Drum Level Indication Disagrees After a Transmitter Outage
A 150,000 lb/hr, 750 psig watertube boiler has been back on line for three weeks since a six-day outage. During the outage the differential-pressure drum level transmitter was removed, its reference leg was drained, and the sensing lines were back-flushed. Since restart the DCS has displayed drum level about 2.5 inches above the normal waterline, but the direct-reading gauge glass mounted in the same vertical plane sits near the bottom of its visible range. The feedwater control valve is in automatic and running about 12% more open than the steam-flow signal alone would explain. The unit is at 70% load and climbing; the plant cannot take it off line. The gauge glass was blown down an hour ago and water returned promptly and settled. The board operator wants to 'trim' the transmitter to match the glass and move on.
1. What is the safest immediate action, and which indication governs the decision? Explain the risk if the operator is wrong about which instrument is faulty.
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The direct-reading gauge glass governs until it is proven wrong. It is the only indication that does not depend on sensing lines, a reference leg, a power supply, or compensation math. Before acting on it, verify the glass itself: blow it down, confirm both the top and bottom connections are open (water should drop, then return smartly and settle without slowly creeping), and confirm the glass is not isolated or partially valved off.
If the glass is sound, then the DCS is reading high and the actual drum level is low - near the bottom of the visible range at 70% load is a genuine low-water condition. The correct action is to take the feedwater control valve to manual, increase feed to restore level in the glass, and watch the glass continuously while the discrepancy is investigated. Do not trim the transmitter to the glass while running: that masks the problem, defeats the low-water trip logic that uses the same transmitter, and could leave the burner permitted to fire with the drum actually low. If the glass proves faulty, the reverse risk applies - the operator could overfill and carry water over into the superheater. Either way, the decision must be made on verified evidence, not on which display looks more convenient.
2. The transmitter was drained and refilled during the outage. Explain the most likely root cause of the 2.5-inch error and how the reference leg works.
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A DP drum level transmitter measures the difference between a constant reference-leg head (condensate in the high-pressure leg, connected to the steam drum's steam space) and the variable head in the water leg connected to the drum's water space. The reference leg must be completely full of condensate and must stay that way; the condensing chamber at the top is there to keep it full as steam condenses.
If the reference leg was drained during the outage and not properly refilled and stabilized, it will be partially filled or contain air or steam pockets. A light reference leg reduces the measured differential pressure, so the transmitter interprets it as a lower water level - but the direction of error depends on which leg was affected and how the transmitter's zero was set when it was returned. A transmitter zeroed while the legs were empty or equalized will read high afterward. A second common cause is that the density compensation constant still reflects the outage condition rather than 750 psig drum pressure, which shifts the indicated level by exactly this order of magnitude. Partially plugged taps and a flooded or leaking condensing chamber produce the same signature.
3. Outline a verification and correction plan that does not require a shutdown, and state what must happen before the loop is returned to automatic.
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With the boiler on line, run the transmitter against the verified glass at steady load: hold load steady, record glass level and indicated level together, then repeat at two other loads to establish whether the error is a fixed offset or changes with pressure/load. Check the reference leg condensing pot for level and temperature, refill or re-establish the condensate seal per the manufacturer's instructions, and confirm both sensing lines are clear by comparing response times after a small feedwater change. Verify the density compensation input is set to current drum pressure and that the suppression/zero is correct for the actual tap spacing.
If the error is a fixed offset with a stable reference leg, a documented recalibration with a certified pressure source is acceptable on line. If the error moves with load or pressure, the loop is not trustworthy and should stay in manual.
Before returning to automatic: the glass and transmitter must agree within the plant's acceptance band across the load range, the low-water trip setpoints must be proven to act on a trustworthy signal, the work must be recorded as a management-of-change item with the instrument shop and the inspector informed, and the operator must be told what was found. Trimming a display to hide a disagreement is not a calibration.