Can't wrap my head around pressure and flow rates....

[phdinfunk]

So, I'm an electronics guy and this problem has me stumped.

If there's a cylinder of gas at 150 Bar, and I have a regulator on that cylinder. I'm pumping gas from 150 Bar to Atmospheric Pressure. What pressure does the regulator need to be set at in order to get 15 Liters per minute as my flow rate?

I'm not even sure exactly how to set up this problem. It's a bit embarrassing, but it got me on this forum.

Thanks!

PS: Also, just so we can add some extra calculus here: Assuming a single-stage regulator, at what rate would my flow rate drop off as gas is being released?

I love a chance to use second derivatives.

So, again assuming a single-stage regulator, after half an hour, what would the new flow rate be, of my 16 Liter tank which was originally at 150 Bar pressure and set for 15L per minute?

PPS: REALLY, thanks for the help.

Edited May 20, 2016 by phdinfunk

[studiot]

Electronic guy huh?

 

OK regard your gas tank as a large charged capacitor.

Now connect a load through a voltage regulator.

 

The regulator will maintain the voltage at the load, regardless of flow rate (current) until the voltage at the capacitor falls below the regulator input requirments.

 

Note that many different current rates are available with this arrangement; you cannot set the current and the voltage simultaneously.

 

Now the gas pressure regulator works in much the same way as the voltage regulator and you cannot use it to set the flow rate, that is determined by the hydraulic load.

 

Mechanical constant flow devices are available like constant current regulators, but they are not pressure regulators.

They are known as positive displacement devices.

 

Final note.

 

In my analogy I did not include a battery or generator.

In hydraulics a pump is the equivalent so saying 'pumping gas' is equivalent to saying there is a pump in circuit, which I don't think you meant.

I am expanding gas or supplying gas to (at) atmosphere would appropriate terminology.

 

Does this help?

Edited May 20, 2016 by studiot

[xavierjevon]

Actually, I would like to understand this too. I think what phdinfunk is asking is a specific answer -

"I'm pumping gas from 150 Bar to Atmospheric Pressure. What pressure does the regulator need to be set at in order to get 15 Liters per minute as my flow rate?"

What should the regulator setting be for this?

[studiot]

 

Actually, I would like to understand this too. I think what phdinfunk is asking is a specific answer -

 

"I'm pumping gas from 150 Bar to Atmospheric Pressure. What pressure does the regulator need to be set at in order to get 15 Liters per minute as my flow rate?"

 

What should the regulator setting be for this?

 

 

 

I thought the answer was quite specific.

 

You can't do this.

 

 

Now the gas pressure regulator works _{in much the same way as the voltage regulator and} you cannot use it to set the flow rate, that is determined by the hydraulic load.

[xavierjevon]

From my understanding, I am sure you can. There is lots of explanation and talk about setting a regulator so that it releases pressure at a certain level. Maybe it requires a double-stage regulator in order to achieve this...

But at the very least, even though the cylinder pressure is going to decrease over time, as a starting point, you can still set a regulator at a particular setting so that it BEGINS at releasing 15 L per minute. That question can be answered.

[John Cuthber]

From my understanding, I am sure you can.

Your understanding is wrong.

Also, if the OP understands electronics then Studiot's explanation will probably help greatly.

Edited May 27, 2016 by John Cuthber

[studiot]

Good job you are not a diver, then.

 

Here is example 1

 

A scuba diver has a gas tank a pressure regulator and a breathing mouthpiece.

Now as the diver performs various activities he breathes a variable quantity of gas, the harder he swims the more he needs to breathe.

But it is vital for the life of that diver that the pressure he breathes that variable gas flow at is set at fixed value by the regulator.

That fixed value is equal to the pressure of the surrounding water he is swimming through.

 

Now this was a simple system

 

Tank - pressure regulator - diver.

 

Example 2 is a gas burner on a camping stove.

 

Here the system is one item more complicated.

 

Tank - regulator - burner manifold - exhaust.

 

I don't know if you understood the comparison to electrical circuits but the introduction of the burner manifold is the gas equivalent of introducing a large value resistor in series so that it swamps the load and turns a constant voltage supply into effectively a constant current one.

 

In this situation the manifold characteristics may well allow a pressure setting at the regulator to produce a set flow rate.

But we have not been told the characteristics of this manifold and in any case I did say that it is the characteristics of this extra item that actually controls the flow, in conjunction with the set pressure.

 

You would not ask

 

What speed do I need on my speedometer to drive 50 miles?

 

The question is incomplete.

Edited May 27, 2016 by studiot

[xavierjevon]

"What speed do I need on my speedometer to drive 50 miles?"

