Cylinder bodies are pressure containing vessels. Cylinder cracking is a safety concern from a gas containment issue, as well as a catastrophic failure of the cylinder body.
You may suspect a cylinder leakage by the presence of a gas smell near the unit. Another possibility is that the cylinder is underperforming compared to historical data for that unit. (This may also be caused by other issues, most notably valve problems).
The next step is to verify that your cylinder is the problem. This requires taking that unit down, and performing a visual inspection of the cylinder and it’s associated parts. It is important to note that not all cracks in cylinders can be seen with a visual inspection. You may need to perform non-destructive tests (for example, magnetic particle test) to confirm the presence of cracks.
In the event that you discover cracks in your cylinder walls, water jackets, etc., they may be able to be repaired, if minor. In most cases, however, the cylinder will likely need to come out of service.