- Email: [email protected]
Guides to estimating a pumps performance, if its hydraulic sizes are given
APPENDIX B
Guides to estimating a pumps performance, if its hydraulic sizes are given Pump hydraulic design is a combination of art and science. Quick and simple rules that claim to accurately establish pump performances should be viewed with great suspicion. However, very rough rules giving broad estimates are sometime useful. Such a rough guide is detailed below, and is based on easily obtained hydraulic measurements. The following data is needed see Fig. 3.21 - Impeller outer diameter - Impeller outlet passage width - Shaft rpm
STEP 1 From the impeller diameter, use Fig. B.1 to estimate the likely head b.e.p if the pump was to run at 2950 rpm.
Fig. B.1 Head generated per stage as a function of impeller diameter, running at 1450 rpm. Chart applies to machines up to Ns 2000, which covers most commonly found pumps. Above this value the chart will provide an underestimate.
365
366
Appendix B: Guides to estimating a pumps performance, if its hydraulic sizes are given
STEP 2 Correct the estimated head to the actual running speed. H ¼ Head from Fig. B.1 ‘h ¼ Head at actual running speed N ¼ 2950 RPM ‘n ¼ actual rpm ‘h ¼ H [n/2950]2
STEP 3 From the outlet width ratio chart, Fig. B.2 estimate the pump specific speed
STEP 4 By re-arranging the specific speed Eq. (3.1) Flow @bep ¼ Ns ½h0.75 =rpm0.5
STEP 5 Use Fig. 3.36 for a rough estimate of efficiency. For a more approximate approach, consider the process outlined in Chapter 3; [performance estimate, unknown pump]
Fig. B.2 Specific speed related approximately to impeller width ratio. As outlined in Chapter 3, pumps having the same design point performance, and hence same specific speed will exhibit a range of width ratios. This reflects the curve shape e in particular the ratio of head at design flow to that at zero flow e the so-called Head Rise To Shut Off.
Guides to estimating a pumps performance, if its hydraulic sizes are given Pump hydraulic design is a combination of art and science. Quick and simple rules that claim to accurately establish pump performances should be viewed with great suspicion. However, very rough rules giving broad estimates are sometime useful. Such a rough guide is detailed below, and is based on easily obtained hydraulic measurements. The following data is needed see Fig. 3.21 - Impeller outer diameter - Impeller outlet passage width - Shaft rpm
STEP 1 From the impeller diameter, use Fig. B.1 to estimate the likely head b.e.p if the pump was to run at 2950 rpm.
Fig. B.1 Head generated per stage as a function of impeller diameter, running at 1450 rpm. Chart applies to machines up to Ns 2000, which covers most commonly found pumps. Above this value the chart will provide an underestimate.
365
366
Appendix B: Guides to estimating a pumps performance, if its hydraulic sizes are given
STEP 2 Correct the estimated head to the actual running speed. H ¼ Head from Fig. B.1 ‘h ¼ Head at actual running speed N ¼ 2950 RPM ‘n ¼ actual rpm ‘h ¼ H [n/2950]2
STEP 3 From the outlet width ratio chart, Fig. B.2 estimate the pump specific speed
STEP 4 By re-arranging the specific speed Eq. (3.1) Flow @bep ¼ Ns ½h0.75 =rpm0.5
STEP 5 Use Fig. 3.36 for a rough estimate of efficiency. For a more approximate approach, consider the process outlined in Chapter 3; [performance estimate, unknown pump]
Fig. B.2 Specific speed related approximately to impeller width ratio. As outlined in Chapter 3, pumps having the same design point performance, and hence same specific speed will exhibit a range of width ratios. This reflects the curve shape e in particular the ratio of head at design flow to that at zero flow e the so-called Head Rise To Shut Off.