Pumps/pumping double-suction design

Fig. 8. Single-stage between-bearings double-suction pumps (a) axially split design and (b) radially split design.

Typically, for single-suction pumps, suction-specific speed above 11,000 is considered excellent. Below 7000 is poor and 7000-9000 is of an average design. Similarly, for double-suction pumps, suction-specific speed above 14,000 is considered excellent, below 7000 is poor, and 9000-11,000 is average. 

Use a double. suction impeller. Convert an end suetion centrifugal pump into a split ease horizontal design. 

Howe er, most conventional pump impellers receive the fluid into the impeller eye, at the center or inside diameter of the impeller. There are single suction impellers, and dual or double suction impellers with two eyes, one on each side. Dual suction impellers are mostly specified for low NPSH applications because the eye area is doubled (it ean reeeive twice as much fluid at a lower velocity head). Dual suction impellers arc mostly found on split case pumps where the shaft passes completely through the impeller. But they can afso be found mounted onto the end of the shaft in some special pump designs. 

By the use of double-suction impellers almost perfect balancing is accomplished. The design of a single-stage double-suction impeller pump is simple, see Fig. 12. Here the suction branches out so as to embrace the casting, and the water enters from both ends and is discharged in the center. Multi-stage pumps with double-... 

Centrifugal pumps constitute the most common type of pumping machinery in ordinary plant practice. They come in a number of types other than the simple volute machine shown in Fig. 8.8a. A common type uses a double-suction impeller, which accepts liquid from both sides, as shown in Fig. 8.8h. Also, the impeller itself may be a simple open spider, or it may be enclosed or shrouded. Handbooks, texts on pumps, and especially the catalogs of pump manufacturers show many types, sizes, and designs of centrifugal pumps. 

Pump manufacturers have established guidelines to ensure each pump they supply is not exposed to conditions that result in cavitation. The design standard is called NPSHR or net positive suction head required. The NPSHR takes into account any potential head losses that might occur between the pump s suction nozzle and impeller thereby ensuring the liquid does not drop below its vapour pressure (bubble point). The NPSH is a measure of the proximity of a liquid to its vapour pressure, and must exceed the pump manufacturer s pump NPSHR. There are two process variables that can be adjusted, in case the available NPSH is less than the NPSHR raise the static head and lower friction losses. Conversely, the NPSHR can be reduced by using a larger, slower speed pump, a double suction impeller, a larger impeller inlet area, an oversized pump and a secondary impeller placed ahead of the primary impeller. 

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