Why does the pressure remain unchanged after the pipeline branches?
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The original poster works at a power plant, and has noticed that in many of the pipelines there the pressure remains unchanged even after the pipes branch off from the main pipes. It is said online that pipeline design takes into account the optimal flow rate, with the flow rate being kept relatively constant – this is an empirical value. I would like to seek confirmation from other experts here, as I am not from a chemical engineering background; this issue has been bothering me for a long time, and I hope experts can provide a detailed explanation. Also, are the flow rates the same, and are the pressures the same as well? (Avoiding factors such as pipeline losses) If the flow rate is the same and the pressure is the same, what determines that? Is it inferred from Bernoulli’s equation?
In many pipelines, the pressure remains unchanged even after branching off from the main pipe? It depends on where the flow-limiting valve is located; if flow limitation occurs at the root of a branch, the pressure downstream may decrease. However, the design pressure takes into account the possible maximum pressure, which is the same for each branch as it is for the main pipe.
During this design process, the total amount of medium and the pressure should be taken into account when determining the main pipes and branch pipes, including the impact of resistance
Does that mean that if the design flow rate of the pipelines is the same, then the pressure in those pipelines will also be the same? Or is it not this kind of relationship, but rather caused by design requirements? I hope experts can provide a detailed explanation
One can take a look at the characteristics of the branch pipelines in chemical engineering principles: Total flow rate = sum of the flow rates in each branch； The energy loss in each branch is equal ； The relationship between flow velocity and pressure can be seen from Bernoulli’s equation.
You mean that, in terms of design, the flow rate remains roughly constant after the pipeline branches. Then, according to Bernoulli’s principle, the pressure remains unchanged, right?
z1+p1/ρg+u1^2/2g+hf1=z2+p2/ρg+u2^2/2g+hf2; if frictional losses are ignored, that is, if hf values for each pipeline are set to zero, then when u1=u2 and the relative height difference between the two pipelines is small, the pressures will also be essentially the same.
If the flow rate remains unchanged, the pressure will not change either. When the flow rate increases, the pressure increases; when the flow rate decreases, the pressure decreases. It’s similar to squeezing a hose to spray water
After reading everyone’s replies, I think it’s better to first consider the situations in which Bernoulli’s equation applies. This is a case of flow division, and a comprehensive calculation is needed; in situations where there is one pipe and one faucet, both have the same size. If the valve is opened near the water inlet, the water flow will be strong, while it will be weak further away from the inlet. We encounter this situation every day at noon when using the sinks in the cafeteria for washing dishes. Sometimes, putting the formulas aside and considering the actual conditions helps things become much clearer.
The entire piping system is calculated based on the process requirements. If necessary, it is also possible to make the flow rate and pressure of each faucet the same when using them for eating and washing in the cafeteria, but this makes installation more complicated, and there is no need for it.
If the flow rate increases, doesn’t that mean the pressure decreases? The water we obtain by squeezing the pipe actually has increased kinetic energy; in other words, the pressure difference increases while the static pressure decreases.