Liquid chlorine has a very high coefficient of thermal expansion. Relatively small increases in liquid chlorine temperature can create enough pressure to cause a blocked-in section of piping to rupture or leak at the connections. A temperature rise from 14°F (-10°C) to 32°F (0°C), will expand liquid chlorine by 2%. However, in a contained volume, the same temperature rise will increase the hydraulic pressure by more than 2400 psi.
Users shall have means to prevent trapping liquid between valves. One or more of the following options may be implemented:
- Installation of expansion bottle
- Installation of pressure relief
- Use of operating procedures and administrative controls (such as cable seals, warning tags or locks on valves), for small trapped volumes and where valves are within close proximity (e.g. arm’s length, valves within sight, etc.) is acceptable. Operating procedures should indicate nonprotected lines must be drained or contain some vapor before closing the valves.
Liquid expansion chambers, as shown on Cl Drawing 136-8, are commonly used where liquid chlorine can be trapped between two closed valves. Although the intent is to reduce risk, in some cases, small expansion bottles can increase the risk of leaks because they add complexity and are themselves subject to failure. For this reason, for small trapped volumes (as noted above) use of an administrative isolation procedure may be preferable to the installation of a small expansion chamber.