25-01-2013, 04:53 PM
Pipe Networks
Pipe Networks.ppt (Size: 3.73 MB / Downloads: 170)
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
Tips for EGL/HGL
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.
Tips, continued
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 sub-atmospheric 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.
EPANET Free Software
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
Description
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.
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.