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Written by Eric Hahn

How LPG Gas Bottles-Cylinders are Refilled

There are two ways that LPG gas bottles are typically refilled.

The first technique is by weight, using digital scales.

The second approach is by decanting, relying on a bleeder screw.

We’ll look at the details and the pros & cons of both methods…

Current Inspection Date Required

No matter which of the refill methods are used, the gas bottle must have a current inspection date.

It is the responsibility of the refill technician to check the inspection date stamped into the neck ring of the gas bottle.

Out-of-date gas bottles cannot be filled until they are re-inspected and stamped with a new inspection date.

Refilling Using Digital Scales

Using digital scales helps ensure proper and safe filling, based on weight. (see the image to the right)

The scales are set for each individual gas bottle, based on its empty (tare) weight, as well as the desired gas content weight, when full.

When the total weight equals the empty weight plus the appropriate gas fill weight, the process is completed.

How the Scales Work

The tare weight (empty weight) is stamped into the neck ring by the bottle manufacturer.

The operator inputs the tare weight of the gas bottle into the scale’s keypad. 

After inputting the weight, the technician connects the bottle to the LPG supply.

The bottle is then filled until automatically stopped by the digital scale, after reaching the target weight.

This ensures very accurate filling and avoids dangerous overfilling or lost value through under filling.

The filling accuracy achieved is why the use of digital scales is the preferred method for LPG refilling.

Not surpisingly, this is how major suppliers, like SWAP'n'GO^®, do their refilling.

Faster Filling with Carousels

A side benefit of using scales for filling is speed.

The LPG supply line can be pump driven for faster filling.

This is not possible with decanting, as the operator might be too slow to stop the process, resulting in dangerous overfilling.

Digital scales and pump driven LPG also allow for the use of high speed filling carousels, like the SWAP'n'GO carousel in the accompanying image.

In fully automated plants, with filling carousels, even the keypad step can be automated.

The carousel digitally reads an RFID chip on the cylinder that contains the tare weight, as well as the rest of the manufacturing specifications of the gas bottle.

The Decanting Method

Decanting is the process seen if you have ever watched a gas bottle refilled at a service station.

After connecting the gas bottle to a larger LPG storage tank, the trained operator fills the bottle until some gas starts leaking out of the loosened bleed screw.

The operator is supposed to stop the filling as soon as liquid LPG starts coming out of the bleeder opening.

The liquid LPG escaping is unmistakable, as it looks like a white cloud.

This intentional venting of gas into the atmosphere is avoided with the digital scale method.

Decanting can also result in either overfilling or under filling, as well as correct filling, depending on the gas bottle and the operator filling it. 

For example, if the operator is slow to stop the process when the liquid LPG starts to bleed out, the gas bottle could easily become overfilled.

As previously mentioned, overfilling is a significant safety hazard.

Dip Tube Variations

In addition to operator performance variances, there can be inconsistencies in the dip tube lengths. 

Dip tubes can even be missing or have fallen out of the bottom of the valve. 

The accompanying picture shows some examples.

Dip tubes lengths are important because they are connected to the bleed screws. 

The refill operator opens the bleed screw and fills the cylinder until the level of LPG reaches the bottom of the dip tube, at which point liquid LPG will start to bleed out. 

This is his signal that the cylinder is full. 

Long dip tubes can result in under filling.  

Short or missing dip tubes will cause overfilling. 

A missing dip tube is the worst case scenario, as the entire ullage space would be filled, allowing no room for expansion. 

This is one of the reasons that swap is a much safer alternative, as dip tube variances have no effect when filling by weight.

20% Oversized for Expansion

LPG vapour pressure rises with temperature.

If a gas bottle is exposed to higher temperatures, the gas will expand.

LPG bottles are designed to be full while allowing 20% of the space, called “ullage”, for the natural expansion of the LPG.

Properly filled bottles, with the required 20% ullage, should never have a problem.

Overfilling is Unsafe

When overfilled, the gas bottle has less than 20% ullage, creating the possibility of the unwanted release of gas to the atmosphere, through the pressure relief valve.

The pressure relief valve is incorporated into the main gas valve on the bottle.

It’s actually a valve within a valve that prevents dangerous overpressure situations.  

So, overfilling combined with heat can result in gas venting from the pressure relief valve, which creates a significant safety hazard. 

This is especially true in the warm Australian climate.

Refilling is NOT a DIY Procedure

Only trained technicians should attempt to refill by either method.

This is not a do-it-yourself handyman procedure and should never be attempted by untrained personnel.

The trained operators know all about the appropriate procedures, what personal protective equipment (PPE) to wear, the importance of the proper type of clothing (including material), proper cylinder grounding, static electricity hazards, ignition sources, fire protection and more.

In-depth safety training is mandatory for these technicians.

Doing it incorrectly could result in cold burns (liquid LPG is -42°C), fire or even an explosion, in addition to dangerous overfilling.

The simple message for people who are thinking about trying to refill their own gas bottles…

Just DON’T do it!