In steam generators, the Benson (once-through) boiler operates at supercritical pressure and does not require steam–water separation within a drum. Identify how many drums are required in a Benson boiler construction.

Introduction:
The Benson boiler is a once-through, supercritical-pressure boiler widely discussed in power plant engineering. Unlike subcritical drum boilers, the Benson design avoids steam–water separation hardware. This question tests your understanding of why the Benson boiler requires no steam drum and how that affects operation and control.

Given Data / Assumptions:

• Benson boiler operates near or above the critical pressure of water.
• Once-through flow path from feedwater inlet to superheated steam outlet.
• No phase-separation hardware is needed along the flow path.
• Modern controls manage sliding pressure and variable load.

Concept / Approach:
At or above the critical pressure, the distinction between liquid water and steam disappears; there is no two-phase mixture with a free surface. Consequently, there is no need for a steam drum to separate steam from water. Even when operating slightly below critical pressure (sliding pressure at lower loads), a Benson-type once-through arrangement still does not rely on a fixed drum for separation; moisture control is achieved via throttling, separators, or bypass arrangements in ancillary systems, not a conventional steam drum within the boiler circuit.

Step-by-Step Solution:
1) Recognize that drum boilers use a steam drum to separate steam from water under subcritical conditions.2) In a Benson boiler, water passes through economizer → evaporator zone → superheater in a single pass.3) Operating near/above critical pressure removes the need for gravity separation; therefore no drum is required.4) Final answer: the Benson boiler construction requires no drum.

Verification / Alternative check:
Control philosophy and thermal-hydraulic analyses for once-through units consistently show absence of a steam drum; transient separators, if any, are outside the boiler pressure part classification.

Why Other Options Are Wrong:
One/two/three drums: these are characteristic of subcritical water-tube designs (e.g., Babcock–Wilcox, Lancashire shell type uses two large flue tubes but drum context differs).

Drum with external separator: not integral to the once-through principle and not required.

Common Pitfalls:
Confusing Benson with subcritical drum boilers; assuming every boiler must have a steam drum for level control.

Final Answer:
no drum