Plumbing question

[Maurice TaylorParticipant @mauricetaylor82093]

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 KyteParticipant @martinkyte99762]

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.

regards Martin

Edited By Martin Kyte on 11/12/2019 16:27:05

[SillyOldDufferModerator @sillyoldduffer]

Posted by Maurice Taylor on 11/12/2019 16:25:35:

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.

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…

[HopperParticipant @hopper]

Posted by Howard Lewis on 11/12/2019 12:14:16:

Folks, are we not making mountains out of molehills?

…

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

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 2Participant @robertatkinson2]

Posted by SillyOldDuffer on 11/12/2019 16:43:19:

Posted by Maurice Taylor on 11/12/2019 16:25:35:

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.

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…

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.

Robert G8RPI.

[martin perman 1Participant @martinperman1]

Gentlemen,

Arent we looking at a venturi **LINK** which happens to work with liquids and gases.

Martin P

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