It’s a common field question:
“My outdoor equipment and gas booster aren’t running, but the static pressure is much higher than the booster’s set point—why?”
The answer lies in physics: the Ideal Gas Law and the effect of thermal expansion.
The Science Behind It: Ideal Gas Law
The Ideal Gas Law tells us that a gas’s pressure, volume, mass, and temperature are all interconnected:
P × V = n × R × T
Where:
P = Absolute Pressure
V = Volume
n = Mass of gas (moles)
R = Gas constant
T = Absolute Temperature
In a closed piping system:
Volume (V) is fixed
Mass (n) is constant (no gas is flowing)
Gas constant (R) is unchanged
That means temperature and pressure rise together.
What Thermal Expansion Looks Like In Practice
When natural gas heats up inside a closed pipe, the pressure rises—even if no gas is flowing.
Example calculation:
- Initial condition: 50 °F, 10” w.c. (gauge pressure)
- Final condition: 80 °F
In absolute terms:
- 50 °F = 509.7 °Rankine
- 80 °F = 539.7 °Rankine
That’s about a 6% temperature increase.
The same 6% increase applies to pressure.
So the 10” w.c. starting pressure becomes:
~35.7” w.c. at 80 °F.
Why It Feels Bigger Than Expected
There are two main reasons engineers often overestimate or underestimate the effect:
- Units of temperature – We think in °F, but the law uses absolute °Rankine.
- Gauge vs. absolute pressure – Pressure gauges measure above atmosphere, but the gas law applies to total (absolute) pressure.
These differences can make the rise look more dramatic than it really is.
When You’ll See It Most
Thermal expansion effects are most noticeable during:
- Seasonal swings (spring/fall) when outdoor temperatures shift rapidly.
- Idle equipment periods where gas sits in closed piping without flow.
In Summary
If you see higher-than-expected static pressure when your booster is idle, it’s usually not a system fault—it’s thermal expansion at work.
- Temperature increases → Pressure increases.
- No gas flow is required for this to happen.
- Proper system design can account for and manage these swings.
ETTER engineers can help troubleshoot pressure irregularities and recommend the right design adjustments for stable, reliable operation.
