Selecting Centrifugal Pumps S1-201
Operating Parameters Head/Capacity Temperature Viscosity Density Vapor Pressure Solids Content Chemical Compatibility Multiple conditions
Remember The system controls the pump.
Head/Capacity THE SYSTEM As the rate of flow is increased through the system, friction losses increase. The losses increase discharge head and reduce suction head. This increases Total Dynamic head (TDH) as flow is increased. THE PUMP The pump is a machine that is capable of producing a maximum amount of head at shutoff. As more of the pump s energy is diverted to producing flow, less head is produced by the pump.
Head/Capacity 2 THE SYSTEM The positive relationship between flow and head is the basis for the upward slope of the SYSTEM CURVE. THE PUMP The tradeoff between head and flow is the basis for the downward slope of the HEAD/CAPACITY CURVE for the pump.
Head/Capacity 3 THE SYSTEM The system often changes. Open/close a valve. Add/remove restrictions. Levels in tanks change. Filters get clogged. Things get moved around. THE PUMP The pump is usually fixed. Impeller trim is set at assembly (can only be changed by disassembly or increased with new impeller.) Motor speed is fixed (unless VFD is employed.)
Temperature THE SYSTEM It can change the density, viscosity, and vapor pressure of the pumped fluid, thus changing the shape of the system curve. The above factors can also impact NPSHa. THE PUMP It can exceed the design limits of the pump or impact material selection. It can reduce lubricity of the pumped fluid. It can trigger the need for certain options. It can impact required clearances.
Viscosity THE SYSTEM Increases friction head Increases required pipe size. Or is the cause of problems if size is not increased. May or may not be constant in the system compared to at rest. THE PUMP Shifts head/capacity curve down. Shifts power curve up. Reduces efficiency. Above 500cps has large impact on efficiency. About 1000cps is practical limit.
Density THE SYSTEM Used to calculate head if only pressure is given. Impacts velocity head. THE PUMP No impact on head/ capacity. Impacts power requirements proportionally.
Vapor Pressure THE SYSTEM Can be important factor in determining NPSHa. Often not given in data Need to know VP if other factors point to low NPSHa. Need to know VP if fluid is unfamiliar. Need to know VP if temperature is high. THE PUMP Pump will cavitate if NPSHa is less than NPSHr. In severe situations, can vapor lock the pump so nothing gets pumped.
Solids Content THE SYSTEM Clogging will change shape of system curve. Abrasives may damage components, changing the shape of the system curve. Strainer can effect NPSHa. THE PUMP Solids must fit through the smallest passage. Abrasives may impact material choice. Abrasives may dictate slower running speed. Can build up in close clearances or can jam the pump causing damage.
Chemical Compatibility THE SYSTEM Material selection. Corrosion can reduce pipe diameter, thus changing the shape of system curve. THE PUMP Material selection. Seal selection. Seal support system selection.
Multiple Conditions THE SYSTEM Can be on one system curve. (Different flow through the same system.) Usually not on same system Curve. (System has been changed.) THE PUMP If on same system curve, a variable speed drive may allow multiple conditions with one pump. If not, may not be possible with one pump and bypass loops, control valves, and/or VFD may be required.
System One pumps Using Curves
Remember The system controls the pump.
Shape is determined by physical elements of the system and the pumped fluid. Always parabolic. Determined by any two points on curve. Operation can only be on curve unless system is changed. The System Curve
The Head/Capacity Curve Shows tradeoff between head and flow. Each curve is for a fixed pump speed and impeller diameter. The pump can only operate on the H/C curve unless speed or diameter is changed.
The pump must operate somewhere on the H/C curve. The system controls where on the H/C curve the pump will operate. The actual duty is at the intersection of the two curves. The Rated Duty
Selection Example Design Flow, Confirm Units Maximum at design flow Maximum, Rated, Minimum Through pump, not at rest At rated temperature If present, get size, hardness, concentration Get name of fluid to verify other data and make material selection. 135 GPM (30.7 m³/hr) 42 ft TDH (12.8m) 68 F (20 C) 1 cps S.G. = 1 No Solids Water
Curve 1
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Curve 3
Curve 4
Curve 5
Curve 6
Curve 7
Application Information to Pump Datasheet
A Quote for Your Quotes: Default values have been used for all data not provided to Blackmer. Please review Pump Performance Datasheet to confirm that default values used are reasonable for the intended service.
System one pumps General Arrangement Drawings
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