The equation that calculates the minimum wall thickness needed to contain the pressurized
fluid is given as follows: In this equation we have
the design pressure of the fluid in PSI, and we'll talk more about this later,
the outside diameter of the pipe in inches, which should not include any coating thickness,
and the allowable stress of the pipeline material,
also in PSI.
AWWA Manual of Practice 11 (or just M11 for short) states that we should assume a maximum
of 50% of the allowable stress.
This means that we should also insert here a factor of safety of 2 into this equation.
Please note that the allowable stress is a function of the specific type of Steel used
as determined by the appropriate ASTM standard.
Those listed in M11 include ASTM A36 steel as well as, A283, A570, and A572.
When I was getting paid for doing these calculations we typically specified ASTM A570, Grade 30
(meaning 30 ksi as the maximum allowable stress).
Realizing that not everywhere uses ASTM standards to define the properties of a given type of
steel, the key lesson you should learn is that whatever maximum allowable stress you
use in the calculation is directly tied to the material of the pipe.
Therefore, when writing your design specifications, you should expressly note which material you
are selecting so that the contractor purchases the correct steel.
Later when the contractor shows you what he bought, you can check his shop drawing against
the calculation assumptions you made earlier.
Now that you are all bored to death, let's move on!
Design pressure.
If we look at this simple pumping hydraulics schematic, you may initially think perhaps
the design pressure is merely hydrostatic head shown here as the difference between
Z2 and Z1.
But they you would quickly correct yourself, because that is only true when the pump is
off and the water is simply flowing down the hill.
When the pump is on, we add energy into the system to overcome the headloss due to friction
and fittings.
So when the pump is running the maximum pressure occurs just downstream of the pump,
and can be calculated by converting the static head and the dynamic head into pressure units
by multiplying by the unit weight of the fluid.
However, even this calculation is not good enough because it does not account for the
Surge pressure.
Surge pressure is a transient or non-steady condition that can be cause by a number of
factors.
The most common are when the pump turns on, when the pump shuts off, or when a valve closes
in the system.
Let's assume that you have water travelling down a pipe at velocity "v".
In a short time a valve closes in the pipeline causing the water in the pipe to crash into
the valve.
Depending upon the velocity, the pipeline reacts
by expanding slightly and then creating a return wave headed in the opposite direction.
This return wave then bounces its way back through the system, pounding every pipe fitting
like a hammer - which is why it is often called water hammer.
In another video I cover the details of hydraulic transients and the effect known as water hammer
a little more, but you should know now that this needs to be accounted for in our
equation for the design pressure.
A distribution system is divided by pressure zones, and given the guidelines applied to
water systems the minimum operating pressure is
150 psi for one pressure zone.
Because surge analyses are quite complicated and require that much of the system be in
final design before they can even be remotely accurate we often simply add
an additional 50 psi to handle the surge pressure.
What this means is, for example, that a system pumping only one pressure zone, will have
a design pressure of 200 psi.
A given water agency will specify a maximum and minimum static head, and the difference
between these two numbers is the additional pressure needed to pump up to the next pressure
zone.
In this example, this is 50 psi.
Therefore, a water system pumping to two pressure zones at a time, with this same static head
criteria, will need to account for 200 psi of operating pressure with the same
50 psi of surge; making the design pressure in this example, 250 PSI.
A similar procedure can be applied to systems that pump 3 pressure zones at a time.