Thermal expansion is the tendency of matter to change in shape, area and volume in response to a change in temperature. Peter explains the problems that thermal expansion can cause in a closed sprinkler system and how the PCA can help manage the problem.
Where does this happen?
When a substance, in this case water, is heated, the kinetic energy of its molecules increase, the molecules begin vibrating/moving more and usually maintain a greater average separation. The relative expansion divided by the change in temperature is called the material's coefficient of thermal expansion and generally varies with temperature.
In a closed system, the extra water volume that results from heating puts stress on other parts of the plumbing system and can result in broken pipes, leaky taps, relief-line leaks, etc. If the water pressure increase is severe, there can be even more serious damage, possibly posing a safety hazard. For reasons of safety, the Plumbing Code of Australia (PCA) requires both residential and commercial plumbing systems to have ways to accommodate water heater thermal expansion. But what do you do when you have a monitored fire sprinkler system? You cannot allow expansion relief as any water movement could trigger the alarm system and the next thing you know is the fire brigade is at your door.
What causes the problem?
Simple physics tells us that when a fluid heats, it expands in volume. This is a distinct problem in any closed water system, which can be identified by any system that has a one-way valve (such as a backflow valve, check valve, or pressure-reducing valve) designed to prevent water from expanding back into the Network Utility Operators water supply. Fire sprinkler lines are examples of closed loop systems separated from the Network Utility Operator's supply.
Is thermal expansion widespread?
The problem is more likely to occur in the tropics and northern parts of Australia and there is empirical evidence of burst pipe joints and leaking valves. The incidence is almost always when the sprinkler line is close to the uninsulated roof or an uninsulated western wall.
What can be done to solve this problem?
Protect the pipe. Either relocate it away from the source of heat or protect it with insulation.
Explanatory information in PCA 2016, BP1.2 - Unintentional Heating of Cold Water Services includes outlines possible solutions to avoid the problem (see below). This is also covered in AS/NZ 3500.1 - 2018 Clause 5.4.1 - Notes (4).
So if you want to avoid an unwanted visit from the fire brigade, the best solution is to not expose the system to unnecessary thermal expansion.