Transcript Chapter 4: Pipe Networks

Chapter 4: Pipe
Networks
December, 2011
Have you been doing the
homework?
Student who did not keep up with homework after
seeing final class grade!
EGL, HGL
• Energy drives water through pipes. Visual tools to assist
with overview of pipeline systems are the
• Energy Grade Line: V2/2g+z+p/ ρ g since we have a
figure, the pipe is located at z so the energy grade line is
the sum of the velocity and pressure heads above
(sometimes below) the pipe
• The energy grade line (EGL) gives a visual picture of
energy loss/gain in the system. Where is most of the
head loss occurring? Something important to an
engineer
• Hydraulic Grade Line: z+p/ρg which is one pressure
head away from the pipe; as such it gives a visual
measure of the pressure in the system; pressures too high
rupture pipes and exacerbate leaks
• Negative pressure can lead to cavitation
Figure 4.1
Figure 4.1
Figure 4.3 Energy grade line and hydraulic grade line of a pumping station
Figure 4.3 Energy grade line and hydraulic grade line of a pumping station
Tips for EGL/HGL
Note: switch back and forth to figure on next slide
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4.
Some hints for drawing hydraulic grade lines and energy grade lines are as follows:
By definition , the EGL is positioned above the HGL an amount equal to the
velocity head. Thus if the velocity is zero, as in a lake or reservoir, the HGL and EGL
will coincide with the liquid surface.
Head loss for flow in a pipe or channel always means the EGL will slope downward
in the direction of flow. The only exception to this rule occurs when a pump
supplies energy (and pressure) to the flow. Then an abrupt rise in the EGL occurs
from the upstream side to the downstream side of the pump.
In point 2, we noted that a pump can cause an abrupt rise in the EGL because
energy is introduced into the flow by the pump. Similarly, if energy is abruptly
taken out of the flow by, for example, a turbine, the EGL and HGL will drop
abruptly as in Fig. 5-8. Figure 5-8 also shows that much of the velocity head can
be converted to pressure head if there is a gradual expansion such as at the
outlet. Thus the head loss at the outlet is reduced, making the turbine installation
more efficient. If the outlet to a reservoir is an abrupt expansion, all the kinetic
energy is lost; thus the EGL will drop an amount of aV2/2g at the outlet.
In a pipe or channel where the pressure is zero, the HGL is coincident with the
water in the system because p/g= 0 at these points. This fact can be used to
locate the HGL at certain points in the physical system, such as at the outlet end
of a pipe, where the liquid discharges into the atmosphere, or at the upstream
end, where the pressure is zero in the reservoir.
Example used in tips
Efficiency: round reservoir exit, larger/smoother pipe, round exit
Tips, continued
5.
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For steady flow in a pipe that has uniform physical characteristics (diameter,
roughness, shape, and so on) along its length, the head loss per unit of length will
be constant; thus the slope of the EGL and HGL will be constant along the length
of pipe.
If a flow passage changes diameter, such as in a nozzle or a change in pipe size,
the velocity therein will also change; hence the distance between the EGL and
HGL will change. Moreover, the slope on the EGL will change because the head
loss per unit length will be larger in the conduit with the larger velocity.
If the HGL falls below the pipe, p/g is negative, thereby indicating subatmospheric pressure. If the pressure head of water is less than the vapor pressure
head of the water (approximately -33 ft at standard atmospheric pressure and (T
=60°F), cavitation will occur. Generally, cavitation in conduits is undesirable. It
increases the head loss and can cause structural damage to the conduit from
excessive vibration and pitting of the conduit walls. If the pressure at a section in
the pipe decreases to the vapor pressure and stays that low, a large vapor cavity
can form leaving a gap of water vapor with columns of water on either side of
the cavity. As the cavity grows in size, the columns of water move away from
each other. Often these columns of water will rejoin later, and when they do, a
very high dynamic pressure (water hammer) can be generated, possibly rupturing
the pipe . Furthermore, if the pipe is relatively thin walled, such as thin-walled steel
pipe, sub-atmospheric pressure can cause the pipe wall to collapse. Therefore,
design engineer should be extremely cautious about negative pressure heads in
the pipe.
Practice Locations
• Develop a pump and pipeline design that
addresses a particular set of physical conditions of
your own devising.
