Pipeline flow rate and pressure difference?
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A metal tube float flow meter is installed in a pipeline. The pressure in the pipeline ahead of the meter is 1 MPa, and the pressure in the pipeline behind the meter is also 1 MPa; the flow rate is displayed normally. I have the following questions and would appreciate it if experts could help answer them: 1. Isn’t it said that flow rate is generated by pressure difference? If a flow meter indicates flow, does that mean there must be a pressure difference before and after it? 2. As fluid flows through a pipeline, must there be a pressure difference at every cross-section of the pipeline? If there is no pressure difference, how can there be flow? Thank you!
Your differential pressure unit is too large; it’s impossible to tell the difference! Flowmeters are also components that create resistance; when the fluid flows through them, a pressure difference is generated. The smaller this pressure difference, the better. In the case of float flowmeters, the pressure difference is not negligible, being on the order of KPa. There’s no superconductivity in this.
Moderator: I’ve been quite confused and would like to ask the following: In a typical pipeline, when the pressure is constant and the flow rate is stable, if two sections that are very close to each other are chosen, are the pressures at these two sections equal? It seems they should be equal; if they are equal, there is no pressure difference, so how can there be flow? Isn’t that contradictory? :lol
Metal tube float flow meters are designed using the variable area measurement principle, and they are manufactured with the aid of modern high-tech methods and components. A flow meter is mainly composed of three basic components: 1. The measurement tube ； 2. Conical float ； 3. Indicator. You mean the orifice plate flow meter, right!
Let’s set aside the flow meter and talk directly about pipeline pressure difference and flow rate. The question is as follows: In a pipe where the pressure remains constant and the flow rate is steady, if two sections that are very close to each other and adjacent to one another are chosen, are the pressures at these two sections equal? It seems they should be equal; if they are equal, there is no pressure difference, so how can there be flow? Isn’t that contradictory? :Lol, is it a paradox?
You may not have understood what I mean yet. Imagine an ideal pipeline with constant pressure and constant flow rate; if you choose any two cross-sections that are extremely close to each other, will the pressures at these two cross-sections be equal? If they are equal, there is no pressure difference; how can there be flow then? Isn’t this a contradiction? “Is “pressure difference generates flow” wrong?
You can think of it as a series circuit, where all the components are connected in series; even a wire has resistance. Therefore, as long as there is current, there will be voltage. In other words, once there is flow, there is also a pressure difference. What you call being infinitely close is, in essence, the same point. Since it’s the same point, there’s no need to discuss the issue of pressure difference flow for that same point. You’ve trapped yourself; go back and sleep in peace.
Yeah, ever since I started thinking about these theoretical issues, I find myself getting stuck over and over again: lol
Curious, but also prone to getting stuck in details; P
There must be a pressure difference; whether it can be measured depends on how precise your differential pressure gauge is :)