By Larry Bachus, The Pump Guy
I read your article featured in the December 2011 Flow Control e-newsletter (“The Relationship Between Pressure & Head”). In that article, you stated, "The term head is the constant for the pump manufacturer. A pump that generates 90-ft. of head can elevate any liquid to 90-ft. above the surface level of the liquid’s source.” You also say, “If I use the word ‘head’, the liquid’s density is not important."
As a professional engineer, I say this is incorrect. I have always understood that if a pump will generate 90-ft. of head, that is actually 90-ft. of water ¾not any and all fluids. Whenever I size a centrifugal pump for a client in feet of head, I use a density correction to determine how high the pump will lift the liquid.
For example, if a pump will generate a head equal to 90-ft. of water, it will lift a liquid with a specific gravity of 0.9 a total of 100 ft. (90/0.9). Please clarify this for me.
Roswell T., PE
Wow! Don’t feel left out. You are in a large group. Many engineers, technicians, pump users and even pump company employees misunderstand this concept.
The pump industry is guilty of not properly explaining this concept to the pump users. Engineering textbooks and professors of “Fluid Mechanics” are guilty of leaving too many engineering students with doubts regarding this concept. This is what happens after industry downsized the older engineers and abolished “mentoring” as a teaching tool.
Pump companies rate their pumps in feet (or meters) of head because the pump company normally doesn’t know the liquid that will move through the pumps. Let me explain this with an example:
A pump manufacturer in Ohio sells 20 centrifugal pump models to their distributor in Louisiana. The 20 pumps are rated for the chemical process industry. Louisiana has many applications for mid-frame, back pullout chemical process pumps.
The 20 pumps accommodate impellers up to 10-inches in diameter. The 20 pumps have a shaft that measures 2-inches in diameter and bearings that can handle the loads generated by the shaft and impeller. The performance curve for each pump is rated at 1,750-rpm and indicates best efficiency at 78 percent, when pumping 90-ft at 600-gpm.
The sales manager at the pump distributorship in Louisiana wants to give good service to the customers. He put these pumps on his shelf for immediate delivery. The sales reps are under instructions to move these pumps as much as possible.
One sales rep sells three pumps to the water bottling plant in New Orleans to pump potable water. Water has a specific gravity of 1.0. The pumps will elevate potable water 90-ft., or overcome 90-ft of resistance in the pipes. At best efficiency, the differential pressure across the pumps will be about 39-psid (90-ft/2.31 x 1.0 = 39-psid). The pump manufacturer in Ohio doesn’t know three pumps were sold to pump water in New Orleans.
Another sales rep in Baton Rouge sells two pumps to the dairy to pump milk. Whole milk has a specific gravity of 1.07. The pumps will elevate milk 90-ft., or overcome 90-ft of resistance in the pipes. The differential pressure across the pumps at best efficiency will be about 42-psid (90-ft/2.31 x 1.07= 42-psid). The pump manufacturer in Ohio doesn’t know two pumps are in Baton Rouge moving milk.
Another sales rep sells three pumps to the chemical plant in Lake Charles to pump sulfuric acid. Sulfuric acid has a specific gravity of 2.0. The pumps will elevate sulfuric acid 90-ft., or overcome 90-ft of resistance in the pipes. The differential pressure across the pumps will be about 78-psid (90-ft/2.31 x 2= 78-psid).
With a specific gravity of 2.0, these pumps will require a motor with twice the horsepower rating. The technician will mate, and align larger motors to these pumps before shipping the pumps to the customer. In Ohio, the pump manufacturer might never know that three pumps are moving sulfuric acid in Lake Charles.
Another sales rep sells one pump to the local paint factory in Metairie to pump paint thinner. The paint thinner has a specific gravity of 0.87. The pump will elevate the paint thinner 90-ft., or overcome 90-ft of resistance in the pipes. The differential pressure across the pump at best efficiency will be about 34-psid (90-ft/2.31 x 0.87 = 34-psid). The pump manufacturer in Ohio will never know this, unless there is a problem while the pump is under warranty.
Let’s return to the dairy in Baton Rouge. The pumps were bought to pump milk. And, whole milk has a specific gravity of 1.07. However, skim milk, chocolate milk, half & half, evaporated milk, coffee cream and ice cream mix all have different specific gravities. If a pump moves more than one liquid, the pressures and the motor’s power (to drive the pump) will vary by the specific gravity of the liquid.
But 90-ft. is always 90-ft. And frequently, this is all the pump manufacturer knows. So, the pump manufacturer prints a curve that shows feet or meters of head.
Roswell, I ask you to review your university “Fluid Mechanics” Textbook. Also, review some recent pump company literature. You will find most literature states “feet (or meters) of head” without specifying water or a named liquid. The specific gravity becomes important as you convert feet of head into pressure in psi. The specific gravity is important as you size the motor to the pump.
Head is energy. The units of ‘head’ are feet or meters. Centrifugal pump head is determined by two principal factors:
1. The impeller’s speed (rpm), and
2. The impeller’s diameter.
The liquid’s weight or specific gravity (density relative to water) is not a component of the term “head.”
Let’s go back in time a few centuries. The scientist Aristotle had theorized that the acceleration of gravity was proportional to an object’s density. Aristotle said a three-pound weight would accelerate toward earth three times as fast as a one pound weight. Another scientist disproved Aristotle’s theory.
|Photo credit: iStock|
In 1589, Galileo purportedly dropped two similar balls together from the Leaning Tower of Pisa in Italy, a height of 183-ft. The two balls were of different density (weight). The balls fell toward earth together and struck the ground at the same instant. The acceleration and time of descent was independent of their mass.
With this experiment 426 years ago, Galileo proved that gravity’s rate of acceleration is a constant, defined today as 32.16-ft./s2 (9.8-m./s2). With time and technology, gravity’s acceleration is clocked slightly faster (32.25-ft./s2) at the earth’s poles compared to the acceleration at the equator (32.09-ft./s2). This is called centrifugal relief.
It has also been revealed that an object accelerates faster from higher elevations (like maybe 40,000-ft.) compared to the acceleration from lower elevations (a skyscraper for example). This is due to the atmosphere’s increased density closer to earth.
In 1971, the Apollo 15 astronauts confirmed Galileo’s work on the moon by dropping a hammer and a feather together in the absence of air. The hammer and the feather struck the moon’s surface at the same instant.
There is a 1.5-minute video of this experiment at YouTube. Enter: “Apollo 15 Hammer and Feather.”
So, if the acceleration of gravity is a constant independent of an object’s mass, then accelerating (elevating) an object against gravity (or counteracting gravity) is also a constant. The energy to elevate a liquid against gravity is a constant, independent of the liquid’s mass or density relative to water. Pump companies call this “Head,” which is a measure of energy. The units of energy are “feet” or “meters” of head against gravity.
When a pump company shows feet or meters of head on a pump curve, the liquid’s specific gravity (or density relative to water) is not important. When you read a pressure gauge or the kilowatts (amps) of an electric motor, the liquid’s weight or density is very important.
Larry Bachus, The Pump Guy
Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tennessee. Mr. Bachus is a retired member of ASME and lectures in both English and Spanish. He can be reached at email@example.com.
Larry Bachus and Flow Control magazine will present the Pump Guy Seminar, June 16-18, in Indianapolis. For more details, visit FlowControlNetwork.com/PumpGuy or contact Matt Migliore at firstname.lastname@example.org.