Generator Propane Tank Sizing

[crokett™]

didn't want this to get lost in the other thread.  I made 2 calls today, one company is supposed to call me back.  I have a 10Kw generator and I'm in the RTP area of NC. One of the companies is recommending a 325 gallon tank. Is that excessive to get 3 or 4 days' runtime on the generator? Based on an earlier comment of 50 hours for 150 gallon tank I think that is about right. I'd be looking at the initial charge as basically a 1-time fee, since the propane will just sit until/if I ever need the generator, except for the occasional test fire.

[blackhat]

Typically run time is not the critical factor. It's the ability of the tank to vaporize enough liquid at the coldest expected temperature and still maintain line pressure. I'll check my charts later and get back to you.

[crokett™]

ok. fwiw we've had nights here the last few winters down in the low/mid teens although that's unusual.

[blackhat]

That 325 should handle temps down to just below 0 and give you better than 100 hrs of run time.  That's based on Generac's consumption numbers but they'll be close enough for planning purposes.

[crokett™]

thanks, I will go with the 325. I appreciate the input.

[frule]

What the gas supplier told me: a 325 gallon tank, filled to 80% (they won't fill past 80% here, and recommend not going much below 20% full) would be about 195 available gallons.

My Generac 8kW uses about .9 gallons per hour at 50% load. So that's .9x24x4=86.4 gallons for 4 days. After 5 days without power and very cold temps (well below freezing all 5 days in East Tennessee), I estimated that I used around 110-130 gallons of LP to run the generator and an LP furnace. Your generator should use around a gallon an hour give or take a little either way.

I would think that a 325 gallon tank is appropriate for your slightly larger generator. Plus, LP fuel is very inexpensive right now. I paid $2.89 in 2013/14/15....and $1.39 a few weeks ago. The previous posters comment about sizing regarding fuel vaporization in cold temps is very important.....larger tanks do better.

Fred

[Herb G]

A 325 gal tank filled to 80% is 260 gals, not 195.

[TDKPE]

A 325 gal tank filled to 80% is 260 gals, not 195.

But is a 325 gallon tank actually 325 gallons?  Or is it 406 gallons, filled to 80%?

Not as dumb a question as it might sound, since "20 lb" propane tanks are actually 25 lb capacity if filled to the top (WC=48lb [as stamped on the ring], and propane SG=.525, so propane would be 25 lb, and 80% of that is 20 lb).

Edit: I see that 500 gallon tanks refer to the total capacity, and at 80% they're good to 400 gallons, so presumably the 325 gallon size is similarly the total (full) capacity, and would be filled to only 260 gallons.

[blackhat]

That minimum 20% is to maintain enough wetted surface inside the tank to absorb heat for vapourizing the liquid at a rate sufficient to maintain the minimum outlet pressure.

[DieselDennis]

A 325 gal tank filled to 80% is 260 gals, not 195.

I believe the poster said the propane companies would not fill it past 80% full, but wanted to keep it at least 20% full.  So you have (80% - 20%= ) 60% of the 325 gallon tank to use.  And 60% of 325 gallons is 195 gallons. 

Are you buying this propane tank strictly to run a backup generator?  Do you have any other appliance that uses propane?