Bottom line: on a 2×135 MW CFB unit, 20 SIPOS5 intelligent actuators showed correct local indication and correct multimeter readings, yet the DCS (SIMATIC PCS-7) could not display valve-position feedback above roughly 30%–40% opening. The root cause was a mismatch between the feedback-signal voltage and the DCS module input allowance (AI module 6ES7331-7KF02-0AB0 allows only 2.5 V DC between input and analog ground MANA). Two workable fixes: (1) photoelectric isolators on the feedback, or (2) short-circuiting the valve-position command negative to the actuator analog ground — the project adopted option (2), saving the cost of 20 isolators and the associated secondary wiring; all actuators have run well since.
These 20 SIPOS5 actuators are intelligent integrated regulating actuators. Both input and output signals are standard 4–20 mA signals. The actuators feature excellent performance and satisfactory operation. However, problems of incorrect valve position display occurred at the interface with DCS.
During the initial commissioning of the actuators, local and remote operation of the actuators was normal, and the valve position feedback was displayed correctly on the DCS screen.With the trial operation of the units, problems gradually emerged. For example, the valve position feedback could be displayed within a small opening range, but the DCS failed to correctly display the feedback over the full stroke.
That is:
The DCS output could control the local actuator over the full stroke.
The valve position feedback displayed on the actuator itself was consistent with the DCS output command.
The feedback current measured by a multimeter was balanced with the command.
When the valve position feedback reached 30%–40% or above, the DCS could not display it correctly.
Similar problems had occurred on some actuators during commissioning of other units, and were solved by installing photoelectric isolators at the valve position feedback terminals.To identify the cause, the following steps were taken:
The six troubleshooting steps — “isolate the variables first, then check the grounding structure on both sides, then measure”:
| Step | Action | Result / notes |
|---|---|---|
| 2.1 | Disconnect the DCS feedback loop from the DCS; manually operate the actuator 0→50%→100% | DCS screen still shows wire break; multimeter reading matches the DCS command — disconnecting the feedback loop does not affect actuator operation |
| 2.2 | Install a photoelectric isolator at the actuator valve-position feedback | Full-stroke feedback displays correctly on the DCS |
| 2.3 | Review the DCS hardware manual | AI module 6ES7331-7KF02-0AB0: channels optically isolated from bus and control power; allowed DC voltage between input and MANA (analog ground) and channel common-mode are both 2.5 V |
| 2.4 | Review the actuator manual and measure | The DCS current-output negative is NOT tied to the actuator MANA, while the actuator feedback negative IS tied to MANA; the DCS output negative is tied to the DCS MANA — a potential difference exists between the feedback negative and DCS MANA, which can disturb sampling |
| 2.5 | Measure with a multimeter (voltage mode) | Voltage between the actuator feedback terminals and DCS MANA exceeds the module allowance |
| 2.6 | Short-circuit the DCS valve-position command negative to the actuator MANA at the actuator, without isolators | Full-stroke feedback displays correctly on the DCS |
From the above tests, it was preliminarily concluded that:Voltage mismatch between the actuator feedback signal and the DCS module input caused the DCS to fail in correctly displaying the valve position feedback.
Editor’s note: “Table 1” of the source article (voltage comparison before/after isolators and after short-circuiting, for the low-pressure bypass actuator) was not preserved in the early site migration; the narrative text governs.
To further verify the cause, voltages between the feedback signals of multiple actuators and DCS MANA were measured.Identical measurements were carried out:after installing photoelectric isolators on the actuator feedback;after short‑circuiting the actuator command negative terminal to actuator MANA.
A large amount of data was obtained.Analysis and comparison of the data confirmed the above judgment.Table 1 shows the voltage comparison of the low‑pressure bypass actuator before and after installing the photoelectric isolator, and after short‑circuiting the command negative terminal.
4.1In the 2×135 MW units of the Xinxiang Technical Renovation Project, the SIPOS5 actuators could display valve position feedback within a small opening range, but the DCS could not correctly display the feedback over the full stroke.
Through measurement of the voltage between feedback signals of multiple actuators and DCS MANA, tests with photoelectric isolators, and tests with short‑circuiting the command negative terminal to the actuator analog ground, together with data analysis and comparison, the root cause was identified:Voltage mismatch between the actuator feedback signal and the DCS analog input module caused incorrect valve position display on DCS.
4.2Through analysis and comparison, the final solution adopted was to short‑circuit the negative terminal of the actuator command to the actuator analog ground.
The two options compared:
| Option | Method | Effect / cost |
|---|---|---|
| A. Photoelectric isolators | Install a photoelectric isolator on each actuator feedback (proven on other units) | Full-stroke display OK; 20 isolators required — extra investment and secondary wiring |
| B. Short command negative to MANA (adopted) | Short-circuit the DCS valve-position command negative to the actuator analog ground at the actuator | Full-stroke display OK; saves the 20 isolators and the secondary wiring, speeding up commissioning |
This method:
Saved equipment investment for 20 photoelectric isolators;
Avoided secondary wiring for installing isolators;
Accelerated the unit trial operation.
4.3
Since the implementation of this improved measure, all actuators have been operating well, and the above problems have not recurred.
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