Does water pressure ôgo to the endö of a pipe? 5

[gandytable]

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.

 

[bvi]

One simple rule is that the direction of flow is always from the higher pressure to the lower pressure.

 

[jte]

gandytable-

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

 

[Montemayor]

gandytable:

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

 

[MortenA]

I seems to me that you are referring to a circulation system?

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

 

[25362]

Having said that the highest pressure in a header is at the source (pump) the pressure distributions along a manifold may show a discharge profile that doesn't follow conventional wisdom. See Perry VI fig. 5-57, in which we see that maximum discharge is at the end of the dead-ended distributor. It is possible for pressure to rise rather than fall in proceeding along a manifold, depending on the relative diameters of manifold and takeoff outlets.

 

[abeltio]

the reason why the pipe diameter is decreased is to maintain the velocity of the water inside the pipe more or less constant...
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.

 

[25362]

In a dividing manifold with equally sized outlets, as the first outlet or branch is fed, the velocity in the main header drops and the static pressure may rise (Bernoulli's equation) in the streamflow direction. The pressure drop across the side branches increases progressively with the distance of the branch from the inlet to the manifold. This results in maldistributed flows. I still recommend Perry VI for an in-depth explanation on the balance of acting forces and the empirical rules to keep the flow maldistribution at circa +/- 5%.

 

[PEDARRIN2]

What we need to remember is that fluids follow bernouli's(sp) equation. So it is a little inaccurate to say that pressure would be higher at the beginning of a system as compared to the end (i.e. the example with the open elevated pipe) without describing the other energy components.

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.

 

[quark]

Pedarrin is right on. We should actually consider total energy of the system when analyzing flow. But generally in pumping systems the flow is always from high static pressure to low static pressure fields.

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.

 

[gandytable]

What is Perry VI? I see that 25362 mentioned it in his post and it is mentioned in several other posts of his, but I cannot find reference to it on Google or on the few engineering book sites I visited.

 

[quark]

You didn't answer my question. Anyhow, Perry VI is the classic Perry's Chemical Engineers Handbook 6th Edition.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[TBP]

The fundamental thing to remember, is that if there is no pressure drop, there is no flow. Pressure drop, GPM and velocity in your water system are all linked. Change one, change them all. The greater the pressure drop, the more GPM you'll move, the higher the velocity. It is possible in liquid systems (and in some badly designed/installed systems, common) to have a large flow stop suddenly, when something like a solenoid valve closes. In cases like that, you can indeed get a momentary (and usually highly undesireable) pressure spike generated from water hammer at the end of a line that will be greater than the pump pressure.

 

[gandytable]

Quark, I apologize for not responding sooner to your request to explain what I meant 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?"" I had already posted when I saw your question. I will explain the best that I can. I am relatively new at a company that designs and manufactures commercial dishwashers. I did not understand the explanation here of why manifolds are designed a certain way. The manifolds leading to multiple spray arms are always tapered and therefore get smaller toward the end where the last spray arm is located (each spray arm feeds multiple nozzles). I was given the explanation above as to why the feeding manifolds are tapered. The conventional wisdom here is that is that the smaller cross section of the manifold would limit flow and therefore equalize the gpm (spray pattern) in all the arms, because “the pressure goes to the end and then comes back, so the highest pressure is at the end”. It just appeared to me that it would be cheaper to manufacture straight manifolds as opposed to tapered arms and that the tapering was not necessary. I am told that tests have been done in the past with both types of manifolds and that the tapering is what works in practice. I am considering some “pipe with holes” tests at home to see for myself what happens with the flow from equally sized holes placed at various locations. It is just that it bugs me when my understanding of something is so far off from what I am told is correct. I just want to understand what is actually going on and why. I thank everyone for their replies and really appreciate Eng-Tips.

 

[25362]

I found -with the help of Google- a site that may be helpful:

http://www.coolingzone.com/Guest/News/NL_MAR_2002/Inres/Inres_02.html

 

[quark]

There are two methods, generally practised, two maintain proper flow through multiple outlets when they are spanned in short distances. One method is called the Decreasing Supply Header and Increasing Return Header and the other method is Reversed Returned Header(this is for closed loop systems).

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.

 

[quark]

I have to confess that I didn't see 25362's link before giving my last reply(in other words, that was not just cut and paste reply). One correction I would like to make in this context vis-a-vis the above posted link is that in our case the flow will be maximum at the opening closer to the main inlet (in the recirculatory system it is the farthest point as noted in the paper)

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[pmover]

ok fellow engr's,

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

 

[tr6]

But, then equal, or nearly equal, flow distribution along a constant diameter pipe with equally sized holes is also a function of the pressure drop across the hole and the velocity head down the pipe. A velocity head of 10.26 times the hole pressure drop will produce a flow variation of 5% from the first hole to the last.

 

[25362]

To tr6. Since the velocity would be dropping along the direction of flow, do you mean the 10.26 factor should be taken at the pipe entrance ? Is this factor applicable to all fluids, liquid and gases alike ? I'll appreciate your considered answer.