Imagine you are standing in a swimming pool, and you place your hand flat on the surface
of the water and then you push your hand rapidly, down into the water.
If you have ever done this, you may know well that trailing your hand, as you plunge it
into the water, is a stream of bubbles.
The fact that your hand created a cavity in the water where air pockets were able to form
is one definition of cavitation.
Here is an impeller, and similar to the motion of your hand, the blades of the impeller are
pushing on the water.
On the trailing side of each impeller blade lies the potential for cavitation.
This definition of cavitation, by itself, is not a problem in pumps.
The problem starts as the next impeller blade, that is trailing the first where the cavitation
started, is - for lack of a better term - catching the bubbles that are created.
On the surface of the impeller blade the bubbles can be compressed until the point of collapse.
At the moment of collapse, a micro-jet of water forms and strikes the surface of the
impeller at a velocity exceeding 100 m/s!
As you can imagine this process, though happening on a micro level, can begin to pit the metal
surface of the impeller blade when you consider that these bubbles are forming and exploding
at the rate of hundreds or thousands of mini-explosions per second.
If we return to our first thought experiment - the one where we were standing in the pool.
But this time we start with our hand severl inches beneath the water surface and then
we pull our hand downward, we will see that not nearly as many bubbles are generated by
the motion of our hand.
This is due to the fact that as the static pressure helps keep the water together and
this prevents the cavities from forming and the numbers and size of the bubbles decrease.
Therefore, in order to prevent the formation of the bubbles in the first place, we - as
civil engineers who design the piping systems that supply water to the pumps - must ensure
that there is sufficient pressure head in the fluid to prevent cavitation.
The minimum pressure head our systems provide is called the Net Positive Suction Head.
The net positive suction head is the absolute minimum pressure required to prevent cavitation.
In any given system, because of the orientation of the reservoir to the pump, or the diameter
and roughness of the pipelines, there is a minimum NET POSITIVE SUCTION HEAD AVAILABLE
in the system.
This we simply abbreviate as NPSH(A).
It is calculated as the potential energy between the reservoir and the pump elevation.
Written this way you can see that if the pump is higher than the reservoir then there is
a negative potential energy that the pump must overcome, while if the pump is below
the reservoir, then the potential energy is helping our NPSH(A) calculation.
Next we account for the atmospheric pressure head.
This is a function of the elevation we are, above sea level.
We must also account for all friction and minor losses between the reservoir and the
pump suction.
Finally we must also account for the vapor pressure head of the fluid itself.
The difference in elevation between the reservoir and th epump is an engineering design choice
we must make, which also has an impact on the pipe diameter, length and minor losses.
We have tables that we use to lookup the atmospheric pressure at different elevations as well as
the vapor pressure as a function of water temperature.
When we calculate the friction losses we must use the Darcy-Weisbach headloss formula because
unlike the Hazen-Williams headloss formula, the Darcy-Weisbach formula t accounts for
temperature variations in the fluid, and can be applied to fluids other than water.
So if you are pumping Oil, you can still use Darcy-Weisbach.
Every pump that is designed is tested to define its performance limits.
One of the performance considerations is the NET POSITIVE SUCTION REQUIRED by the pump
to avoid cavitation.
Therefore plotted on the pump performance curve is often an additional line that represents
how much NPSH(R) as we call it is needed at a given operating point.
In theory, the NPSH(A) simply needs to be greater than or equal to the NPSH(R).
However in practice - at least in water distirbution systems, the NPSH(A) needs to be greater than
the NPSH(R) plus 5 feet.
This additional head acts like a buffer against the uncertainties of pumping water.
By comparison, When we are pumping wastewater or sludge the buffer is 7 feet.