| Larry Bachus
(a.k.a. "The Pump Guy")
Hello Pump Guy,
I have a question after reading your previous article in Flow Control (“Rocket Science,” July 2014, page 40). Can you explain further how the head will not be affected by changing the liquid handled? The head and pressure are both dependent on the specific gravity of the liquid. This is shown in the equation:
Head = pressure x 2.31/sp.gr.
Interesting observation! You are not alone. Pressure and head are frequently misunderstood.
Head and pressure are different words. The words have different meanings. “Head” is a measure of energy. “Pressure” is a force applied to an area, such as a pound of force applied to a square inch of area.Notice the word “force” is a component of pressure. Speaking of force, gravity is a natural force.
Can we agree that gravity’s rate of acceleration is constant? Most science teachers tell a story of the Italian scientist Galileo. In 1589, Galileo purportedly dropped two similar balls together from the Leaning Tower of Pisa. The two balls were of different density (weight). The balls accelerated together as they fell toward Earth. The two balls struck the ground at the same instant. The time of descent was independent of their mass.
Galileo’s experiment contradicted the prevailing wisdom of the 16th century. Another prominent scientist, Aristotle, theorized that heavier objects would fall faster than lighter objects in direct proportion to their mass. Aristotle proposed a 2-pound ball would fall toward the Earth twice as fast as a 1-pound ball.
With this experiment, Galileo proved that gravity’s rate of acceleration is a constant. An object will fall (accelerate) toward Earth at 32.16-ft./second squared. This is the force of gravity. You can prove this if you can find an adequate building or height from which to drop two similar objects of different density.
Although gravity is a constant, the denser (heavier) object will strike the Earth with a force relative to its density. It will do more damage than a lighter object when it hits the ground, or leave a deeper crater.
So if gravity is a constant, then accelerating (elevating) an object against gravity (or counteracting gravity) is also a constant. Does this make sense?
Pump people call this “head.” Head is a measure of energy. The units of energy are feet of head against gravity.
Let’s apply head to a pump and liquids. I have an impeller of a given and fixed diameter. If I rotate this impeller at a given speed, the impeller will add a certain quantity of energy to the liquid. If I release or channel the liquid’s energy into a vertical pipe, I can measure this energy as feet of elevation (head) against gravity.
One element of head is the diameter of the impeller. The other element is the velocity or speed of the impeller. These two factors determine head or elevation against gravity.
Let’s say the impeller diameter is 10 inches. Let’s say the rotational speed is 1,750 RPM. Let’s say these two factors (speed and diameter) make the liquid rise 100 feet into a vertical pipe against gravity.
A pump person would say, “This pump develops 100 feet of head. The elevation against gravity is independent of the liquid’s density. The liquid can be water, kerosene, cow’s milk, sulphuric acid, or liquid mercury. 100 feet is 100 feet. The liquid’s density or specific gravity plays no role in head. I’ll offer some examples.
A pump manufacturer ships 15 pumps to a distributor. Each pump has an impeller measuring 10 inches in diameter. Each pump rotates at 1,750 RPM. Each pump develops 100 feet of head. The distributor sells the pumps in a given geographical area.
One distributor salesman sells two pumps to the local water authority to pump water. Each pump can elevate the water 100 feet.
Another salesman sells the pumps to the oil refinery to pump kerosene. Each pump can elevate the kerosene 100 feet, or overcome 100 feet of resistance in a pipe system.
Another salesman sells two pumps to the paint factory to pump white house paint. Each pump can elevate the paint 100 feet, or overcome 100 feet of resistance as the paint goes through a filter.
Another salesman sells four pumps to the dairy to pump milk. Each pump can elevate the milk 100 feet or overcome 100 feet of resistance as the milk is homogenized.
Another salesman sells four pumps to the chemical plant to pump caustic soda. Each pump can elevate the caustic soda 100 feet.
The pump manufacturer doesn’t know the ultimate destination of the pumps. The manufacturer only knows the pumps will develop 100 feet of head.
Earlier in this article, I wrote (exact words), “Although gravity is a constant, the denser (heavier) object will strike the Earth with a force relative to its density.” Pressure is a force applied to an area, such as a pound of force applied to a square inch of area, the famous PSI.
A cubic foot of gasoline, a cubic foot of water, and a cubic foot of liquid mercury would all occupy the same area (a square foot of space) on the floor. The gasoline would apply less force than the water. The liquid mercury would apply more force than the water.
Therefore, the liquid’s specific gravity (density) is a component of pressure and required to convert head into pressure. If the liquid were water, the specific gravity (or relative density) is 1.00.
For water (sp.gr. 1.00), 100 feet of head is 43.3 PSI of differential pressure across the pump. For gasoline (sp.gr. 0.75), 100 feet of head is 32.5 PSI of differential pressure across the pump. For sulphuric acid (sp.gr. 2.00), 100 feet of head is 86.6 PSI of differential pressure across the pump. For liquid mercury (sp.gr. 13.6), 100 feet of head is 589 PSI of differential pressure across the pump.
I know some readers are thinking, “This is logical, but there is still one dangling, unresolved factor.” To resolve this factor, go back to the same paragraph mentioned before. I said (not an exact quote), “Although gravity is a constant, the heavier object will do more damage than the lighter object when it hits the ground. It will leave a deeper crater.” So what’s the difference?
When elevating against gravity, the power (horsepower or kilowatts) of the driver (electric motor or steam turbine) is a function of the liquid’s weight or density. This is called the “specific gravity.” The driver’s power is also proportional to the liquid’s density.
Head is independent of the liquid’s specific gravity or density-relative-to water. Power and pressure are directly proportional to the liquid’s density (sp.gr.).
You cannot rate a pump by pressure unless you know the specific gravity of the liquid. Therefore, most pump manufacturers rate their pumps by head. Head is the rating constant for the pump manufacturer. Specific gravity is not a component of head.
On rare occasion, a pump manufacturer will rate the pump by pressure when the liquid is given or known. Here are two examples:
- Let’s say a pump manufacturer makes a pump to be mounted on a fire truck to extinguish fires with water. This pump will never pump diesel fuel, or vegetable oil. This pump is designed, sold and installed to pump cold water on a fire truck. The manufacturer might say the firewater pump can develop 200 PSI.
- Let’s say a pump manufacturer makes a special pump to be used in a computerized blood chemistry analyser. This pump is only designed to pump human blood with anticoagulant. This pump will never pump ink or tomato sauce. The manufacturer might say this special pump can develop 2 PSIG, knowing the liquid is ambient human blood.
Specific gravity plays a role in converting “head” into “pressure,” and “pressure” into “head.” But specific gravity is not part of head. A pump that develops 40 feet of head can elevate any liquid 40 feet against gravity or overcome 40 feet of resistance in a pipe system.
I hope this explanation helps answer your question?
The Pump Guy
The Pump Guy Seminar is scheduled for the following trainings: Sudbury, Ontario, CA, Sept. 15-18; Durban, South Africa, Nov. 3-6; Dubai, UAE, Nov. 10-13. For more information, contact email@example.com.
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, Tenn. Mr. Bachus is a retired member of ASME and lectures in both English and Spanish. He can be reached at firstname.lastname@example.org.