I am trying to establish the flow rate and time to discharge a compressed gas from a small pressure vessel from a small orifice/valve (3mm dia). It’s not for the want of trying. And I have been researching for some time and only came across a relatively complex formula to work out the initial surge which is beyond my understanding at this stage. I am after some pointers to which laws/formulas to study to help me start to plot this in a spreadsheet. Any help would be appreciated tbh..
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flow rate and time to discharge gas from a simple pressure vessel 1
- Thread starter zulu4
- Start date
Yes somewhat complex but that's the way it is ...
Here's what you need to look for.
Flow will most likely be choaked until tank pressure is very very low.
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Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.
Here's what you need to look for.
Flow will most likely be choaked until tank pressure is very very low.
Link
Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.
LittleInch
Petroleum
zulu4
This is a transient thing, i.e. things change with time, hence there is no simple formula.
The programs that do this can be complex.
Alternatively you can spread sheet this and just choose a suitable time period when things don't change very much ( say pressure doesn't change by more than 5%) and asusme steady state conditions for that time period.
As my fellow poster above notes, assuming you're venting into atmospheric conditions, any pressure more than 1 barg will be choked flow.
So velocity is fixed, but mass flow rate varies with pressure.
So calulae how much mass you have in your vessel at time =0. calcualte how much leaves in say 30 seconds. That mass or volume then reduces the pressure by a certain amount. Subtract that from your start mass and repeat until pressure gets down to whatever you want or less than 1 bar you need to move to non choked flow.
The trick is to konw when to stop and the time period as this will be an exponential decay.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
This is a transient thing, i.e. things change with time, hence there is no simple formula.
The programs that do this can be complex.
Alternatively you can spread sheet this and just choose a suitable time period when things don't change very much ( say pressure doesn't change by more than 5%) and asusme steady state conditions for that time period.
As my fellow poster above notes, assuming you're venting into atmospheric conditions, any pressure more than 1 barg will be choked flow.
So velocity is fixed, but mass flow rate varies with pressure.
So calulae how much mass you have in your vessel at time =0. calcualte how much leaves in say 30 seconds. That mass or volume then reduces the pressure by a certain amount. Subtract that from your start mass and repeat until pressure gets down to whatever you want or less than 1 bar you need to move to non choked flow.
The trick is to konw when to stop and the time period as this will be an exponential decay.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
- Thread starter
- #4
Great.. thanks chaps. I’m a little way out of the rabbit hole now.
Yes, looking into this it is choked – and when not should not affect what I am trying to achieve I dont think. I will time step this accordingly on a spreadsheet like you say.. I’m going to assume this could get more complex when you introduce heat into the equation.. One step at a time.
Yes, looking into this it is choked – and when not should not affect what I am trying to achieve I dont think. I will time step this accordingly on a spreadsheet like you say.. I’m going to assume this could get more complex when you introduce heat into the equation.. One step at a time.
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1
- #5
LittleInch
Petroleum
Heat will affect the pressure, but maybe not as much as you think as you need to work in absolute temperature (degrees K). So what sounds like a big increase, say 20C to 80C is actually only 20% more pressure for a fixed volume of gas.
The easiest way is probably to work out how much gas you have in your vessel at standard conditions, then calculate volume converted to standard conditions lost per time period from the mass flow equation and subtract it. then calculate pressure drop due to that lost volume and start again.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
The easiest way is probably to work out how much gas you have in your vessel at standard conditions, then calculate volume converted to standard conditions lost per time period from the mass flow equation and subtract it. then calculate pressure drop due to that lost volume and start again.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
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