Restriction orifice and fluid law of continuity

[firefastball]

As far as I know a restriction orifice functions as 'choked flow' which limits the flow exiting the orifice.
In other words, the flow before and after the orifice would be different.
Wouldn't that violate the law of continuity?

 

[LittleInch]

Why would the flow be different? The density and volume might be different if the liquid vapourises for example, but mass flow in equals mass flow out. An orifice can operate below choked flow, but at choked flow the volumetric flow rate is fixed and mass flow rate fixed for a fixed upstream pressure.



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[MortenA]

@firefastball, as you identify it yourself - thats not a description of an orifice to say that the flow is different upstream and downstream. And, BTW, flow across an orifice need not to be choked (e.g. a flow measurement orifice). As littleinch obeserves, density might change (usually for a gas or multiphase) and actual volume flow might thus change - that that does not vialote any law of continuity (mass or energy).

 

[firefastball]

Let's say fluid flows upstream of a restriction orifice at 100 m3/hr.
Law of continuity state that flowrate would remain at 100m3/hr after it passes the orifice - the velocity is higher than before, and pressure drops.
Where does the term 'choke' in a restriction orifice fits in?

 

[LittleInch]

Depending on the size of your orifice and the respective pressures upstream and downstream, at some point, with liquid, regardless of any increase in pressure upstream or decrease in pressure downstream, no more flow will go through the orifice, hence it will then be "choked". Gas is a bit different where the increased upstream pressure results in the same volumetric flow at the speed of sound, but mass flow rate will increase due to increased gas density.

I'm sure Wikipedia can help you a bit more...

Is this an idle query or do you have a practical issue to address?

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Also: There's usually a good reason why everyone does it that way

 

[firefastball]

Its a problem that I'm trying to investigate.
There is a header pipe (DN 200) with four pipes (DN 100) attach to it. These four pipes are equally distributed among them.
Two out of the four pipes have orifice in them with 30mm hole.
The four pipes function are to inject sodium hypochlorite to our seawater intake to suspend bacterial growth.

I've been observing that the pipes that have orifices in them does not inject the sodium hypochlorite compared to the other two.

 

[LittleInch]

Seems strange. A diagram or schematic would help a lot with spot pressures and flows if available and which ones have the orifice. Any knowledge of why the orifice plates were installed in the first place?

If this is a one way header and the first two have orifice plates, it sounds like an attempt to distribute flow more evenly across the four pipes that may not be working as intended. Are all the pressures downstream exactly the same?

how do you know nothing is getting through the orifice plates?

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Also: There's usually a good reason why everyone does it that way

 

[JohnGP]

I agree with LittleInch in that the orifices have likely been installed to balance flows in the pipes. You would need to review the complete system to determine the intent. An orifice in the line will reduce flow by increasing pressure drop, sort of like partially closing a valve in the line. This will likely raise header pressure slightly, so flows in the unrestricted pipes could increase, but if the system is being fed by a centrifugal pump, then it would move back on its curve as it sees the higher pressure in the header.

The whole system would balance out though depending on the resistances in the four lines.

If you are getting zero flow in the lines with orifices though, is it possible for the orifices to be blocked?

Cheers,
John

 

[firefastball]

I have attach a file regarding the problem.

 

[LittleInch]

Assuming all the pipes go to the same height/ length etc it looks to me like the orifice plates are in the wrong pipes(!) you need to look at flowrates, pressure drops across the orifices etc, but on the basis that the flow comes in through that central pipe which then splits evenly, but reduces in diameter, the friction losses in the two central ones will be lower than the two outer ones and hence probably take the vast majority of the flow.

Has anything changed since the original design (less flow?) Orifice plates can work well to balance flows, but need fixed conditions. The fact they all work when only one valve is open suggests that either they are only designed to work with one valve open or the flow rate through the system has changed. I still think they are in the wrong pipes as you have tagged them.

That vertical drop to the channel looks quite interesting. Is there some sort of nozzle on the end restricting flow or is it open? The pressure in that header might be less than atmospheric unless your flow rate is very high or there is something providing a pressure drop at the end equal to the head of the pipe drop.

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Also: There's usually a good reason why everyone does it that way

 

[firefastball]

Yes. Your guess is as good as mine. I am trying to figure out why the orifices are there in the first place.

The vertical drop goes down 12m and ends with an end plate. There are holes 15mm in dia at few intervals along the pipe. No nozzles.

 

[LittleInch]

Equal distribution of flow in headers like that are notoriously difficult to achieve without active flow control and feedback from flow meters. It will depend how much you can experiment to achieve equal flow and how much you really want / need to get equal flow between your four pipes.

first off I would swap the pipes with and without orifice plates and see how that works.....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way