BIRMINGHAM — In the article, “'Head' and 'pressure' in pumps,” featured in a previous issue of Water Technology, Larry Bachus, contributing writer, discusses the terms, “pressure” and “head,” and why it is important to know how these terms relate to each other.
In the article, Bachus relays a story of when he was employed as an apprentice mechanic in the mid-1960s and one of the water pumps failed in the cooling ponds. He was told by his boss to go get a water pump that pumps 30 psi (pounds per square inch) at 400 gpm (gallons per minute). A sales rep at an industrial supply house told Bachus to get a pump that generates 70 feet of head at 400 gpm.
“I said, ‘Wait a minute! I don’t need 70 feet of head; I want 30 psi at 400 gpm. What is 70 feet of head?’ I thought the sales rep was trying to do a ‘bait-and-switch’ on me,” says Bachus. “The requisition chit clearly stated 30 psi. I wondered why the sales rep used different terms. Indignantly, I walked away and went to a competing industrial supply house — where I repeated the same verbal exchange with their sales rep.”
Bachus continues by informing that the misunderstanding in regards to “head” and “pressure” happens all of the time. “Pump users want pressure; pump manufacturers supply feet (or meters) of head,” he adds. “In the final analysis, they are the same, just expressed from two different points of view. As someone who specifies and/or installs pumps, you need to know how these terms relate to each other.”
In the article, Bachus reports a brief history on how the terms pressure and head related to water flow came to be, with pipes carrying the water by gravity and how it was understood that force — rated in units of energy — was required to elevate a quantity of water against gravity. He notes that today’s pump companies continue to rate a liquid’s force as a unit of energy against gravity, and by applying this same force in another direction, such as against a pressurized tank’s interior sidewall, we would use the term “pressure.”
“In simple terms, the mathematical constant 2.31 converts a unit of energy against gravity into a unit of force against any other area,” explains Bachus in the article. “This constant converts a foot of head of water into pressure: Head in feet of water divided by 2.31 equals pressure in psi, and pressure in psi times 2.31 equals head in feet.”
He adds, “The constant 2.31 comes from the following: A square foot of area contains 144 square inches; a cubic foot of ambient-temperature water weighs 62.38 (62.4) pounds per cubic foot at 70 F at sea level. If I poured one pound of water into a tall, narrow vessel that occupies one square inch of floor space, I would fill that vessel to 2.31 feet of elevation.”
Bachus reports in the article that the sales rep back in 1965 was indeed correct, as 30 psi multiplied by 2.31 equals 70 feet.
You can read the entire article on head and pressure in water pumps here.