Which statement best describes the function of the stop valve and stop-check valve when isolating main steam?

• A. They provide two-valve protection
• B. They regulate temperature
• C. They vent steam to atmosphere
• D. They drain condensate

Answer: (A)

Isolating main steam relies on two-valve protection: using a stop valve in series with a stop-check valve creates two independent barriers to prevent steam release during maintenance. The stop valve provides a tight, positive shutoff, while the stop-check valve adds backflow protection by stopping steam from flowing back toward the boiler if pressure on the isolated side changes. With both valves closed, even if one valve isn’t perfectly sealed, the other still blocks the steam path, keeping the system safe. This setup isn’t about regulating temperature, venting to atmosphere, or draining condensate; it’s about ensuring a secure isolation to protect personnel and equipment.