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I found this article written by Larry Bachus recently (Jan 2008) and find it is pretty simple but useful. It gives us a simple idea the difference between pump pressure and head.
Larry Bachus (a.k.a. "Pump Guy")
Cheat Sheets: Pump Secrets Lost in Time
The Relationship Between Pressure & Head
Early in my career I worked in a steel mill. One day, my boss gave me a purchase chit, put me in a company truck, and told me to go into town and buy a pump for cooling water. He said to get a pump that develops 30 PSI.
At the pump shop, the clerk showed me a pump that develops 70-ft of head. I thought, “Who cares about 70-ft? I need 30 PSI.” I didn’t know the relationship between head and pressure. I’m not alone.Pump industry people use the term “head”;maintenance people use the term “pressure”.
What is head? What is pressure?
In simple terms, they are the same. The terms head and pressure are interchanged in conversations regarding pumps. But they are different, with different definitions.
Head is a measure of energy. The units of energy are expressed in feet or meters. The term pressure is a force applied to a unit of area, such as pounds per square inch [lbs/in2 (or PSI)], or kilograms per square centimeter (kgs/cm2).
The term “head” goes back to the beginning of civilization. Ancient civil engineers built giant water troughs (aqueducts) to carry drinking water from mountain streams and lakes, down into the cities below. The water would spout forth at a public fountain. Housewives and servants would carry the water home.
The water’s flow was measured in barrels and jugs. The water’s force (pressure) was a function of gravity, the elevation differential from the source (the mountain lake) to the fountain (along with the water’s velocity and resistance losses).
This force was measured in units or lengths of elevation. Ancient engineers understood the hydraulic laws that govern today’s modern pumps.
Early pump builders (the Archimedes Screw, the Egyptian Noria, the Persian waterwheel) adopted the term head to express the elevation and force of water. And 3,000 years later, today’s modern pump manufacturers rate their pumps in feet or meters of head.
The term head is the constant for the pump manufacturer. And because head is the measure of energy, a pump that generates 90-ft. of head can elevate any liquid to 90-ft. above the surface level of the liquid’s source. As long as I use the word head, it does not matter what the liquid is.
You see, a pump manufacturer will ship some pumps to a distributor. The distributor will sell the pumps to the end-user. The distributor might sell a specific model of a pump to a dairy, a fuel storage company, a paint manufacturer, a chemical plant, and a pharmaceutical company. The pump manufacturer doesn’t know the ultimate service of his pumps. He only knows that his pumps will develop 90-feet of head. His pumps will elevate chocolate milk, kerosene, red paint, acid, caustic soda, and cough syrup 90 feet. The pressures will be different for each liquid, but the heads will all be 90 feet.
For this reason, the engineer, operator, and mechanic must understand head to converse with the pump manufacturer, and to interpret pump and system curves. This is also the reason that too many pumps are sold without adequate gauges. Not understanding head, pressure, and pump curves is the reason too many pumps suffer mysterious vibrations, with rapid bearing and seal failure. We’ll bring head and pressure together in a future cheat sheet.
You now know what Archimedes and Pi (π) had figured out 3,000 years ago. Rip these pages, copy and share them, or store this edition of Flow Control in a safe place for future reference. This is your CHEAT SHEET of useful pump information.
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 member of ASME and lectures in both English and Spanish. He can be reached at larry@bachusinc.com or 615 361-7295.
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