Why does oil not flow from elevated tank

[weedkiller]

I have a Rayburn 308K oil burning stove (uses concentric wicks) It is fed from a tank which is around 3m (10ft) above the inlet of the stove. I have it around 20 years. Ever since I got it, if I run out of oil, the stove will not light after the tank is refilled. The only way I can get the oil to flow again is to disconnect the section of oil pipe between ball valve shutoff and fire valve and use compressed air to force contents of the pipe back in to the tank. Close ball valve and remove airline. When the ball valve is opened again the oil then flows freely.
My question is why does head of oil in the tank not push everything - air bubbles - whatever- through the pipe as soon as the tank is filled with oil? I am getting too old for fiddling with spanners valves and oil pipes in the confined space beside my stove.

 

[Last Plumber]

It must be a bad pipe run.
What size pipe?
How long a run?
Does it rise and fall over the run?

 

[SJB060685]

Old Steel tanks are notorious for gunking up and depositing this crap into the filters and fuel line, this is one of the reasons old steel tanks were installed slightly slanted to the rear with a drain off valve, which when opened could drain any crap or water that might have got in through condensation or other reasons. My advice to you is simple, don't let it run out of oil completely, otherwise you will get this problem and will have to continue doing what you are. Remember with your tank elevated like it is, which is also required, then when it runs out of oil it will pull crap into the outlet and system until the oil level in system is just below the metering stem outlet in the OCV.

 

[weedkiller]

What size pipe?
How long a run?
Does it rise and fall over the run?

White pvc covered copper pipe 10mm
About 20-25m overall
drops straight down from tank which stands on 3m elevated concrete platform into ground then levelish underground until rise to subfloor level. Then buried in floor screed for about 5m and emerging close to stove. So it should be pressurised for entire run.
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Tank is 1000l plastic - changed last year. There is probably about 20 /30 litres storage below the outlet drawoff. Would expect debris water etc to lie in that area

 

[SJB060685]

In your original post you said you've had it 20 years and every time you run out of oil you've had to force air and crap back through the pipe to free up the flow. Now you're saying the tank is a year old. Whenever you run out of oil you draw air in and risk drawing in crap as well, especially when your tank is elevated for a vaporizing burner. If you're adamant this isn't happening and all strainers, filters and valves aren't blocked then ok, I'll let someone else advise.

 

[weedkiller]

Yes I've had the stove 20 yrs. All pipework is that age also. Plastic tank was replaced last year as I began to have doubts about the old one. I don't think there was ever any debris in the pipe, only air. The filter at the tank has been checked and always clean.
So the essential question is can bubbles of air block the pipe and prevent oil flowing down elevation of 3.3m. (Not actually a lot of pressure 3.3m of water is 4.7psi - Kerosene 80% density of water so approximately 3.76 psi)
The olive on the compression fitting at the shut off valve has been so compressed I will need to replace the short section of pipe leading to the fire valve.
Suggestions please for three way valve with extra leg that I can uncap and open to either apply pressure or suction to restart flow if it happens again and not have to keep undoing pipe connections

 

[John.g]

I would think that there is at least 100 litres of unusable oil below the outlet draw off, mine is more like 125 litres in my new 1000 litre tank so very unlikely getting debris into the pump, my filter is all ways spotless (kerosene) on inspection, even after 28 years.