It’s more important to pump design than you think.
By Ed Butts, PE, CPI
This edition of “Engineering Your Business” will return to one of the fundamental subjects it has always concentrated on—pumps and pump theory. On numerous occasions I have discussed many of the primary and vital pump performance characteristics such as capacity, head, horsepower, and NPSH. One factor, however, just as important to the design and selection of a pump but often ignored or overlooked is the specific speed.
Pump Impeller Types
Before we can gain an understanding of specific speed, it is important to fully comprehend the types of impellers specific speed applies to. Specific speed is a method used by pump people to classify velocity pumps according to the type of impeller used plus the flow rate, head, and rotational speed the impeller has been designed for or is capable of.
Velocity pumps function by imparting kinetic energy (energy in motion, water being pumped) from potential energy (energy at rest, water from a static condition). The medium used to convert this energy is usually a liquid, such as water.
Three classifications of pumps we are concerned with in this column—standard centrifugal, vertical turbine, and submersible— are examples of velocity pumps. Velocity pumps can be further broken down into three pump types: radial flow, axial flow, and mixed flow.
Radial flow pump: Often referred to as a volute or diffuser pump, this is the most popular and common style of centrifugal pump in use today. The direction of fluid flow through the impeller is 90° normal to the shaft. Simply stated, this means the flow enters the impeller parallel to the pump shaft through an entrance opening called the impeller eye. More simply, the eye travels through the impeller and is then discharged from the impeller at a perpendicular right angle, or 90°, from the pump shaft.
Axial flow pump: This is more commonly referred to as a propeller pump. Flow through the impeller is routed straight through the impeller, or at an angle of 180° normal to the shaft. The pump’s primary use is in high capacity and low head applications, such as high volume fluid transfer for dewatering, stormwater pumping, and tailwater recovery systems.
Mixed flow pump: The mixed flow impeller is a compromise of design between the radial and axial flow impeller where the flow through the impeller is oriented at an angle between 90° and 180° normal to the pump shaft. This impeller combines the design characteristics of the two other impellers in order to achieve a system both efficient and capable of delivering a high output of flow and head. This style of impeller is commonly used for applications in vertical turbine and submersible pumps.
Specific Speed Introduction
“Specific speed” is a pump term often thrown around as a general term or applying to an actual single speed of a pumping unit. The term is actually quite important when defining a class of pump—or more specifically, the impeller in the pump.
Specific speed is a term used to describe the geometry (shape) and nomenclature of a pump impeller. Individuals responsible for the selection or design of a pump can use this specific speed information to:
- Select the shape of the pump curve
- Determine the efficiency range of the pump
- Anticipate motor overloading problems
- Predict NPSH requirements
- Select the lowest cost pump for the specific application.
Since I often deal in various pump types (standard centrifugal, axial flow, and mixed flow), I occasionally work with specific speed (1) as a factor for specifying a pump for purchase, (2) as a general descriptor when working with a pump manufacturer or supplier, or (3) to provide a specificity to a needed pump characteristic such as capacity or head.
Understanding the meaning of specific speed and the different pumps classified with differing numbers can be a real benefit when trying to work across the various types of dynamic pump classes and the extreme differences that can be encountered in pump flow or head. A knowledge of specific speed is also important when trying to understand the subtle and sometimes not so subtle variances in pump impellers and shapes. Table 1 provides general guidance for classifying most velocity pumps (Figure 1).
Specific Speed Definition
Two design factors associated with impellers frequently misunderstood and misapplied are specific speed and suction specific speed.
The specific speed (NS) of an impeller or pump is a dimensionless index number used to relate the hydraulic performance of a centrifugal pump to the shape and physical proportions of its impeller. Where the specific speed rating of a pump is used to help define the discharge characteristics of a pump, the suction specific speed is used to define the suction characteristics of the same pump.