This is very simple. If the speedometer was reading in KILOMETERS and not in miles... and someone asked, "What speed do I need the speedometer at to drive at 50 miles an hour?" That is a legitimate question.

In the same way, for a regulator that does not list itself in terms of liters per minute, to ask, what should the setting be to at least get an INITIAL flow of 15 litres per minute? Is a valid question.

There is no need to avoid this question. If you don't know, say you don't know, guys.

Edited May 27, 2016 by xavierjevon

[studiot]

 

"What speed do I need on my speedometer to drive 50 miles?"

 

This is very simple. If the speedometer was reading in KILOMETERS and not in miles... and someone asked, "What speed do I need the speedometer at to drive 50 miles?" That is a legitimate question.

 

 

 

I rarely heard so much rubbish.

 

I can drive 50 miles whatever speed I have on my speedometer, it just takes a different itme at different speeds, since

 

distance (in miles) = speed (in mph) x time in hours

 

In the same way a pressure regulator regulates pressure which is not measured in litres per minute.

 

I took the effort to put some proper thought into replying to your added question, but you obviously haven't done me the same courtesy.

[xavierjevon]

Let me make clear that I understand everything you are saying about the DROP in pressure, and how the fluctuation works.

But that still doesn't mean the question of INITIAL FLOW RATE cannot be answered.

"In the same way a pressure regulator regulates pressure which is not measured in litres per minute."

And this is expressly incorrect. Balloon Helium Gas Manufacturers, for example, speak about litres per minute as flow rate in order to make sure that there is a tempering on the amount of gas coming out so that balloons can be filled easily.

[phdinfunk]

Wow, I see a lot of activity on my post!

I understand what you're saying in the analogy you're trying to make. But with a circuit, you will always end up with a loop, and every node is going to obey Kirchoff's laws. If you ain't got a loop, you can't really start talking about electronics.

With my original question: The resistance load is that I specified dumping the cylinder into atmospheric pressure. Or if I had specified a vacuum, that would have been a different resistance to the flow.

So, if I've got the single stage regulator, what is going to create the initial flow rate of 15 L / minute?

Edited May 27, 2016 by phdinfunk

[xavierjevon]

Let me clarify even further. A Cylinder with 2 Cubic Meters of Gas, at 150 Bar Pressure, needs the regulator set at what in order to get an initial flow of 15 liters per minute?

[phdinfunk]

The thing is, in my original question I asked how to set up the problem. Had that been shown, I could easily solve for any number of situations.

But I don't even know how the units and formulas interact here. Is it an irreversible pressure drop equation? How do my units need to be set up?

I've never seen one of these problems worked out before. If I've left out a variable, then please help me set up the equations and show me the space where I didn't specify. THAT would constitute a very good answer to the question.

So, without anyone having suggested any useful equation, as I'm looking at the formulas I can find, it looks like fluid dynamics requires an aperture size.

2mm.

[John Cuthber]

Let me clarify even further. A Cylinder with 2 Cubic Meters of Gas, at 150 Bar Pressure, needs the regulator set at what in order to get an initial flow of 15 liters per minute?

Thank you for further clarifying your lack of understanding.

 

Pick a number. Specifically, pick a number that you think might be the "right answer".

Imagine you set the pressure regulator at that pressure.

And then imagine that I have connected the output from the regulator to a length of copper pipe -well able to hold that pressure.

 

But, since I'm awkward that way. I have folded the end of the pipe over and soldered it closed.

No gas can possibly flow through it because it is sealed shut.

 

Please explain how you can have a flow rate of 15 litres per minute in these circumstances.

 

 

OK so now I drill a very small hole in the pipe.

That lets a little gas through.

What pressure does it take to drive 15 litres per minute through a "very small hole"?

 

Obviously, the answer depends on how big "very small" is.

 

 

Do you now understand why you were wrong?

 

phdinfunk

essentially you need to know the "resistance" of the system- the size of the "very small hole".

Without that it is equivalent to asking "what voltage do I need to get a current of 15 milliamps?"

 

If I was a bit more nearly sober I could answer the question for the case of " a 2 mm hole"

https://en.wikipedia.org/wiki/Choked_flow

Edited May 28, 2016 by John Cuthber

[studiot]

What John was showing you is that the flow rate is not controlled by the regulator valve.

 

It is controlled by whatever you are feeding the gas into.

 

Connect some pipework etc

Set the gas pressure valve to any value you like

Measure the flow rate

Unconnect the pipework and connect different pipework

Measure the flow rate again, with the same setting on the gas pressure valve.

The flow rate will be different.

 

I have already told you how to control the flow rate,

 

Did you look it up?

 

You connect another device (not the pressure regulator).

Once such is called a metering pump.

These are extensively used in industry and hospitals.

Edited May 28, 2016 by studiot