reliability, and lifespan. For most water service applications, a hard material (like ceramic or silicon carbide) is used for the stationary face, while a softer, self-lubricating material is used for the rotating face. Carbon (in the form of carbon-graphite) is the industry standard for the rotating seal face for the following surveyor-grade reasons:
1. **Self-Lubricating Properties / Low Coefficient of Friction**: