Q. We are operating a centrifugal pump, and a recent change in the system has resulted in a deficiency of about 5 to 10 percent in the rate of flow. Can anything be done to the pump to increase the flow to meet our needs?
A. Yes, but two important questions must be answered.
1. Is the system head curve primarily static head, as found in boiler feed applications, or is it primarily due to friction head, as found in systems that circulate liquid? See Figure 1.3.6.1.1a. If the latter is true, the steeply rising system head curve will diminish any increase in the pump rate of flow.
2. Any increase in rate of flow will require a comparable increase in the power input. Does the pump driver have the ability to supply the increased power required by the pump without overloading?
[[{"type":"media","view_mode":"media_large","fid":"236","attributes":{"alt":"Pump performance curve versus system curve","class":"media-image","id":"1","style":"float: left;","typeof":"foaf:Image"}}]]There is no standard method for increasing rate of flow from a pump, but pump designers know what methods can be used successfully. Assuming that the system curve is relatively flat, the rate of flow from the pump can be increased by enlarging the open area between the vanes at the location where the flow exits the space between the vanes. This area is perpendicular to the flow and is usually increased by filing or grinding metal from the underside (concave side) of the impeller. This filing must extend back to the area between the vanes and blend smoothly with the interior of the waterway between vanes. This must only be done with the advice and under the direction of the pump manufacturer.
To allow the increase in rate of flow to exit the casing, the casing throat area must be enlarged comparably by removing metal from the volute tongue on the side away from the impeller. This too must be done with the advice and under the direction of the pump manufacturer. Please refer to ANSI/HI 1.3 Rotodynamic (Centrifugal) Pumps for Design and Application for more information on this subject.
Q. For slurry applications, we have observed that centrifugal pumps that develop higher heads are limited in their operation. Can reciprocating pumps be used in these applications? If so, are any special design features necessary to handle slurries?
A. Reciprocating pumps are often used for slurry service with in-plant process and pipeline applications. Basic construction may be different from that for clear liquid applications. The differences may be in valves, addition of surge chambers and fluid injection into the inner portion of the stuffingbox or material for wearing parts. ANSI/HI 6.1-6.5 Reciprocating Power Pumps for Nomenclature, Definitions, Application and Operation contains additional information on this subject.
Hydraulic passages should be sized so that the lowest velocity of the fluid will be above the critical carrying velocity of 4- to 6-ft/s. The highest velocity should be below that which causes excessive erosion.
Lubrication and flushing of the packing are extremely important. Metered, clear, external injection-timed to the position of the plunger during its stroke-or continuous flow injection is required. How dilution affects the process must be considered.
Valves for use in slurry service are designed for velocities between 6- and 12-ft/s to reduce erosion and abrasion of the valve seat and other valve components. Valve construction usually has replaceable valve inserts made of an elastomer or polymer. Metal to metal ball valves may also be used.
To facilitate the starting and stopping of a reciprocating slurry pump, it should be fitted with adequate connections so the liquid end passages can be flushed with clear liquid to remove the slurry.
Rod and plunger packing requires special consideration when pumping abrasive materials. In piston pumps, the piston moves in a renewable metal cylinder or liner. The liners are made of abrasion and corrosion resistant metals. Piston rods and plungers are coated to resist wear.
[[{"type":"media","view_mode":"media_large","fid":"238","attributes":{"alt":"Circular wet pit with sloping walls","class":"media-image","id":"1","style":"float: left;","typeof":"foaf:Image"}}]]Q. Pumps operating in wet pit applications often suffer from the accumulation of grit or other solid material near the pump inlet. This requires draining of the pit or removal of the pump to remove the solids. Is there a better way?
A. It is often practical to build the pit with sloping sides to minimize the horizontal floor area. See Figure 9.8.15 for an example.
The sloped walls should be a minimum of 60-deg from the horizontal for concrete or 45-deg for steel, plastic and other smooth surfaces. This design allows the solids to collect at the pump inlet where they can be swept away with the flow. The pump should be periodically operated at its maximum rate of flow to more effectively carry away the solids. If two or more pumps are in the pit, the one farthest from the inlet to the pit may be run alone to remove the solids. Verify the NPSHA to ensure sufficient margin to operate at the high rates of flow. ANSI/HI 9.8 Pump Intake Design includes considerable detail on this design.
Pumps & Systems, April 2008