Does water pressure ôgo to the endö of a pipe?
Does water pressure ôgo to the endö of a pipe?
(OP)
Does water pressure go to the end of the pipe and lessen as you head back toward the pump? The application is a spray system with heavy flow. The water is pumped into pipes and dispersed by orifices in the pipes. The conventional wisdom here at work is that the greatest pressure and flow is near the end of the pipes, and that it therefore needs to be tapered toward the end to equalize the flow (or the orifices made smaller). It appears to me that the greatest pressure in a consistent ID pipe with equal size orifices would be nearest the pump. Can someone explain what the pressure does or direct me to something that explains simple rules of thumb for water flow.





RE: Does water pressure ôgo to the endö of a pipe?
RE: Does water pressure ôgo to the endö of a pipe?
Simple enough to prove the assertion: Take a garden hose and punch some holes in it. With low flow, the holes closest to the hose bib will squirt higher while those at the end of the pipe will squirt lower or not at all. Turn up the water and watch the last hole squirt just as much as the first.
Even easier would be to go to a lawn sprinkler setup which has multiple zones. If you only turn one on, it functions well (provided, of course, that it was well designed to begin with). Now manually (you'll have to turn the valve, a timer won't let you open multiple valves) open the valve for another zone. Continue turning on zones until you see that the sprinkler system doesn't work well at all.
The trick is to get enough flow into the system such that the pressure is essentially constant within the piping. Reducing the diameter of the pipe at the end of the run shouldn't help a system which has too low of a supply volume (gpm). What it will do is keep the flow rate (ft/s) more constant and save on the piping cost. Making the orifices smaller will simply reduce the gpm each orifice is passing such that the supply gpm can better keep up with the losses.
jt
RE: Does water pressure ôgo to the endö of a pipe?
How can it be any other way? It can't.
You cannot have flow - be it liquid, gas, electricity, solids, asteroids, etc. without a driving force. In fluid flow, that driving force is pressure. In order to have a driving force, you must have a difference in the pressure(s). In other words, the downstream pressure must be less than the upstream pressure. Otherwise, there is no flow - just a static condition. This applies to conventional fluid flow within a conduit.
You can also have fluid flow due to convection - there, the driving force is density (energized or activated by temperature difference).
The answer to your question is: Of course, the pressure is differntially decreasing as the fluid flows through the pipe! This pressure loss is due to resistances, friction, change of direction and velocity head losses.
Art Montemayor
Spring, TX
RE: Does water pressure ôgo to the endö of a pipe?
In any pipe with flow the flow must be from the point of high pressure towards low pressure. That without a doubt. I have however the following example where measurements would prove me wrong:
You have a vertical pipe filled with water say 10 m high. Its open to atm pressure at the top - and pressure must therefore here be 0 barg.
At the bottom there is a small hole. If you however insert a pressure gauge it will read 1 barg due to the static head of water in the vertical pipe (flow is quite low and frictional pressure drop insignificant.
Now we have a situation wher there is flow from one point where the pressure must be 0 barg towards another point where its 1 barg - magic
Best regards
Morten
RE: Does water pressure ôgo to the endö of a pipe?
RE: Does water pressure ôgo to the endö of a pipe?
in distribution systems, like irrigation, the pipe near the discharge of the pump has all the flow (100%)... as the water is distributed to the different services, the remaining flow needs a smaller diameter to maintain velocity... otherwise the velocity becomes so small that the fluid is almost stagnant.
many designers (specially in barge heating systems) choose to have a return pipe (very small diameter) to equalize pressures between the pump discharge and the end of the line so all take-off's have almost the same pressure.
this kind of "ring" design will avoid having good pressure near the pump discharge and very little at the end of the line... (like in the houses that u have good pressure in the bathroom on the 3rd floor, but if someone opens a tap at the kitchen in the ground floor... you found yourself all wet and soaped and screaming)
HTH
saludos.
a.
RE: Does water pressure ôgo to the endö of a pipe?
RE: Does water pressure ôgo to the endö of a pipe?
The total energy of the water or whatever fluid at the begiinning will equal the total energy at the end of the system. It just changes the state or form of energy.
RE: Does water pressure ôgo to the endö of a pipe?
What 25362 mentioned in his last post, is one of the methods we use to design air ducts. (called as static regain method)
Gandytable!
Can you pl. explain what do you exactly mean by The conventional wisdom here at work is that the greatest pressure and flow is near the end of the pipes, and that it therefore needs to be tapered toward the end to equalize the flow?
Regards,
Eng-Tips.com : Solving your problems before you get them.
RE: Does water pressure ôgo to the endö of a pipe?
RE: Does water pressure ôgo to the endö of a pipe?
Regards,
Eng-Tips.com : Solving your problems before you get them.
RE: Does water pressure ôgo to the endö of a pipe?
RE: Does water pressure ôgo to the endö of a pipe?
RE: Does water pressure ôgo to the endö of a pipe?
http://www.coolingzone.com/Guest/News/NL_MAR_2002/Inres/Inres_02.html
RE: Does water pressure ôgo to the endö of a pipe?
When you maintain constant header size, when the flow reduces after an opening, velocity reduces and so the velocity pressure. By Bernoulli's principle the static pressure increases. (For pumped fluid systems, the flow is created by the static pressure difference). So pressure difference across the opening will be more than the pressure difference between the header before the opening and the header after opening. This tends to short circuit the fluid across the opening and flow in the rest of the header will be reduced. That is why we reduce the header size after each opening and maintain constant velocity more or less.
