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Tuesday, February 17, 2009

Pump selection and quality considerations

The following conditions should (explicitly or implicitly) be known in view of correctly selecting a pump:

  1.  task of the pump in the system.
  2. The system pressure and temperature.
  3. Data for rated performance: QR, HR,tot. Often rated performance equals the guaranteed point Qg, Hg. The rated and/or guaranteed performance may beidentical to the BEP (but this is not necessarily so).
  4. The NPSHA of the plant at rated, guaranteed or BEP conditions and, as necessary,at other operation conditions.
  5. Performance data for other specific operation points (if necessary).
  6. The maximum and minimum flow rates in the domain of continuous operation.
  7. The maximum and minimum flow rates during short-term operation or in transient conditions, e.g. during a switch-over of parallel working pumps, at load rejection or other.
  8. . For pumps operating in parallel the maximum flow rate (run-out) is determined by the operation of a single pump. At run-out the available NPSHA must be sufficient to prevent excessive cavitation.
  9. When pumps are installed in series, their interaction has to be analyzed with regard to control and upset conditions such as one pump falling out of service.
  10. The type and the chemical composition of the medium to be pumped, in particular corrosive substances.
  11. The physical properties of the pumpage if it is any other than water or a common, clearly defined medium. In this case the vapor pressure must be correctly specified in order to ensure that the effects and risks of cavitation can be assessed.
  12. Viscosities appreciably above that of cold water need corrections for Q, H, P,η and NPSH .
  13. Possible inclusions of free gas or dissolved gases that might separate from the liquid in the suction pipe.  The available NPSHA must be selected so that the volume fraction of free gas at the impeller inlet is below typically 2 to 4% at low suction pressures.
  14. Possible inclusions of solids (abrasion).
  15. The type of driver (electric motor, turbine, combustion engine).
  16. Fixed or variable speed? Speed range, if applicable.
  17. Is a gear box necessary?
  18. What kind of control is intended?
  19. How much standby capacity is required (e.g. 2x100% or 3x50% pumps)?
  20. Operation mode: Continuous or short-term operation? Cyclic operation with frequent start-ups and shut-downs?
  21. Installation conditions: Horizontal or vertical arrangement?
  22. Approach flow or suction conditions: Open or closed circuit? Open pit?
  23. Fluid level variations in the suction and discharge reservoirs or pressure variations on the suction and discharge side of the pumping system.
  24. The system characteristic or at least its static part Hstat resulting from the geodetichead differences and/or the pressure differences between the suction tank and the discharge vessel.
  25. Are there any special requirements concerning the head-capacity characteristic (steepness, head rise, shut-off pressure)?
  26. The maximum admissible shut-off pressure with the allowed tolerance, if applicable.
  27. For correctly sizing the driver, the maximum power consumption must be determined; with a small specific speed it occurs at about the maximum flow rate, with a medium nq near the BEP, and with very high specific speeds at shut-off.
  28. Are there any special requirements regarding vibrations or noise? Have limits been specified for the sound level?.
  29. What tolerances are permitted for manufacturing and measurements? Which standard is to be applied for the acceptance test?
  30. The guarantee and acceptance conditions, including possible penalties on efficiency or power consumption
  31. The operation period per year and the energy costs (e.g. $/kWh) or an assessment of the capitalized energy costs (e.g. $/kW). Minimization of the energy costs per year according to the intended operation scenarios.
  32. Safety considerations, explosion protection, zero-leakage to environment, ecological aspects.


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