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I'm running a gas line for a garage heater in a cold northern climate. At times the pipe could pass through air as cold as 0°F.

Does pressure drop due to reduced pipe temperature present a concern with respect to pipe size, or will the supply pressure compensate? In other words, would pipe size need to be increased to provide additional storage volume?

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  • Pressure would only decrease if the system were closed. In an open system where more gas can flow in and equalize the pressure. Colder gas packs more molecules in at a given pressure, so colder gas has more capacity at a given pressure, not less. In reality this is likely a very small difference, since the temperature is measured in Kelvin, and 250K isn't a lot different than 290K. I'd also expect the gas wouldn't really cool down much while it's actually flowing in the pipe. – Steve Sether Jan 11 at 18:24
  • Your points about pressure and volume are taken, but I think you're off the mark with the temperature units thing. The numbers don't matter--gas pressures vary quite a bit between 70F and 0F. See also any child's balloon in the winter. Also, that all should probably be an answer. – isherwood Jan 11 at 19:03
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    As long as you have a drip leg prior to your gas valve it won’t make much difference. When the pipe goes to a cold area the pipe can condense the moisture on the pipe walls and when it warms it is now liquid water traveling along the pipe following the flow or Mr gravity. A drip leg will catch the water and keep the liquid water and rust flakes from messing the regulator up. – Ed Beal Jan 11 at 19:51
  • @isherwood The relevant physics here is the ideal gas law. PV = nRT. P is pressure, V is volume, n is number of molecules, T is temperature (in absolute scale like Kelvin), and R is the ideal gas constant. If pressure and volume remain the same, but temperature goes down, the number of molecules in that volume will be proportional to absolute temperature change. i.e. n = PV/RT – Steve Sether Jan 11 at 21:18
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The child's balloon is a great analogy for the gas piping system in a house. But rather than having been tied off with a knot, the neck of the balloon is still pressed to the lips of a parent with infinitely large lung capacity. (The gas piping is not a closed vessel, but is connected through the gas meter to a vast reserve of natural gas in the supplier's distribution system.) If the parent steps outdoors into a cold ambient, using their diaphragm to maintain steady air pressure inside the balloon, the balloon won't shrink. It's true the air inside the balloon will contract -- it'll become more dense as it cools -- but some of the infinite supply of air in the lungs will flow in to make up the difference and maintain the volume or size of the balloon.

What would happen if the child's balloon were not a round globe, but instead a very long tubular balloon used for making art? If just the end of the balloon were put out through the window, would the outdoor part shrink while the indoor part remained unchanged?

Pressure in a closed gas pipe system, ie where the meter is hooked up but at the moment there's no flow, will be equal at every point in the system regardless of the temperatures of various sections. The density of the gas will vary with temperature but its pressure will not.

But static pressure is not the factor that causes us to use varying sizes of pipe.

When the appliance valve at the end of the pipe opens and gas starts flowing, then yes, the strict engineering answer is that ambient temperature affects the pressure drop. Engineer's Edge provides the following equation for calculating pressure drop of a fluid flowing in a circular pipe:

pressure drop equation

The relevant piece here is rho, the density of the fluid. The density of natural gas is inversely proportional to temperature; as the gas gets colder its density increases. Because rho is in the numerator, an increase of density caused by low temperature leads to increased pressure drop in the pipe. That could make necessary an increase of pipe size.

Fortunately, for small buildings matters like lumber, wire, and pipe sizing are often handled by following prescriptive tables rather than by developing site-specific engineered solutions. Though the pipe sizing tables don't explicitly state it, I believe we are to assume that the prescriptive tables include allowances for worst-case conditions of cold temperatures, rough and rusty pipe walls, and so on.

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