Orifice-Plate Flow Reading Disagreement in a Water Treatment Plant
A water treatment plant measures filtered water flow with a 200 mm orifice plate and a differential-pressure transmitter. The design maximum flow is 100 L/s at 25 kPa differential pressure. The transmitter is calibrated 0–25 kPa to a linear 4–20 mA output, and the PLC applies square-root extraction. The plant magnetic flowmeter, calibrated last month, reads 72 L/s. The orifice-plate channel reads 61 L/s, and the DP transmitter indicates 9.5 kPa. The impulse lines are 8 m long, run upward to the transmitter, and have no automatic vent. The plant has recently changed coagulant, and there are visible deposits in the sight glass. The operator wants to adjust the transmitter span to make the readings agree. The orifice plate was installed 18 months ago and the pipe is reportedly full.
1. Calculate the DP expected at 72 L/s and the flow corresponding to the indicated DP. Diagnose the discrepancy and explain why changing the span is wrong.
Read the worked answer
For an orifice plate, Q = Qmax × sqrt(DP / DPmax). At 72 L/s, expected DP = 25 × (72 / 100)² = 12.96 kPa. At the indicated 9.5 kPa, the flow should be 100 × sqrt(9.5 / 25) = 61.6 L/s, which matches the channel reading. So the PLC scaling and square-root extraction are consistent; the DP signal itself is low. Likely causes are air or gas trapped in the impulse lines because they rise to the transmitter, partial blockage by deposits from the changed coagulant, a partially closed isolation valve, or a transmitter zero shift. Changing the span would force agreement at one flow but would make every other flow wrong, and it would hide the root cause. The magnetic meter is an independent, recently calibrated check, so the DP installation should be investigated, not re-ranged.
2. Describe the loop check and state the expected mA values and acceptance criteria.
Read the worked answer
Expected mA at 9.5 kPa = 4 + 16 × (9.5 / 25) = 10.08 mA. At the expected 12.96 kPa, mA = 4 + 16 × (12.96 / 25) = 12.29 mA. Procedure: isolate the transmitter, vent both impulse lines, refill with process water, and check zero at equal pressure. Zero should be 4.00 mA ±0.08 mA; apply 25 kPa and check 20.00 mA ±0.08 mA; apply 12.5 kPa and check 12.00 mA. Compare the transmitter display, the loop current measured with a calibrated mA meter, and the PLC engineering value. Acceptance for calibration is ±0.5% of span (0.08 mA), but the process indication should also agree with the magnetic flowmeter within about 2–3%. Inspect the orifice plate for deposits and correct orientation. If zero shifts after venting, re-zero the transmitter. Do not adjust span until the impulse lines are proven clear.
3. Recommend permanent changes to prevent recurrence, considering the coagulant change and maintenance access.
Read the worked answer
Relocate the transmitter below the tap, or slope the impulse lines continuously down to a drain, so gas cannot collect at the transmitter. If the transmitter must stay above the tap, fit a high-point vent and a regular venting procedure. Install diaphragm seals or flush connections if the new coagulant produces deposits, and check that the wetted materials are compatible. Add isolation valves and a bypass so the orifice plate and impulse lines can be cleaned without shutting the plant. Increase flushing frequency after the coagulant change, and trend the DP flow against the magnetic meter so fouling is detected early. If deposits continue, consider a magnetic flowmeter for this duty, because it has no impulse lines and is less affected by suspended solids. Record the calibration and venting in the maintenance schedule.