Every Miura steam boiler includes two electrode-type low-water cutoff functions. The 2023 controlled summary describes L2 as the automatic-recovery function and L as the manual-reset safety lockout.

Device	Function	Response	Factory & Startup Test
L2 probe	Automatic-recovery LWCO	After water falls below the probe for 10 seconds, combustion stops and the boiler enters standby. Operation may resume when level recovers.	Lower water level below L2 and verify shutdown within 10 seconds.
L probe	Manual-reset LWCO	After low level persists for 30 seconds, the boiler shuts down and locks out with Feed Water Alarm 1. Water must recover and the controller must be manually reset.	Drain the liquid volume control chamber below L and verify shutdown/alarm within 30 seconds.

Low Water Cutoff Reset Sequence

Operator acknowledges alarm and manually cycles boiler operation off and on at the boiler.
Power restored to boiler controller and integrated burner safety interlock circuit.
1. If water is not present at L or L2, combustion is prohibited due to controller interlock, call for feedwater on.
2. If water is present at L and L2 but not M, purge is allowed, combustion is prohibited, call for feedwater on.
3. If water is present at L, L2, and M or higher, LWCO interlock is released, combustion is permitted if other burner safety interlock conditions are also met.

On reset, burner safety circuit remains interlocked, and combustion is not permitted until water level is restored sufficiently above the LWCO probes.

Water Level Detection Equipment

Pressure Vessel Diagram Showing Chambers for Conductivity Probes

*In some designs, L2 probe is placed directly within a water tube and only one external chamber is used.

Diagram of Chambers with Conductivity Probes

Real vs Average Water Level

Miura once-through water tube boilers do not have a fixed water line and the horizontal firing burner produces a variable half-cone shaped water line. The placement and length of the water probes is specific to each pressure vessel model to account for this "average" water level and are placed in an unfired portion of the pressure vessel (known as the liquid volume controller or LVC) to limit the impact of boiling water.

Water Level Control Example

Steaming lower water level below probe. On delay timer starts.
On delay timer completes. Boiler calls for feedwater.
Refill
Water level rises above probe. Off delay timer starts.
Off delay timer completes. Boiler stops call for feedwater.

Variable Water Level Control Range

Water level control range is set by the controller in real time based on burner firing rate, steam pressure, and other factors. The range is controlled by adjusting feedwater on and off delay timers.

Related to

Low Water Cutoff and Water Level Control