Pressure increasing at the end and then coming back is also a wrong notion. Just assume an end opening with an elbow instead of a tapping just before the header blank off. In this case no pressure building up.
Your sparger(pipe with equidistant and equally sized holes) idea is a good practical to do. Just measure the jet height from each hole and you will find that the height reduces across the length of the pipe. Just keep the inlet pressure low for better accuracy.
Regards,
Eng-Tips.com : Solving your problems before you get them.
RE: Does water pressure ôgo to the endö of a pipe?
Regards,
Eng-Tips.com : Solving your problems before you get them.
RE: Does water pressure ôgo to the endö of a pipe?
although not likely a factor in this case, but want to appraise you all that elevation differences (if substantial) are a contributing factor to higher pressures. case in point, liquid pipelines with significant elevation differences.
other than that, good post/responses.
-pmover
RE: Does water pressure ôgo to the endö of a pipe?
RE: Does water pressure ôgo to the endö of a pipe?
RE: Does water pressure ôgo to the endö of a pipe?
In deriving this factor, it is assumed that the static head remains constant as the fluid travels down the pipe. Not quite true, but our design was based on relatively short pipes, maximum of 60".
RE: Does water pressure ôgo to the endö of a pipe?
The law of conservation of energy leads to seemingly odd observations of pressure increasing as the flow proceeds downstream. While this sounds impossible, it is thermodynamically correct, as pressure changes inversely to velocity to conserve energy (Bernoulli).
Although friction may affect the pressure developed in a gradual change of diameter, a sudden expansion in a pipe system may show downstream pressure gage readings being higher than upstream. As an example, for water flowing in a 4" pipe at 10.1 fps, then into an 8" pipe at 2.54 fps, the pressure rise would be:
This is also a known effect in the diffusers of vacuum ejectors.
Another common example are centrifugal pumps which impart energy in the form of high velocity to fluids. A velocity which is then reduced by expanding the flow area both in the pump casing and frequently by expanding the pipe size on the pump outlet.
The word force, or better energy, would be indicated to replace the word pressure in your statement or, otherwise, the addition of: "in constant cross-section ducts or pipes" would be needed. Do you agree ?
RE: Does water pressure ôgo to the endö of a pipe?
It is a simple application of Bernoulli equation.
Brian
RE: Does water pressure ôgo to the endö of a pipe?
As you see Bernuli rules here, but in your application you have one more factor to consider, the spray arm is spinning, with the spin comes centripedal acceleration, which will result in a higher head at the tip of the arm than at the center. With that you can run the numbers for flow, friction loss, pressure as a function of distance from the center, I can see this will be lots of number crunching.
One other item, since that arm is spinning, the lighter it can be at the end from both pipe mass and water mass, the signficantly less forces on the arm and support structure when in motion. I believe this is the primary reason for the preffered shape in common use.
Hydrae
RE: Does water pressure ôgo to the endö of a pipe?
mdshydroplane
RE: Does water pressure ôgo to the endö of a pipe?
Brian
RE: Does water pressure ôgo to the endö of a pipe?
No, I am not creating energy. Look at the physics of the spray arm.
First lets consider the arm fixed. Water enters the pipe and travels to the first spray point, along the way the pressure is reduced by the friction loss of all the water moving though the pipe. a small portion of the water exits the pipe and the rest continues to the second spray point, if the pipe had a constant cross sectional area, the pressure drop per unit length is less than in the first section of pipe because there is less water flow though it. The response by designers has been to reduce the cross section area to reduce material costs, similar to HVAC designers reducing duct size. At each spray point pressure is converted to velocity to perform the work required in accordance to Bernulli.
Now let the arm spin,
It spins because the thrust of the last spray point is directed to the side imparting a torque on the arm. The arm will accelerate until the friction losses of the arm flowing through the air/water medium it is in and other friction losses equals the thrust created by the last spray point. (the other spray points also produce this thrust but they are counteracted by the bearing holding the spray arm and impart no movement to the arm).
Now consider that the arm has no spray points and is spun by an external mechanism. The water is pressurized by the supply pump to a pressure P1. If you measure the pressure at the end of the arm while it is spinning it will read a pressure greater than P1, just like a centrifuge imparts G forces on objects it will also impart a greater pressure on the water inside the pipe.
to quote Gandytable on March 24 "The conventional wisdom here at work is that the greatest pressure and flow is near the end of the pipes, and that it therefore needs to be tapered toward the end to equalize the flow?"
Now I agree to the 'conventional wisdom' but not the 'therefore'. With heavy objects on the end of a spinning device the forces are large when the items are heavy, if you can make it lighter the forces will be less.
Now to get back to your comment on creating energy, the only energy source is flow of water under pressure. All the energy supplied minus friction losses is converted to water having a high velocity under zero pressure. The energy used to spin the arm is converted from the last spray point imparting its velocity to the side, now this water has less absolute velocity because the spray arm is spinning away from the point of exit, reducing the energy in the water and putting it in the spray arm. ( a Pelton turbine works on this principle and when perfectly designed all the velocity is imparted to the arm (bucket) and none is left in the water) the energy added to the spray arm by the last spray point results in increased pressure inside the pipe but not so much pressure the total energy is increased.
As for water flowing uphill in an open channel, there head is coverted to velocity then back to head, minus the friction loss. In closed pipes, the same thing happens with reductions in cross sectional areas, pressure is converted to velocity then after the reduced area the pressure rises again when the velocity is conveted back to pressure (minus friction losses of course)
Hydrae
RE: Does water pressure ôgo to the endö of a pipe?
An interesting problem - your explanation has me convinced - it is the "absolute" velocity that is the key.
Brian