Although often mistaken as a sole speed factor, the index is often used to classify pumps within a group related to its rotational speed, head, and capacity and includes virtually all classes and types of commonly used centrifugal pumps— starting at 400–500 for the relatively low capacity radial flow style of impeller up to 15,000 for high capacity, axial flow units. The formula used to calculate specific speed is:
Suction Specific Speed Defined
There are actually two different types of specific speed in reality: the version first defined and suction specific speed. The suction specific speed (S) is also a dimensionless rating number used to evaluate the relative ability of a centrifugal pump to operate under conditions of low net positive suction head–available (NPSHA). Depending on the impeller design, suction specific speeds range from below 4000 to more than 11,000 with the higher values indicating lower net positive suction head–required (NPSHR). The equation used to calculate this factor is:
where S = Suction specific speed, dimensionless N = Rotational speed of impeller in revolutions per minute (RPM) Q = Capacity at the best efficiency point (BEP) in gallons per minute (GPM) NPSHR = Net positive suction head–required by the maximum diameter impeller at the BEP in feet
Unlike specific speed, a suction specific speed is not a type of number to simply rate a pump’s performance, but a criterion of a pump’s performance with regard to cavitation potential. The suction specific speed (S) rating of a pump indicates how capable the pump’s impeller inlet design is (i.e., how low is the NPSHR for a given pump speed and BEP flow rate). Higher “S” values mean lower NPSHR and therefore greater NPSH margins. The range in Table 2 shows how pumps are rated according to their suction specific speeds.
An example of an application using suction specific speed follows here.
Nominal rotational speed: 1800 RPM
Capacity at BEP: 1500 GPM
NPSHR at BEP: 12 feet
Or conversely, another application where the suction specific speed from a pump is provided:
S = 9000
Nominal speed: 3600 RPM
Capacity at BEP = 125 GPM
To find approximate NPSHR of pump:
Approximate total dynamic suction lift at BEP (at sea level) = 32 feet – 7.37 feet = 24.63 feet
Typical Specific Speed Ranges
It is important to note there are no universal or established categories for low, moderate, or high head or capacity pumps. Each individual who works with pumps has their own personal definition of what constitutes a low or high head or capacity pump.
Table 3 has been prepared with the author’s general interpretation of these classes based on the industry’s generally accepted ranges. The most critical element of using this table is to divide pumps based on the relative head or flow rates used to determine the specific speed, not an arbitrary classification. Most standard well pumps of 6 inches or greater diameter fall into a given range of specific speed (NS) between 2000–5000.
These ranges can help determine the type of impeller and class of pump you are working with or intend to buy. Obviously, the information contained in the table should be used as an approximation only, but in many cases this data can be extremely helpful during an analysis of a potential pump purchase.
Specific Speed Impact on Developing Pump Curves and Performance
The use of specific speed is not only helpful when examining a particular pump, it is also a valuable tool often used when developing a pump curve. When using this value to compare or evaluate pumps, it must be remembered the specific speed is a number calculated at the pump’s best efficiency point (BEP) at maximum diameter for both head and capacity.
This is a singular value that applies to all pumps, and trying to adjust or shift the value to retain it for trimmed impeller diameters will not work. This value defines the basic geometric path of the impeller with lower specific speed pumps, usually with larger outside diameters and narrower radial (right angle) flow passages, and higher specific speed pumps equipped with smaller outside diameters and larger more axial (straight through) flow passages. Pumps of the same specific speed can be scaled up or down for greater or less flow and head requirements.
Specific speed also dictates the shape of the head-capacity curve with low specific speed pumps having flatter or drooping head-capacity curves, and higher specific speed pumps having steep curves (Figure 2). The pump’s shutoff flow brake horsepower also increases with increasing specific speed values, exceeding the best efficiency flow input power requirements at NS values above 4500 to 5000.
NS also dictates the maximum obtainable efficiency of a pump with the maximum obtainable efficiencies occurring at specific speed values between nearly 2000 and 3000 (Figure 3). This is the area where single stage pumps begin to meet their effective limit for head and multistage units start to occur for smaller diameter (< 8-inch) pumps.
If the application calls for a low, single stage specific speed value of 500–2500 or lower, such as those often found on 4- inch and 6-inch submersible pumps, the specific speed value of the overall pump is increased by adding these additional impeller stages, which reduces the head produced by each stage. This results in an increase in the efficiency of the pump. This is one of the advantages of vertical turbine or submersible pumps, which can vary up or down the number of impeller stages and also helps to explain why efficiency deductions are often needed for some multistage vertical turbine pumps with only one to three stages.
The use of specific speed and the suction specific speed for various classes of pumps is a time-honored tradition and an established method to compare different types and groups of pumps. Even if you don’t use these definitions on a frequent basis, knowing the derivation of each term and understanding how each one relates to the unique characteristics of pumps is an important ability.
Until next month, work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at firstname.lastname@example.org.