Open impellers

Semi-open (afso called Semi-enclosed), and 

Most totally open impellers are found on axial How pumps. 

This type of impeller would be used in a somewhat conventional appearing pump to perform a chopping, grinding, or macerating action 

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Open Impeller Semi-open Impeller Closed Impeller... 

FIG. 10-30 Performance curves for a range of open impeller pumps. 

Impeller-to-case or head clearances are excessive (open impeller design). 

Excessive wear ring (closed impeller) or cover-case clearance (open impeller). 

Open impeller has slight rub on casing. Most often occurs in operations from 250 to 400 F due to piping strain and differential growth in the pump. 

Replace wear rings or adjust axial clearance of open impeller. In severe cases, cover or case must be replaced. 

David and Colvin [Am. Inst. Chem. Eng. J., 7, 72 (1961)]. Continuous heat transfer between kerosine and water unbaffled vessel. Open impellers (paddles and propellers) are better than closed (centrifugal and disk impellers) at the same tip speed. 

With the pump di.sa.ssemblcd in the shop, with open impellers, the damage is seen on the leading edge of the impeller blades toward the eye of the impeller, and on the blade tips toward the impeller s OD. With enclosed impellers, the damage reveals itself on the wear bands between the impeller and the volute casing. See the illustration (Figure 3-2). 

To correct this condition with an opened impeller, it s nece,s,sary to perform an impeller adjustment to correct the strict tolerance between the blades and the volute. Some back pullout pumps arc designed with jack bolts on the power end of the bearing housing to easily perform this adjustment without pump disassembly. 

Single Stage. Multi - Stage. Open Impeller. Closed Impeller. 

A semi-open impeller has exposed blades, but with a support plate or shroud on one side. Some people prefer the name semi-enelosed. These types of impeller are generally used for liquids with a small percentage of solid particles like sediment from the bottom of a tank or river, or crystals mixed with the liquid (Figure 6-17). 

The scratch marks on the circumference of these close tolerance rotary parts will correspond to. scratch marks on close tolerance stationary parts at approximately 60 around the volute from the cutwater. These marks will be vi.sible on the back plate with open impellers, or on the wear rings of pumps with enclosed impellers, or the ID bore of the restriction bushing at the bottom of the seal chamber where the shaft pa.sses through, or the ID of the seal chamber bore at the back end of the mechanical seal (Figure 9-6 and Figure 9-7, next page). 

The internal volute wall and/or hack plate on pumps with open and semi open impellers. 

Centrifugal compressor impellers, with the exception of the open impeller, are thrust unbalanced. The machine also has a requirement for a location device to maintain axial clearances. For these reasons, all centrifugal compressors use some form of thrust bearing. 

While the axial position of multistage impellers to their diffusers is not critical, they should line up reasonably well. Impellers are not extremely sensitive to leading-edge dings and minor damage, but anything, such as erosion on the exit tips, that tends to decrease the effective diameter of the impeller is more serious. Front shroud clearance on open impellers should be maintained close to the design values to minimize capacity loss. 

Milk-of-lime transfer pumps should be of the open impeller centrifugal type. Pumps having an iron body and impeller with bronze trim are suitable for this purpose. Rubber-lined pumps with rubber-covered impellers are also frequently used. Makeup tanks are usually provided ahead of centrifugal pumps to ensure a flooded suction at all times. Plating out of lime is minimized by the use of soft water in the makeup tank and slurry recirculation. Turbine pumps and eductors should be avoided in transferring milk of lime because of scaling problems. 

Figure 3-15. Impeller types. Open impeller for corrosive or abrasive slurries and solids. (Courtesy Goulds Pumps, Inc.)...

Figure 11-60. Typical closed and open impeller designs for use in a mechanical expansion turbine. (Used by permission Bui.

Figure 12-46G. Open impeller with back-leaning blades, Type SR. (Used by permission Bui. Centrifugal Compressors Single Stage. A C Compressor Corporation.)...

Impellers can be further classified with regard to their physical design a closed impeller has shrouds or sidewalls enclosing the fluid flow, an open impeller has no shrouds or sidewalls, and a semiopen impeller is a mix of the closed and open design. 

Reducing the pressure acting on the fluid behind the impeller can be accomplished by two different methods, or a combination of both, on an open-impeller unit. One method is where small pumping vanes are cast on the backside of the impeller. The other method is for balance holes to be drilled through the impeller to the suction eye. 

The impeller (Figure 8.20) consists of a series of curved vanes so shaped that the flow within the pump is as smooth as possible. The greater the number of vanes on the impeller, the greater is the control over the direction of motion of the liquid and hence the smaller are the losses due to turbulence and circulation between the vanes. In the open impeller, the vanes are fixed to a central hub, whereas in the closed type the vanes are held between... 

Open impellers. Notice that tne impellers at left and right are strengthened by a partial shroud, (Worthington Pump, Inc.)... 

KOpen Impeller Semi-open Impeller Closedimpeller... 

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