CHE3166 Chap.5 Pumps, Hydraulics and Compressors
Pumps, Hydraulics and Compressors
A centrifugal-pump operating point is the intersection of the pump curve and the installed system curve. Build the system requirement from static and frictional terms, then check the actual operating range, control method and suction margin. NPSH available belongs to the system, while NPSH required comes from vendor data at the relevant flow and speed.
A pump curve relates head, flow, efficiency and power for a defined speed, impeller and fluid basis. The system curve combines elevation or pressure difference with line, fitting and valve losses. Their intersection determines the actual steady duty; a catalogue flow on its own is not a selection. Changing a throttle valve, speed, line resistance or vessel pressure moves one of the curves and therefore the operating point.
Check the full range, not only normal demand. Minimum flow, maximum flow, driver power, control authority, parallel operation and start-up can expose recirculation, vibration, overload or unstable operation. For suction design, keep absolute pressure, static contribution, line loss and vapour pressure on separate signed lines.
Compare available NPSH from the system with required NPSH from vendor data at the same flow and speed, then apply the project margin. Repeat the calculation for low source level, warm liquid, fouling and maximum demand. Compressor work adds gas-property and temperature effects. Define suction composition and reference conditions, required pressure ratio, staging, cooling and operating range.
Dynamic compressors need surge protection and recycle logic; all compressors need review of liquid carryover, backflow, blocked discharge, shutdown inventory and relief. Distinguish values calculated by the process team from limits that require vendor confirmation.
What this chapter covers
- 01
Pump types and selection
- 02
Pump and system curves
- 03
Static head and frictional loss
- 04
Control by throttling and speed
- 05
NPSH available and required
- 06
Compressors and liquid-system safety
Worked example · free
Check an available NPSH margin
- +3NPSH available = 8.0 + 2.5 − 1.4 − 0.3 = 8.8 m.
- +2Displayed margin = 8.8 − 5.6 = 3.2 m.
- +3Verify datum, absolute pressure, temperature, operating flow and required design margin.
Key terms
- System curve
- The head required by an installed system as flow changes.
- Cavitation
- Vapour formation and collapse caused when local liquid pressure approaches vapour pressure.
Pumps, Hydraulics and Compressors FAQ
Why can a selected pump miss its quoted flow?
The catalogue curve must intersect the actual system curve. Different elevation, pressure, piping resistance, speed or impeller changes the operating point. The selected equipment and installed piping determine the result.
How does higher liquid temperature affect suction margin?
Higher temperature commonly raises vapour pressure, which reduces NPSH available when the other suction conditions remain unchanged. Check the worst credible temperature and source level together.
Where does a centrifugal pump's operating point come from?
Construct the system curve from the pressure, elevation, velocity and friction head required along the installed flow path, then place it against the pump curve. Their intersection is the operating point. Because valve resistance, speed, impeller size and system conditions can move either curve, verify the selected pump across the intended operating range rather than at one catalogue flow.
Assessment move
Sketch both curves before calculating. Label every head term and sign, locate the operating point, then check suction conditions and off-design operation rather than stopping at one duty point. Rebuild a system curve from static head and a flow-dependent loss term, then predict how throttling, speed change or a new pipe affects the intersection before calculating.
Read efficiency, power and NPSH required at the chosen duty and state the speed and impeller basis. Write suction pressure as absolute head, include elevation with a declared datum, subtract line loss and vapour-pressure head, and test the most demanding credible case. For parallel pumps, combine capacity at equal head and examine one-pump and two-pump operation.
For compressor questions, list suction composition, temperature, pressure and reference flow before considering pressure ratio, power or discharge temperature. Add surge, recycle, liquid separation and trip behaviour to the review. End every selection with operating-range and vendor-data checks, not just a numerical design point.
Build a numerical pump case from elevation, vessel pressures, pipe lengths and fitting losses, then calculate several points on the system curve. Overlay a plausible pump curve and locate the intersection graphically before interpolating a duty. Predict how throttling, speed reduction and a parallel pump move the point and change energy use. Check driver power, preferred operating region and minimum-flow requirements.
Recalculate available NPSH from absolute suction pressure, declared elevation datum, losses and vapour pressure at the relevant temperature. Test a low-level, warm-liquid, fouled-strainer case rather than quoting only normal margin. For compressors, convert all capacities to a stated suction reference and examine pressure ratio, discharge temperature, staging and cooling.
Sketch the operating map, surge boundary and recycle path, then follow shutdown inventory and possible backflow. Mark every number that requires vendor confirmation. Compare the rotating-equipment choice with its P&ID safeguards and layout access. Finish each practice by stating the physical change that moves the operating point and the equipment consequence that makes the change acceptable or unacceptable.
Maintain one data table with source pressure basis, elevation datum, liquid temperature, vendor curve revision, speed and impeller. Reusing that table across calculations prevents a correct equation from being fed incompatible operating cases. Keep the final recommendation tied to the stated basis, and record the observation that would require a revised engineering decision.
