Weil-McLain P-368 Sight-glass level drop as steam pressure falls?

JMC3

I have a 1994 Weil-McLain P-368-S oil-fired steam boiler serving a one-pipe steam system. The boiler is rated for 355 sq ft EDR, with approximately 237 sq ft of connected radiation.

I've observed an interesting water-level phenomenon for many years and would appreciate an explanation of the underlying physics.

During extended firing, the water in the sight glass oscillates approximately one inch. When the Pressuretrol cuts out at 1.7 psi, the burner stops and the water level gradually falls as pressure decays.

By the time pressure reaches 0.5 psi, the water level has fallen close to the low-water cutoff operating point.

When the burner refires and pressure begins increasing, the sight-glass water level rises promptly toward its normal operating position.

When the thermostat is satisfied and the burner shuts down for an extended period, the water level returns to its normal resting position, slightly above the midpoint of the sight glass, in approximately one minute.

This behavior is longstanding. During a comprehensive boiler overhaul this summer, I disassembled and cleaned the LWCO, gauge column, Pressuretrol pigtail, and connecting fittings. All were remarkably clean, with no apparent restrictions.

I've considered whether this is caused by delayed condensate return, but the rapid water-level recovery when firing resumes makes me question that explanation.

I've also wondered whether vigorous steam generation changes the apparent water level through internal circulation and steam-bubble formation, followed by shrinkage when firing stops.

My question: What physical mechanism would explain the water level falling as boiler pressure decays and then rising again as steam generation resumes?