David W. Spitzer
David W. Spitzer

A centrifugal pump is designed to flow 100 lpm of water at a pressure of 10 bar at full speed. Using the Affinity Laws, what is the approximate flow and pressure generated when a variable speed drive operates the pump motor at 70 percent speed? Approximately how much less energy is required at the lower speed?

A.    70 lpm and 7 bar using 70 percent less energy
B.    70 lpm and 7 bar using 30 percent less energy
C.    70 lpm and 5 bar using 70 percent less energy
D.    70 lpm and 5 bar using 30 percent less energy
E.    70 lpm and 3 bar using 70 percent less energy
F.    70 lpm and 3 bar using 30 percent less energy
G.    None of the above

Commentary
The Affinity Laws for centrifugal loads state that:
1.  Flow is proportional to pump speed.
2.  Pressure is proportional to the square of pump speed.
3. Power is proportional to the cube of pump speed.

READ ALSO: QUIZ CORNER–Why Was a Control Valve Installed Upstream of a Flowmeter?

Industrial Automation Insider 300x250 Therefore, the calculated flow, pressure and power consumption at 70 percent speed using the Affinity Laws is approximately:
1. 70 lpm (0.7 * 100 lpm)
2.  5 bar (0.7 * 0.7 * 10)
3.  34 percent (0.7 * 0.7 * 0.7 * 100 percent)

Energy consumption of 34 percent represents a 66 percent energy savings, so the closest answer is Answer C.
 
Additional Complicating Factors
The calculated 70 lpm flow rate may not be possible in many installations. This is because the pump must overcome the static pressure in the system before water can flow. For example, if the static pressure that the pump needs to overcome is 6 bar, the flow will be zero because the pump discharge pressure is only 5 bar.
 

David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation Insider.  Mr. Spitzer can be reached at 845 623-1830 or www.spitzerandboyes.com. Click on the “Products” tab to find his “Consumer Guides” to various flow and level measurement technologies.