When exploring and learning about pumping systems, I was amazed at the information available online today in 2021. For a non-engineer, there are numerous sites that provide incredibly easy to read and understand information. The U.S. department of energy has done an excellent job with their tear sheets. I’ve provided the information that they teach, as it’s an incredibly reliable resource:
Most pumps operating today were selected to meet a maximum system demand, or potential future demands. This means that most pumps are oversized, rarely operating at their full design capacity. In addition, pumps are often installed in systems with multiple operating points that coincide with process requirements. A throttling valve is usually employed when the process ﬂow requirement is less than the ﬂow at the pumping system’s natural operating point.
Throttling valves control ﬂow by increasing the system’s back pressure or resistance to ﬂow. This increase in pressure or head requirements shifts the pump’s operating point to the left along its performance curve, and, typically, away from its best efﬁciency point. The result is a loss in efﬁciency.
Adjustable speed drives (ASDs) provide an efﬁcient ﬂow control alternative by varying a pump’s rotational speed. These drives are broadly classiﬁed as mechanical (ﬂuid or eddy current) drives and variable frequency drives (VFDs). Today, the VFD is the most frequently speciﬁed type of ASD, and pulse-width-modulated VFDs are the most commonly used.
In centrifugal applications with no static lift, system power requirements vary with the cube of the pump speed. Small decreases in speed or ﬂow can signiﬁcantly reduce energy use. For example, reducing the speed (ﬂow) by 20% can reduce input power requirements by approximately 50%.
In addition to energy savings, VFDs offer precise speed control and a soft-starting capability. Soft-starting reduces thermal and mechanical stresses on windings, couplings, and belts. Also, VFDs reduce voltage ﬂuctuations that can occur in starting up large motors. Induction motors with across-the-line starting draw as much as six times the full-load current during start-up. During acceleration, a VFD-controlled motor’s locked rotor current is limited to one and one-half times the full-load current.
Operating at reduced speeds results in other benefits, as well, such as lower bearing loads, reduced shaft deﬂection, and lower maintenance costs.
We can use the afﬁnity laws to predict the performance of a centrifugal pump with little or no static head at any speed, if we know the pump’s performance at its normal operating point. The afﬁnity law equations are as follows:
Q2 / Q1 = N2 / N1
H2 / H1 = (N2 / N1)2
P2 / P1 = (H2Q2) / (H1Q1)
= (N2 / N1)3
Q = fluid flow, in gallons per minute (gpm)
N = pump rotational speed, in revolutions per minute (rpm)
H = head, in feet
P = brake horsepower (hp)
Q1, H1, P1, N1 = pump performance at normal (initial) operating point
Q2, H2, P2, N2 = pump performance at final operating point
The afﬁnity laws show that the pump head decreases signiﬁcantly when the pump speed is reduced to match system ﬂow requirements (see ﬁgure). Pump shaft horsepower requirements vary as the product of head and ﬂow or as the cube of the pump’s speed ratio. Note, however, that the afﬁnity laws will not provide accurate results for systems with static head. In that case, constructing a system curve to calculate new duty points is essential.
ASDs are ideally suited for variable-torque loads from centrifugal pumps, fans, and blowers when the system load requirements (head, ﬂow, or both) vary with time. Conditions that tend to make ASDs cost-effective include the following:
- High horsepower (greater than 15 to 30 hp)—The higher the pump horsepower, the more cost-effective the ASD application.
- Load type—Centrifugal loads with variable-torque requirements (such as centrifugal pumps or fans) have the greatest potential for energy savings. ASDs can be cost-effective on positive displacement pumps, but the savings will generally not be as great as with centrifugal loads.
- Operating hours—In general, ASDs are cost-effective only on pumps that operate for at least 2,000 hours per year at average utility rates.
- High utility rates—Higher utility energy charges provide a more rapid payback on an investment in an ASD.
- Availability of efﬁciency incentives—Where they are available, electric utility incentives for reducing energy use or installing energy-saving technologies will reduce payback periods.
- Low static head—ASDs are ideal for circulating pumping systems in which
- The system curve is deﬁned by dynamic or friction head losses. They can also be effective in static-dominated systems—but only when the pump is carefully selected. A thorough understanding of pump and system interactions is critical for such applications.
Consider using adjustable speed drives in pumping applications that range from 1 to 1,000 horsepower (hp).
About Jonathon Bell
Jonathon Bell is an entrepreneur, focused on building his family's legacy in the industrial pump market. Currently, he is focused in Latin America, building Dynapro Pumps Mexico from the ground up while contributing in Canada & the United States with Sales & Marketing efforts.
His commitment is developing teams through individual and partnered coaching, to bring out the best in each team member and giving them the tools to help them reach their goals. Guiding and teaching the core values of passion, evolving, and team communication, his teams and members become top performers in their respective fields.
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References: DOE/GO-102007-2229 January 2007 Pumping Systems Tip Sheet #11
Industrial Technologies Program Energy Efficiency
and Renewable Energy
U.S. Department of Energy Washington, DC 20585-0121 www.eere.energy.gov/industry
Improving Pumping System Performance: A Sourcebook for Industry, U.S. Department of Energy, 2006
Variable Speed Pumping: A Guide to Successful Applications, Hydraulic Institute, 2004