Why are there percussive sounds and vibrations in the boiler drum or pipe of a coal-fired boiler? (w

Question details (Performance):

The steam pipe vibrates violently and has a dull impact sound.

Answer:  fault reason: 

There is air in pipeline  ; automatic air vent valve is not turned on; or it is out of order.

Troubleshooting: 

Check all the air release valves on the water supply pipe and the air release valves on the top of the boiler tube for gas storage. If the valve fails, it should be replaced immediately.