Heavy pipe knocking issue

[Mitsui]

Hi everyone!
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!!

[D Winsor]

Sounds like there is a problem venting the pipe and with the condensate in the pipe so warn it will flash off into steam with the reduction of the pressure and cause a water hammer in the pipe.
I can not be certain but I expect the discharge pipe is submerged in water inside the feed tank in order to prevent excessive turbulence and the pipe may be venting through the drain line. So in order to drain the line you will either have to drain the feed tank below the level of the discharge pipe or find and open the vent plug at the highest point of the pipe.

[hamal]

I am not familiar with your boiler, but it looks like while your tube is emptied, some steam is leaking into the tube (the steam inlet valve is not holding). The heavy hammering occurs while the steam is condensing into the tube. Another reason that may be independent or together with the condensing, is that while into the tube a vacuum is created some liquid is entering via the same steam inlet valve (condenssed steam) and the hammering is happening due to vacuum + 55 m head.