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Active Contributor
Engineering Fields - Piping Design Engineering - New Member

Join Date: Oct 2008
Location: Chennai, India
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What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/03/2009 2:13 AM

Dear all,

I had a discussion with my co-worker about the pressure inside the LPG filled cylinder (domestic used at home for cooking). The ratio of LPG mixture being 70% of Propane and 30% of Butane. I told that the pressure inside the cylinder would be equal to the vapour pressure of the propane and butane at that particular temperature (say at 40 degree celsius). But my co-worker argues that the pressure is around 7 Bar and he is also not willing to provide the backup for his calculations. Can anyone help me out from this confusion and also plese let me know what will be pressure inside the cylinder.

Please also include the calculations while you post your answer.

Many Thanks in advance,

Nayek.

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Anonymous Poster
#1

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/03/2009 2:33 AM

You are right - the pressure will be the vapor pressure at that temperature:

http://www.engineeringtoolbox.com/propane-butane-mix-d_1043.html

Which is about 9 bar

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Active Contributor
Engineering Fields - Piping Design Engineering - New Member

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Location: Chennai, India
Posts: 15
#8
In reply to #1

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/04/2009 11:20 PM

Thank you all for answering my question.

I am from India. As per IS:4576 (indian standards) code for LPG Specification. In that the vapour pressure of propane and butane have been given as 1550 kpa for propane and 520 kpa for butane at 40 degree celsius. I am giving below my calculations, Please let me know whether the calculations are right or not.

Total vapour pressure of a mixture = partial pressure of vapour 1 + partial pressure of vapour 2

As I told in my question, the mixture contains 70% propane and 30% butane.

So the pressure of LPG mixture = (0.7 x 1550) + (0.3 x 520) = 1241 kpa

Converting it into bar, 1241 kpa = 12.41 bar @ 40 degree celsiuis.

Hence the pressure inside the cylinder will 12.41 bar @ 40 degree celsius.

Please let me know whether the above calculation right or not.

Thanks in advance,

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Join Date: Jan 2008
Location: Durban, South Africa
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#9
In reply to #8

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/05/2009 2:00 AM

The pressure is self regulating! You compress the gas and it liquifies - if you compress the liquid then the bottle will burst otherwise it depends upon vapour pressure! You cannot incease the pressure inside the bottle as long as their is a vapour space. Over the critical temperature then it is another story!!!

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Associate

Join Date: Dec 2006
Location: Manchester - England
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#10
In reply to #8

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/05/2009 7:57 AM

For your calculation you are using Raoult's Law, which is really only accurate for low pressures. Also I assume that the 70% Propane and 30% Butane are liquid weight percentages. In order to calculate partial pressures you need to work in mole fraction and not weight percent. Also the pressure gauge on your tank will give a gauge pressure while the vapour pressures will be absolute pressures.

Assuming 100 kg of liquid LPG

then you have 70 kg C3 Mol Wt = 44.1

Moles C3 = 70/44.1=1.587 kgmoles/kg

and 30 kg C4 Mol Wt = 58.1

Moles C4 = 30/58.1 = 0.516

Total Kgmoles/kg = 2.103

Mole Fractions C3=1.587/2.103=0.755 & C4=0.516/2.103 = 0.245

Raoult's Law

Bubble Point Pressure = 0.755x1550 + 0.245x520 = 1297.65 kPa (absolute)

Gauge Reading = 12.97-1.01 = 11.96 Barg

You can download a more accurate vapour/liquid equilibrium set of calculations at:

http://www.che.udel.edu/thermo/basicprograms.htm

Using the VLMU program you get 10.99 bar absolute at 40 deg centigrade = 9.98 barg

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Active Contributor
Engineering Fields - Piping Design Engineering - New Member

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#12
In reply to #10

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/06/2009 11:37 PM

Thanks Tesser,

This is what I wanted. Thanks again for very detailed answer.

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Power-User

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#11
In reply to #8

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/05/2009 4:49 PM

Excellent, well thought out & accurate answer. It's refreshing to hear from a true professional !

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Anonymous Poster
#14
In reply to #8

Re: What is the pressure inside the LPG cylinder vessel used for domestic cookin

11/11/2009 3:00 AM

good. keep it up.

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Guru

Join Date: Mar 2007
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#2

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/03/2009 6:45 AM

Why the guests do not unguest them. I miss frequently losing my valuable GAs.

