Gas Tank valve letting small amount of gas through when closed on habitational check. ( Letby )
Please note ( Update December 2020 ): In over 30 years of selling our German made outlet taps in the many hundreds of thousands, we have never had to supply a replacement tap or remove and change a outlet tap or tank due to the gas not shutting off ( in or out of warranty ) as its a simple case of either cleaning the valve ( does not require it removing from the bottle or tank ) or simply making sure the tank is not over filled with liquid LPG.
When you close the manual or electric valve of any gas bottle or gas tank the gas should shut off but on the very odd occasion there might be a very small amount of gas that gets past the seal on the shut valve and this is known in the industry as Let by. This letby is not normally visible to the eye or ear and it might not be picked up with leak detection spray but when carrying out an habitational check the gas engineer might be able to see it when they are doing their tests.
In all the many years we have been selling gas bottles and gas tanks we have never experienced a manufacturing issue with the german made valves ( by the many, many thousands every month ) that we fit and sell and this Let by has always been found to be either be
1). Caused by the dirt in the gas or bottle/tanks getting on the valve seat and causing it not to close 100%. LPG, which is the same gas that's in a refillable as a exchange type bottle, is inherently dirty especially when it evaporates from a liquid to a vapour (because its a member of the Petroleum family and produces oils and something called heavy ends). You only have to ask anyone with an autogas car about the number of filters in their gas system plus you see the likes of GAS IT and Truma who have for a number of years supply filters to clean up the gas from normal gas bottles and refillables.
2) The other cause to take in to consideration is the tank or bottle has been over filled and the liquid in the tank is coming out of the valve and either the valve seat has frozen or the pipes are filled with liquid gas which will take longer to burn if you turn the tap off ( giving you the impression it won't shut off )
There is also the mechanical dirt issue getting into the tank or bottle which is caused by physical dirt from the previous refueling vehicle carry mud, dirt and grit on its fillpoint which then transfers from the the fill gun and being pushed into your gas bottle or gas tank. This is why the regulations state you have to have a dust cap on your fillpoint to stop dirt passing from one vehicle to another.
Now as we said earlier, it is very rare that a valve on any gas cylinder would let even a small amount of gas by - we only hear of it on a few gas bottles or tanks a year during habitational checks which is very small then you take in to consideration the many hundreds of thousands of valves we have sold over the years which are still in use.
Now what is the solution to fixing this let by issue?
Well there are a few options but we have found over the last 25+ years that a 100% fix is quite easy as it clears the dirt off the seat of the valve, on anyone's gas bottle or gas tanks, and this is by suggesting that the valve be fully opened and fully shut 20 to 30 times. This opening and shutting fully resolves the issue of the dirt that was on the valve seat and when retested works every time.
Should the above not work, then the next step would be to try and remove the dirt from the seat by opening the tap in a couple of short - but strong enough bursts with no pigtail attached to blow the dirt off the seat, but obviously this can only be done when the gas bottle has an inert gas in it like Co2 or Nitrogen as we can't recommend doing it that way with LPG gas in it due to flammable issues of releasing LPG.
Now let's assume though that the outlet tap valve has become really damaged ( this can be caused by physical dirt wearing it away, its very old and had a lot of use, its sustained a lot of liquid LPG damaging on a vapour only valve or there has been some kind of mechanical damage. Worse case if the valve is a manual tap version then the whole outlet tap valve will need replacing and a new one fitted.
If its over filled then the only option is to remove the pigtail and remove the over filled gas from the tank or bottle which will not take long ( we can't recommend just letting the liquid go by opening the tap as there is a risk of fire and explosion - seek a engineer who will carry this out safely and controlled )
If you have a electronic valve then that's even simpler as the plunger can be removed within a minute, it can be physically cleaned and so can the valve seat and if the plunger seal is damaged then a new service plunger can be slotted in place and away you go - in less than 5 minutes.
Obviously the replacing or the valve and servicing or cleaning electronic valves plunger is beyond a normal person's abilities but a qualified and trained gas engineer, specifically a Autogas engineer, will be more than able to carry out this work or call GAS IT and we can do it for you here at GAS IT HQ.
So to conclude - If you have Letby on any gas valve fully open and shut the valve 20 to 30 times to remove the dirt on the seat and if this doesn't work you might need the valve or plunger replacing ( we've yet to need this doing in over 30 years ) or drain the over filled tank or bottle but only call a trainiend gas person in to do this work.