DP Transmitter Dry Leg and Wet Leg Level Measurement: ABB 266DH and Yokogawa EJX110A Configuration Guide

Dry Leg vs Wet Leg — Choosing the Right Configuration

DP transmitters measure liquid level by comparing hydrostatic pressure at the vessel bottom (HP tap) against a reference at the top (LP tap). Use a dry leg when the process fluid is non-condensing or when the operating temperature keeps vapor above its dew point. The LP line remains filled with vapor — no liquid column accumulates, simplifying the LRV/URV calculation.

Use a wet leg when the process fluid condensates readily, when the LP tap sits in a steam environment, or when the process is a boiler drum above 1 MPa. A condensate pot at the LP tap maintains a constant liquid-filled reference column. This introduces a fixed pressure offset that engineers must account for in the span calculation. Missing this offset is the most common cause of systematic level errors in steam drum applications.

LRV and URV Calculation: Dry Leg Configuration

The ABB 266DH HP port connects to the vessel bottom tap. The LP port vents to vapor space through an open impulse line. The transmitter measures net hydrostatic pressure of the liquid column above the HP tap.

Formula: DP_URV = H × SG × 9.81 kPa  |  DP_LRV = 0 kPa (HP tap at zero-level datum)

Example: H = 3.0 m, SG = 0.85. DP_URV = 3.0 × 0.85 × 9.81 = 24.99 kPa. Configure ABB 266DH: LRV = 0.00 kPa (4.00 mA), URV = 24.99 kPa (20.00 mA). On Yokogawa EJX110A, set H_RNG = 24.99 kPa and L_RNG = 0.00 kPa in the calibration menu.

If the HP tap sits below the zero-level datum by distance X meters, adjust: LRV = X × SG × 9.81 kPa. This ensures 4.00 mA corresponds to empty vessel.

LRV and URV Calculation: Wet Leg Configuration

Wet leg configuration fills the LP impulse line with a reference liquid (condensate or sealing fluid). The condensate pot maintains the LP column at a fixed height above the LP tap, creating a permanent pressure on the LP side that subtracts from the HP-side hydrostatic pressure. The transmitter output shifts toward negative DP at low level — often requiring a negative LRV configuration.

Variables: H_vessel = maximum level above HP tap (m); SG_process = process fluid specific gravity; H_wet = height of wet leg condensate column above HP tap (m); SG_ref = specific gravity of reference fluid (usually 1.0 for water condensate).

• DP at URV (full vessel): DP_URV = (H_vessel × SG_process × 9.81) − (H_wet × SG_ref × 9.81)
• DP at LRV (empty vessel): DP_LRV = 0 − (H_wet × SG_ref × 9.81) = negative value

Example (Boiler drum): H_vessel = 1.2 m, SG_process = 0.74 (saturated water at 3 MPa), H_wet = 2.5 m, SG_ref = 1.0. DP_LRV = −24.53 kPa. DP_URV = 8.72 − 24.53 = −15.81 kPa.

Configure Yokogawa EJX110A: L_RNG = −24.53 kPa (4.00 mA = empty drum); H_RNG = −15.81 kPa (20.00 mA = full drum). Both values are negative. Many engineers incorrectly enter positive values, resulting in a reversed output. Confirm correct assignment by increasing the process level and verifying the transmitter output increases toward 20.00 mA.

HART Commissioning Procedure

• Step 1: Connect a HART communicator to the 4–20 mA loop. Apply a 250-ohm resistor in series. Verify loop supply voltage at the transmitter terminals — minimum 12 VDC required under 250-ohm load.
• Step 2: Read the current PV value. On ABB 266DH, navigate to Configure → Basic Setup → Sensor → Range. On Yokogawa EJX110A, go to Device Setup → Output Setting → Range.
• Step 3: Enter the calculated LRV value first. Confirm the display accepts the negative value if using a wet leg configuration. Some transmitter firmware versions require LRV entry before URV to correctly calculate the span.
• Step 4: Enter the URV value. The transmitter automatically calculates the span (Span = URV − LRV). Verify the calculated span matches your hand calculation within ±0.1 kPa.
• Step 5: Simulate the 4 mA and 20 mA endpoints using a portable dead-weight tester or pressure calibrator. Apply the LRV pressure to the HP port and confirm 4.00 mA ±0.02 mA. Apply the URV pressure and confirm 20.00 mA ±0.02 mA.
• Step 6: Write the loop tag, engineering unit, and process connection data to the transmitter memory using HART command 22 (Write Long Tag). This ensures configuration traceability without relying on external records.

Impulse Line Design Rules

For dry leg installations: slope the HP impulse line continuously downward from the process tap to the transmitter HP port, maintaining a minimum slope of 1:12 (83 mm drop per meter of horizontal run). This prevents condensate from accumulating in the HP line. Use 12 mm O.D. stainless tubing with Swagelok compression fittings. Avoid pockets, sags, or horizontal runs longer than 0.5 m without adequate slope.

For wet leg installations: slope the LP impulse line continuously upward from the transmitter LP port to the condensate pot. Mount the condensate pot at least 300 mm above the LP tap on the vessel. Insulate the LP line to prevent thermal gradients that could boil off the reference fluid in high-temperature applications.

For both configurations: maintain impulse line length below 15 m. In outdoor installations, heat-trace impulse lines handling high pour-point fluids — paraffin crystallization at 4°C can completely block a 12 mm impulse tube within 12 hours in a cold snap.

Four-Fault Diagnostic Matrix

• Fault 1 — Impulse line partial blockage: Symptom: level reads low and responds sluggishly. Diagnosis: disconnect the HP impulse line at the transmitter and measure static pressure with a calibrated gauge. A discrepancy greater than 2 kPa confirms blockage. Action: rodout or hot-water flush the blocked line. Install a root valve with a flush connection for future maintenance.
• Fault 2 — Wet leg condensate loss: Symptom: level trend drifts progressively downward over days or weeks without actual level change. Diagnosis: check the condensate pot sight glass. A depleted pot reduces LP-side pressure, causing the transmitter to falsely read higher level. Top up the condensate pot with demineralized water and investigate the root cause.
• Fault 3 — Process fluid density change: Symptom: level reads consistently high or low across the full range after a process change. Diagnosis: obtain a current lab sample of process fluid SG. If SG differs from the design value by more than 0.02, recalculate URV and update transmitter configuration. For Yokogawa EJX110A, update the density compensation parameter in the advanced configuration menu.
• Fault 4 — Gas pocket in HP impulse line (dry leg): Symptom: level reads lower than actual, typically a constant offset regardless of level. Diagnosis: isolate the HP root valve and vent the HP impulse line at the transmitter bleed valve. If gas bubbles vent before liquid, a gas pocket exists. Action: redesign the impulse line slope to eliminate the low point where gas accumulates.

Conclusion and Action Advice

DP transmitter level measurement remains one of the most cost-effective and robust technologies in process plants — when the installation mechanics and engineering calculations are executed correctly. The difference between a successful installation and a persistent calibration problem is almost always in the LRV/URV calculation (especially for wet leg configurations with negative spans) and the impulse line slope.

For ABB 266DH applications, verify the minimum 12 VDC terminal voltage before HART commissioning. For Yokogawa EJX110A, confirm the H_RNG and L_RNG polarity matches the wet leg arithmetic before accepting the configuration. Build a one-page calculation sheet for each DP level loop in your plant — documenting H_vessel, H_wet, SG_process, and SG_ref alongside the as-configured LRV and URV values. This sheet halves the diagnostic time during the next commissioning call.

Author: Zhang Hua is an industrial automation engineer with over 10 years of experience in PLC, DCS, and control systems.