In steam boiler practice, which fitting directly controls the flow of steam from the boiler shell to the main steam line and can shut off steam completely when required?

Introduction / Context:
Boilers are equipped with mountings to ensure safe operation and control. Among these, a device must regulate the delivery of steam to the plant and also isolate the boiler when needed. Recognizing each fitting’s function is a foundational skill in boiler operation and examinations.

Given Data / Assumptions:

• A standard fire-tube or water-tube boiler with a main steam outlet.
• Requirement: control of steam flow and complete shut-off capability.
• Normal codes and practices for boiler mountings apply.

Concept / Approach:
The main steam stop valve (often called the stop valve) sits on the steam outlet nozzle of the boiler. It throttles or isolates steam to the plant header. Other fittings have different purposes: blow off cocks discharge sludge/water from the mud drum, fusible plugs provide overheat safety, superheaters raise steam temperature but do not shut off flow, and feed-line check valves prevent backflow into the feed system.

Step-by-Step Solution:
Identify the function: regulate and shut off steam to the main line.Map functions to fittings: only the stop valve fulfills both control and isolation at the steam outlet.Select the correct option: “stop valve”.Confirm: others either protect or condition but do not perform outlet isolation.

Verification / Alternative check:
Piping and Instrumentation Diagrams (P&IDs) show a main stop valve at the boiler outlet, commonly followed by strainers, stop-check valves, and headers.

Why Other Options Are Wrong:
Blow off cock: drains sludge/water, not steam outlet control.

Fusible plug: melt-safety device only.

Superheater: heat transfer unit raising temperature; no isolation function.

Non-return on feed line: prevents reverse flow of steam/water into feed, unrelated to steam outlet control.

Common Pitfalls:
Confusing the stop valve with stop-check (downstream) or globe valves located elsewhere in the system.

Final Answer:
stop valve