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!

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

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.

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.

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.

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

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

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.

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

For the same pressure differential between inlet and outlet, the limit on flow will, be the smaller pipe.

The longer the small diameter section is, the more the flow will decrease, because of the friction with the pipe wall. This will apply to all fluids.

If a gas is at pressure at the inlet to the minor diameter, and there is minimal back pressure at the outlet, the gas will expand, decrease in temperature. If it expands, the volume will increase.

The drop in temperature during expansion is used to liquify gases, reducing it to below its critical temperature, where recompression can liquify it.

Witness the nozzle of a CO2 fire extinguisher, which will become coated with "frost", because of the drop in temperature, condensing and freezing moisture in the surrounding air.

Gases will behave differently, depending on whether they are above or below the critical temperature, and not like liquids which are only VERY slightly compressible, when subjected to high pressures.

Howard. .

As a picture is worth a thousand words:

If the tap on the open tank is cracked open, the amount of water flowing out of the tank will be constrained. If the tap is fully opened, then more water flows but the volume is limited by the water pressure and the diameter of the outlet pipe.

In the pressured tank, something rather different happens. If the piston in the large cylinder is pushed down a fixed volume of water is transferred to the small piston, which moves much further inside its cylinder. And in reverse, pushing the small piston down moves a fixed volume of water back into the large cylinder, which pisto moves a small distance. While the presence of a choke in the pipe makes no difference to the flow, or to the movement of either piston, it does make a difference to the pressure!

Open tanks are typical of domestic water systems: water pressure at the tap is usually obtained by gravity feed from a reservoir on higher ground, not from a directly connected force pump. As a system gravity fed water is cheaper and more controllable. Most of the system runs at low pressure, up to about 10bar.

Pressured systems are typical of heating systems and Hydraulic systems like power steering and road diggers. In a hydraulic system small pipes are used to transfer energy as high-pressure water. The high-pressure is easily produced by a force pump, and the relative movement of different diameter pistons can be exploited to multiply force at the expense of movement or vice versa. The small pump on my Hydraulic engine crane allows me to lift a 1/3rd ton lathe with no trouble at all. Hydraulic systems run at much higher pressures, say 500 or 600 bar. The very high water pressure available from a simple force pump – little more than a lever operated ram in a plain cylinder – is excellent for testing boilers. Although the pressure is much higher than that produced by steam, there is very little energy stored in it. If a water pressured boiler gives way, it won't explode.

Testing a boiler by pressurising a gas is very dangerous because gas acts like a spring. If the boiler breaks, it could well explode because compressed gas contains stored energy that can be released almost instantaneously.

Gases and liquids follow the same basic rules, but the compressibility of gases cause considerable side effects compared with liquids. But designing a ship's propeller is much the same mathematically (I'm told) as designing an air-screw. They're not interchangeable though – large bladed ship propellers turn slowly in a thick fluid, while slim aircraft propellers spin fast in a thin fluid. Propellers have to be optimised to the working fluid.

Dave

Edited By SillyOldDuffer on 11/12/2019 15:24:49