Sudden pressure rising in a pipe of an old plant transporting 33% ammonia solution We pump 33% ammonia solution along a single pipe line before we introduce it to the mix. We've had two occasions now where the ammonia pressure has rapidly risen in the pipe. It happens when the fluid is at rest, meaning all valve shut and no pumps running. The pressure just escalates. The pipe was rinsed with recycled water, but this is a weak solution of what the ammonia is mixed with anyway.

Is there any material/condition that the ammonia solution could react with to cause this sudden over pressure?

Please note:


The ammonia tanks are outside (+1–2 degrees Celsius).
The internal temperature is (+15–19 degrees Celsius).
There is no pressure release system.
The system is 8yrs old.
Ammonia is more soluble at lower temperatures so the temperature change between the outside of the factory and the inside could raise the pressure inside the pipes.

Gas dissolved in water are metastable systems most of the times. Nucleation points in many cases suffice to promote the release of the gas from the solution (have you ever tried to put some salt in your beer?). It is possible that somewhere in the old pipes there are some areas with higher roughness (e.g. rusty connections) where some nucleation points promote bubble formation and so the ammonia can be released.