purging lines when converting from lp to natural?

[just robert]

Hello, longtime hvac tech/electrician in the industry here.
We have a 14 unit apartment complex that was rehabbed by us 5 years ago with all new hvac equipment for each apartment.
The local utility was to provide natural gas service so we piped everything for that day.In the meantime all furnaces/boilers/ H W tanks were converted to LP.
We finally have the natural gas switchover scheduled for next week and I have all the conversion kits to convert back to natural gas.

My question is : do I need to purge the lines of lp before introducing the natural gas(after the conversions of course)?

My initial reaction would be no but a couple of local hvac guys said the 2 gases might not get along well. So I would be better off purging the lp before introducing the natural gas. I can do that but it would consume a lot more time.
I googled the question on the internet but didn't get a single hit.
Thanks , Bob

[second opinion]

Hello, longtime hvac tech/electrician in the industry here.
We have a 14 unit apartment complex that was rehabbed by us 5 years ago with all new hvac equipment for each apartment.
The local utility was to provide natural gas service so we piped everything for that day.In the meantime all furnaces/boilers/ H W tanks were converted to LP.
We finally have the natural gas switchover scheduled for next week and I have all the conversion kits to convert back to natural gas.

My question is : do I need to purge the lines of lp before introducing the natural gas(after the conversions of course)?

My initial reaction would be no but a couple of local hvac guys said the 2 gases might not get along well. So I would be better off purging the lp before introducing the natural gas. I can do that but it would consume a lot more time.
I googled the question on the internet but didn't get a single hit.
Thanks , Bob

IFGC 406.7 Purging. Purging of piping shall comply with section 406.7.1 through 406.7.4

NFGC NFPA 54 Purging 7.3

Which ever code you are under should be followed.

[just robert]

Those codes talk about purging lines for the first time to get the air out.
Nothing about the need to purge out the LP when converting to natural gas or vice versa.

[deerwhistle]

purging gas piping...
hard to believe there is any quarrel regarding purging lp or natgas piping. ask yourself this...'what do i have to lose by purging and how much am i looking forward to failing equipment or worse, failing with fire'. there is no room with this issue to guess. purge always.

[RoBoTeq]

The worse that would happen is the burners would burn a little hot for a minute or so until the LP gas is burned off. Up to 20% of LP is mixed with natural gas during really cold weather in some areas.

[deerwhistle]

with respect roboteq..you need to hit the books friend.

[RoBoTeq]

with respect roboteq..you need to hit the books friend.

Please elaborate with real life, real conditions information....with all due respect...

[just robert]

I didn't mean to offend anyone with the question.

I truly wanted to know if anyone had knowledge or actual experience with this type of situation of the two different gases not "getting along well" for the 60 seconds or so while the natural gas makes it's way through 40-60' of 1" pipe to each of the furnaces.

Of course the new pressure to the equipment is 6 or 7" w.c. not the old pressure of 11" w.c. and the burner orifices will all have been changed.

Keeping in mind that these are existing lines that have been in service for years now, I would feel comfortable just converting the units and firing them up if I didn't hear from you guys that it would NOT be ok to do that.

I don't have a problem purging the LP out of the lines,it would require some logistics to safely vent it out of the 14 individual enclosed furnace rooms,but if it isn't necessary why expose the tenants to the potential odor situations.

As always I appreciate any input/knowledge that you can share!
Thanks , Bob

ps- With all the storm damage from Sandy the Utility Company has postponed the conversion switchover date by 2 weeks.

[RoBoTeq]

I didn't mean to offend anyone with the question.

I truly wanted to know if anyone had knowledge or actual experience with this type of situation of the two different gases not "getting along well" for the 60 seconds or so while the natural gas makes it's way through 40-60' of 1" pipe to each of the furnaces.

Of course the new pressure to the equipment is 6 or 7" w.c. not the old pressure of 11" w.c. and the burner orifices will all have been changed.

Keeping in mind that these are existing lines that have been in service for years now, I would feel comfortable just converting the units and firing them up if I didn't hear from you guys that it would NOT be ok to do that.

I don't have a problem purging the LP out of the lines,it would require some logistics to safely vent it out of the 14 individual enclosed furnace rooms,but if it isn't necessary why expose the tenants to the potential odor situations.

