Air Flow - calculation challenge.

[W&KO]

I’m struggling to work out the air flow out of a pressure vessel.

90m3 vessel @ 120kpa

4” outlet, fully opened

How do I work out the rate of air flow out of the vessel (to atmosphere) in either m3/min or CFM.

[Milton477]

Good luck. We only deal with leaks here, the oily type.

[Old Farang]

I’m struggling to work out the air flow out of a pressure vessel.

90m3 vessel @ 120kpa

4” outlet, fully opened

How do I work out the rate of air flow out of the vessel (to atmosphere) in either m3/min or CFM.

Good luck with that!

Gas Discharge Rate Atmosphere From a Pressure Vessel | Engineers Edge | www.engineersedge.com (https://www.engineersedge.com/pressure,045vessel/gas_discharge_rate_14170.htm)

[ramblingboy42]

I don't think you can.

Quite a few years ago an engineer where I was working tried to measure the open flow rate of steam on I think an 8" line.

he fired up the boilers and got full head of steam and then opened a v/v with a flow meter some 12" or so in front of the open v/v.

all that happened was that within minutes all the boilers ......and there were a number of them....were at full burn , the stacks were belching black , the Weir pumps were working like jumping jacks , huge amount of vapour going into the sky and we nearly all fell around laughing.

btw , this was Australia's largest cotton textile plant, so there was serious boiler power.

[Blknight.aus]

I’m struggling to work out the air flow out of a pressure vessel.

90m3 vessel @ 120kpa

4” outlet, fully opened

How do I work out the rate of air flow out of the vessel (to atmosphere) in either m3/min or CFM.

Absolute or gauge?

[W&KO]

Absolute or gauge?

Er more info

Compressor is turned off

De-pressurizing

Need to know max air flow leaving vessel.

[Old Farang]

Er more info

Compressor is turned off

De-pressurizing

Need to know max air flow leaving vessel.

Err, I am inclined to think that I can probably fart more than what you are asking. 120 Kpa is 17.4 psi. Through the size outlet you are quoting the pressure will equalise within a few seconds, that is if you can fully open the orifice instantly.



If you neglect viscosity, Bernoulli's equation (just Navier-Stokes without frictional or stress terms) will get you into the ballpark:

Pg+12ρgv2g=PaPg+12ρgvg2=Pa
fluid dynamics - Calculate flow rate of air through a pressurized hole - Physics Stack Exchange (https://physics.stackexchange.com/questions/131032/calculate-flow-rate-of-air-through-a-pressurized-hole)

[W&KO]

Err, I am inclined to think that I can probably fart more than what you are asking. 120 Kpa is 17.4 psi. Through the size outlet you are quoting the pressure will equalise within a few seconds, that is if you can fully open the orifice instantly.



If you neglect viscosity, Bernoulli's equation (just Navier-Stokes without frictional or stress terms) will get you into the ballpark:

Pg+12ρgv2g=PaPg+12ρgvg2=Pa
fluid dynamics - Calculate flow rate of air through a pressurized hole - Physics Stack Exchange (https://physics.stackexchange.com/questions/131032/calculate-flow-rate-of-air-through-a-pressurized-hole)

Keep in mind it’s a 90m3 vessel, 40 foot long by 3.2 meters round. It probably takes 10min to de-pressurize. For the first few minutes the airflow comes out at a rate of knots and I need to clean the air. The current systems are undersized, while limiting how much we open the valve this increases de-pressurize time.

Kinda hoped there would be a simple calculation.

[Old Farang]

Keep in mind it’s a 90m3 vessel, 40 foot long by 3.2 meters round. It probably takes 10min to de-pressurize. For the first few minutes the airflow comes out at a rate of knots and I need to clean the air. The current systems are undersized, while limiting how much we open the valve this increases de-pressurize time.

Kinda hoped there would be a simple calculation.
Cannot be a simple calculation as the pressure is continually falling.

What is "3.2 metres round"? Is it circumference, diameter or what? You also need to use the same units to even calculate the volume of air in it.

[W&KO]

Cannot be a simple calculation as the pressure is continually falling.

What is "3.2 metres round"? Is it circumference, diameter or what? You also need to use the same units to even calculate the volume of air in it.

Yep, just need the max. Air flow at the start, understand it’s constantly dropping as it does get to zero.

Sorry 3.2m diameter x 12m long, only gave the above measure to give an understanding of size as it doesn’t equalize instantly. I believe you only need

capacity (volume) 90m3 and
pressure 120kpa

[Blknight.aus]

For this calculation the shape of the vessel is more or less immaterial.

we know that it has 90M3 behind it. and its going to be a 4 inch hole.

lets make a couple of other assumptions...

1. its coming out laminar (it wont be)
2. the valve opens instantly
3. its dumping directly to atmosphere with nothing after the valve
4. we are only interested in the split second after the air gets moving on the way out and the interial delay can be ignored
5. its plain dry compressed air

if we're working with absolute pressure the first 101.3 KPA of your 120 KPa is taken up by the external atmospheric pressure so you really only have 18.7kpa to work with, thats about 2.7psi...

if thats the situation then your air velocity will be in the order of 958l/s with a velocity of 1.2m/s and with a flow mass of around 1.17kg/s

IF youre working in gauge....... it get a little more energetic about things as youve got the full 17.4psi of head pressure

you have the air coming out at 2403l/s with a velocity of 3.04m/s with a flow mass of nearly 3kg/sec

****edit*****

forgot you wanted CFM, multiply by 2.12 to go l/s-cfm

so 2030CFM if your pressure is absolute

and

5095 if its in gauge

now I've done the fun bit...

what the hell is it for, those pressures and volumes sound sussly like a decompression chamber

[W&KO]

Needing some numbers to back up my case with the OEM on damage to its dust collector.

[Blknight.aus]

IF you're talking about damage caused by the discharge through the 4 inch hole then I suspect that you're likely to be more interested in the mass flow numbers rather than the flow velocity.

you have my curiosity.

Whats such a large pressure vessel for with such a low pressure?

[austastar]

Hyperbaric chamber??
Cheers

[ramblingboy42]

if it was a recompression chamber, you would or should already have all that information.

I'm with you Dave.

[Old Farang]

IF youre working in gauge....... it get a little more energetic about things as youve got the full 17.4psi of head pressure

you have the air coming out at 2403l/s with a velocity of 3.04m/s with a flow mass of nearly 3kg/sec

(https://www.aulro.com/afvb/members/13085.html)
Blknight.aus (https://www.aulro.com/afvb/members/13085.html) : The numbers that you have come up with may have a bit more credibility if you were to show at least some of the factors used to arrive at them.

Why confuse the question by asking if it is absolute or gauge pressure? The shape of the vessel may be "more or less immaterial", but in fact the volume is
closer to 96.5 cu metres than 90. In view of later comments made by the OP it appears that he may be involved in some form of legal dispute, where by quoting even accurate numbers without proof could lead to more problems. There are too many unknown variables to arrive at even an estimated number.

OP: do yourself a favour and go and buy a cheap anemometer, more so if you are in dispute over equipment.

https://www.amazon.com/General-Tools-DAF800-Digital-Airflow/dp/B002OGVUF2

[W&KO]

No legal dispute, more about me getting a better understanding so I can engage in the conversations.

Just trying to get the set up right. We are somewhat limited on the weight of the ancillary equipment.

The vessels official capacity is 90m3 as registered with Queensland WHS, the measurements were from memory.

No biggie....I’ll get it sorted.