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• http://www.ce.utexas.edu/prof/maidment/gishydro
/ferdi/webedu/utflow/utflow.html
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• http://www.ce.utexas.edu/prof/maidment/ce319f/
webtools/visiflow/visiflow.htm
quiz
Figure 4.7 Multiple reservoirs connected at a junction
We will not solve network
problems by hand
Figure P4.6.7
A network problem
Networking Example
EPANET Free Software
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http://www.epa.gov/nrmrl/wswrd/dw/epanet.html
Image: Water tower. EPANET tracks the flow of water in each pipe, the pressure at each node, the height of the
water in each tank, and the concentration of a chemical species throughout the network during a simulation
period.
EPANET
Software That Models the Hydraulic and Water Quality Behavior of Water Distribution Piping Systems
* Description
* Capabilities
* Applications
* Programmer’s Toolkit
* Multi-Species Extension
* Support
* Downloads – updated 5/27/08
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Description
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Developed by EPA's Water Supply and Water Resources Division, EPANET is software that models water distribution
piping systems. It is a Windows 95/98/NT/XP program that performs extended-period simulation of the hydraulic
and water quality behavior within pressurized pipe networks.
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Pipe networks consist of pipes, nodes (pipe junctions), pumps, valves, and storage tanks or reservoirs. EPANET
tracks the flow of water in each pipe, the pressure at each node, the height of the water in each tank, and the
concentration of a chemical species throughout the network during a simulation period. Chemical species, water
age, source, and tracing can be simulated.
Water Hammer
Water Hammer Examples
• http://www.youtube.com/watch?v=Kb6feUfHgAU
• http://www.youtube.com/watch?v=PUyuaOKXYY8
• http://www.youtube.com/watch?v=oZbguheiVs4&f
eature=related
Home Fix
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Water Hammer Description
Engineered Water Hammer Arrester
© 2007 Sioux Chief Manufacturing Company, Inc.
Water Hammer (hydraulic shock) is by far the loudest and most common plumbing noise
problem in the home. You hear it in a home having high water flow rates (around 10 feet per
second) when a faucet or water valve is shut off quickly. Older homes have (or should have)
what is called an "air chamber" located on each hot and cold water line at or near each
faucet or water inlet valve. The purpose of the air chamber is to act like a shock absorber for
water when it is flowing at high speed under pressure. Since air compresses (it's a vapor) and
water doesn't, the air chamber allows the water a place to temporarily expand into and softens
the blow of the water shock wave when the faucet is turned off quickly.
Air chambers are often fabricated on-site by the plumber and installed at the faucet's water
supply. They typically consist of a vertical length of capped pipe about 12" long or longer and
are the same diameter as the water supply pipe. The problem with these things is that they are
sometimes made too short and undersized and eventually fill up with water and you have no
more shock absorber. The result? Water hammer!. If you have a water filled air chamber it can
be easily fixed by draining and recharging the plumbing system.
A better solution to alleviate the problem is to have a mechanical or engineered water hammer
arrestor installed. These mechanical plumbing devices are charged with air or gas and will not
fail like typical air chambers.
http://homerepair.about.com/od/plumbingrepair/ss/pipe_noises_3.htm
Irrigation Fix
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The easiest method to fix water hammer is to lower
the water pressure for your entire irrigation system. It
will not get rid of all of the water hammer, but it will
sometimes reduce it to a level you can live with.
If you have an automatic system, you may be able
to rid water hammer by simply changing the order
in which the valves operate. Simply find out which
valve uses the least water. This will probably be the
one with the least amount of sprinklers, but not
always. Once you figure it out, rewire the controller
so that the valve that uses the least water is the last
valve to run.
Try a water hammer arrestor. This mostly only works
with just water hammer caused from
washingmachines and dishwashers. But it is worth a
try with sprinkler related water hammer. Install it per
the instructions on the package and try putting it on
a hose bib to the point where the irrigation system
connects to the house water. If you have a hose
bib on the irrigation system mainline, that is an even
better place. Or you can tap into the irrigation
mainline to install it. If it does not work, remove it
and return it to the store where you purchased it.
An air chamber will
not drain properly if it
is clogged. Remove its
cap and ream out the
accumulated scale
inside the chamber.