relay back up safety system help

[photoman290]

i am running my waste oil burner from a peristaltic pump. works fine so far. i am using a 555 timer and a relay to pulse the pump on for around 4 seconds every 30 seconds or so. this gives a nice flame and nothing is getting too hot. bit like an aga. i am feeding the waste oil from a 25 ltr drum.
now being somewhat concerned about what could go wrong the most likely thing is the relay points sticking and the pump not turning off. this would not be good as the pot would fill with oil and i would have a major problem as the flame height is very dependent on the oil level.

i am trying to think of a way of preventing this happening. i could use a SSR but they tend to fail short circuit. the best i can come up with is a pot that limits the amount of oil with an overflow back to the reservoir,something like the constant level chicken waterer idea if the pump runs continuously the oil level cant get to high.

strange, being puzzling over this all evening and as soon as i write down the problem i think i may have the answer. only thought of the pot idea after i started writing this post.

[knighty]

why not put 2 relays in series ?

if one fails closed/on, the other one will still turn the pump off ?


p.s.

probably doesn't apply to you, but I've always thought pot type oil burners should have a conical bottom... because the oil only burns on the surface... and the amount of surface area determines the burn rate....
with a conical pot, the fuller it is, the more surface area it has, so the faster it burns ?


p.s.s.

wire some LEDs into the relays, so you know when they're on/off and you'll know if one has failed ?

you could also add a thermostat, so if it gets up over X'C it cuts off the pump ? - would have to do it via a relay, so you need to push an on switch to turn it on, then if temp gets too high it shuts if off ?

[julesandtash]

Why not use a power transistor as opposed to a mechanical relay, far less likely to fail closed and no annoying clicks.

[therecklessengineer]

High temperature safety cutout?

Similar to a dryer - if T becomes too high, kills power to the circuit/pump. You'll probably want to kill power to the fan as well, but keep the water pump going.

[photoman290]

probably doesn't apply to you, but I've always thought pot type oil burners should have a conical bottom... because the oil only burns on the surface... and the amount of surface area determines the burn rate....
with a conical pot, the fuller it is, the more surface area it has, so the faster it burns ?

the roger sanders design works like that. he  uses a conical aluminum disc. good idea but they burn very dirty. i tried using the disc in the spike design but didn't find it helps very much.  it burns ok with just a flat bottomed pot. the difference between a nice steady burn and a raging inferno is about 10 mm of oil. any more than that and it gets very hot, very quickly. as someone pointed out somewhere it is a controlled chip pan fire.

[greasemonkey]

There must be some kind of cut out timer available. You know the pump only runs for 4 seconds, so if it runs for 5, it cuts out.

Another way, and I'm not to sure about this, is have the pump running when the relay is open circuit, so your going to need some kind of electric trickery to get that to work. If it goes open circuit for too  long, then it cuts the power to the whole lot. Some kind of countdown timer that automatically resets.

[photoman290]

There must be some kind of cut out timer available. You know the pump only runs for 4 seconds, so if it runs for 5, it cuts out.

Another way, and I'm not to sure about this, is have the pump running when the relay is open circuit, so your going to need some kind of electric trickery to get that to work. If it goes open circuit for too  long, then it cuts the power to the whole lot. Some kind of countdown timer that automatically resets.

i

have puzzled over all the different ways to do this. i think the  best suggestion so far is the pot with a microswitch or hall effect sensor,it is likely to be the most reliable. gravity doesn't let you down. thinking about it, the water container on the dehumidifier works like that except they use a float with.a magnet on it.i will use a thermal fuse as as last resort back up. just need to find the best place to fit one so i get the right temperature one. i use a fan to get the flue hot so must take that into account.t dont want to use a float as i dont what is in the oil. thinners petrol god knows what else.
i

[photoman290]

after blundering around the web for a few days i have found a elegant solution to feeding my oil  burner safely. no pumps one valve. its called a bell siphon. pretty simple and easy to make and with an overflow to a container. i cant see anything to go wrong. if it blocks up the oil flow stops and the oil runs into the overflow container. just got to work out how much oil i need to dump per flush so it doesn't overwhelm the burner. a valve from the feed tank will adjust the time between flushes. its the same idea as those automatic urinal cisterns. i would like to think i got the idea when i was in the pub toilet,but i didn't.