How low can UG propane tank reach before pressure dropping out?

sunlight33

I have a 500-gal underground propane tank (on a lease) supplying fuel for my boiler. When it was at 21% I called my propane company and they told me I will be getting delivery soon because I am on their automatic delivery system with the remote motoring. Now it's 13% and the company reassured me again that I will not be running out of propane, but they wouldn't tell me the delivery date. I use about 1% (5 lb) of propane daily. I have been using this company for eight years and they had always been reliable, but they never wait the tank to be this empty before refueling. Perhaps the drivers are calling sick due to Covid but at what point should I start to worry? 5%?

Jamie Hall

Propane delivery is much much more a matter of temperature than fill. What drives the propane out of the tank is the vapourization of the liquid -- and the pressure that exerts is entirely dependent on its temperature.

jumper

Jamie Hall said:

Propane delivery is much much more a matter of temperature than fill. What drives the propane out of the tank is the vapourization of the liquid -- and the pressure that exerts is entirely dependent on its temperature.

A good reason to bury propane tank? Also will ground reduce venting in summer?

Jamie Hall

Yup

Zman

I would be more concerned about the accuracy of the gauge. With an underground tank, the temp will be stable and you should be able to go to 1% (unless the gauge is sticking).
Both of my local propane companies have closed their local offices in the last few years. Dealing with their websites and call services has been a major PITA.

Solid_Fuel_Man

I would say 5% is the real minimum I'd ever go. If you are using 1% daily, then I surmise you don't need to worry much about wetted surface for vaporization. That usage is low for a 500, which is helpful in that you can get away with going lower. 

This is all assuming your gauge is accurate, which most generally are. 

sunlight33

Finally got the refill today. Even the driver himself was surprised at how low the tank has gotten to, well he said the reason for this lag was due to shortage of truck drivers.

hot_rod

Dave Yates just wrote a great article on this in PM Mag.