Whilst a case has been made for the safety of carrying an LPG tank in a vehicle elsewhere on this website, (see 'Is my vehicle more dangerous when it has an LPG tank?') it may be useful to run through the safety aspects of LPG systems in general, so that anyone involved in or attending an accident will know what to do.

The most significant risk is the same as it is with Petrol or Diesel fuels, i.e. fire resulting from an accident, so we'll centre around what the Fire Service will need to know about LPG safety features.

In most installations, the LPG tank will be mounted in the boot or load space.

Isolating the LPG Tank

Even the most basic LPG systems have provision for automatic cut off of the gas supply to the engine. Protection is also provided for gas pipe(s) that may be severed in an impact. The mechanism has two main parts, i.e electric Solenoids placed on (or very close to) both the actual tank outlet at one end of the gas supply pipe and gas vaporiser at the other end * and a switching unit which controls solenoid operation.

* Generally speaking, the tank is normally placed in the rear of a vehicle whilst the vaporiser is at the front, but this not always so. The main exceptions are Commercial vans and the 'People carrier' type of vehicle.

Solenoids are simply an electrically operated tap or valve. They are normally held open by a 12 volt current being supplied to them through wires from the ignition circuit. Switch off this current and the Solenoid will close, cutting gas supply. The current itself is only supplied when the engine is running. The switching unit (usually close to the driver's position and in the same box as the LPG gauge) 'knows' whether the engine is running or not because it receives a signal from the vehicle's ignition system. (It relies on an impulse from the engine's ignition system to tell it that the engine is running, and is said to be 'sensing' engine RPM.) If the engine is not running, no RPM signal will be produced and the switching unit will switch off the current to the solenoids.

In far less words;

No engine RPM = Solenoids switched off by the switching unit and no gas is supplied from the tank to the engine.

In the event that the LPG supply pipes are severed

Following on from above, there is an interesting 'failsafe' here. If the vehicle were to suffer impact that severed the LPG feed pipe to the engine (normally running front to back) then the engine will stop very quickly due to lack of fuel. When the RPM signal is lost the solenoids will cut off the gas supply, limiting any potential leakage to a short burst only.

Please note that the LPG filler pipe (normally at the rear of the vehicle and leading from the filler point to the tank itself) is NOT normally protected by a solenoid valve. It is protected by a mechanical 'check' valve (normally a ball bearing and seating arrangement). If the vehicle is hit from the side or rear and the filler pipe is ruptured or severed between the filler and the tank, gas will only escape until the pipe itself is empty (as with the supply pipe).

The LPG pipes themselves are most commonly made out of thick-walled copper tubing. 6mm outside diameter (OD) copper tubing is mostly used for the tank to engine supply pipe and 8mm OD is used for the filler to tank pipe. Both have a tough plastic external coating.

Other pipe materials used may be a tough flexible plastic / fibre composite, although they are normally only used for the filler pipe.

In all cases, however, the material (whether copper or plastic) has been chosen because it has properties that resist cracking or fracturing when deformed by any impact. Copper pipe, for instance, may be bent and straightened many times before the metal work hardens and begins to crack. In addition, a high resistance to corrosion caused by the fuel inside or the atmosphere outside is afforded by both materials. They also do not tend to become brittle over time.

Manual isolation of the LPG Tank

If the tank must be isolated but the solenoids have failed to shut off the gas supply (this is possible if a car crashes but the engine does NOT stop) there is a further backup. Most mulitvalve designs have a manual isolator valve situated on the tank outlet itself, normally placed inside the airbox. This valve will completely isolate the LPG tank, as it closes of both the filler entry port and the gas supply to the engine (through the front to back pipe). To close this valve, turn it clockwise (screw it in) until it reaches its seating. The tank will now be completely isolated.

Interesting side-benefits of the manual isolator valve are (a) that it can be used if the vehicle or the gas system are not used for a long period (closing the valve is recommended in these circumstances, e.g. if you are going on holiday) and (b) that it may be used as a very effective anti-theft device. Your car will not go far if gas is selected on the switch and the isolator is closed!

The manual tank isolator valve is closed by turning it clockwise (screwing it in).

Go to our item 'What you should know about your LPG system' to see what the fuses and manual valve look like, and find out where they are likely to be placed in your vehicle.

