What does liquid propane look like

 

Boiling point / evaporation



Propane and butane differ mainly in their different boiling points, i.e. the temperature point at which evaporation begins.

The boiling point is at Propane above - 42 ° C
  at Butane above + - 0 ° C

These boiling points mean that at a temperature below -42 °, propane remains liquid. Butane remains liquid at temperatures below + - 0 ° C.

Since the gas can and must only be used from the gas phase and never from the liquid phase for the use of the devices, the boiling point is a decisive value.

Theoretically, propane could be transported in an open vessel at temperatures below –42 ° C. But theory and practice are vastly different and, of course, they play a major role in safety.

The evaporation starts when the temperature rises. One could use water as an example. If water is heated, evaporation starts at + 100 ° C. With propane this point is just at (minus) –42 ° C and with butane at + -0 ° C. The heat is supplied to the liquid gas from the outside through the container wall. Even at sub-zero temperatures, the ambient temperature is still a source of heat with propane, but not with butane.

The boiling point, which influences evaporation, also plays a major role in the pressure in the pressure vessel / liquefied gas cylinder. Again the water for comparison. When you bring it to a boil, the pressure gets higher.

The pressure conditions for liquid gas in relation to the temperature can be read from a vapor pressure curve. The temperature can therefore be determined using the pressure.

It is therefore very important to know that liquid gas expands when it is heated and, at the same time, the pressure in a container is increased if there is no possibility of expansion in the container. Regardless of the amount in a container. This explains that the filling quantity in a container cannot be measured using a pressure measurement.

A pressure measurement would only show two things:

  1. the ambient temperature, due to the different pressure when the temperature changes

  2. whether there is propane or butane in the container, due to the different pressure of the two gases.
The filling quantity cannot be determined by measuring the pressure.

However, this also means that a gas phase must always be present in addition to the liquid phase in a container so that the possibly increasing pressure in the gas phase can be absorbed when the temperature increases. Otherwise the pressure vessel could burst.

The gas phase in a pressure vessel is therefore an essential safety aspect when using it. The gas phase is always above the liquid phase. The pressure is therefore absorbed by the gas phase when the ambient temperature rises. In plain language it means that when filling, the pressure vessel / gas cylinder must never be completely filled so that a gas phase remains.

In general, it can be said that the gas container may only be filled to 80% with the liquid phase.



Since it is not possible to determine the filling quantity by measuring the pressure, filling only takes place according to weight, i.e. gravimetrically. Especially since you cannot see through the pressure vessel / gas container. At the approved filling stations, filling is always carried out using a scale and then a checkweigher.

The easiest way for the user / operator of a system to determine the quantity in a liquid gas cylinder is with a so-called bag scale. The tare weight that is stated on the nameplate is subtracted from the total weight.

Devices that determine the amount of content with an ultrasonic measurement have also recently appeared on the market:

A Self-filling of gas bottles with unauthorized transfer hoses that are offered on the market is not only prohibited for the aforementioned reasons, but also absolutely dangerous to the public. Anyone who does something like this is obviously not clear about the scope of his action. Unfortunately, there are always some who think they know better. However, accidents prove the opposite. According to the Product Liability Act, the seller of such an article is also liable for damage if this article has been delivered to unauthorized persons / non-specialists.

Examples of incorrect filling:

  1. Incorrect filling of blue GAZ bottles with propane without a safety gas cushion immediately after the fall of the Wall in the former GDR on a boat led to the spring pressure valve being blown out on several butane gas bottles when the temperature rose. Apart from the noise, no major damage was incurred by chance. Only the mast of the sailboat was pierced by a valve.

  2. in an accident in Spain in 1973, the tank of a tanker truck burst due to overfilling with 3.8 t. The safety gas cushion was missing here. When filling at dawn, all safety conditions were clearly ignored. In the midday heat, the container could no longer withstand the pressure and burst. Unfortunately, the gas ignited on the motor vehicle and rolled over a campsite as a burning roller, with a devastating effect.

Unfortunately, the connection threads of the German gas cylinders are not identical to the connections of foreign gas cylinders. Almost every country has its own ideas here. A thread standardization only in the EU is currently hardly to be expected or feasible. When filling German propane bottles abroad, it can therefore be very helpful to take so-called intermediate pieces with you on the trip:



With these respective spacers, the threads can then be bridged by the foreign filling stations if the filling station is not set up for German users.

Conversely, there are also intermediate pieces on the market to possibly connect a foreign gas cylinder to a German system / pressure regulator:



However, there is a growing trend towards safety in other countries too, and with it new filling regulations that are not always beneficial for travelers. Sometimes you come across real problems when you want to fill them, which are also handled differently.

So the pressure in a container is very important in terms of safety.

In the case of propane, a change in temperature results in the following approximate values ​​for pressure:

at 1 ° C increase in temperature - pressure increase = 0.3 bar
i.e. at a temperature increase of 10 ° C - pressure increase = 3.0 bar
thus extrapolated at an ambient temperature of  
+ 20 ° C - pressure = 7.0 bar
+ 30 ° C - pressure = 10.0 bar
+ 40 ° C - pressure = 13.0 bar

It turns out that these pressures are not a problem if they rise due to solar radiation. Unless the solar radiation is intensified by glass or mirrors or other circumstances. Of course, this information cannot be used for the temperature conditions in warmer zones. This is why butane is mainly used there, because the pressure with butane is significantly lower due to the higher boiling point:

Butane + 20 ° C - pressure = 1.2 bar

This lower pressure, due to the higher boiling point, is the reason why the blue butane GAZ bottles can be thinner-walled. Therefore it should never be filled with propane.

In plastic gas lighters there is always butane because of the low pressure. When it's cold, the lighter only works with the heat generated in your pocket.

Serious accidents can occur if the gas cushion / gas phase was not taken into account when filling.


Here are the values ​​at more irregular Filling:

at 1 ° C increase in temperature - pressure increase = 7.0 bar
i.e. every 10 ° C increase in temperature - pressure increase = 70.0 bar

The gas containers such as propellant gas tanks and gas cylinders, which are approved according to the pressure vessel ordinance, would hardly withstand the pressure load if filled incorrectly. The containers would burst at a pressure of around 100 bar. Of course not, as is often falsely claimed, to explode. The liquid gas could only explode if an ignition source were present at the same time after the bursting in order to ignite the entire quantity at once.

Compared to other countries, every German propane gas cylinder has a safety extraction valve. This valve is set in such a way that it blows off at approx. 30 - 35 bar in the event of an uncontrolled increase in pressure. The safety valve to be screwed in to use a butane GAZ bottle blows off at approx. 14 bar. After the pressure has stabilized back to normal, the safety valve would close again.

However, the safety valve is not intended to blow off if an incorrect filling takes place and could not respond so quickly. The function of this safety valve is given when the pressure rises slowly and also not when the pressure rises, where the container would suddenly be exposed to a fire. In practice it has happened that gas cylinders were left in the fire during a fire. The strength of the steel jacket decreases sharply in the event of a fire, while the pressure in the bottle increases at the same time. At the weakest point, the steel jacket tears open and the gas escapes. There is an additional fire, of course, but no explosion. The steel bottle remains as a flat sheet metal or if the gas bottle tears open on the bottom, it can happen that the filling acts like a propellant and the bottle flies around.

As already stated, the gas phase in every container not only plays a very important role in terms of safety and filling, but the "fuel" gas can be taken from the gas phase for combustion purposes, i.e. to operate the devices.