Regulator's pressure meter of 15 ltr. LP gas cylinder showing RED!!!

Willen · Jun 10, 2018

While buying the regulator the seller just said red is danger, green is normal and yellow is low. When I used new cylinder today, it shows Red! What it means? The gas is more than a limit inside? Or there are some leakage? And what I need to do now?

JimB · Jun 10, 2018

Not enough information.

Red yellow and green on the pressure gauge means nothing.
What is the actual pressure (psi or Bar or ??)

The pressure in an enclosed tank with liquid and gaseous propane will be mostly dependant on the temperature.
There will be some variation depending on what other gasses are in there as well. (eg Butane, ethane , methane).

Have a look at the graphs here:
https://www.engineeringtoolbox.com/propane-vapor-pressure-d_1020.html

Measure the temperature of your propane tank and read the actual pressure shown on the gauge on the regulator.
Compare the readings with what is shown on the graph.

Note that I am assuming that your LPG is propane and not butane.
Also that the gauge on the regulator is scaled for the corrct gas.

JimB

unclejed613 · Jun 12, 2018

it depends on the way the gauge is meant to be used, is the gauge before (between the bottle and the regulator) or after the regulator, and is the red at the top of the scale (right side) or bottom of the scale (left side)?

if the gauge is after the regulator, and the red is the top of the scale, you have the regulator set too high.

if the gauge is before the regulator, maybe the bottle has been overfilled.

when you buy gas, be absolutely positive about what you are buying. there is one flammable gas that should NEVER be filled overpressure, and that's acetylene. if acetylene is filled above 200PSI (13ATM), it can spontaneously detonate (yup, that triple valence carbon bond is unstable).

Willen · Jun 12, 2018

Hi, The pressure level meter is a part of regulator (attached). The red means full of the scale and Yello is low scale and green for 50%. The meter do not have numbers about pressure unit. It was showing red (overpressure) two days ago which was pretty hot day (40 degree C). So I guessed because of high temperature. So I used water to cool down the cylinder few minute. Then meter dropped to green.

Is it normal with LP gas?

unclejed613 · Jun 12, 2018

yes, and that is fairly warm. keep the cylinder out of direct sunlight, and away from surfaces that radiate a lot of heat like asphalt road or parking lot surfaces. i'm guessing Nepal at this time of year is kind of a hot climate.

Willen · Jun 13, 2018

unclejed613 said:
yes, and that is fairly warm. keep the cylinder out of direct sunlight, and away from surfaces that radiate a lot of heat like asphalt road or parking lot surfaces. i'm guessing Nepal at this time of year is kind of a hot climate.

I never have thought about storing temperature of the cylinder. Nepal has any type of climate always. I am living around 150km south from the tallest mountain Mt. Everest and here days are hotter like 38 to 40°C. Just 150km north in there is snowfall always.

gophert · Jun 14, 2018

Propane is mostly in the liquid state inside the tank. The space above the liquid is propane gas and the pressure of the gas is proportional to the temperature of the tank. A pressure gauge is not helpful for liquidified gases like propane. You must either weigh the tank or check the temperature of the tank from top to bottom to feel where it is cool (the surface of the liquid cools when propane evaporates - only works while gas is flowing out of the tank).

Willen · Jun 14, 2018

Hi,
And since the gas is liquid, how there's pressure created inside?

Maybe gas above the liquid creates pressure but why there's liquid propane and gas propane both created in a same tank?

JimB · Jun 14, 2018

At room temperature and pressure (20 degC and 1 barA), propane is a gas.

When the gas is compressed it becomes a liquid.

When the liquid is contained in a sealed container, with free space above the liquid, the liquid will evaporate (boil) until a pressure equilibrium is reached.
The pressure will depend on the temperature, the pressure/temperature relationship is given in the graphs which I linked in post#2.
This pressure is known as the Vapour Pressure.
Look at the second graph, at 20 degC, the pressure will be about 8 bar.

JimB

large_ghostman · Jun 16, 2018

Acetylene cylinders contain a honeycomb material and the acetylene is dissolved in acetone rather than as a compressed gas, precisely to stop them going bang if dropped. Modern ones the filler prevent over filling as the gas is mixed with a solvent.

ronsimpson · Jun 16, 2018

Willen said:
Maybe gas above the liquid creates pressure but why there's liquid propane and gas propane both created in a same tank?

Here is a bottle half full of liquid and half of gas. (well probably air which is CO2 and many things) The gas could be water vapor like a cloud. You can have water as a liquid and as a gas at the same temperature and pressure.

Get a bottle with a vacuum in it. (nothing there, like space) Fill it 1/2 full of water. About 0.1% of the water will turn to gas to fill up the vacuum. Now you have a bottle with 49.1% liquid water and 50.1% water gas. By weight the bottle is 1/2 full. If you tried to put more liquid water in the bottle the more pressure will force some of the gas back to liquid. If you removed some of the water the reduced pressure will allow more liquid to move to gas.

unclejed613 · Jun 27, 2018

https://www.propanecarbs.com/propane.html

about half way down the page is a section describing how liquid propane behaves in a pressurized container.

ronsimpson said:
Here is a bottle half full of liquid and half of gas. (well probably air which is CO2 and many things) The gas could be water vapor like a cloud. You can have water as a liquid and as a gas at the same temperature and pressure.

