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Plumbing question

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John Hilton09/12/2019 14:03:05
96 forum posts
20 photos

I wonder if any of you clever people out there can help me with a question.

Imagine a pipe 3/8 dia, which for a length is reduced to 1/4 dia, and then back to the original 3/8 dia .

My question is would the flow at the end of the pipe be the same as if it were 3/8 all the way through? Let's assume only water pressure from a tender.

Originally I thought not, but someone has confused me now by suggesting that the volume would be the same in either example, because the pressure or speed would rise in the smaller pipe to compensate.

Is this reasonably correct?

Many thanks in advance

.

John

.

not done it yet09/12/2019 14:06:55
3791 forum posts
15 photos

No, of course not. Would a 1/4 diameter pipe provide the same flow as a 3/8 diameter pipe? Just forget the two combined for a few seconds!

John Haine09/12/2019 14:16:14
2787 forum posts
140 photos

Suppose it was 12 inches diameter reduced to 1/4 - kinda makes it obvious.

BCPROF09/12/2019 15:00:24
116 forum posts

No. I it was true the water companies could fit reducers to water mains and run 1mm diameter pipes

Bill Davies 209/12/2019 15:25:29
142 forum posts
10 photos

Perhaps the other person has missed the important point you mention, i.e., the flow would be the same in both cases.

He or she is right that the flow on either side is the same as in the narrow part of the tube, but rather like electrical circuits, there is a resistance. The higher the resistance, the lower the current flow. Old electrical books make the analogy between water and electrical flow, hence use of the word 'current'.

In the case of no hole in the smaller tube, there would be no flow, so could we call that an insulator?

John Hilton09/12/2019 15:49:21
96 forum posts
20 photos

Thanks all.

I guess the confusion is that it would be ok the other way, ie small pipe to big and back to a small again. In that case the flow would be the smaller pipe rate.

Thanks

John

Speedy Builder509/12/2019 16:24:36
1865 forum posts
130 photos

Re work the question. Is a 1/4" pipe sufficient to deliver water from a tender to the ........ whatever.

BobH

not done it yet09/12/2019 16:42:31
3791 forum posts
15 photos
Posted by John Hilton on 09/12/2019 15:49:21:

Thanks all.

I guess the confusion is that it would be ok the other way, ie small pipe to big and back to a small again. In that case the flow would be the smaller pipe rate.

Thanks

John

Noo, not even that is correct. Imagine half a mile of 1/4 pipe on the ends of a foot long piece of 3/8 diameter pipe. Now consider the lengths reversed. The difference in flow would be like considering a 2 foot length of small pipe with a half mile length under the same pressure. One may not notice much difference with short lengths of pipe but considering extremes of length should make it obvious that there will be a difference, however small.

Brian Oldford09/12/2019 16:45:37
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596 forum posts
4 photos

Those with the slightest knowledge of electricity will recall water flow being analogous to electrical flow. Now apply Ohm's Law to your pipework.

Robert Atkinson 209/12/2019 17:13:20
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463 forum posts
21 photos

It depends on what the water source is.

If it's constant pressure source like a head of water or non -positive displacement pump then the flow will be less with the smaller section in place.

If it is a constant flow source e.g. a positive displacement pump, then the flow will not reduce with the restriction in place. However the pressure upstream of the restriction will be higher. This works up to the point where the pipe bursts or pump fails.

Robert G8RPI.

not done it yet09/12/2019 17:36:39
3791 forum posts
15 photos

Yes, Robert, but here we are following the OP’s “Let's assume only water pressure from a tender” scenario in his first post. As Brian so rightly says, use the Ohm’s Law analogy. In this case the potential difference across the ends of the pipe is constant.

Edited By not done it yet on 09/12/2019 17:37:19

Robert Atkinson 209/12/2019 19:22:01
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463 forum posts
21 photos

I covered the tender (non-constant flow) case in my post, what is your point?

If we are restricting this to tender ppework why are you talking about about half mile long pipes? If point is picking at other peoples posts, there is no "potential difference" (voltage) across the pipe and even the pressure drop is not constant, it will vary as the water level in the tender drops, reducing the available pressure head and flow rate.

Maurice Taylor09/12/2019 21:33:26
52 forum posts
4 photos

If the flow into the pipe at 3/8 end is 10 litre/min ,the flow through the 1/4 pipe will be 10 litres/min,as will the flow out of end of 3/8 pipe.Prividing there is enough pressure to get it through ,think fire hose same flow out of 1/2inch nozzle as 3 inch hose.

Richard -09/12/2019 21:40:29
49 forum posts
7 photos

Why throughout hydraulics do me make use of flow restrictors to slow components down ?

Paul Lousick10/12/2019 00:48:48
1264 forum posts
509 photos

It is true that "If the flow into the pipe at 3/8 end is 10 litre/min ,the flow through the 1/4 pipe will be 10 litres/min,as will the flow out of end of 3/8 pipe". The 1/4" pipe is the restrictor that controls the volume thru the pipe. Inclease this and the flow will be greater then 10 litres/min.

