Propane gas flow to a generator regarding pipe size issues

[frule]

I have a Generac 8KW standby generator that runs on propane gas. The manual says to use 3/4" pipe
to the generator. From my 1,000 gallon LP tank, I have 1" black iron pipe that transitions to 3/4" pipe. But the last 6-7 feet of this run is only 1/2" pipe. So I have a run that is 60 feet of 1', then 40 feet of 3/4", then 6 feet of 1/2" black iron pipe.

I have been unable to get a good answer as to how the 1/2" pipe affects the flow  to the generator (volume and pressure). Does it make the whole pipe system act like 1/2" pipe? Or is it more like the 3/4" pipe? Or somewhere in between?

I found this quote on a plumbing forum: "In the end, it is the total resistance between the source and the appliance that matters (as well as the available pressure). A short section of 3/4" pipe mixed in with a long run of 1" pipe will have significantly less resistance (and higher resulting flow volume) than the same total length of 3/4" pipe". Is this true?

Thanks! 
Fred

[fredhargis]

Blackhat (or someone) may be along with the definitive answer, but I'll offer my opinion. You didn't say where the regulator is, and that's makes a difference. if the 1/2" is on the high pressure side (upstream from the regulator), I'm sure you're OK...my 22KW generator is fed with 1/2" copper on the high pressure side, then goes to 3/4" black iron after the regulator (that part is only 5' in my case). Not answering your question about the fluid dynamics, but it sounds like the concern is whether the generator has enough gas to run. Even if the 1/2" is on the low pressure side (after the regulator,) I suspect you are still good to go. Also, is anything else drawing gas from this line? That also could be a factor.

[frule]

Blackhat (or someone) may be along with the definitive answer, but I'll offer my opinion. You didn't say where the regulator is, and that's makes a difference. if the 1/2" is on the high pressure side (upstream from the regulator), I'm sure you're OK...my 22KW generator is fed with 1/2" copper on the high pressure side, then goes to 3/4" black iron after the regulator (that part is only 5' in my case). Not answering your question about the fluid dynamics, but it sounds like the concern is whether the generator has enough gas to run. Even if the 1/2" is on the low pressure side (after the regulator,) I suspect you are still good to go. Also, is anything else drawing gas from this line? That also could be a factor.

Fred,

My LP tank has a regulator on the tank, then a secondary regulator where the copper line enters my crawl space. There it transitions to the 1" black iron pipe to feed all gas appliances in my house (water heater, gas range, gas logs(never used), and the generator. And yes, my concern is enough gas to run the generator. The line to the generator was installed 20+ years ago, before I owned the house.

I can change the 1/2" to 3/4" if needed, but it will be a lot of work. 

Thanks, 
Fred

[fredhargis]

They are all like that (one regulator at the tank, and a second at the house normally). The tank regulator is just a first step, and past that the pressure is still high. It's the second regulator that steps the pressure down to a usable level for the appliances. My generator has a dedicated line, so my situation is quite a bit different than yours. But the answer (I think) will be to add the BTU requirements for everything hooked to the line. Then there are charts that list the BTU capability of the 1/2" line, and they are a function of the length...which is fairly short in your case. One more question, does the 1/2" line just feed the generator?

[Roly]

I won't be a problem for a few feet of 1/2" pipe.   If you have doubts, run the generator under a large load to try out.    I would think if it was a problem it would have been corrected in the last 20 years.   Roly

[frule]

I won't be a problem for a few feet of 1/2" pipe.   If you have doubts, run the generator under a large load to try out.    I would think if it was a problem it would have been corrected in the last 20 years.   Roly

Roly,
The previous owner had no idea what generator was there as it had been removed and the line capped off when he bought. But I think you are still right.



Fred,
Yes, the 1/2" only feeds the generator.

[blackhat]

I don't have ready access to my sizing charts right now, I will later today and I'll crunch the numbers for you.  First blush says its a bit light because of the length.

[frule]

I don't have ready access to my sizing charts right now, I will later today and I'll crunch the numbers for you.  First blush says its a bit light because of the length.

I don't know if this will help, but here are some specifications from Generac for my generator:

at 50% load  it uses 0.87 gallons/hour of propane
at full load it ises 1.47 gallons/hour of propane.

Generac allows 165 feet of 3/4" black iron pipe and 570 feet of 1" pipe.

Thanks!

Fred

[Roly]

I don't know if this will help, but here are some specifications from Generac for my generator:

at 50% load  it uses 0.87 gallons/hour of propane
at full load it ises 1.47 gallons/hour of propane.

Generac allows 165 feet of 3/4" black iron pipe and 570 feet of 1" pipe.

Thanks!

Fred

Didn't you say you only had 6' of 1/2" pipe, the rest was 1" and 3/4".   Just curious as to what size pipe is internal of the unit.  Roly

[frule]

Didn't you say you only had 6' of 1/2" pipe, the rest was 1" and 3/4".   Just curious as to what size pipe is internal of the unit.  Roly

Correct. IIRC, the unit has a 1/2" inlet. But I don't know what the internal pipe is.