Steam drum holding time 1

[curve3104]

Does anyone know how to calculate the drum holding time for an industrial watertube boiler?
I have seen requirements for boilers to maintain "X" minutes of drum hold time before the low water alarm trips.
Any ideas how this is calculated?

Thanks

 

[davefitz]

The X holdup time is typically based on the measured liquid volume between normal water level NWL and the lo-lo water level trip .

The lo-lo level is the min level needed to ensure the water flowing down the downcomers does not entrain excessive steam and thus avoid upsets to the natural circulation. This might be 12" above the drum invert, but varies according to downcomer size and water velocity into the downcomer bellmouth.

The NWL may be set Z" below the hi water level, as needed for water level measurement accuracy and margin to control fedwater flow, perhaps a 30 sec - 1 min volume between hi level and NWL.

The volume of water contained in the drum at these different heights can be calculated using simple formulas for the area of a circular segment.

 

[GenB]

you may find your answers in the ASME CSD1 controls Code.
ER