Someone said, “A zoo is a special park in many cities where animals come from all over the world to stand around and look at the people.” Most people think it’s the other way around.
I’m sitting in a hotel room in the Caribbean. Tomorrow I will go into a chemical plant to see a sick ammonia pump. Hopefully, I’ll be able to solve the pump’s problem. The TV is on some movie channel, and there’s an old movie about a cowboy who whispers to horses. He coerces the horse to do man’s bidding without “breaking” and dominating the horse. He’s the “Horse Whisperer.”
There is a similar TV program starring some man who whispers to dogs, the “Dog Whisperer.” The dog whisperer changes the dog’s behavior without resorting to force or punishment — or so it seems.
Actually, he changes the dog owner’s behavior. I admit that the dog reacts differently by the end of the program, but only because the owner changed his attitude toward the dog. And this brings me to this installment of “The Pump Guy.”
|James Watt’s centrifugal governor on “Old Sally,” a 150-Hp steam engine installed in 1881 at Boulder, Co. A shot of steam drives the balls (top, center of photo). As the balls spin faster, they stand out by centrifugal force and levers raise a valve stem that opens to relieve excess pressure and governs the engine’s velocity. This mechanical device is the history of instrumentation.|
People think the Pump Guy is clairvoyant with cast iron. They think I can diagnose and cure a “bad actor” pump. I really hate that phrase, because the pump is not the bad actor. Think of the zoo and the dog whisperer. My real mission is to change the engineer’s attitude toward pumps.
In this installment of the Pump Guy, I’m going to offer some horse (common) sense to understanding horsepower and kilowatts applied to pumps. First, a little history.
James Watt was born in 1736. He was raised and educated in the coal and iron mining region of Scotland. In school, James was interested in math and studied instrument repair. In 1755 a customer brought him a boiler to repair. James studied the old boiler and thought of ways to improve it. James is credited with developing the modern boiler, the centrifugal pressure regulator, the centrifugal governor, and the rotary steam engine. He also invented devices used in civil engineering and surveying. Every student of electricity knows James. After all, the electric unit “Watt” is named in his honor.
Mining was a labor intense industry, and still is today. For menial labor, children were sent into the mines, because they had small lungs and didn’t consume the air (oxygen) that a full-grown miner would consume. For work beyond the ability of a grown man, the mining industry bred especially small horses that could go into the mines and do the pulling and hauling that a grown man could not do.
Because most mines are dug below the water table, and sometimes below sea level, many mines would eventually flood if it were not for the children and horses that removed the water. With pity toward the children and the horses, James Watt decided to replace the human and animal labor in the mines with machines.
Just as an artist would begin a painting by establishing perspective, James Watt began by defining terms and establishing standards. He defined “energy” as the capacity to perform work. James defined “work” as a force exerted or multiplied over a distance. And he defined “power” as work performed within a certain time frame.
James began his quest by establishing what a horse could do. He harnessed a work horse to a support frame with a pulley and a platform with different weights. The horse lifted 550 pounds of weight a distance of 10 feet in 10 seconds.
So James Watt declared that 550 foot-pounds per second is one horsepower. It is because of Mr. Watt’s scientific effort that all electric motors, internal combustion engines, turbines, boilers, jet and rocket engines, etc. are rated in horsepower, as opposed to iguana power or ostrich power.
James went to his grave with an important part of the formula though. He never told anyone what the size of his test horse was, so we don’t know if it was a Shetland pony or a Clydesdale.
Because the first pumps pumped water, the output or work of a pump is called Water Horsepower (WHP). The formula is:
3,960 is a factor that converts horsepower into pump terms. Enter this into your cheat sheets, and remember it. I’ll explain.
Horsepower is expressed in foot-pounds per second, and pump output is expressed in gallons per minute (GPM). We need to convert horsepower terminology into pump terminology. One HP is 550 foot-pounds per second. Multiply this by 60 seconds in a minute and we have 33,000 foot pounds per minute, or horsepower-minutes. Next, a gallon of water (at sea level and 70 F.) weighs 8.333 pounds. Divide the 33,000 ft.-pounds by 8.333 pounds per gallon and we have 3,960. The “3,960” is a horsepower expressed in pump terminology.
In the early days of pumps, water was the first and only liquid moved in bulk. Other liquids like beer, paint, lamp oil, milk, acid, and whiskey were made in bottles, buckets, or casks and carried from one place to another. As technology advanced and pumps were applied to move other liquids, another element, the specific gravity, was incorporated into the basic horsepower formula. The formula for water horsepower incorporating specific gravity is:
The result of this equation is a good number, but it’s not very useful because we never need to know this number. Frequently, we need to know a variation of this number. We need to know the BHP or what size motor we are going to use on this pump. The BHP considers the pump’s efficiency. Now we have:
This is a relatively easy formula to understand. Lets work with it a little. Imagine that we need a pump for a system that requires 600 GPM of water, while generating 40 PSI. The 40 PSI in pump terminology is the same as 92 feet of head. Let’s say our pump is 77 percent efficient. The specific gravity of water is 1.0. To apply the formula, the H is 92, the Q is 600, and the efficiency is 77 percent. Now we have:
This pump would burn 18.1 horses or 13.5 kilowatts (1-Hp = .746-kw). We’d have to install a 20 hp motor onto this pump because no one makes an 18.1 hp motor.
Many pump specifications don’t mention the efficiency. They’re mostly concerned with meeting the needs of the system, meaning head and flow. What if we could save $800.00 by buying another comparable pump that meets the needs of the system. If the cheaper pump were 60 percent efficient, we’d have some different horsepower figures:
Now we must buy a 25 HP motor to operate this other pump with the reduced efficiency. And it’ll consume 17.3 kilowatts. The difference is 5.1 horses or 3.8 kilowatts (1 HP = .746 Kw) compared to the first pump.
If the pump runs for a year, this would mean an additional $3,332 dollars per year in electricity to run the inefficient pump at 10¢ per kilowatt/hour. And this $3,332 would be on top of the additional cost for the 25 HP motor. How do you feel about your $800.00 savings now?
An even cheaper pump might perform the same work and be only 50 percent efficient. This pump would burn 27.8 horses of electricity, and it would require an even larger motor. If it ran 24 hours per day, this would be an additional $6,340 in the electric bill per year compared to the first pump.
The lesson to be learned is: Always buy the most efficient pump, especially these days with rising energy costs.
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 email@example.com or 615 361-7295.