A Guide For Pump Efficiency Prediction – Attainable Efficiency in Rotodynamic Pumps
The pump industry faces a formidable challenge of improving efficiency to align with the ever-increasing need to reduce energy consumption and mitigate carbon emissions.
Category: Blogs August 8, 2023
By Alex Moser, Senior Engineer, Hydraulic Institute
The pump industry faces a formidable challenge of improving efficiency to align with the ever-increasing need to reduce energy consumption and mitigate carbon emissions. While governmental regulations have acknowledged the significance of enhancing efficiency in commercial and industrial clean water pumps, as well as circulator pumps, the understanding of a pump’s achievable efficiency is often overlooked. In the current state of rotodynamic pump technology, the efficiency that a pump can attain is contingent upon factors such as pump type, size, orientation, and operating speed.
Attaining optimal efficiency in rotodynamic pumps holds paramount importance for maximizing energy efficiency and performance. To address this imperative, the Hydraulic Institute (HI), a standards development authority in the pump industry, has developed a standardized method for predicting achievable efficiency levels at the best efficiency point (BEP) for a wide range of rotodynamic pumps. The HI 20.3-2020 Program Guideline for Rotodynamic Pump Efficiency Prediction (available as a free download) offers an empirical approach that considers factors including BEP flow rate, total head per stage, and rotational speed to estimate the expected attainable efficiency. This guideline serves as an invaluable tool for pump designers, specifiers, end users, and professionals in the pump industry.
The HI 20.3 guideline encompasses a diverse array of pump types commonly employed across various industries, providing comprehensive coverage (Figure 1). It offers optimal specific speed (Ns) ranges for each pump type and highlights the factors that exert influence on pump efficiency.
The attainable efficiency levels of rotodynamic pumps are influenced by several major factors detailed in the guideline. In addition to the hydraulic requirements, factors include the pump type, design standard, surface roughness, and internal clearances. The guideline illustrates that different pump types and designs exhibit varying efficiency characteristics, with an optimal specific speed for maximum efficiency. It also illustrates the influence surface roughness and internal clearances have on efficiency, and how these particularly affect the efficiency of low specific speed pumps.
When designing internal clearances, hydraulic design considerations, compliance with industry standards, material properties, and specific application requirements are considered. The guideline uses an alternative definition of specific speed, which incorporates the flow rate per impeller eye (Q’) rather than the total flow rate (Q) for double inlet impellers. This alternative value is utilized throughout the guideline to adjust efficiency based on the deviation from the optimal specific speed for a particular pump type. It is also applied to estimate efficiency improvements resulting from surface finish upgrades and efficiency reductions caused by increased clearance in wearing rings. The alternative equation for specific speed is presented in Equation 1.
Various factors and design considerations impact the efficiency of a pumping unit. Mechanical losses from components like bearings, seals, and packing contribute to decreased efficiency. While these losses are more significant in small pumps, regular maintenance and high-quality components can help minimize their impact. Pumps handling liquids with solids need special attention, and modifications like pump-out vanes and larger clearances can improve solids handling at the expense of efficiency.
Design choices, such as optimizing the impeller for suction performance or curve shape selection, can also influence efficiency. For vertical turbine diffuser-type pumps, bowl efficiency may be reduced due to the staging effect where the inlet hydraulic losses are not spread over multiple stages. The staging effect efficiency decreases with the increase in the number of stages and is generally not a concern when considering designs with four or more stages. Each application requires careful analysis for determining the necessary corrections to determine attainable efficiency.
Procedure To Predict Attainable Efficiency
To effectively utilize the HI 20.3 guideline, a straightforward procedure is provided to predict attainable efficiency levels at the BEP for specified pump types. The procedure involves identifying the pump type, the BEP flow rate, and determining the baseline predicted attainable efficiency from a comprehensive multi-curve chart that encompasses all pump types listed in Figure 1. The baseline efficiency is then further adjusted based on the specific speed, using an efficiency reduction chart provided in the guideline. Subsequently, the calculated efficiency is modified as needed, considering additional factors such as surface roughness (Figure 2) and internal clearances (Figure 3). Finally, an additional chart provides the expected normal deviation for the predicted efficiency at the BEP.
Now, the outlined procedure is applied to an example calculation to determine the attainable efficiency of an API double-suction process-type pump using U.S. customary units.
A Hypothetical Example of How to Use the Guideline
A design engineer is tasked with determining the normally attainable efficiency of an API-type, double suction process pump. The pump has a rotational speed of 1780 revolutions per minute (rpm) and is pumping clear water at a temperature of 68 °F. The BEP total rate of flow (Q) is 5,000 gallons per minute (gpm), and the total head (H) is 420 feet (ft).
