Natural gas pipe size/flow rate

[cobra46]

As part of my back yard project, I plan to build a BBQ Island with a medium sized BBQ. The BBQ I have tentatively selected requires about 65000 BTU's. My house has a 1/2" gas stub sticking out of the back of the house and need to know if this will provide the required amount of gas. I asked the home builder this question and they were clueless.

I went to the web and found charts that have flow for various pipe sizes and length. The charts tell me that a 1/2" pipe will flow 119 cubic feet per hour (CFH) at 20ft and 82 cubic feet per hour at 40ft. Part of the problem is that I don't know how long the 1/2" pipe is inside the wall before it changes to a 3/4".

Without the required data I thought it might be best if I just measured the flow. It turns out my gas meter has dials for 1/2 cubic foot and 2 cubic foot flow. For my test I just opened the valve all the way and measured the flow. I measured 3 cubic feet in 33 seconds. It was a nice windy day so I wasn't too worried about releasing the gas into the air. My measurement yields 327 CFH. Since natural gas has about 1000 BTU's per cubic foot it appears that my pipe flows was more than enough to run my desired BBQ, however, the chart for 1/2 pipe only show 174 CFH at 10 ft. So makes me suspicious of my technique.

To test my technique I took a measurement with only the hot water heater running and compared my result to the data plate on the heater. It matched almost perfectly.

Is it possible that a 1/2 pipe can flow 327 CFH? Maybe there is a 3/4" line just inside the wall and the 1/2" is just the stub out. Are there any other means to test my flow?

I normally would have asked this question at the BBQ Galore forum but it's been down for a while.

Thanks,

Kevin

 

[EskimoPie]

Well the flow is going to vary by pipe size as well as pressure... I'm no LNG expert, but I thought gas pressures can vary significantly... When I bought my grill the BBQ Galore sales person told me that for the area of town I'm in I should not use the regulator because the pressure is regulated at the meter while in other areas (with a different gas company) it's not. Perhaps your pressure isn't regulated at your meter and so you're getting a lot more flow through the pipe than you expect due to higher pressure.

 

[mas985]

Often times the tables for recommended flow rates include a significant amount of margin. So while you may be able to get the flow rate desired with the pipe you have, most plumbers would probably not recommend it. Also, with several gas devices on, the pressure at the meter will drop and the flow rates to the BBQ will also decrease. Normally, you want to design for worst case which is probably where the table recommendations came from.

 

[cobra46]

I posted this same question over at one of my engineering websites and found a very good answer. It turns out that the tables I was looking at are for .5" W.C. and PG&E pressure is about 7" W.C.. I found another website that has a calculator that allows one to enter in all the relevant information and it spits out the answer. It gave me almost the exact same flow I measured.

I'm now confident my new BBQ will work just fine.