If the tank is isolated by the solenoids or manual valve but being heated in a fire

Clearly the risk of explosion exists. The tank is designed to hold fuel at a pressure of around 7.5 bar (105 Lbs per sq. inch or psi) but before it is approved for road use it must be tested to an internal pressure of 48 bar (672 psi). In addition to this bulit in safety factor, a preset safety valve is (should be) included in the outlet area of the tank. This will release pressurised gas long before internal pressure rises to a critical level. Initially, it will be released into the airbox and then vented out underneath the vehicle but there may be limitations here.* Some airboxes are plastic and might have melted before this happens. Better quality tanks may have an Aluminium airbox, which will melt at a higher temperature. That aside, the most severe limiting factor is that the vent pipe(s) themselves are almost always made out of plastic.

However, there is still a great deal of benefit in the safety valve system. If the tank becomes heated it will release its contents in one direction only, and do so in a controlled manner. It is very unlikely that it would explode and spread its flammable contents far and wide ( e.g. as a petrol tank would at much lower temperatures and internal pressures)

The safety valve

* Please note that it is very unlikely that underslung LPG tanks or tanks otherwise not fitted inside the vehicle will have either an airbox or vent pipes - These are only fitted on tanks mounted inside the vehicle's load or passenger compartments because they are only intended to conduct escaped gas from inside the vehicle out to the atmosphere. If the tank is mounted outside the vehicle, e.g. underneath, the venting is not required.

Beware of possible tank venting in the summer months.

A vehicular LPG tank has (should have!) a safety valve, designed to release the contents of the tank if the pressure within it rises for any reason. The prime objective of this valve is to allow the fuel to escape in a controlled manner should the tank be heated by a fire in the vehicle, thus avoiding a large explosion.

The safety valve can also release some gas if a certain set of circumstances arise. This account of a recent ocurrence will help to illuminate the cause, prevention and cure -

In mid June, one of our customers recently filled his LPG tank at around 6 AM before going to work. His vehicle (Range Rover Classic) has its tank mounted in the rear load space.

As the day wore on the weather become quite hot and the sun was beating down on the black tank (the parcel shelf was not in place).

Black is the colour that will derive the most heat from any light that falls upon it. At around midday the tank began to vent LPG, doing so for several minutes.

Why did this happen and what could the owner have done to avoid the situation?

The answers are quite simple.

First of all, the tank was filled when the ambient temperature was quite low, in the early morning. This was fine until the Sun heated the tank. The contents expanded (became less dense) and the tank pressure exceeded the pre-set limit of the safety valve which reacted by venting some gas. When the pressure within the tank reduced below the pre-set valve's maximum level it ceased venting.

OK, so this could be at least very worrying for an owner or others, and is potentially dangerous, so what could have been done to stop this happening?

Well, first of all, if the tank had not been completely filled this may never have happened as more expansion space would have been available. Secondly, if it had been filled during the hottest part of the day, it also may not have happened because the pump would have stopped at the maximum permissible pressure under the higher temperature conditions.

Given that the tank was filled at the time it was, could the owner still have avoided the situation?

Yes.

Avoidance / preventative measures

He could have parked the car in such a way that the Sun did not heat the tank, it is then unlikely that any venting would have occurred. If he could not do this, he could have covered the tank, insulating it from the Sun's heat. The lighter the colour of the cover, the more efficient it would have been. Silver reflective material is best at reflecting light (and therefore reducing solar heating of the tank). One of those reflective windscreen blinds would be perfect for this job.

Dealing with the occurrence (venting).

If the tank is venting, the first thing to do is to reduce the effect of the heat source, the Sun in this case.

A cold wet towel, sheet or blanket could be put over the tank, or if possible, cold water run directly over it. As the LPG content cools down it will shrink and the pressure within the tank will reduce, stopping the venting. At this time it (when venting has ceased) it would be wise to move the vehicle to shady spot and maybe even leave the engine running to further reduce the amount of fuel in the tank.

Notes -

Don't worry that the whole tank contents will be expelled if venting occurs - Once the pressure within the tank has reduced sufficiently the safety valve will close again.

Don't be put off using LPG as a fuel just because venting could occur one day - If you heed the advice given above the it'll never happen, it is quite a rare occurrence in any event.

Don't be misled into thinking that venting is an additional risk over petrol either - Petrol tanks can and do vent but this often remains unseen as the vapour expelled is largely transparent and not at any significant pressure.

Don't ever remove, or let anyone else remove or tamper with the lid of the airtight box around your LPG tank's multi valve (includes helpful souls, meddlers and the kids!). If the lid is left off the airbox any vented gas could be released inside the vehicle, the risks of which are clear.