Auxiliary Systems, Pumps, Compressors, Piping and Valves
Auxiliary-system questions test whether you can match equipment to duty and recognize operating limits. Pumps, compressors, piping and valves are common sources of vibration, cavitation, water hammer and relief failures. Expect NPSH, affinity laws, surge, expansion and valve-selection reasoning.
Pumps and compressors
- Centrifugal pumps produce flow against system resistance; the operating point is where the pump curve meets the system curve. Throttling moves the operating point along the pump curve; speed change follows the affinity laws.
- Positive-displacement pumps must not be run against a closed discharge without a relief path. Centrifugal pumps can be deadheaded briefly but will overheat if run at shutoff for long.
- NPSH available must exceed NPSH required. Cavitation causes noise, vibration and erosion and is worsened by high suction temperature, low suction pressure or a restricted suction line.
- Centrifugal compressors are vulnerable to surge at low flow and high pressure ratio. Anti-surge valves recycle flow; never rely on a check valve alone.
Piping and valves
- Piping expansion must be accommodated by loops, offsets, expansion joints or anchors. Supports carry weight and control movement; spring hangers allow thermal movement.
- Water hammer results from sudden velocity change, often from fast valve closure or steam-line condensation. Slow closure, proper draining and air vents reduce it.
- Gate and globe valves suit isolation and throttling respectively; check valves prevent backflow; safety and relief valves protect pressure. A safety valve is for steam/gas service under ASME Section I; a relief valve is generally for liquid service.
Worked example
A centrifugal pump runs at 1,200 rpm and delivers 100 m3/h at 40 m head with 20 kW absorbed. At 1,500 rpm, flow = 100 x (1,500/1,200) = 125 m3/h; head = 40 x (1.25)^2 = 62.5 m; power = 20 x (1.25)^3 = 39.1 kW. The system curve may limit actual flow, so the calculated values are the pump-curve values at the new speed.
Traps that cost marks
Using nominal pipe size for velocity or loss calculations; ignoring NPSH margin; throttling a positive-displacement pump discharge; and treating a relief valve as a control valve. Also watch for valve installed backwards, missing check valves and unrestrained thermal expansion.
Key points: pump operating point is the intersection of pump and system curves; affinity laws change flow, head and power with speed; positive-displacement pumps need relief protection; compressor surge occurs at low flow; piping expansion and water hammer must be designed for.