BOILER DRUM LEVEL SELECTION & IMPLEMENTATION

Selection of Boiler's steam drum level set points for alarm and safe level trip interlock limit for failsafe operation of boiler to avoid pressure parts failure is primary responsibility of OEM & Design team for designing of adequate steam drum sizing at the time of low drum level & low-low drum level operation in boiler.

OEM vast experience and design knowledge failed to safeguard boilers from extreme damages when OEM try to save money and time in boiler commissioning by manipulating the steam drum level control and other essentially required control logics.

In Boiler, Steam Drum Sizing Selection mainly depends on level set points for alarm and safe level trip interlock, Water Level holding time to reach upto low limit in Boiler feed Pump / feed water supply stopped condition.

Boilers face Pressure Parts failure during commissioning, operation and just after maintenance due to low drum water level and starvation.

SELECTION OF WATER LEVEL IN STEAM DRUM

The Trip Level i.e. Low-Low Water Level (LLWL) and High-High Water Level (HHWL) is usually depend on steam drum diameter and by the highest point at which downcomers are entering in the steam drum / top of steam drum cyclone.

Minimum 50 mm visibility is required in local field gauge glass at Low-Low Drum Level (LLWL) for safe operation and for tripping on level transmitters also. Water Holding time from NWL to LLWL in minute = (Total Water Volume in m3 at NWL - Total water volume in m3 at LLWL) / Boiler Evaporation Rate in m3/minute

Boiler Evaporation Rate in m3/minute = Boiler Steaming Capacity x Specific Volume of Water at Drum Operating Pressure

Calibration, Dumping in transmitter, Density correction factor, Level Scaling in Level transmitter and use of Compensated Drum Level for control with average transmitter level value selection should be recorded and maintained on regular intervals

EXAMPLE ON STEAM DRUM SIZING w.r.t. DRUM LEVEL TRIP LIMITS

OEM Design & Consideration: To fulfill the customer requirement of minimum Two Minutes storage capacity at boiler rated steam mass flow, Steam Drum Length is required 14 meter

OEM wants to reduce costing, hence Steam Drum Supplied Length is restricted to 11 meter by manipulating the drum water low level alarm and low-low level trip interlock limit

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