As always I appreciate any input/knowledge that you can share!
Thanks , Bob

ps- With all the storm damage from Sandy the Utility Company has postponed the conversion switchover date by 2 weeks.

Don't worry about us bickering. It's all part of our teaching and learning process. Deerwhistle is correct if we only go by code regulations which are designed to prevent LP being used on systems designed for natural gas and vice versa. It is better for regualations to not allow for possibilities of abuse by just nixing what could be a potential issue.

LP is part of natural gas before processing and is seperated during the processing of natural gas for sale as a Liquid Petroleum product. In fact, LP is considered a green gas, while natural gas is not. The two do mix well and as I noted before, natural gas suppliers add a certain amount of LP to natural gas during colder months.

It would be disasterous to continually operate a furnace designed for natural gas, on LP. However, the amount of LP gas left in the gas lines is not going to do any harm to furnaces converted over to natural as long as the natural gas is coming up right behind. The furnaces may fire off a little noisy and hot, but no damage is going to occur from such a short usage of LP gas.

[carmon]

purge them.... appliances will be happier.... or don't.... all will be fine....

[beenthere]

The two do mix well and as I noted before, natural gas suppliers add a certain amount of LP to natural gas during colder months.

Yep, or they wouldn't be able to maintain gas pressure in their main lines.

[RoBoTeq]

Yep, or they wouldn't be able to maintain gas pressure in their main lines.

I didn't even think about that aspect. I always thought they did it just to raise the Btu rating of the gas a bit. Maintaining line pressure makes a lot more sense.

Also, it's not like many of us have not accidentally fired up furnaces with LP before we changed out the orifices or adjusted the pressures....NOT ME....OR YOU....of course.....

[beenthere]

I didn't even think about that aspect. I always thought they did it just to raise the Btu rating of the gas a bit. Maintaining line pressure makes a lot more sense.

Also, it's not like many of us have not accidentally fired up furnaces with LP before we changed out the orifices or adjusted the pressures....NOT ME....OR YOU....of course.....

[thermofridge]

Maybe I can help clear this up. First, we all know that propane has more heat potential than natural gas at the same pressure. The reason for this is that propane has a specific gravity of 1.562 at 70 degrees. Natural gas has a specific gravity of .667 at 70 degrees. When propane is introduced into natural gas lines, it is first mixed with air to bring its specific gravity down. So, when you open up the lines to reconnect to the natural gas meters, some air will enter the lines bringing the specific gravity of the remaining propane closer to that of natural gas. There will be no ill effects to the appliances.

[RoBoTeq]

Maybe I can help clear this up. First, we all know that propane has more heat potential than natural gas at the same pressure. The reason for this is that propane has a specific gravity of 1.562 at 70 degrees. Natural gas has a specific gravity of .667 at 70 degrees. When propane is introduced into natural gas lines, it is first mixed with air to bring its specific gravity down. So, when you open up the lines to reconnect to the natural gas meters, some air will enter the lines bringing the specific gravity of the remaining propane closer to that of natural gas. There will be no ill effects to the appliances.

Very well put with more information for us to understand. Thanks.

[snupytcb]

could the lenght and size of the gas lines make a differance?

[MicahWes]

Just start your furnaces. It won't hurt a thing.

I'm curious about this supposed adding of propane to natural gas for any reason? They wouldn't add it for increasing pressure...That is what the utilities compressor stations are for. I could see adding it to increase BTU/unit, but that seems like an impossible feat. NG feeders and mains run at at least 200psig and higher, so the propane would be a liquid in that instance and would be incompatible with the gas system. Even at the lower pressures of street-level mains (usually at least 60psig), propane would condense when the line temperature dropped below the mid 30's.

[Cooked]

This is a very interesting thread that I just came across. I have a very special lady friend engineer, smart as a whip, (waaay smarter than me but that ain't sayin' much) that builds and installs NG compressor stations. I will ask her, she'll explain to me in gory detail.

[RoBoTeq]

Maybe the addition of LP to natural gas during the colder days of winter is a Northern thing.

[Freezeking2000]

I have heard in the past that many gases may be introduced to natural gas systems during peak demand to maintain supply.