Centrifugal pump selection and specification

Reliability and efficiency go hand-in-hand in many aspects of pump selection because generally, a pump that has been selected and controlled properly for the normal operating points will operate near its best efficiency point (BEP) flow, with low forces exerted on the mechanical components and low vibration &#; all of which result in optimal reliability.

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Pumping applications come in all shapes and sizes, and the requirements and consequences of not meeting the requirements or premature pump failure vary greatly for each application. Therefore, it is not possible to state generically how a pump should be selected or specified for design and performance, thus this article focuses on selecting a pump to operate in the preferred operating region (POR) &#; with net positive suction head (NPSH) margin above the net positive suction head required (NPSHR) and with acceptable vibration &#; to meet the goal of reliable and efficient pumping. Three American National Standards that should be referenced provide general guidelines or acceptance levels regarding a pump&#;s NPSH margin, POR and allowable vibration as follows:

• ANSI/HI 9.6.1 Rotodynamic Pumps &#; Guideline for NPSH margin
• ANSI/HI 9.6.3 Rotodynamic Pumps &#; Guideline for Operating Regions
• ANSI/HI 9.6.4 Rotodynamic Pumps for Vibration Measurements and Allowable Values

In addition to the general guidance provided here, certain industries such as the oil and gas market and chemical process market have design standards with stated requirements and appropriate standards that should be followed. 

Preferred operating region

The POR is a range of rates of flow to either side of the BEP flow within which the hydraulic efficiency and operational reliably of the pump are not substantially degraded. The POR for a rotodynamic pump varies based on its design; therefore, to determine the POR for a specific pump, refer to ANSI/HI 9.6.3.  

To select a pump to operate in its POR, first identify the maximum and minimum process flow requirements and the corresponding system heads. System curve(s) for the process should be developed that represent the system head requirements over the normal operating conditions. The system curve is a graphical representation of the system&#;s static (i.e., source to destination level or pressure difference) and frictional (loss due to fluid velocity) head requirements. The system head is expressed in feet (ft) of liquid being pumped and is synonymous with the "pressure required" of the pump at a certain flow rate for the liquid being pumped.

The system curve is important because the pump will operate at the flow rate where the pump curve intersects the system curve. The system curve can change based on static head changes, such as level changes between the source and destination, in open systems (see Figure 1), resulting in an operating region instead of a single point.

Net positive suction head

The net positive suction head available (NPSHA) over the range of system conditions should be determined. NPSHA is the total suction head absolute at the suction of the pump, over the vapor pressure of the liquid pumped at its operating conditions. NPSHR is a minimum NPSHA given by the manufacturer, which is required for a pump to achieve a specified performance at a specified rate of flow, speed and pump liquid.  

The NPSHA to the pump will generally decrease, and the NPSHR of a pump generally increases with flow rate as shown in Figure 2.

The first in a series of articles on the principles of pump operation.

This four-part series of articles is intended to explain and develop the basic principles of pump operation and, except for sizing, allow the optimum selection of centrifugal pumps.

This section concentrates on centrifugal pumps. The following conditions are used for purposes of discussion only:

• Internal pump losses will be ignored.
  
• Water will be used as the liquid.
  
• Only single-stage, open systems will be discussed.
  
• This section is limited to commercial products that are easily available.

A centrifugal pump is a nonpositive displacement, dynamic machine that transfers  mechanical energy into a practical and useful form called pressure. The pump first converts this energy from water into velocity with a circular (centrifugal) force and then into head or elevation (pressure energy) as it passes from the eye of the pump, through the impeller and finally through the casing and volute. A typical centrifugal pump is illustrated in

A schematic illustration of the flow pattern of water inside a pump when spun by the impeller is illustrated in

In basic terms, a centrifugal pump&#;s performance is measured by a volume of delivery against  how high it will lift a column of water, with consideration given to its efficiency. The flow of water is almost always designated as foot of head vs. gallons per minute because this gives the best general description of pump operation. Refer to

below on for a conversion of various quantities into U.S. gpm.

Key Pump Definitions

The affinity laws describe the relationships between capacity, head, brake horsepower and impeller diameter for any particular pump. Capacity, head, net positive suction head (NPSH) and horsepower change as a result of speed change. Capacity, head, NPSH and horsepower change as a result of impeller diameter change.

The relationships are:

Pump gpm varies directly as a change of impeller speed or impeller diameter ratio.

Pump head and NPSH vary directly as the square of speed or impeller diameter ratio.

Pump horsepower varies directly as the cube of the speed or impeller diameter ratio.

is a reference point where the pump operates at about 80%  to 90% of the shutoff head only with regard to the shutoff head displayed on the general pump curve. The pump head is shown by convention on a vertical scale at the left side of a manufacturer&#;s pump graph. This is a general rule and not a law.

The best efficiency point is a term that can be defined in several ways. The one most often used is a general term applicable to all pumps that occurs when the operating condition of a generic pump design has it highest efficiency. The second is for a specific pump that has its theoretical BEP where the incidence angle of the fluid entering the hydraulic passages best matches with the vane angle of the impeller. The third is where the system curve and pump curve for a specific application intersect. A general rule (not a law) is to have a pump operate at about 85% of the shutoff head using the system curve as reference.

This occurs when the atmospheric pressure acting on a liquid is below its vapor pressure, causing it to boil and create bubbles. When bubbles in the liquid pass into a region of lower pressure they collapse or implode with a resulting shock wave that could cause damage to any interior, wetted surface.

The rotational speed, in rpm, that causes a natural frequency of vibration in a pump. Pump horsepower is the power necessary to drive the pump with the required flow and head.

