Wet-Leg Level Transmitter Reads High After Service
A closed condensate receiver at a paper mill is measured by a DP transmitter calibrated 0–2.0 m. The lower tap is 0.4 m above the vessel bottom and the upper tap is 2.4 m above the bottom, so the level span is 2.0 m. The transmitter is mounted level with the lower tap. The high side connects to the lower tap; the low side is a wet leg filled with 1.10 SG glycol solution and connected to the upper tap. The process liquid is condensate with SG 0.96 at operating temperature. The transmitter is calibrated in kPa for 0–2.0 m. During operation the DCS indicates 62% while the certified sight glass shows 40%. The wet-leg fill valve was serviced two weeks ago, and no transmitter zero check has been done since. The area is classified, and the receiver cannot be taken out of service without bypassing the level interlock.
1. Calculate the calibrated lower-range value, upper-range value, span, and the differential pressure expected at 40% level. State any assumptions.
Read the worked answer
Use DP = P_high - P_low. Assume the transmitter is at the lower-tap elevation, the wet leg is full, both taps see the same vessel vapor pressure, and g = 9.81 m/s².
At 0% level, the high-side head above the lower tap is zero, so P_high = 0 kPa. The low-side wet-leg head is: P_low = 1.10 × 9.81 × 2.0 = 21.58 kPa LRV = 0 - 21.58 = -21.58 kPa
At 100% level, the high-side head is: P_high = 0.96 × 9.81 × 2.0 = 18.84 kPa URV = 18.84 - 21.58 = -2.75 kPa
Span = URV - LRV = -2.75 - (-21.58) = 18.84 kPa
At 40% level, the process height above the lower tap is 0.40 × 2.0 = 0.80 m: P_high = 0.96 × 9.81 × 0.80 = 7.53 kPa DP = 7.53 - 21.58 = -14.05 kPa
The negative LRV is normal for a wet-leg installation: the low side always carries the glycol head, so the transmitter must be calibrated with a suppressed zero.
2. The DCS reads 62% while the sight glass says 40%. Show whether this is consistent with a wet-leg loss, and calculate the approximate loss of wet-leg height. What else would you check?
Read the worked answer
At a true 40% level the expected DP is -14.05 kPa. A 62% indication corresponds to: DP = LRV + 0.62 × span = -21.58 + 0.62 × 18.84 = -9.90 kPa
The indicated DP is therefore about 4.15 kPa more positive than expected. If wet-leg liquid is lost, the low-side pressure falls and DP rises. For a loss of height h of 1.10 SG glycol: ΔDP = 1.10 × 9.81 × h = 10.79h h = 4.15 / 10.79 = 0.38 m
A loss of roughly 0.38 m of wet-leg height would explain the high reading. This is plausible because the fill valve was recently serviced.
Do not stop at that conclusion. Also check for a transmitter zero shift: a positive zero shift of about 4.1 kPa would raise every reading by about 22% of span and produce a similar symptom. Check the fill valve for leakage, verify the glycol concentration, confirm the wet leg is fully filled and air-free, and compare the DCS indication with the sight glass at more than one level. A blocked high-side impulse line would usually drive the indication low, not high.
3. Describe the safe correction and revalidation sequence, including the interlock and classified-area constraints.
Read the worked answer
Obtain the required permit and management-of-change approval, because the level interlock must be bypassed. Coordinate with operations, put the level controller in manual, and apply the approved interlock bypass with a time limit and independent oversight. Close the transmitter root valves, isolate and depressurize the wet leg, and vent it safely. Do not open impulse fittings while the leg is pressurized.
Drain and refill the wet leg with the specified glycol solution, bleed air from the low side, and leak-test the fill valve and fittings. Confirm the fill height and glycol SG. Rezero or recalibrate the transmitter using a pressure standard or the proper wet-leg static method, then check at 0%, 25%, 50%, 75%, and 100%. Verify the DCS reading against the sight glass after the receiver is returned to service.
Restore the interlock and remove the bypass under the permit, then document as-found and as-left conditions. In the classified area, use appropriate PPE, avoid hot work, bond and ground equipment, and prevent glycol or condensate spills from reaching drains.