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Power Eng 4th Class 4A study materials
Power Eng 4th Class 4A study resources

Power Eng 4th Class 4A: Piping, Valves, and Fittings

Power Eng 4th Class 4A · Work through your exam, one topic at a time.

All guide sections

Piping, Valves, and Fittings

What this domain tests

This domain covers pipe sizing, materials, expansion, supports, and the valves and fittings that control flow. Expect questions on valve selection, pressure ratings, steam trapping, and code requirements for piping systems.

Core rules to have at your fingertips

  • Pipe sizing: flow velocity limits - steam typically 25–40 m/s in mains, water 1–2 m/s in suction, 2–3 m/s in discharge. Use Q = A·v to size.
  • Valve types: gate (on/off, low pressure drop), globe (throttling), ball (quick on/off), butterfly (large diameter, throttling), check (prevents backflow), safety/relief (overpressure protection).
  • Steam traps: thermodynamic, thermostatic, and mechanical (float and thermostatic). Each removes condensate while retaining steam.
  • Expansion: pipe expands with temperature; ΔL = L·α·ΔT. Expansion loops, bellows, and sliding supports accommodate movement.
  • Pressure ratings: fittings must match or exceed pipe rating; derating applies at elevated temperature.
  • Code: ASME B31.1 (Power Piping) governs power plant piping; ASME B31.3 for process piping.

Worked example

A steam main carries 5 kg/s at 1 000 kPa and 250 °C. Specific volume v ≈ 0.20 m³/s (from steam tables).

Volume flow Q = 5 × 0.20 = 1.0 m³/s. If velocity limit is 30 m/s, required area A = Q/v = 1.0/30 = 0.0333 m². Diameter D = √(4A/π) = √(4 × 0.0333/π) = 0.206 m, i.e. about 200 mm (use 200 mm nominal).

If the pipe runs 30 m and heats from 20 °C to 250 °C with α = 12 × 10⁻⁶ /K, ΔL = 30 × 12 × 10⁻⁶ × 230 = 0.083 m - an expansion loop or bellows is needed.

Traps that cost marks

  • Using gate valves for throttling - they erode and lose control.
  • Forgetting to derate valve pressure rating at high temperature.
  • Ignoring the difference between safety valve and relief valve terminology.
  • Sizing steam lines on liquid velocity limits.
  • Omitting drain points at low points and before valves.

Exam strategy

For valve selection, state the duty (on/off vs throttling), the fluid, and the pressure/temperature rating. For pipe sizing, show Q, A, and D with units.