# Natural Gas Pressure



## Michael217 (Apr 23, 2011)

Hello all, first post here.

I own an electrical contracting business and one of the services we offer is the installation of natural gas generators. We install all the electrical requirements and sub out the gas install. Before we commission the genny we complete a gas pressure test with a Yellow Jacket Gas pressure test kit. I usually complete this test and test the genny without a load first and under a full load after. My question is why does the meter fluctuate under a full load but reads steady under no load? Asked the mechanical grew and they could not state the reason why the meter jumps under a load, which is usually from 5 to 8 inches of water column.

TIA


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## flashheatingand (May 3, 2008)

just an "educated" guess, but the generator deals with alternating current, and the demand on the engine fluctuates. Something to that nature.


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## DuMass (Feb 6, 2008)

This is only on the outlet side of the gas valve right? If it’s fluctuating on the inlet/supply side, I’d say you may have a regulator issue.


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## beenthere (Mar 5, 2009)

Either the gas meter is not large enough, or the line feeding the gen is too small.


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## fireguy (Oct 29, 2006)

There is a chart in the code book on sizing gas pipe. But, I usually wimp out and ask the gas company for help. The can also check the input and output pressure of their gas meter. Every time I have called the gas company, they are more than willing to help, at least on the tech level. Flagging them down is a good way to talk to someone who can helpl.


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## Michael217 (Apr 23, 2011)

DuMass said:


> This is only on the outlet side of the gas valve right? If it’s fluctuating on the inlet/supply side, I’d say you may have a regulator issue.


Yes it is on the load side, (outlet side) of the gas valve but within gas pressure speculations. The last test I did was yesterday and with no load on the genny the meter was reading a steady 7 inches of water column, with a full load it was fluctuating between 5 to 9 inches of water column.


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## flashheatingand (May 3, 2008)

Did you call the manufacturers? Was the inlet side within manufacturer specs? A furnace or water heater operates with a steady flow (with some exceptions). I don't know, but an engine may be different in the way it consumes gas.


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## Michael217 (Apr 23, 2011)

flashheatingand said:


> Did you call the manufacturers? Was the inlet side within manufacturer specs? A furnace or water heater operates with a steady flow (with some exceptions). I don't know, but an engine may be different in the way it consumes gas.


 
I commissioned a number of generators and always get the same results with the gas pressure meter. With no gas load on the genny, (just the genny running alone and no other gas loads) the meter reads steady. Add other gas loads, like hot water and furnace, the meter fluctuates.


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## Tinstaafl (Jan 6, 2008)

Michael217 said:


> Add other gas loads, like hot water and furnace, the meter fluctuates.


As long as we're guessing...

With only one load and a relatively short supply line to it, the regulator can do its job pretty much free of any fluctuations in the small "reservoir" in that line. With extra devices (and their supply lines) connected, there is a larger quantity of gas in the system that has already passed through the regulator and is no longer under its control, in a sense.

Intriguing question. I hope you do find an authoritative answer and we get to see it here. :thumbsup:


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## Michael217 (Apr 23, 2011)

That sounds a bit logical to me, thanks!

While I'm here, second question: Generic gas inlet to the genny is 3/4 inch, but they supply a flex line with the genny and it is sized at 1/2 inch. Seems like it would be simpler to supply a 3/4 inch flex line. Is this for some kind of restriction in gas pressure?


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## fdbservices (Feb 27, 2011)

what are the RPM's on gen doing when under full load?


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## Michael217 (Apr 23, 2011)

fdbservices said:


> what are the RPM's on gen doing when under full load?


Don't know. I did not measure the RPM's.


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## fdbservices (Feb 27, 2011)

if the RPM are not steady the fuel demand will change with RPM's... just a thought


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## beenthere (Mar 5, 2009)

What CFH does the generator require?


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## Michael217 (Apr 23, 2011)

beenthere said:


> What CFH does the generator require?


The last install was a Generic 8KW and at 1/2 load 77CFH and at full load 139 CFH.


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## beenthere (Mar 5, 2009)

Do you run off a tee at the gas meter? Or just tap into the same line as the other house appliances are on?

How long is the 3/4" pipe run to the generator?


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## Michael217 (Apr 23, 2011)

beenthere said:


> Do you run off a tee at the gas meter? Or just tap into the same line as the other house appliances are on?
> 
> How long is the 3/4" pipe run to the generator?


