"It's just a section of pipe filled with liquid... What could possibly go wrong?"
More than you might think. In the process industry, we devote a great deal of attention to high pressures. Compressors, reactors, pumps—everything is carefully designed to withstand the expected operating conditions. But sometimes, the biggest source of pressure isn't a pump at all.
Sometimes... it's simply the sun.
An apparently harmless situation
Imagine a section of pipe isolated between two closed valves. During maintenance, the line is completely filled with liquid, and both valves are shut. Everything seems perfectly safe. Until the sun decides to join the process. As the ambient temperature rises and solar radiation heats the pipe, the trapped liquid begins to warm up. And that's where the problem starts. Liquids expand when they get hotter. Normally, that's not an issue. There's room for the liquid to expand. But in a completely isolated, liquid-filled pipe, there isn't.
No air pocket..
No expansion vessel.
.
No escape route.
Just... pressure.
The smallest PSV in the Plant
That's why you'll often find a small valve on many installations that hardly anyone notices: the Thermal Relief Valve (TRV). It's not impressive. It's not large. But it may be one of the smartest safety devices in the plant. Unlike a conventional Pressure Safety Valve (PSV), a TRV doesn't have to relieve a large process flow. It only needs to discharge the small volume created as the trapped liquid expands due to heating. Small valve. Big impact.
What I find fascinating about topics like this is that they remind us that process safety isn't only about the process itself. Sometimes, the greatest hazard comes from outside.
From the environment.
From the weather.
From something nobody on the night shift is thinking about.
The sun doesn't appear on a P&ID. But it's still part of the process.
Engineering Is About Thinking Ahead
Hopefully, a Thermal Relief Valve will never actually be needed.
And that's exactly the point.
Good engineering isn't just about solving problems. It's about preventing problems that you hope no one will ever realize you anticipated.
Or, as I like to say:
"The best safety device is the one everyone thinks is unnecessary... until the day it isn't."
Ing. T