SITRANS LR250 FEA Selection for Hot Acid Tank Level Measurement

SITRANS LR250 FEA Selection for Hot Acid Tank Level Measurement

Replacing a discontinued XCT-8 ultrasonic sensor on a 145 °C hot-acid tank requires careful review of the antenna material, controller I/O, and pump-control topology. This reference walks through the field-proven combination of the SITRANS LR250 FEA radar transmitter, the SITRANS RD200 remote display, and the appropriate Siemens pump controller (LUT400, MultiRanger 200, or RD300) for an 8 m open tank.

1. Application Overview

The original installation specified an XCT-8 ultrasonic transducer for continuous level measurement on an open tank containing hot acid at a maximum process temperature of 145 °C. The XCT-8 was phased out, and three Siemens product families are typically evaluated as substitutes:

2. Why the XCT-8 Is No Longer Viable

The XCT-8 ultrasonic transducer was a proven solution for chemical tank gauging because its CPVC / ETFE housing tolerated many aggressive media. It has been removed from the active Siemens portfolio, and ordering has been closed out for new installations. Replacing it with a technology that does not rely on acoustic coupling through hot, acidic vapor removes a long-term spares risk and broadens the choice of controller on the HMI layer.

The most direct replacement path for a hot-acid radar sensor is the SITRANS LR250 with a Flanged Encapsulated Antenna (FEA), where the PTFE lens isolates the radar horn from the process.

3. SITRANS LR250 FEA: Antenna Selection for Aggressive Media

The LR250 is a 2-wire 25 GHz pulse radar transmitter designed for liquid level measurement in storage and process vessels. The FEA variant is the correct model for hot acids:

• Antenna construction. The horn is mounted behind a flat PTFE (Teflon) lens so the wetted face of the antenna is pure PTFE.
• Wetted materials. PTFE lens, stainless-steel flange — no metallic horn is exposed to the process.
• Process temperature. The PTFE lens is rated for hot-acid service well above the 145 °C application envelope. Verify the exact temperature rating on the ordered code; the LR250 FEA datasheet covers up to 200 °C depending on the O-ring / gasket selection.
• Measurement range. Up to 20 m for liquids — the 8 m fill height is well within the linear range.
• Output. 4–20 mA with HART 7 protocol, allowing digital trim from a HART handheld or via the HMI.

Refer to the official SITRANS LR250 catalog sheet (A5E33481367) for the antenna variants, process connections, and pressure / temperature derating curves.

3.1 LR250 FEA Wiring

• 2-wire loop-powered at 24 V DC nominal; load limits per HART network.
• Recommended cable: shielded twisted pair, drain grounded at one end.
• Loop voltage budget: 14 V at the terminals is the minimum for HART communication; verify the power supply and 250 Ω HART resistor drop.

4. SITRANS LUT400 Compatibility Constraints

The SITRANS LUT400 operating instructions (A5E35857004) describe a dedicated ultrasonic level and pump controller. Two hard constraints must be understood before specifying the LUT400 as the pump controller in this application:

1. No 4–20 mA input option. The LUT400 does not accept an external analog level input. The analog input card is reserved for an EchoMax ultrasonic transducer signal.
2. Transducer-bound operation. The LUT400 drives and processes an EchoMax ultrasonic transducer. It cannot interpret a radar 4–20 mA signal from an LR250.

Consequences for the proposed architecture:

5. Alternative Controllers: MultiRanger 200 / HydroRanger 200

If the radar path is retained, the controller must accept a 4–20 mA analog input. Both the SITRANS MultiRanger 200 and the HydroRanger 200 support an optional analog input card:

When the optional mA input is fitted, the MultiRanger 200 / HydroRanger 200 can:

• Accept the LR250 4–20 mA loop directly.
• Scale the mA signal to engineering units (m, %).
• Drive its internal relays for pump-up / pump-down control.
• Re-transmit the level on a 4–20 mA output to the RD200 or to a PLC / DCS analog input.

The Pump Control Wizard in the HMI menu guides the engineer through relay assignments, alternation, and start-delay logic. The relay control scheme is functional but has a steep learning curve — field experience suggests budgeting 3–4 hours for first-time configuration, even for an engineer familiar with Siemens controllers.

6. SITRANS RD200: Remote Display and Pump Control

The SITRANS RD200 is a loop-powered remote indicator that can be inserted anywhere in a 4–20 mA (or HART) loop. Two product versions exist:

For this application, the RD200:

• Displays the 4–20 mA level value from the LR250 in metres, %, or user-defined units.
• Triggers relays at setpoints for pump-up, pump-down, high-high, and low-low alarms.
• Supports pump alternation — equalizing runtime across duty and standby pumps.
• Is configured entirely from the front panel or via HART handheld — no separate engineering tool is required.

