Time to fill a pipe

[cwright]

Hello Everyone,

It has been a long time since I have performed a calculation such as what I am about to describe. I have been out of school for a while now, and just haven't had to use any of these skills in the work place. In any event, here are the parameters that are known at this point.

Length: 2,355 feet
Pipe Pressurization: 20 psi
Diameter: 20-inch
Compressor Flow Rate: 11.5 cfm
Inlet Hot Tap size: 1-inch (This is the size of the tap we will be putting on the pipe to hook our pressure lines to).
Product: Air
Piping: Carbon Steel

What we are looking to figure out is how long it will take to pressurize this 2,355-foot pipeline to 20 psi. The pipe will be at atmospheric pressure once the hot taps are done and will be open to the atmosphere until the air lines are connected. For some reason I am thinking I need to go through the calculations via Hazen-Williams and then figure out pressure drops, but I am in the dark at the moment. I could be completely wrong at this point. I broke out my open channel hydraulics book for some pipe flow calculations, so I should be able to follow along.

Thanks

 

[SteamKing]

I think you have a filling problem rather than a flow problem to solve here. You are looking to use a pump to add air to this pipeline to increase internal pressure. Rather than Hazen-Williams, PV = nRT will come into play in determining how long this process will take.

In order to make the calculation more accurate, you need to specify whether the final internal pressure is 20 psi gage or 20 psi absolute. Also, 20-inch nominal pipe sizes refer to the OD of the pipe. In order to calculate the internal volume to be pressurized, you will also need to know the ID or wall thickness of the pipe.

 

[cwright]

Steam, you would be correct in needing the ID of the pipe. I was attempting to not confuse anyone by just stating the OD, but the ID of a 20-inch is 18.81 inches. We would be talking about gage pressure in this case. As far as I know, the piping is underground in New York City and the entire pipe would need to be pressurized to 20 psi.

We will be using an air compressor to pressurize the line. It is just a hydrostatic test using air as opposed to water.

Thanks

 

[Travis_King]

The air compressor should have a pressure gauge. When the pressure reaches 20 psi. Stop.

 

[SteamKing]

Using one compressor with 11.5 CFM capacity, I estimate it will take about 9 hours to pressurize the given line to 20 psig. I have assumed that T = 60 F for the air. No analysis was done on the air line since its length was unknown, so I don't know if choked flow will occur. If choking occurs during pressurization, the amount of time to reach 20 psig will increase.