A cylinder shall ________ if it is NOT suitable for continued service.

A cylinder shall not be filled if it is deemed unsuitable for continued service. This safety measure is crucial because filling a damaged or improperly maintained cylinder can lead to catastrophic failures, risking explosions, gas leaks, and severe injuries. It is essential to ensure that any propane cylinder is in good working condition before being refilled to prevent any hazardous situations.

In practice, if a cylinder shows signs of significant wear, corrosion, leaks, or any other compromising conditions, it's essential that it is taken out of service to protect both the users and the environment. By prohibiting the filling of such cylinders, safety regulations aim to minimize the risk of accidents in handling propane gas.

While repair, marking, or discarding may be appropriate depending on the circumstances, the immediate action directly tied to its unsuitability for service is to refrain from filling it.