It is a basic fact that when gas(vapour) and liquid co-exist, the pressure of the system will be the vapour pressure of the liquid.

The GA is for the link, i too frequently visit the site, but it took a quite a few steps to reach the page from home(when I refused to follow the link)

And the guest must have been more enterprising - since I know the data is in the site and he didn't (or did he google ?)

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Anonymous Poster
#3

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/03/2009 11:52 PM

with my experience in a lpg plant in honduras, i see the manometers in the place of refilling, the pressure are 150 psi to 5 galons cilinder, and others is equal.

i hope my info will be good for you.

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#4

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/04/2009 2:01 AM

This a liquified gas - very little pressure is needed to get below freezing.

I have had this question as to what happens to an aerosol can (with lpg propellant) when it is heated.

Vapour pressure matters until above critical temperature - I believe it will then be a compressed gas and pressure will increase. This is what happens when you have a BLEVE (Boiling Liquid Expanding Vapour Explosion)

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#6
In reply to #4

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/04/2009 9:54 AM

Non sequitur !

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Power-User

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Location: Olympia, WA
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#5

Re: What is the pressure inside the LPG cylinder vessel used for domestic cookin

01/04/2009 9:13 AM

Greetings.

Use a manometer to get an accurate reading of output.

I believe 27.71 inches of water column equals 1 pound per square inch.

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Power-User

Join Date: Oct 2008
Location: Meherrin Virginia
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#7

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/04/2009 1:36 PM

Pressure at about 70 deg. F is about 225 PSI. higher with higher temp. lower with lower temp., The outlet of the regulator is held at about 11 inches water column.

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Anonymous Poster
#16
In reply to #7

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

02/28/2010 8:51 AM

You "failed to follow through". Are these just pretty words, because that's what has been expressed! What regulator are you referring to? If it's in the contex of the original question, you are in error by roughly 220% ! The LPG manufacturer worker may be able to confirm this; the bottle outlet is 10 psi + - 1.5 psi.

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#17
In reply to #16

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

02/28/2010 10:39 AM

What am I missing in the following;

Propane/LP Gas

All propane and LP Gas applications require the use of a regulator. Because pressures in propane tanks can fluctuate significantly, regulators must be present to deliver a steady flow pressure to downstream appliances. These regulators compensate for tank pressures in excess of 200 psig and commonly deliver 11 inches water column for residential applications. Propane regulators differ in size, delivery pressure and adjustability but are uniform in their purpose to deliver a constant outlet pressure for downstream requirements. As is the case in all regulators, inlet pressure is higher than outlet pressure.

from wikipedia,

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Anonymous Poster
#20
In reply to #17

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

05/09/2010 9:22 PM

11" water gauge is the usual working pressure for LPG domestic appliances. It is not the standing pressure.

The pressure at the regulator outlet is quite a bit higher (sorry I have gotten what the actual pressure is) to allow for multiple outlets to be used at once and loss of pressure through drag through the pipework etc. All appliances should also have their own regulator to allow finer adjustments if required.

cheers,

Terry

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Anonymous Poster
#13

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

01/28/2009 1:46 PM

that is equal to around 10 bar that is vapour pressure of 70 + 30 butane and propane. vapour pressure is that pressure at which lpg transforms to vapour form from liquid foam.

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Anonymous Poster
#15

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

02/18/2010 6:51 AM

0 PRESSURE IS INSIDE THE CYLINDER, BECAUSE PRESSURE COMES IN EXISTANCE WHEN MEDIUM ( ATMOSPHERE ) ARE DIFFERENT , BUT INSIDE THE CYLINDER MEDIUM ( ATMOSPHERE ) IS SAME , SO THAT IS WHY THER IS NO ANY PRESSURE

PRESSURE WILL COME WHEN GASS COME OUT FROM CYLINDER , BECAUSE OUTER MEDIUM ( ATMOSPHERE ) IS DIFFERNET FROM INSIDE THE CYLINDER.

OK

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Anonymous Poster
#18

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

03/24/2010 4:02 PM

So what maximum pressure can LPG vessels cope with then ?

DCB

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#19
In reply to #18

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

03/24/2010 5:22 PM
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Anonymous Poster
#21

Re: What is the pressure inside the LPG cylinder vessel used for domestic cooking

10/30/2010 12:21 AM

we are working on the topic of LPG cooling we need the temp. & pressure inside & outside the cylindr while working ,please reply me to www.suyogpatil.mech@gmail.com

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