Bernoulli Equation For Combine/split Flow, Flow direction in combine/split flow

Hi,

I would appreciate any help if possible as it is racking my brains out.

1) Firstly, I have a main flow at say 2 bar with velocity at 5m/s flowing in a pipe(3inch). Then, I decide to connect a secondary pipe to the main pipe and decide to introduce flow into the pipe at say 1 bar with 10m/s(2inch controlled by a flow gauge).

2) In this scenario, what will possibly happen?. Will the secondary stream join the main stream and flow together? Will either flow reverse flow into another liquid?
Please explain.

Hi,

Anyone please?

The reason for me asking this is that I have to connect a secondary line to my main line at work. How do I know for sure that there will be no backflow in either direction?

What are the parameters that will me whether the liquid will backflow or not? for example pressure, velocity etc?

Please assist if possible.

Ok, from fluid dynamics to plumbing in one fell swoop. I can handle this one:

Don't think about the math, just add a check valve to your parts requisition.

More information is required; such as are the pipes at equal heights?

My immediate response would be that if the pipes are comparable heights then it will depend on what the pressure on exit is. If the flow is to an open exit and is not through any particularly small pipes there shouldn't be any backflow.

If the pipes lead to a tap I would imagine that turning the tap off (stopping the flow) would inevitably lead to back flow in one of the pipes. Can't you put valves at the junctions?

(I am a novice at plumbing and just trying to apply logic to the problem as it appears you are desperate for help - so don't rely too much on my info!!!)

Hope this helps.

Hi,

The pipes are at the same height.

The combined flow is towards a tank which is atmospheric pressure. Thus, can I safely assume that both streams will combine and end up at the atmospheric tank?

Basically, I have another more important question is that if for example the main pipe flow is at 2 bar and flowing through a 3 inch pipe at 10m/s. I decide to introudce another flow via a 2inch pipe to the 3 inch pipe at 1 bar and 5m/s.

What will the above introduction of the 2 inch pipe flow do to the 3 inch pipe pressure and velocity? Will it hinder the flow of the 3 inch pipe fluid and result in higher pressures?

Sorry for my lack of understanding in fluid mechanics. It is this split flow that is killing me.

Do you want to describe the system mathematically, or do you want a specific result? My previous response wasn't meant to be flippant. There exist anti-backflow and check valves of all sizes and materials, and metering/mixing manifolds, all from the shelves of a supply house near you.

The height of the tank will now play a role. There will be no backflow until the water level in the tank builds up enough to overcome the pressure from the pipes. There will come a point where the water in the tank will be too heavy and you will start to get back flow in one of the pipes. Eventually the system will reach equilibrium and will just flow from one pipe to the other (without water escaping from the tank). At worst you will need a pressure release valve on the tank. One-way valves on the pipes before the junction would be better.

The introduction of a secondary flow will increase the pressure of the water in the main pipe, yes.


It sounds like Sapo could be better at giving you a practical solution!

This post has been edited by bm1957 on Sep 24 2007, 04:25 PM

Thanks, bm1957. I'll take approbation when I can. I'm better at cabinetry than plumbing, though...

Hi Sapo and bm1957,

Thanks for the help and explanation.

The system parameters that I give was just an example, not the real case parameters but close enough.

Also, the check valve is also part of the consideration, just in case things go wrong.

Also, let the pressure of the main flow is P1 and velocity is V1. After the introduction of the secondary flow, the pressure of the secondary flow is P2 and velocity is V2, does the pressure and velocity of the main flow remain as P1 and velocity as V1?
Or the system will reach an equilibrium and thus the pressure and velocity of the main pipe will change to accomdate the secondary flow.
From my little knowledge of benoulli, I understand that the P1 +V1^2/2g = P2 + V2^2/2g when the secondary line is introduced to the main line.

Can you explain using benoulli equation to reflect the changes in pressures and velocities of the main pipe?

Sorry to sound so confusing as I am interested to find out how will pressures and velocity of the main pipe change when the secondary flow is introduced.

I appreciate all the help.

Hi everyone,

We had a good discussion of this topic in this previous thread. I think this should answer choychoy's question.

Hope that helps!

--Stuart Anderson