# Physics(?) question re LP and NG



## JackTheHack (Aug 15, 2012)

Can someone explain to me why an LP appliance requires a smaller orifice than a NG appliance?

In the interest of saving time, I understand the following:
LP has higher btu per cu ft than NG.
LP requires higher pressure than NG.
LP is heavier than air and so wants to settle in a low spot, NG is not heavier.
Farm-tap NG has a higher btu rating than "domestic" NG.

Seems to me the smaller orifice/higher pressure (of LP) is a tradeoff versus the larger orifice/lower pressure (of NG) - the objective is to get the same btu's to the burner, no? 

What am I missing?


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## SLSTech (Sep 13, 2008)

It is all about complete combustion - in order to achieve that the manufacturers will specify which fitting you need to accomplish this & of course all the other steps required


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## JackTheHack (Aug 15, 2012)

I understand the desire for complete combustion. If we have the same btu's reaching the burner, why the need for two different orifice/pressure setups?

Is it just a safety issue to help prevent someone from pushing too much propane into a NG appliance?


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## flashheatingand (May 3, 2008)

higher pressure with lp.  If you were to use a ng orifice on a lp settings, you would likely have a fire like a m.f..


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## JackTheHack (Aug 15, 2012)

flashheatingand said:


> higher pressure with lp. If you were to use a ng orifice on a lp settings, you would likely have a fire like a m.f..


So you're saying it's simply a safety issue, designed to allow for the difference in BTU/scf?


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## JackTheHack (Aug 15, 2012)

flashheatingand said:


> higher pressure with lp. If you were to use a ng orifice on a lp settings, you would likely have a fire like a m.f..


Let me put it another way: If LP and NG had the same BTU/scf, one orifice/pressure setting would work for both gases?


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## hdavis (Feb 14, 2012)

JackTheHack said:


> Let me put it another way: If LP and NG had the same BTU/scf, one orifice/pressure setting would work for both gases?


No. Different gasses have different flow rates through the same orifice at the same pressure.


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## Oconomowoc (Oct 13, 2011)

LP and NG are two totally different things.

100,000 BTU water heater will use about 100 Cubic Feet of natural gas in 60 minutes of continuous running.

100,000 BTU water heater will use about 40 Cubic Feet of propane in 60 minutes of continuous running.

Clearly, given this example it becomes obvious the two can't possibly share a similar means of delivery.

Mike


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## JackTheHack (Aug 15, 2012)

Oconomowoc said:


> LP and NG are two totally different things.
> 
> 100,000 BTU water heater will use about 100 Cubic Feet of natural gas in 60 minutes of continuous running.
> 
> ...


Your example above tells us what we already knew - that propane has about 2.5 times the btu's per cu ft as compared to NG. Why can this disparity not be overcome by increasing the (NG) pressure?


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## JackTheHack (Aug 15, 2012)

hdavis said:


> No. Different gasses have different flow rates through the same orifice at the same pressure.


Interesting. So given the same orifice and pressure, what is the difference in the rates of flow? Is there a chart that would tell me?


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## hdavis (Feb 14, 2012)

N2 is the normal reference gas, and gas flow rate can be calculated knowing the gas's specific gravity:

Flow = {Flow Rate of N2 at the same pressure} * sqrt(1/Specific Gravity)


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## Oconomowoc (Oct 13, 2011)

Why not just use Google and figure this out?


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## CarpenterSFO (Dec 12, 2012)

Are you trying to figure out how to set a pressure regulator in order to use an appliance that's set up for the other gas?


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## JackTheHack (Aug 15, 2012)

CarpenterSFO said:


> Are you trying to figure out how to set a pressure regulator in order to use an appliance that's set up for the other gas?


On occasion, I need to be able to switch a furnace from one gas to the other without a lot of effort. The furnace would run primarily on NG.

I would like to have two separate regulators (one for each gas), and switch between them with a 3-way valve. Sometimes (normally at 2AM when it's 0 degrees outside), my NG will freeze at the wellhead and flow stops. Then my house gets  cold.

When that happens, I want to turn a valve and send propane to the furnace.


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## flashheatingand (May 3, 2008)

That seems like a gas delivery problem with the meter. There isn't a uick & easy method to do what you want. Except, perhaps make sure everything is cool with the gas to the meter (call gas company), and install a drip-leg immediately at the outlet of the meter and/or furnace and w/h.


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## CarpenterSFO (Dec 12, 2012)

JackTheHack said:


> So you're saying it's simply a safety issue, designed to allow for the difference in BTU/scf?





flashheatingand said:


> That seems like a gas delivery problem with the meter. There isn't a uick & easy method to do what you want. Except, perhaps make sure everything is cool with the gas to the meter (call gas company), and install a drip-leg immediately at the outlet of the meter and/or furnace and w/h.


Your own well?


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## JackTheHack (Aug 15, 2012)

CarpenterSFO said:


> Your own well?


Sort've.


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## JackTheHack (Aug 15, 2012)

flashheatingand said:


> That seems like a gas delivery problem with the meter. There isn't a uick & easy method to do what you want. Except, perhaps make sure everything is cool with the gas to the meter (call gas company), and install a drip-leg immediately at the outlet of the meter and/or furnace and w/h.


The freeze-up happens on the high pressure (1500 psi) side of things. It's not cost effective to separate the water vapor on the high side. I have drip legs on the low side. On occasion, I'll open a 2" valve on the wellhead and let it blow for a few minutes to help clear the water vapor - probably vent off enough gas in three minutes to heat your house and mine for the next 100 years.

I have other options as far as remedy to the freeze-up, I just never understood why you couldn't tweak the gas pressures and just use one orifice.


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## 91782 (Sep 6, 2012)

JackTheHack said:


> Sort've.


Ah, so its yours until the neighbor finds out?
:laughing::laughing::laughing:


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## CarpenterSFO (Dec 12, 2012)

JackTheHack said:


> The freeze-up happens on the high pressure (1500 psi) side of things. It's not cost effective to separate the water vapor on the high side. I have drip legs on the low side. On occasion, I'll open a 2" valve on the wellhead and let it blow for a few minutes to help clear the water vapor - probably vent off enough gas in three minutes to heat your house and mine for the next 100 years.
> 
> I have other options as far as remedy to the freeze-up, I just never understood why you couldn't tweak the gas pressures and just use one orifice.


Edit: DaveyLad answered this question with more authority in a later post.

At lower pressure and volume, LP may waterfall out of the orifice and not ignite - ignition and proper combustion require a decent jet. Well, it will ignite eventually, maybe after your basement fills up with LP, no joking. Or, incomplete combustion equals bad CO problem. Or maybe just horrible inefficiency.

I would consider it to be a dangerous alteration. I would not do it if it were my home.

People do this sort of thing all the time, and you wouldn't be the first. You also wouldn't be the first to burn his house down, family inside. I'd look at other ways to keep the well-head from freezing.


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