Pumps double suction

Figure 3-4. Centrifugal Pump, double suction single-stage impeller. (Courtesy Allls-Chalmers Mfg. Co.)...

Single-suction pump Double-suction pump Rating... 

For double suction pumps, using the HI convention, is taken as the total pump flow, although some pubHcations use half-flow, ie, flow per impeller eye. For multistage pumps, the developed head must be taken per stage for the NS calculation. By definition (eq. 9), high head, low flow pumps have low specific speed low head, high flow pumps, such as turbine and propeller pumps, have high specific speed. 

According to HI convention, for double-suction pumps, Q is half of the total pump flow, ie, taken per impeller eye. The value of S is calculated at the best efficiency point (BEP) at maximum impeller diameter. 

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

Inlet Piping Configurations. An incorrect piping configuration can create undesirable flow distortions that, especiaUy for double-suction pumps, means turbulence. Air pockets trapped in the pipe can cause uneven flow, vibrations, and pump damage. 

The vapor pressure may be dependent on the amount of the dissolved, not the entrained, air in the Hquid. This issue is important to appHcations of cooling-water double-suction pumps (58,59). Because of the unknowns, a safety margin is always recommended for use to minimise the effects of cavitation. 

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. 

Q)nvcrt to a pump with a double suction impeller. Double suction impeller pumps are for low NPSH applications. 

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. 

Wliere N = the speed of the pump/motor in revolutions per minute Q = the square root of the flow in gallons per minute at the Best Efficiency Point BEP. For double suction pumps, use A BEP Flow. NPSHr = the net positive suction head required by the pump at the BFiP. 

Water above 250° F. Single or double suction. This is usually boiler feed service at high pressures requiring multistage pumps. Closed except for very small capacities... 

Figure 3-50. Typical centrifugal pump characteristic curve vrith auxiliary specific speed curve. Double-suction, single-stage, 6-in. pump, operating at 1760 rpm constant speed. (By permission, Karassik, I. and Carter, B., Centrifugal Pumps, McGraw-Hill Book Co., inc., 1360, p. 197.)...

Pump impellers are further classified as to the inlet flow arrangement such as single suction (which has a single inlet on one side) and double suction (which has a double inlet on each side of the impeller). 

Figure 3-65 gives the upper limits of specific speeds of single-stage, single and double-suction centrifugal pumps handling clear water at 85°F at sea level [22]. 

Table 3-26 gives the specific speeds for various centrifugal pump types. Table 3-27 gives the suction specific speed ratings for single-suction and double-suction centrifugal pumps. These tables are for pumps handling clear water. 

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Double-Suction Single-Stage Pumps These pumps are used for general water-supply and circulating service and for chemical service when liquids that are noncorrosive to iron or bronze are being handled. They are available for capacities from about 5.7 mVh (25 gal/min) up to as high as 1.136 x lO" mVh (50,000 gal/min) and heads up to 304 m (1000 ft). Such units are available in iron, bronze, and iron with bronze fittings. Other materials increase the cost when they are required, a standard chemical pump is usually more economical. 

For double suction pumps, 0 is one half the pump output. 

Recommendations also are made by the Hydraulic Institute of suction specific speeds for multistage boiler feed pumps, with S = 7900 for single suction and 5 = 6660 for double suction. Thus the required NPSH can be found by rearrangement of Eq. (7.14) as... 

Figure 7.9. Some types of centrifugal pumps, (a) Single-stage, single suction volute pump, (b) Flow path in a volute pump, (c) Double suction for minimizing axial thrust, (d) Horizontally split casing for ease of maintenance, (e) Diffuser pump vanes V are fixed, impellers P rotate, (f) A related type, Ihe turbine pump.

Figure 7.14. Range of applications of various kinds of pumps, (a) Range of applications of single and double suction pumps (Allis-Chalmers Co.), (b) Recommended kinds of pumps for various kinds of head and flow rate (Fairbanks, Morse, and Co.).

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