Pump Primer 1 $100 - 8 Hours - Technical Learning College

Water Pressure
The weight of a cubic foot of water is 62.4 pounds per square foot. The base can be subdivided
into 144-square inches with each subdivision being subjected to a pressure of 0.433 psig.
Suppose you placed another cubic foot of water on top of the first cubic foot. The pressure on
the top surface of the first cube which was originally atmospheric, or 0 psig, would now be
0.4333 psig as a result of the additional cubic foot of water. The pressure of the base of the first
cubic foot would be increased by the same amount of 0.866 psig or two times the original
pressure.
Pressures are very frequently stated in terms of the height of a fluid. If it is the same fluid whose
pressure is being given, it is usually called "head," and the factor connecting the head and the
pressure is the weight density ρg. In the English engineer's system, weight density is in pounds
per cubic inch or cubic foot. A head of 10 ft is equivalent to a pressure of 624 psf, or 4.33 psi. It
can also be considered an energy availability of ft-lb per lb. Water with a pressure head of 10 ft
can furnish the same energy as an equal amount of water raised by 10 ft. Water flowing in a
pipe is subject to head loss because of friction.
Take a jar and a basin of water. Fill the jar with water and invert it under the water in the basin.
Now raise the jar as far as you can without allowing its mouth to come above the water surface.
It is always a little surprising to see that the jar does not empty itself, but the water remains with
no visible means of support. By blowing through a straw, one can put air into the jar, and as
much water leaves as air enters. In fact, this is a famous method of collecting insoluble gases in
the chemical laboratory, or for supplying hummingbird
feeders. It is good to remind oneself of exactly the
balance of forces involved.
Another application of pressure is the siphon. The
name is Greek for the tube that was used for drawing
wine from a cask. This is a tube filled with fluid
connecting two containers of fluid, normally rising
higher than the water levels in the two containers, at
least to pass over their rims. In the diagram, the two
water levels are the same, so there will be no flow.
When a siphon goes below the free water levels, it is
called an inverted siphon. If the levels in the two
basins are not equal, fluid flows from the basin with
the higher level into the one with the lower level, until
the levels are equal.
A siphon can be made by filling the tube, closing the
ends, and then putting the ends under the surface on both sides. Alternatively, the tube can be
placed in one fluid and filled by sucking on it. When it is full, the other end is put in place. The
analysis of the siphon is easy, and should be obvious. The pressure rises or falls as described
by the barometric equation through the siphon tube. There is obviously a maximum height for
the siphon which is the same as the limit of the suction pump, about 34 feet. Inverted siphons
are sometimes used in pipelines to cross valleys. Differences in elevation are usually too great
to use regular siphons to cross hills, so the fluids must be pressurized by pumps so the
pressure does not fall to zero at the crests.
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