Boiler mountings — principal purpose of the steam stop valve What is the principal function of a boiler steam stop valve located at the outlet of the steam drum/shell?

Introduction / Context:
Boiler mountings are mandatory devices required for safe operation. Among them, the steam stop valve provides positive isolation of the boiler from the steam header and allows controlled admission when starting up. Correctly identifying its purpose distinguishes it from other mountings and accessories.

Given Data / Assumptions:

• Standard shell or water-tube boiler with a main steam outlet.
• Separate devices exist for blow-down, overheat protection (fusible plug), superheating, and condensers.

Concept / Approach:
The steam stop valve controls and isolates steam flow. During start-up, it is opened cautiously to warm the main; during shutdown or maintenance, it is closed to isolate the boiler. Other listed duties belong to different mountings or accessories and must not be confused with the stop valve’s role.

Step-by-Step Solution:
Map functions to devices: blow-off → blow-down valve; low-water fire protection → fusible plug; superheat → superheater; vacuum → condenser system.Stop valve’s function: control and isolation of steam flow.Therefore, option (a) is correct.

Verification / Alternative check:
Operating procedures specify cracking open the stop valve for line warming and closing it for isolation; both confirm its control/isolation role.

Why Other Options Are Wrong:

• (b), (c), (d), and the extra condenser option describe other equipment, not the stop valve.

Common Pitfalls:
Calling any valve on the boiler a “stop valve.” Only the main outlet isolation and control valve is the steam stop valve.

Final Answer:
control the flow of steam from the boiler to the main pipe and to shut it off completely when required