Boiler fittings — purpose of the blow-off cock In steam-boiler practice, what is the primary function of the blow-off cock (blowdown valve) fitted at the lowest point of the shell or drum?

Difficulty: Easy

Control the flow of steam to the main and shut it off completely when required
Empty the boiler when required and discharge mud, scale, and sediments accumulated at the bottom
Extinguish the furnace fire automatically when water level falls to an unsafe limit
Raise the temperature of saturated steam without increasing its pressure
Increase natural draught by throttling flue gases at the uptake

Correct Answer: Empty the boiler when required and discharge mud, scale, and sediments accumulated at the bottom

Explanation:

Introduction / Context:
The blow-off cock (also called the blowdown valve) is a standard safety and maintenance fitting on boilers. Understanding its function is essential for reliable operation, water-side cleanliness, and shell longevity. This question tests whether you can distinguish blow-off duties from other boiler mountings such as stop valves, safety valves, superheaters, and low-water protection.

Given Data / Assumptions:

Conventional shell or water-tube boiler with a low-point blow-off connection.
Presence of suspended solids, sludge, and precipitated scale in boiler water.
Routine operational blowdown and occasional full draining for inspection.

Concept / Approach:
Blow-off cocks serve two related purposes: periodic removal of concentrated, sludge-laden water (surface or bottom blowdown depending on arrangement) and complete emptying of the boiler during shutdown, inspection, or maintenance. By evacuating sediment from the lowest points, blowdown limits under-deposit corrosion, foaming, priming, and tube overheating. The blow-off line discharges safely to a blowdown tank to cool and de-pressurize effluent before disposal.

Step-by-Step Solution:

Identify the component: a robust, quick-opening valve at the boiler bottom connection.State its operation: periodically open to discharge concentrated water and solids; fully open when draining the boiler.Link to outcomes: reduced scaling and corrosion; easier internal inspection; stable steam quality.

Verification / Alternative check:
Boiler house SOPs specify scheduled blowdown based on total dissolved solids and sludge levels. Inspection codes require a functional blow-off device to permit internal examination and hydrostatic testing preparation.

Why Other Options Are Wrong:

Steam stop valve duty: controls steam to the main, not sediment removal.
Low-water fire protection: managed by fusible plug or low-water cutouts, not by the blow-off cock.
Raising steam temperature: done by a superheater, not blowdown.
Draught control: via dampers and fans; blowdown does not influence flue-gas flow.

Common Pitfalls:
Confusing surface blowdown (removes scum/foam) with bottom blowdown (removes mud/sludge); performing excessive blowdown that wastes heat and treated water; discharging without cooling, which is unsafe and non-compliant.

Final Answer:

Empty the boiler when required and discharge mud, scale, and sediments accumulated at the bottom

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Boiler fittings — purpose of the blow-off cock In steam-boiler practice, what is the primary function of the blow-off cock (blowdown valve) fitted at the lowest point of the shell or drum?

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