See below for the following terms: Static Head and Total Dynamic Head.
A method of describing the amount of energy contained in a fluid, by linear measurement of the fluid column height perpendicular to the gravitational field, (the earth). If the fluid density is known then the pressure at any point in the fluid column can be calculated.
The standard US linear measurement of head is Feet, thus Feet of Head. In the SI (Metric) system the height is commonly given as Meters, thus Meters of Head.
Do not confuse head with pressure, they are not the same. Head values can be converted to pressure values (and vice versa), but only if the density of the fluid is known.
Head is a convenient method for designing and analyzing fluid pipe systems, because elevations can be directly added or subtracted in the calculations without conversions. If pressure were used, the elevations in a design could not be calculated without first converting pressure values to head values.
Head has a special relationship to centrifugal pumps. Centrifugal pumps add energy into a fluid by accelerating the fluid. If that accelerated fluid were directed up a column of pipe perpendicular to the earth, the fluid would rise to a specific height in that column. If the test fluid under which the pump performance characteristic is reported by the manufacturer, is free flowing, non-viscous, and non-compressible, then that pump will produce the same head performance for any other similar fluid, regardless of density. Why?
This is simply Galileo's experiment in reverse. Galileo observed that when two objects of different weights but with identical shapes and sizes, were dropped from a height, both objects struck the earth simultaneously. Therefore, Galileo correctly concluded that gravity accelerates objects identically regardless of the densities of those objects. So, in reverse the explanation for a pump would be; If two free flowing non-compressible fluids with different densities, are accelerated by a pump to the same velocity, perpendicular to the earth, both fluids will rise to the same height.
Many object to this conclusion because they know there must be differences, and there are in fact two differences, but not in velocity head. First, the power required to accelerate the two objects will be different, more power is required to accelerate the heavier fluid. Second, the pressure will be higher for the heavier fluid. But the Feet of Head will be identical for the two fluids regardless of their different densities.
Consider mercury (Hg) versus water. Hg is 13.5 times heavier than water. If a pump provides 200 Feet of Head at Shut-Off when pumping water, the pump will also provide 200 Feet of Head at Shut-Off when pumping Hg. However, the power and pressure will be different for the two fluids. If the pump requires 10hp when pumping water to 200 Feet of Head, then that same pump requires 135hp to move Hg to that same 200 Feet of Head. Pressure will also be different. A pressure gauge mounted at the pump discharge nozzle, will read 87psi at Shut-Off when the pump is handling water, but that same pressure gauge would read 1,175psi when mercury is pumped.
Static Head
The energy contained in a system resulting only from the elevation of the liquid within a gravitational field (earth's gravity).
Example
The highest point of a water pipe in a building is 200 feet above the pump located on the ground floor. The pipe is full of water, the pump is off, and there is no water movement in the pipe.
Static Head at the pump would therefore be 200 feet.
Observe that pump head must exceed static head to move water into the system. Therefore on this system just described, the pump must develop more than 200 feet of head at its discharge nozzle before water would move into the system.
Total Dynamic Head (TDH)
This term can be confusing, and actually there may be no "official" definition, even though at least one expert does use the term (Stepanoff). TDH often appears in a context where the writer or speaker wishes to convey the following: "head developed at a given flow rate". Because the term can be confusing, Irrigation Craft does not use the term. Irrigation Craft expresses pump performance as in the following example:
300 gpm @ 200 Feet of Head
or
0 gpm @ 250 Feet of Head
Convert Feet of Head to PSI Water Pressure
Conversion Factor = Feet of Head x 0.433
The conversion factor is obtained by the following method:
Determine the force value (pounds in this instance).
Determine the unit area value (square inch in this instance).
Then take the unit area value and make a cube one unit area on each side. In this case we have "square inches", so we construct a cube with 1-Inch sides.
Determine how much that cube of fluid weighs.
Now multiply that value by the number of cubes required to equal the head value. In this case, water cubes stacked 12 high equals 1-Foot Of Head.
1-cubic inch of water weighs .03608 pounds.
12 cubic inches of water therefore weighs 0.433 pounds. (12 x 0.03608 = 0.433 pounds).
Example:
A pump delivers 100 gpm @ 150 feet of head.
Multiply head by 0.433 to obtain water pressure (150 Feet x 0.433 Pounds = 64.95 psi)
The pump delivers 150 gpm of water @ 64.95 psi.
Reverse the process (convert pressure to feet of head) by simply reversing the math. Divide pressure by 0.433. Generally it is better to avoid division, so the preferred method would be to use the inverse multiplicative of 0.433, in which case the multiplier would be 2.31 (1/.433).
Therefore, the multipliers for clear cool water are:
Head * 0.433 = PSI
PSI * 2.31 = Head
The multipliers for any fluid can be worked out as shown above. However, there is a shortcut if you do not work with the fluid type very often. Just do the calculations for water, then multiply the results by the fluid specific gravity (sg).
Example:
Pump delivers 100gpm @ 200 Feet of Head.
Obtain PSI for the gasoline at 100gpm
Gasoline specific gravity = 0.75
200 Feet of Head x 0.433 = 86.6 psi (water pressure).
86.6 psi x 0.75 = 64 psi for gasoline.
Related Subjects on this Website:
Hydrodynamic Bearing - See Non-Rolling Element Bearings
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