Useful work from a pump

The physicist James Watt is honored in the electrical community for the term Svatt'. He made various advancements and improvements to stationary boilers and steam engines. It is said that the first practical use of the steam engine was in raising (call it pumping) water out of the coalmines. Almost all mines would flood if the water were not pumped from the bilge, out of the mine. Before the steam engine, the miners used children and horses to lift and carry the bilge water.

James Watt developed the terms of energy, work, and power. He defined the following:

■ Energy is the capacity to perform work. Example; I have the energy in my bicep muscle to lift a 100-pound weight.

■ Work is a force multiplied over a distance. Example: If I lift a 5-pound weight one foot into the air, then I've performed 5 footpounds of work.

■ Power is work performed within a certain specified time frame. Power is when I perform 5 foot-pounds of work within a second, or minute.

Many people confuse these terms, but they actually have precise definitions. If I should lift 10 pounds a distance of 10 feet, then I've performed 100-foot-pounds of work (10 pounds x 10 feet = 100). Before the steam engine, the most powerful force to perform work, or exert a force, was a horse.

James Watt, with actual tests, determined that a coal mine draft horse could lift 550 pounds, a distance of one foot, within a second. So, James Watt declared 550 foot-lbs/sec. to be one Horsepower. To this day, this has become the standard definition of a horsepower (1 HP =

10 Pounds

10 Pounds

10 Feet

Figure 5-1

550 ft.-lbs./see.). This is the reason that even today, all motors, whether steam, internal combustion engines, boilers, electric motors, gas turbines, and even jet and rocket engines are rated in Horsepower, and not Ostrich power or Iguana power.

We say that the motor generates horsepower (HP), and that the pump consumes brake horsepower (BHp). The difference between HP (output) and BHp (input) is what is lost in the power transmission; the bearings, shaft, and coupling between the motor and the pump.

We say that the useful work of the pump is called Water horsepower (WHp). It is demonstrated mathematically as:

Where: H = head in feet generated by the pump Q = flow recorded in gallons per minute sp. gr. = specific gravity 3960 = constant to convert BHp into gallons per minute

Horsepower x 60 sees. / min. Weight of 1 gal. of water

If the pump were 100% efficient, then the BHp would be equal to the WHp. However, the pump is not 100% efficient so the BHp = WHp x efficiency, and the formula is:


Figure 5-2

The graph (Figure 5-2) shows the useful work of a pump. Notice that the pump pumps a combination of head and flow. As a general rule, as flow increases, the head decreases.

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