Natural gas pipe size/flow rate

[cobra46]

As part of my back yard construction 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 that the builder said was for BBQ's. I need to know if this gas stub 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 lengths. The charts tell me that a 1/2" pipe will flow 119 CFH at 20ft and 82 CFH 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". I recall seeing a size change someplace in one of the other floorplans before the drywall was installed.

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 and vented some gas to atmosphere 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 1024 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 shows 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 pretty close. Data plate 65000 vs. 61000 measured.

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'm I missing something? As a another piece of data, my neighbor, who is quite non technical, has told me he feels he's not getting enough flow to his large but standalone BBQ. He does have a different floorplan than I.


Thanks,

Kevin

 

[vzeos]

Natural gas flow tables give you pipe capacities at a specified pressure drop, usually 0.5 Inch W.C.

You don’t need to know anything about your house piping. What you need to know is the pressure at the stub connection and the pressure your BBQ requires for proper burner operation. For example, if your house pressure at the stub is, say, 4 Inches W.C. and your BBQ requires 3.5 Inches W.C. to operate, then you need to install a connection between your stub and BBQ that will deliver 65000 BTU/H and cause less than 0.5 Inch W.C. pressure drop. If your BBQ requires more pressure than is available, then you’re out of luck.

Before you take any pressure measurements you should enlist the assistance of your utility or of a competent professional. When you take your pressure measurement, you should turn on all the gas appliances you would expect to be using at the same time you’re BBQing. This would give you a pressure reading under the maximum expected demand of your piping system.

 

[dcasto]

furthermore, you only need 70 CF/hr.

 

[cobra46]

vzeos,

Thanks for the reply.

I would think that both pressure and flow are important for gas volume delivery. If one took two pipes of the same diameter and installed a flow control or fixed oriface on one, the pipes would not flow the same. This is why what occurs behind the wall and out of sight is important.

The common electrical analogy:
Pipe size = Resistance
Pressure = Voltage
Volume flow rate = Amps

 

[vzeos]

cobra46,
If there were a restriction in the ½” pipe then it wouldn’t act like a ½” pipe. However, you proved that you were getting a healthy flow through the pipe stub. The reason that you were getting such a large flow is because the pipe was flowing from about 6 or 7 inch W. C. to 0 inch W. C., somewhat like a short to ground in an electrical system. If you’re going to have a problem it will be due to the available pressure (but I think you'll be ok.) Gas appliances frequently have appliance regulators that help stabilize the flow. You need to find out the pressure required by the inlet of the BBQ and you need to find out the available pressure from your stub connection when you have other loads on the system. If your available pressure from the stub minus the pressure drop through the connecting pipe or hose is greater than the pressure required by the BBQ then you should be OK.

If you want to do a little exercise to see how much gas you should be getting, make a few assumptions and do a calculation. Assume that there is a ½” pipe from your service regulator to your stub connection and that the pipe is 50 feet long (including fitting allowance.) Service regulators are usually set at 7 inch W. C. Calculating the flowrate through this pipe with a pressure drop of 7 inch W. C. (7 inch W. C. to 0 inch W. C.), I get a flow of 338.795 cfh. I have attached a copy of a gas flow calculator. Hope this helps.

 

[cobra46]

vzeos,

I don't have time at the moment to see how you did your calculations but your flow is very close to my measured value of 327 cfh.

I looked at my utilities (PG&E) web site to see what pressure they regulate to for residential service and couldn't find anything. Customer service referred me to there office that does installs but they were already closed yesterday when I called. Hopefully I'll see a service vehicle when I'm out today. I thought I recall that PG&E's residential servie pressure is .5 psig (13.8" W.C.).

Thanks for your input so far.

Kevin

 

[vzeos]

Check out their Application for Service (

http://www.pge.com/includes/docs/pd...rservice/brochuresforms/app_62-0687_9-07.pdf)

and look under Natural Gas Load Information. They say "Natural gas standard service delivery pressure is provided at ¼ psig (7” water column).