Feedwater Flow Reading Disagrees with a Portable Ultrasonic Meter

Flow through an orifice is proportional to the square root of differential pressure: Q ∝ √ΔP. If the DCS reads 61.6% while the reference is 70%, the flow ratio is 61.6/70 = 0.880. The DP ratio is therefore 0.880² = 0.774, meaning the transmitter and DCS are seeing about 22.6% less DP than the reference flow implies.

That is far too large to be explained by the observed zero and span: zero is 4.01 mA (0.06% of span) and span is 20.02 mA (0.125% of span). A zero error of about 1.8 mA would be needed to produce a 22.6% DP error at this flow, and that is not present. The DCS square-root calculation and scaling are still unproven, but a pure scaling error would normally affect the reading by a constant percentage at all flows; the 12% discrepancy at 70% is consistent with a DP that is low.

The more likely causes are in the primary element or impulse piping: a worn orifice bore (an enlarged bore reduces DP), a plate installed backwards, partially blocked impulse lines, gas in the water-filled lines, or insufficient upstream straight run. The 90° elbow only four diameters upstream is a strong candidate because it creates swirl and a distorted velocity profile.

Before touching the DCS span, prove each part of the loop:

1. DCS configuration: confirm the input is scaled 4–20 mA = 0–200 inH2O, the square root is applied once and not twice, and the flow equation uses the correct bore, pipe ID, and units. A double square root would produce a large low reading.
2. Loop calibration: disconnect at the transmitter and inject 4, 8, 12, 16, and 20 mA into the DCS input. Compare the displayed flow with the expected square-root values. This separates DCS error from field error.
3. Transmitter static check: equalize the manifold and confirm 4.00 mA at zero DP; apply a known DP with a portable calibrator and check span. The 4.01 and 20.02 mA readings suggest the transmitter is close, but they do not prove performance under flowing conditions.
4. Impulse lines: confirm both lines are full of water, with no leaks, gas pockets, or elevation difference. A 3 ft elevation difference between wet legs cancels only if both legs are identical; a partially filled high or low leg adds an error.
5. Primary element: measure the orifice bore, check the bevel direction (bevel downstream), verify concentricity and gasket protrusion, and confirm tap orientation. Look for erosion, nicks, or deposits.
6. Installation: compare the upstream straight run with the standard requirement; four diameters after an elbow is far short of typical 10–20 D requirements. Check for swirl or a partially closed valve upstream.
7. Reference meter: the ultrasonic clamp-on is useful but has its own uncertainty; verify pipe wall thickness, liner, and coupling. Compare at several flows.

Do not change the DCS span. Spanning to force agreement would mask the physical cause and make the flow reading wrong at every other operating point; it would also invalidate the boiler feedwater mass balance and could hide a real low-flow condition.

Corrective actions:

• If the plate is worn or incorrect, replace it with a new plate of the correct bore and material, installed with the bevel downstream.
• Relocate the plate to achieve the required upstream and downstream straight run, or install a flow straightener if relocation is impossible. The existing 4 D elbow is unacceptable for accurate measurement.
• Clean and refill the impulse lines, remove gas pockets, and verify both legs are at the same elevation. Pressure-test for leaks.
• Recalibrate the transmitter and loop, then document as-found and as-left values.
• Verify the DCS square-root block and scaling against the final plate data.
• If a temporary correction is absolutely necessary, use a documented engineering factor only after the plate and installation are corrected; otherwise treat the reading as indicative until repairs are complete.