Grappling with the problem of predicting pump reliability versus suction conditions led to the development of the concept of suction energy, ESuction. Starting in the early 1990s the pump industry refined the definition of ESuction to more accurately tie together suction conditions and pump reliability — by using a suction recirculation factor that includes peripheral velocity in the impeller eye at the vane edge maximum radius in its calculation:
ESuction = DEye × N × NSS × SGConditions
where DEye is the impeller eye diameter in inches and SGConditions is the specific gravity of the fluid at operations conditions. If you don't have the diameter of the eye available, approximate it by using 0.9 times the suction nozzle diameter for end-suction pumps and 0.75 times the suction nozzle diameter for double-suction pumps.
Typical values used to define high ESuction pumps are 160 × 106 for end-suction pumps and 120 × 106 for double-suction pumps. Very high ESuction pumps start at 50% higher than this (240 × 106 for end-suction pumps and 180 × 106 for double-suction pumps).
Figure 1 shows the NPSH margin ratio — NPSHA over NPSH required, NPSHR — for "reliable" operation across a wide flow range when suction recirculation is the problem. (Don't use this figure for situations where suction recirculation isn't the problem.)
Reducing ESuction by using a lower pump operating speed dramatically improves pump reliability. In Figure 1 the boundary for very high ESuction pumps has been defined so that pumps at that ESuction level have very poor reliability unless the NPSH margin ratio exceeds one. High ESuction pumps only need a NPSH margin ratio somewhat greater than one.
While not perfect, the ESuction concept, coupled with the general curve shown, reasonably quantifies both the speed reduction needed and the relative benefit achieved in solving existing pump suction recirculation problems. ESuction also provides a useful guideline to estimate pump cost versus feed hydraulic changes for reliable operation for new installations.
Andrew Sloley is a Chemical Processing contributing editor. You can e-mail him at: ASloley@putman.net