The mechanical crew teed off the meter and reduced from 1 inch to 3/4 inch for about 6 feet, then reduced again for the supplied 1/2 inch rubber flex line, then back up to 3/4 inch to genny inlet. Don't know why Generic didn't supply a 3/4 inch flex line.


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## beenthere (Mar 5, 2009)

Check the gas meter next time to see what CFH it is rated for. Either the gas piping is too small, or the ,meter is not big enough.


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## erheating (Apr 21, 2011)

I agree with DuMass. You need to check inlet pressure. If it fluctuates it's probably a pipe sizing issue. If it doesn't fluctuate I would guess it is normal operation.


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## flashheatingand (May 3, 2008)

There is only a six foot section that is in question (at least the way I was reading it). I wouldn't think that section in question is the culprit, but, it certainly isn't an absurd request to ask them to change the 1/2" section to 3/4". 

I aslo would agree that it's important to check the inlet pressure. If it's below specs, then it may be a meter issue. Based on what I read ( 1" teed at the meter going to 3/4" and not too long of a run) I wouldn't think it's a gas piping issue. It will be interesting to find out what was up.


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## Michael217 (Apr 23, 2011)

I also don't think it is a gas meter or piping issue. Every single Generic generator I have commissioned and attached the Yellow Jacket Pressure Meter read the same way. Solid reading with only the Genny running on natural gas, add the hot water tank, furnace or whatever is when the meter will fluctuate.


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## beenthere (Mar 5, 2009)

Michael217 said:


> I also don't think it is a gas meter or piping issue. Every single Generic generator I have commissioned and attached the Yellow Jacket Pressure Meter read the same way. Solid reading with only the Genny running on natural gas, add the hot water tank, furnace or whatever is when the meter will fluctuate.


That would be a sign or a piping or meter size problem.


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## flashheatingand (May 3, 2008)

Wouldn't that be more likely a meter problem and/or a gas delivery issue as opposed to a sizing problem? This past season, I came across several homes that had lower supply pressures than usual. The sizing was within specs.

I suspect the problem is with the gas company piping to the meter. Maybe bad meters, but how often do meters go bad?


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## beenthere (Mar 5, 2009)

The pressure regulators can go bad fairly often.

But since he said it happens on everyone they have installed, its not bad regulators or meters. 

Also, he said its fine, until another appliance is on. Indicates piping size or meter problem. but, since it happens on all of them. More like pipe sizing. Probably not doing the longest run calc right.


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## DuMass (Feb 6, 2008)

Years ago, I used to work on Generac and those back then had to have the frequency adjustment manually set at commissioning. The output frequency on them would dip and rise slightly before stabilizing as the various loads were brought on and removed. I’m wondering if maybe the same type of thing is occurring with the gas pressure on the outlet side of the valve in the OP’s situation.
I wish I still had access to one so I could investigate this situation first hand.
I also have a YJ gas pressure gauge and it’s pretty sensitive, but depending on the model, they come calibrated for either 0-10 In WC or 0-35 In WC, so without knowing which one the OP has, I’m kind of wondering maybe if a good digital manometer might give a more accurate or stable reading in this case.


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## beenthere (Mar 5, 2009)

Probably caused by the uneven draw of gas, the pistons have, showing up on under sized lines/meters.


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## flashheatingand (May 3, 2008)

The op mentioned that they teed directly off the meter. Feel free to stop me if I am way off base, but, let's just say the piping was undersized for the rest of the house, he is coming right off the meter. Wouldn't that more than likely throw off the the pressures on the other appliances.

The generator would be bogart-ing from the other fixtures. The scenario in this discussion has it where the other fixtures are taking from the generators....Maybe if the lines were oversized, I could see, but undersized?


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## beenthere (Mar 5, 2009)

Teed at the meter, to some means shortly after it, not right at its outlet.

Since we don't have pics of the piping, we don't know for sure.

But since it only happens when another appliance is using gas, it implies that the piping to the generator or the meter is not large enough.


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## DuMass (Feb 6, 2008)

So much for my reading comprehension. I totally missed where the OP said this pressure fluctuation was occurring when other gas equipment, like the “furnace and water heater” was brought online.
I originally thought he was referring to when the generator itself would pickup additional electrical load. :wheelchair:


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## beenthere (Mar 5, 2009)

DuMass said:


> So much for my reading comprehension. I totally missed where the OP said this pressure fluctuation was occurring when other gas equipment, like the “furnace and water heater” was brought online.
> I originally thought he was referring to when the generator itself would pickup additional electrical load. :wheelchair:


LOL... Its easy to over look a single statement, and think a problem is originating from something else.


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