Get a bottle with a vacuum in it. (nothing there, like space) Fill it 1/2 full of water. About 0.1% of the water will turn to gas to fill up the vacuum. Now you have a bottle with 49.1% liquid water and 50.1% water gas. By weight the bottle is 1/2 full. If you tried to put more liquid water in the bottle the more pressure will force some of the gas back to liquid. If you removed some of the water the reduced pressure will allow more liquid to move to gas.

interesting experiment:

fill a medical syringe 10% full of water. seal the end the needle goes on. pull the plunger out to the 100% mark. the water boils at room temperature inside the syringe because there is a vacuum above the liquid. the water will stop boiling when enough water vapor is above the liquid to pressurize the inside of the syringe to the point where the boiling point is equal to room temperature. there is still somewhat of a vacuum inside the syringe, but at a particular pressure, equilibrium is reached between the liquid and the vapor, and so it stops boiling.

Diver300 · Jun 30, 2018

In a full cylinder of liquid gas, such as propane, if there is any contamination of gasses with lower boiling points, such as air, methane or ethane, the pressure can come from those being compressed, and the contaminants won't liquefy as easily as the main gas.. It is quite common to get the pressure too high because of those other gasses, and as soon as they have been released, the pressure drops to the vapour pressure of the liquid, which is constant for any amount of liquid in the cylinder, as long as the temperature stays the same. As the gas runs out, the pressure drops when there is no more liquid to vapourise.

bobledoux · Jul 7, 2018

Propane liquid, in a tank, expands and contracts with temperature. Tanks are generally not filled to more than 80% of the liquid capacity. This is to permit room for expansion. An overly filled tank moved to a very hot environment could burst because liquids don't compress. A properly fully-filled tank should be safe at 45 degrees C. The safety increases as the fuel is used up. Your red-yellow-green pressure gauge may be intended to display acceptable fuel pressure for the pressure regulator. As the pressure increases the propane output may exceed the acceptable volume.

I've been a hot air balloon pilot for 38 years so I've gotten very close and personal with propane.

Willen · Jul 7, 2018

bobledoux said:
I've been a hot air balloon pilot for 38 years so I've gotten very close and personal with propane.

Wow! The baloon floats just because of 'hot air'? I think the baloon's material is a hot resistance so it does not melt. What is the temperature inside the baloon while floating?

unclejed613 · Jul 10, 2018

Willen said:
Wow! The baloon floats just because of 'hot air'? I think the baloon's material is a hot resistance so it does not melt. What is the temperature inside the baloon while floating?

it really doesn't take much of a difference in temperature, as long as the difference in density between the inside and outside air provides enough buoyancy to lift the mass of the balloon and it's occupants.

JimB · Jul 13, 2018

While I was out cutting the grass this afternoon, a thought occurred to me...

The pressure gauge on Willens LPG cylinder is almost certainly a simple Bourdon tube type of gauge and will be affected by the atmospheric pressure.

The "standard" air pressure at sea level is 1013mb (760mm Hg)
Kathmandu is 1400m above sea level, so the air pressure will be lower.
According to this calculator: https://www.altitude.org/air_pressure.php
The corresponding air pressure in Kathmandu will be 860mb (648mmHg).

So we would expect a pressure gauge calibrated at sea level to read (1013 -860) = 153mb higher.

This would be in addition to any temperature effects which were discussed earlier.

... amazing the things you think about when doing simple manual tasks.

JimB

unclejed613 · Jul 14, 2018

that would also mean that the pressure differential in the walls of the container are still what is indicated on the gauge, or at least fairly close. that additional 153mb is exerting a force on the walls of the container. that's why there are two common measurements, psia (absolute pressure) and psig (gauge pressure). at sea level, 15psia=0psig. if a container were sealed at sea level, a gauge attached to it would read 0psig, and would read 15psig when placed in a vacuum. at STP, the container has no pressure differential applied to the container walls, but in a vacuum, there's a pressure differential of 15psi, and that force is applied to the walls of the container.

JimB · Jul 14, 2018

unclejed613 said:
that would also mean that the pressure differential in the walls of the container are still what is indicated on the gauge, or at least fairly close. that additional 153mb is exerting a force on the walls of the container. that's why there are two common measurements, psia (absolute pressure) and psig (gauge pressure). at sea level, 15psia=0psig. if a container were sealed at sea level, a gauge attached to it would read 0psig, and would read 15psig when placed in a vacuum. at STP, the container has no pressure differential applied to the container walls, but in a vacuum, there's a pressure differential of 15psi, and that force is applied to the walls of the container.

Correct.

JimB

unclejed613 · Jul 27, 2018

i originally learned that in physics class in high school, and got a refresher in calibration school. so, see there you go, i actually learned something in high school that was useful later in life...

Regulator's pressure meter of 15 ltr. LP gas cylinder showing RED!!!

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