A 1/2" dia garden hose connected to a 1/2" tap which is connected to a 1/2" water supply pipe is another example. When the tap is fully open there is a 1/2" bore thru all items and at 50psi you will get about 30 gallons/min but as the tap is closed (similar to going thru a smaller intermediate pipe) the volume of water out of the end reduces. The pressure will also be reduced slightly because of the extra friction. Adding more bends in the pipe run or reduced bore diameter in connector fittings will also reduce the flow and pressure.

Paul.

Hopper10/12/2019 09:51:48
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3896 forum posts
83 photos

Flow rate is proportional to the fourth power of the pipe radius times the pressure. See Poiseulle's Law.

So if the pipe radius is decreased, flow decreases. Unless, unless, pressure is raised proportionately at the same time. Which depends on the type and capacity of pump used etc etc etc.

So a pump with a regulated output pressure (eg hydraulic system) , or a header tank with a constant head (eg mains water pressure), will flow less through a smaller pipe.

But a pump that is capable of increasing the supply pressure (while maintaining the flow rate) could pump the same volume through the smaller pipe as the larger pipe.

 

 

 

 

Edited By Hopper on 10/12/2019 10:04:18

Andrew Johnston10/12/2019 10:04:38
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5082 forum posts
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Posted by Hopper on 10/12/2019 09:51:48:

Flow rate is proportional to the fourth power of the pipe radius times the pressure. See Poiseulle's Law.

Only true if the fluid is incompressible and Newtonian, and the flow is laminar. Water is a good approximation to the first two conditions, but we don't know if the flow is laminar without knowing the Reynold's number.

Andrew

Hopper11/12/2019 10:04:41
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3896 forum posts
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Posted by Andrew Johnston on 10/12/2019 10:04:38:
Posted by Hopper on 10/12/2019 09:51:48:

Flow rate is proportional to the fourth power of the pipe radius times the pressure. See Poiseulle's Law.

Only true if the fluid is incompressible and Newtonian, and the flow is laminar. Water is a good approximation to the first two conditions, but we don't know if the flow is laminar without knowing the Reynold's number.

Andrew

Assuming for our hypothetical example that all other factors are constant, eg viscosity, pipe length, laminar flow, whatever. Otherwise we will end up in the realm of angels dancing on heads of pins, yet once again. And we certainly wouldn't want that, I'm sure. angel

SillyOldDuffer11/12/2019 10:42:16
5019 forum posts
1062 photos
Posted by Hopper on 11/12/2019 10:04:41:
Posted by Andrew Johnston on 10/12/2019 10:04:38:
Posted by Hopper on 10/12/2019 09:51:48:

Flow rate is proportional to the fourth power of the pipe radius times the pressure. See Poiseulle's Law.

Only true if ...

Andrew

Assuming for our hypothetical example that all other factors are constant, eg viscosity, pipe length, laminar flow, whatever. Otherwise we will end up in the realm of angels dancing on heads of pins, yet once again. And we certainly wouldn't want that, I'm sure. angel

Too late! I thought of asking a moderator to change the thread's title from 'Plumbing Question' to 'Rocket Science', because it's about Hydrodynamics rather than pipework. For example, these constrictions all behave much the same at low pressure and very differently at high pressures:

pipes.jpg

Even in plumbing it can matter because it costs money to pump large masses of water through badly designed pipes. Ever more critical to get flow right in a ship's hull, aircraft wing, jet-engine, and rocket engine. The latter really is complicated. For a given energy content, what is the best form of output nozzle? For example, an over tight nozzle in an ordinary Guy Fawkes rocket will cause it to explode rather than fly, and an over wide nozzle will have it burn out without moving.

Firework rockets can be made 'about right' in a shed by experiment, crude methods are good enough. Designing a rocket motor for a guided missile or a space-craft is much more demanding - truly rocket science!

Dave

Howard Lewis11/12/2019 12:14:16
2605 forum posts
2 photos

Folks, are we not making mountains out of molehills?

The theoretical discussions are interesting, but do not do much to help the OP. The effects of turbulence at the change of section may be interesting, but are hardly vital in carrying a small volume of water down a short pipe, as the case in point.. There have already been references to Rocket Science, which this is not.

We do not need a dissertation on the change of viscosity of water with temperature..

The OP was asking about a pipe between the tender and the locomotive.

The pressure available to promote flow is going to be of the order of a few inches of water, or probably less than 200 mm.

The pressure differential may be increased by the sub atmospheric effects of the injector that it feeds.

But basically, it all comes back to "Will a 1/4" pipe reduce the flow through a 3/8" pipe"?

The simple answer, in these circumstances is a common sense "YES".

As the late Rudi Mischetlager used to say "Common sense is not that common"

Remember the motto of the amateur radio world, KISS, "Keep It Simple Stupid"

Howard

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