|John Hilton||11/12/2019 12:57:22|
|116 forum posts|
Wow. I am reading this with great interest as the OP!
I certainly have enough info to blind my accuser with science!
I did wonder - just to compound the issue - would we expect different results (within reason) if the subject was a gas which could be compressed?
keep them coming!
|Howard Lewis||11/12/2019 13:26:50|
|3375 forum posts|
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.
|5926 forum posts|
Well, it depends. I prefer Einstein's 'Make everything as simple as possible but not simpler'. He also said 'If you can’t explain it, you don’t understand it well enough.'
Personally I incline to argue there is no such thing as common sense. Here's a few definitions to choose from. Common Sense is:
I'm not sure John's question has been answered correctly, though I think Hopper and Robert both correctly identify the main consideration, which is the difference between a tender that's just a tank, and one containing a pump. One assumption is the tender gravity feeds water to an injector, equally valid is a tender containing a force pump for directly injecting high-pressure water into the boiler. Maybe both are wrong, could be the tender is used to fill a tea-urn on the back.
If John wanted a simple practical answer to the effect of a choke on water flowing under gravity, easy enough to do the experiment. The experiment would also find the actual relationship between choke and output volume, useful for answering a question like 'will the choked pipe enough flow to allow an injector to fill the boiler'? A rather harder question is, 'how much longer will it take to fill a boiler with a force pump if the pipe is constrained?' In the latter case, the smaller pipe may be necessary to manage the high-pressure involved, not just a casual feature.
While practical skills are high value, they are strongly reinforced by effective theory. I'm impressed by Lord Kelvin, (physicist as smart as Newton and Einstein) who is alleged to have said: 'if you can't put numbers on it, it ain't worth sh1t!'
|5926 forum posts|
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.
Edited By SillyOldDuffer on 11/12/2019 15:24:49
|Maurice Taylor||11/12/2019 16:25:35|
|95 forum posts|
In the pressured tank example, the pressure will be the same in all areas of the tank.The small cylinder will have the same pressure acting on it as the large one.
|Martin Kyte||11/12/2019 16:26:39|
1901 forum posts
The essential question, with apologies to the original poster, is not will it affect the flow which of course it does, but does it matter. To answer this the require flow rate needs to be known allong with the/any downstream restrictions.
For small flow rates it's going to matter less. Zero flow would give the same static pressure regardless of the 1/4" restriction.
Edited By Martin Kyte on 11/12/2019 16:27:05
|5926 forum posts|
That must be right. So me saying '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!' must be wrong. Oh dear, now I'm confusing myself as well as everybody else.
Sackcloth and ashes AGAIN! Or I could go into politics...
4650 forum posts
Ah. Didn't see the reference to tender in the OP on first reading.
In that case, the answer would be "It depends."
Depends on the abilities of the feed pump or injector. If it has the ability to increase the pressure differential between the two ends of the pipe by a sufficient amount then increased pressure will create increased flow through the smaller pipe, as flow is proportional to pipe radius times pressure, (all other things being equal of course).
But, if - as is often the case - the feed pump or injector is only just up to the job with a 3/8 pipe, and can not provide sufficient extra "suction" or pressure differential, the 1/4" pipe will restrict water flow.
So, disregarding frictional losses which would be somewhat minimal in such a situation, you might as well plumb the whole length in 1/4" pipe if you are going to do one section in 1/4".
The question remains: why would you even need to reduce your feed line from 3/8 to 1/4 in one place anyway? Can't you just make a bigger hole for the 3/8 pipe to pass through?
|Robert Atkinson 2||12/12/2019 07:24:36|
698 forum posts
I think you are both correct, it depends on the condition of the system. The pressure is only constant throughout the system when it is STATIC. If anything is moving there are pressure differentials in the system.
On pressure testing with gas v liquid there is a worse case than gas, using a fluid that is gas at ambient and liquid at test pressure e.g. water over 100 deg C or LPG. With these when the pressure is released the liquid flash "boils" and expands elesing even more energy than the same volume and pressure of gas. There is a name for it "BLEVE" Boiling Liquid Expanding Vapour Explosion. Very nasty indeed especially if the fluid itself is flammable.
|martin perman||12/12/2019 15:25:09|
1835 forum posts
Arent we looking at a venturi **LINK** which happens to work with liquids and gases.
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.