I'm a former seaman,3rd engineer and for the last two years i work as a boiler operator at a 300Mw steam electricity power plant.
Recently i encountered a situation which is very familiar to everyone and, it happened to me from an early stage during my sea practice (although in smaller scale)
The unit comprises of a boiler, steam turbine and generator 300Mw. The unit was down, due to boiler tube failure, which occured at 01:00. Since the unit was off grid, we opened the four boiler water low pressure heaters for inspection, because there was an indication of tube leakage. The heaters are horizontal,two pass, straight tube, steam heated. To give you an idea of the circuit, condensate from the turbine condenser is drawn from the condenser collecting tank and gets pumped through the 4 heaters in series, before it reaches the boiler feed tank, 55 meters above the condenser. There is a check valve before boiler feed tank,so that water does is not allowed to flow back by gravity, from the 55 meters to ground 0 meters. That means, water is stationary in the pipe, when pumps are stopped.
When we opened heater No3,at 11:00, a tube was found leaking into the steam side of the heater, and we decided to plug it at both ends. The problem was that, water at 108 degrees celcius was running out and we couldn't work.
Now comes the problem: a supervisor suggested to open a drain valve, in order to empty the 55m height pipe. After only a few minutes of draining, a periodically banging sound and heavy vibration begun. Even though we immediately shut the drain valve, vibration became so intense that the piping system was about to break apart!!!
What do you think the cause is? Is it metal expansion issue or water turning into steam in the pipe? Was there something we could do to relieve the situation?
I know i wrote a lot, forgive me...i would appreciate your help!!