Natural Gas Piping Pressure Drop

[sauba]

I have a problem I would like a sanity check on if anyone has a few minutes.

I have a natural gas system designed for about 3" W.C. of pressure drop from the meter/regulator to the most remote burner. The system is approximately 1000 total equivalent feet. The total load at the meter is 10000 CFH and I am measuring a pressure of 10.5" W.C. By the time I get to the first drop (a 30MBH unit heater), I am seeing a 5" W.C. pressure drop. It is about 200 equivalent feet of 6" pipe to the first drop. At the second and third drops, which are different branches off of the 6" main, I am seeing pretty much the same pressure. From the first drop to the most remote drop, I am only seeing about 1.5" W.C. of additional pressure drop.

I am almost positive there is a blockage (pipe cap, soda bottle, who knows) in the first 200 equivalent feet causing this pressure drop. Does anyone know of any other reason this may be occurring?

There should only about 1/4" W.C. drop in the first 200 equivalent feet based on my calc, but I would like to make sure I am not completely nuts before I ask a contractor to cut into the pipe and run a camera through it. Also, does anyone know a way to see something, likely plastic, inside a 6" steel pipe without cutting into it?

Thanks in advance.

bas

 

[zdas04]

I'm betting that the problem is pretty subtle, and the techniques to evaluate pipe condition from the outside (like an Ultrasonic Thickness meter) are not very subtle.

I think you are on the right track, for dP to be 20 times theoretical is an indication that something is broke. The equations are not perfect but they are better than that. Were I you, I'd run the camera.

David

 

[trashcanman]

Ditto.

 

[willard3]

The other thing to do is make sure that nobody has put in an orifice to control gas pressure/flow.

Look at fittings/valves/flanges.

 

[CRG]

Is the measured 10.5" W.C. upstream or downstream of the meter.

 

[sauba]

Downstream