Expressed as a percentage, efficiency is the ratio between the total possible output power developed by the motor of the pump compared to the actual input power supplied to the pump, taking into consideration the losses due to heat, vibration, internal friction, distortion and noise. Simply put, a pump&#;s peak efficiency is the point where it does the most work for the least input energy. The efficiency is dependent upon the impeller and volute design and indicates a point where minimum turbulence is encountered. The efficiency is determined by tests done by the manufacturer.

This refers to the friction lost, measured in foot head, at the entrance point of water into the supply pipe of the pump from a water supply. Water from a public main should not be calculated. This supply does not  have any pump entrance loss, but rather, the entrance loss into the supply piping will affect the available NPSH.

The complete frictional head loss, in foot head, that is needed to overcome the resistance of the liquid flowing in the supply piping. It is the sum of the supply pipe friction losses of the pipe walls, meter assembly, BEP, fittings, valves and suction pipe entrance losses. This head varies with the flow, size pipe and character of the liquid pumped. Note: This figure is not to be used for the calculation of the head necessary for the required design head for pump calculations.

This is where the driver of the pump is not overloaded under any operating condition. As with most centrifugal pumps, a higher horsepower motor will prevent overloading under all conditions of pump operation. To achieve this, the operating point is placed on the impeller diameter curve and the horsepower curve is plotted above that  point, but will not intersect it.

This type of system is characterized by having a point of free discharge, such as water supply to a house tank or a cooling tower.

The

is shown on the graph of the performance curve of the pump and is the intersection of the system curve and the performance curve. In the absence of a system curve, the operating point is placed directly on the impeller diameter of the selected pump.

Overloading occurs when the flow increases and the BHP increases to a maximum. There must always be enough horsepower available from the driver to supply the needs of the pump. As the total dynamic head (TDH) falls, the flow increases and so does the BHP, which will overload the driver.

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This curve, often called the &#;H-Q&#; curve, is a downward sloping plot showing the actual well-defined relationship between flow rate and total foot head for a typical, characteristic pump made under carefully controlled conditions. The curve is established by the pump manufacturer. It starts with the shutoff head where all the pump&#;s energy is invested in elevation with zero flow and ends when all the energy is invested into maximum flow with zero head. This relationship is unaffected by water temperature and water density. The &#;H&#; refers to total head and the &#;Q&#; is the flow in gpm. The pump curve permits the use of the system curve as a method of pump selection.

(often referred to as total head) is the sum of all pressure requirements necessary for proper system operation.

This is a point when all the pump&#;s energy is used to raise a liquid up to a vertical point where the weight of the liquid is equal to the energy produced by the pump and zero flow.

Specific speed is a dimensionless design index number primarily used by pump designers. The specific speed has no practical significance to the consulting engineer. Some manufacturers disagree concerning the usefulness of this figure at all.

The specific speed can be calculated using

.

Where:

SP = Specific speed of pump, dimensionless

PS = Pump speed, in rpm

Q = Pump capacity at the BHP, in gpm

H = Pumping head at the BHP, in feet

A generically accepted range is from 500 to 15,000. The lower the speed, the higher the head.

The distance, in feet, from the centerline of the pump inlet to the free discharge with no liquid flowing.

This is the vertical difference, in feet, between the highest liquid-free discharge level and the free level of the supply source, which is the eye of the impeller with no liquid flowing. This also must include the addition or subtraction of the pressure difference of the suction or discharge source, whichever applies.

This term is used when the supply is pressurized or the level of a body of water is higher than the centerline of the pump inlet (eye of the impeller).

This term is used when the supply is lower than the centerline of the pump inlet. The pump must lift water up into the eye of the impeller.

Often called the pump performance curve, this curve is simply a plot of the change of foot-head energy resulting from a flow change in a specific fixed piping system. The system curve starts at zero flow and zero head. It is an upward sloping curve of the actual operating conditions for a specific pump. The original point indicating flow and head requirements is first developed by the engineer and the full curve is extended both ways by the manufacturer from the original point. Since foot head is an expression of energy, a pump curve defined by this term is not affected by water temperature or density.

A graphical representation of the total measure of energy, in foot head,  imparted to a liquid for a specific pump at a specific operating speed and capacity. It is frequently used to refer to pumping head. By convention, the pump head is shown on a vertical scale at the left side of a manufacturer&#;s pump graph.

The absolute pressure of a given liquid that is required to keep it in the liquid state at a given temperature.

Two methods of measurement are used. The first is where the liquid is at rest. The velocity head is the energy needed to accelerate the liquid from a static start, measured in foot head, at the end of the discharge pipe.

The second is if the liquid is moving, and it is equivalent to the vertical distance in foot head, then the mass of liquid would need to fall to attain the same velocity as the liquid traveling in a pipe. Because it is a required force to accelerate the liquid at the origin of the system, it is subtracted from the suction head or added to the suction lift. The velocity head is a factor in calculating the total head, although this value is usually quite small. This head is entirely lost if the discharge pipe is the same size as that of the supply pipe.

Conversion Factors For U.S. GPM (Table 1)

1 pound per minute                 0.12

231 cubic inches per minute    1

1 cubic foot per minute           7.481

1 cubic foot per second           448.8

1.000 pounds per hour                2

1 imperial gallon per minute    1.2

1 million gallons per 24 hours 695

1 barrel of oil per minute             42

100 barrels per hour                    70

100 barrels per 24 hours          2.92

1 liter per minute                     0.

1 liter per second                     5.852

100 liters per hour                   0.44

1 cubic meter per minute         264

1 cubic meter per hour                4.4

1 acre foot per minute             326,000

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