I would like carry extra fuel and have 2 x 20ltr and one 10ltr Chlorine container and would like to join so as to full as one and use a 12v pump to fill the car. What should I use and how do I do it. Thanks.
A dedicated and approved tank would be a better and safer option.
The size and shape would largely depend on where it needs to be fitted.
Joining 20 litre drums will work but there in is the risk of the failure of the connections.
If permanent my advice would be to use a tank and if temporary then use your pump independently or separately on each container rather than to try and join them.
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Most 20 litre containers have a "Bung" in bottom to screw in tap or such - I am uncertain of the thread type - You would need to ascertain thread type/size to screw in a metal or plastic union. If I were making a set up in the bush; I would cut the threaded end off several PVC taps, then bond attachment PVC tube to the threaded ends.
This set up would suffice in a static situation, definitely not suitable for transporting flammable fluids.
If it is more of an issue of weight which is our case, we use 4 x 10L & 2 x 5L (the 5L fit within our second spare wheel). But 10L (9kg with petrol) are easy enough for us to get on & off the roof. 20L would be too difficult.
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When you say "chlorine container" I assume you mean ex pool chlorine or similar?
Diesel is not a volatile liquid nevertheless when it does catch fire it burns pretty well. Around camp I often store spare heater diesel in 2L milk bottles short term but no way would I transport it in anything less than an Australian Standard approved container which, amongst many other things, are UV resistant which I'll bet your chlorine container isn't.
Next, the 12V pump: as mentioned diesel is non volatile but no way would I be using an electrical pump for it which was not approved for such liquids.
I think WAWT's solution is good; a number of 10L AS approved containers, easy to hand pour, easy to fill. if one springs a leak you don't lose it all, their weight can be distributed.
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I carry 4x10 litre approved plastic fuel containers for extra diesel and as WAWT and Mike have said they are easy to handle and store. I used to carry 2x20 litre jerries but they were too heavy and large to handle and store, never put them on the roof.
Thanks for the replies. The reason I was going with the specific size was the ability to fit them in an opening that I have in the rear of my Van. Maybe I should just pump each one empty with the correct pump.