Generator Question

[NC tractor driver]

Hey guys,

If this is the right place to post this, but if it's not please let me know.

I have been offered a Generac 45kW generator which is more than I need for my house by about a factor of two, but the price is right. That doesn't bother me. I am going to have to use Propane at my house as I don't have any other gas and am trying to work through how big a tank I will need.

Questions:
1. Since I will only be using about 1/2 the capacity of the generator can I use a tank 1/2 the size recommended for a 45kW generator? (500 gallon tank vs. 1000 gallons)
2. The vaporization rate drops with temperature. From the chart on this web site (Care and feeding of Propane Fueled Generators « Engineering Radio) I will need around 500 gallons to supply a 20kW generator at 0 degrees F (I am assuming the answer to 1 above is yes). If I bury the tank will this take most of the temperature concern out of the equation since the tank will be below grade and warmer?
3. How difficult is it to bury a tank? I understand you need a coated tank, buried surrounded by sand and the lines have to be wrapped for underground use. Is there anything else?

Thanks in advance.

Mark

 

[firedog]

I'm no generator expert by any means, but I've converted two to run off propane and have done some research into issues at hand. (including a stay @ Holiday Inn Express).

1&2) you'll need to verify the HP of the engine driving your genset. I've seen some oversized for the gen. Also, a 90HP engine will use more fuel at 1/2 load than a 45HP at full load. There is probably some tech. data for you genset that will give you fuel comsumption at specific loads.
3) most if not all direct burial tanks are specifically designed for that purpose. In addition to coating they will have a sacrificial anode for corrosion control.

 

[eepete]

I've got a 45 KW Onan running on propane. The engine is a 300 cu inch (4.9L) Ford in-line 6 cylinder. I had an all electric house, so more generator was cheaper than replacing appliances and getting a smaller generator.

It about drove me crazy trying to figure out fuel use in "gallons per hour". In rough terms, here's how it worked out:

Between doing nothing and 1/4 load, about a gallon an hour.
Half load, about 2 gallons an hour.
Full load, about 4 gallons an hour.

Now for residential back-up, you'll never see full load. Things turn on and off. Back when I got it, I turned on 3 heat pumps (total tonnage 10.5, had one with 5KW resistive electric backup), all the burners on the stove, 2 ovens, the water heater, dryer, well pump, other stuff and all the lights hit 165 amps, I had 15 amps to spare. To insure no problems, we just said you can cook or do laundry, not both when on the generator.

I've had sustained outages in May, and in the winter. I've had 4-5 hour outages in the spring and fall seasons (from thunder storms). During the summer and winter, I ran about 2 gallons per hour, in the spring and fall it was around a gallon an hour. The heat pumps were the determining factor.

So the propane tank size is just "how many days are you OK". A 500 gallon tank holds 400 gallons of propane, and would be good for 8 days at an average draw of around 22 KW. Also remember that it's not when the power comes back, it's when something can come to your house and deliver more propane.

If you house would not exceed a peak load of 20 KW, you might be looking at an average draw of around 13 or so, and might get 10 days on a 500 gallon tank.

I bought the tank from a propane company. I dug the hole for it, they brought it out and coated it and gave me the sacrificial anode. Buried tanks = don't worry about air temperature. Although in NC, it's really not an issue in any event. I ran the 1/2" copper lines inside of a 1" 160 PSI black polyethylene (PE) pipe for protection. I "candy-caned" yellow tape around the outside of the pipe so it was marked as a gas line. They make both yellow pipe and a direct burry yellow propane line too, work with your propane supplier and see what they do.

Didn't use sand, but the clay I got out of the hole was OK (no big rocks). Did the bottom fill-in by hand and alternated between by and and with FEL on the rest of the tank. Left a pile of dirt by it so that after the 1st year when it all settled, I could fill in the depression.

I was able to put the 500 gallon tank in with my B21 (barely). A 1000 gallon tank would take a bigger backhoe. When I got a quote on having someone else do the install, I bought the B21 and did it myself. It was a sacrifice I was willing to make :laughing:.

I've avoided a "Well, I'd just do this". Hopefully this information will help you make a decision.

Pete

 

[NC tractor driver]

Great information guys! Thanks

One other question. There is a pressure reducing valve somewhere. I'm assuming it's at the tank in a standpipe or something. Correct? Then from there to the generator all you need it the properly buried copper line? Is the standpipe integral with the tank, or is that installed separately?

I can buy a used 500 gallon above ground tank for about $800, or if I want an underground tank it has to be new and $1,500. Sound about right?

Mark

 

[firedog]

By reducing valve, I'm assuming you mean a pressure regulator. Depending on the distance from the tank to your gen. you may need only a single stage, that is; one regulator at the tank. However, based on the size of the gen. the pressure drop in the line may be too great, I would probably go with a two stage setup; one regulator at the tank that drops the pressure to about 10psi, and then a regulator at the gen.

In the link in your original post, it talks about a vaporizing regulator, which may be necessary on large units (not sure about yours, though). This runs liquid propane from the bottom of the tank to the vaporizer(mounted at the generator) which has a radiator coolant line thru it to vaporize the propane to run the gen.

$800 for a used 500gal sound about right. I'd do a hard search on craigslist.

 

[eepete]

I have a regulator at the tank, like firedog said it makes about 10 psi. Then I have a 1/2" copper line to the generator, it's not very long, about 15 feet. At the old house, the run was about 75 feet. There is a regulator on the outside of the generator that makes about 2 psi. That goes into the generator on a 1" hose, and on the generator itself has a 3rd regulator that makes about 12" of water pressure (about a .5 PSI). That regulator came with the generator and is very close to the intake for the generator.

We had a generator at the fire station (20KW, nat. gas). It was put on the same 2psi line as the rest of the station, and it was starved for fuel. We had to re-pipe it all with 1" pipe, a new regulator, and the gas company had to put in a new meter. The run was 20 feet long. This generator did not come with a regulator, so the one we put on the generator was a 14" of water sized one, fed with a 1" hose to the engine.

I'd do the 10 PSI at the tank and then get to the generator for the rest. The price of being wrong quickly gets to be more that the price of guaranteed doing it right.

My generator runs of the vapor, does not use the liquid in the tank. No problems with above ground, it's more a matter of aesthetics. I think that pricing is OK, I think I paid about $2500 for the 1000 gallon tank I put in. I'm sure you can ask around and see what the pricing is.

Keep the paperwork that says you bought the tank and have a copy of it handy. When you use propane to heat, you lease/rent the tank since you will be a customer of the propane company. Around here, that means that only the company you got the tank from can fill it. When you buy the tank, it's yours and any company can fill it. But you may need to "convince" a company that you really own the tank and that it's not a lease from another company. The usual local YMMV applies here...

Pete

And oh yeah: firedog, love your avatar!