Machinist’s Mate (MM) "A" School Test 3 Practice 2026 - Free Machinist’s Mate Practice Questions and Study Guide

A stop-check on the desuperheater branch acts as a one-way valve, preventing backflow from the desuperheater into the steam main. It lets steam and any injected water flow toward the desuperheater when the desuperheater needs it, but seals off if conditions try to push flow back into the steam main. This keeps the steam main protected from reverse leakage and maintains proper flow direction.

The other possibilities aren’t the valve’s job: pressure buildup is managed by other protections, steam leakage to the environment is addressed by proper sealing and venting, and condensate drainage is handled by dedicated drainage paths rather than by preventing backflow into the main.