7. SITRANS RD300: Multi-Pump Alternation

For installations requiring control of more than two pumps, the SITRANS RD300 offers:

• Up to 8 relays.
• Up to 8 digital I/O for status feedback from each pump.
• Native pump-alternation logic across all 8 pumps.
• HART and 4–20 mA I/O on the input side.

If the 8 m hot-acid tank feeds up to eight transfer or recirculation pumps and runtime equalization is required, the LR250 + RD300 combination is the most economical pump-control path — no separate PLC or DCS is needed.

8. Recommended System Architectures

8.1 Single-Pump Tank (Recommended Default)

+-------+    4-20 mA + HART    +--------+    4-20 mA / HART    +--------------+
| LR250 | --------------------> | RD200  | ----------------------> | MultiRanger |
|  FEA  |                       | (relays|   optional mA I/O     |   200 HMI   |
+-------+                       |  + opt) |                       | (if needed) |
                                +--------+                       +--------------+
                                    |
                                    | Relay output (pump-up / pump-down)
                                    v
                                Pump contactor

Field practice: the RD200 alone is sufficient for a single-pump duty / standby scheme. Add the MultiRanger 200 only if continuous control, trending, or PLC integration is needed.

8.2 Multi-Pump Tank

+-------+    4-20 mA + HART    +--------+    8 relays + 8 DI/O    +---------+
| LR250 | --------------------> | RD300  | ----------------------> | Pumps 1 |
|  FEA  |                       |        |                          |   through 8 |
+-------+                       +--------+                          +---------+

8.3 Architecture to Avoid

+-------+    4-20 mA + HART    +---------+
| LR250 | -- X ---------------> | LUT400  |   <-- NOT VIABLE: LUT400 has no mA input
|  FEA  |                       +---------+
+-------+

9. Configuration and Commissioning Notes

9.1 LR250 FEA Setup

1. Mount the LR250 FEA on the tank flange. The PTFE lens must remain dry and clean; do not coat the lens with paint or labels.
2. Power the loop at 24 V DC and verify ≥ 14 V at the device terminals with the HART resistor in place.
3. Using a HART handheld (e.g. Siemens SITRANS RD200 HART or 475/375), set the lower range value (LRV) at empty (4 mA) and the upper range value (URV) at the 8 m full level (20 mA).
4. Configure the echo profile; the LR250 self-learns the false echoes in its first 30 seconds.
5. Set the failure-mode current (3.6 mA or 22 mA) per the controller preference.

9.2 RD200 / RD300 Setup

1. Loop-power the RD200/RD300 from the same 24 V supply as the LR250, inserting the RD device in series in the loop.
2. Set the engineering units (m), damping (s), and decimal point to match the LR250 scaling.
3. Configure each relay setpoint, hysteresis, and fail-safe direction.
4. Enable pump alternation; assign pump 1 to relay 1, pump 2 to relay 2, and so on.
5. Force-trip each relay from the front panel to verify contactor wiring.

9.3 MultiRanger 200 / HydroRanger 200 Setup (if used)

1. Fit the optional mA input card into the controller backplane.
2. From the HMI, run the Pump Control Wizard. It will request: number of pumps, setpoints, alternation on/off, start delay, and relay assignment.
3. Re-transmit the level to the plant DCS or PLC on the analog output card.
4. Validate with a manual level test (fill / drain) and confirm relay drop-out timing.

10. Verification and Field Checks

11. Selection Matrix

12. Common Pitfalls

1. Ordering the wrong LR250 antenna. Standard horn antenna is not specified for aggressive chemicals. Specify FEA in the order code.
2. Pairing LR250 with LUT400. The LUT400 has no 4–20 mA input; it cannot accept the radar loop.
3. Ignoring PTFE temperature rating. The lens material must be rated above 145 °C with the expected chemical exposure. Confirm on the corrosion resistance chart in the LR250 datasheet.
4. Forgetting the HART resistor. HART communication requires 250 Ω in the loop. The RD200 inserts this internally, but a stand-alone mA display does not.
5. Pump alternation without digital feedback. Without auxiliary contacts confirming pump run, the alternation counter may drift. Wire the pump contactor auxiliaries into the RD300 DI/O for true duty tracking.
6. Failure-mode current mismatch. If the controller expects 22 mA on burnout but the LR250 is set for 3.6 mA, alarms will read as low level instead of fault.
7. Improper flange gasket. For hot acids, use a PTFE envelope gasket with stainless insert; do not use EPDM or Buna.

FAQ