Step 1: Calculate specific speed (Ns) using Equation 1:
Ns = 959
Note: The alternate definition of specific speed utilizes (Q’), instead of (Q), which is defined as the flow rate per impeller eye for double inlet impellers or double suction pump (5000/2 = 2500).
Step 2: Determine the efficiency at optimum specific speed (Figure 4) at the BEP flow rate of 5000 U.S. gpm. This efficiency will correspond to the optimum specific speed for an API double-suction process-type pump.
Efficiency at optimum specific speed = 84.5%.
Step 3: Determine the efficiency reduction due to the specific speednot being at its optimum level. This reduction can be determined from Figure 20.3.4.2.1b in the HI 20.3 guideline. This step requires using the alternate value of specific speed (Ns= 959), which utilizes Q’, the flow rate per impeller eye.
Efficiency Correction = 6.8% (determined from the chart in the HI 20.3 guideline).
[Predicted efficiency] = [optimum efficiency] – [efficiency correction] = 84.5% – 6.8% = 77.7%.
Step 4: Verify the deviation from the normally attainable efficiency using Figure 20.3.4.2.1e in the HI 20.3 guideline. This is done by considering the normal negative and positive deviation when using the BEP flow rate of 5,000 U.S. gpm. The deviation shown for 5,000 U.S. gpm is ±2.5%. Therefore, the predicted efficiency at the BEP will be: Predicted efficiency at BEP = [predicted efficiency] ± [normal deviation] = 77.7% ± 2.5% = 75.2% to 80.2%.
Therefore, the normally attainable efficiency for the API-type, double suction process pump is estimated to be between 75.2% and 80.2% at the BEP (5,000 gpm and 420 ft) when pumping clear water at a temperature of 68 °F with a rotational speed of 1,780 rpm.
Note: No surface roughness optimization has been made, nor are there wear ring clearances larger than normal (aside from the normally larger API clearances, which are reflected in the HI 20.3 guideline).
Applying the same procedure to a double suction pump, which is not of API design but operates at the same rotational speed, BEP flow rate, and head (5,000 gpm and 420 ft), would yield a calculated predicted attainable efficiency of 81.9% ± 2.5%, resulting in a range of 79.4% to 84.4%. This illustrates that the specific design requirements for the API pump result in an approximate four-point reduction in attainable efficiency.
Note: These estimates are for clean water, and ANSI/HI 9.6.7-2021 Rotodynamic Pumps Guideline for Effects of Liquid Viscosity on Performance would be used to estimate the performance and efficiency reduction for viscous liquids compared to clean water.
Impeller Trimming and Its Effect on Pump Efficiency
The efficiency of a pump is influenced by the specific speed of the impeller when it undergoes trimming. However, it is important to note that the maximum diameter impeller may not always lead to the highest efficiency for the pump.
This relationship between impeller trim and efficiency can be explained in two main cases. In Case 1, the pump’s highest efficiency at the BEP is achieved with the maximum diameter impeller. However, when the impeller diameter is reduced (trimmed), the efficiency at the BEP decreases.
In Case 2, if the pump is equipped with an oversized impeller, it initially operates with suboptimal efficiency at full diameter. However, by reducing the impeller diameter, the pump’s efficiency improves until it reaches its peak at a specific reduced diameter. It is important to note that further reduction in impeller diameter beyond this point leads to a decline in the pump’s efficiency.
The HI 20.3 guideline offers a comprehensive and practical approach to estimating the normally attainable efficiency of rotodynamic pumps at BEP. By considering factors such as specific speed, surface roughness, internal clearances, and design standards, the guideline enables pump designers, specifiers, and end-users to optimize pump performance and energy efficiency. Applying the outlined procedure, engineers can predict efficiency levels and make informed decisions regarding pump selection and operation, ultimately contributing to energy conservation and reducing carbon emissions in various industries.
To learn about how entrained gases, slurries, and viscosity affect pump attainable efficiency, please refer to ANSI/HI 14.3-2019 Rotodynamic Pumps for Design and Application, ANSI/HI 12.1-12.6-2021 Rotodynamic Centrifugal Slurry Pumps for Nomenclature, Definitions, Applications, and Operation, and ANSI/ HI 9.6.7-2021 Rotodynamic Pumps Guideline for Effects of Liquid Viscosity on Performance.
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Originally published in Pump Engineer, August 4, 2023.
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