Instrument Encyclopedia | Do You Know About Instrument Diagnostics and Repair Methods?

When dealing with chemical instrument failures, the first step is to rule out instrument issues and analyze the chemical production process. Only by doing so can the safe operation of the chemical production line be ensured. If it is determined that the fault lies with the chemical instrument, further functional testing should be conducted on the instrument. Once the problem is identified, the faulty component should be replaced. After testing confirms that the instrument meets operational requirements, it can be reinstalled.

In what situations is it an instrument system failure?

• If the instrument’s recording curve appears as a dead line (a line with no changes at all is called a dead line), or if the recording curve originally fluctuated but suddenly becomes a straight line, the fault likely lies within the instrument system.
• Since most recording instruments nowadays are DCS computer systems, which are highly sensitive and can reflect parameter changes very promptly, you can manually adjust a process parameter at this point to observe the curve’s response. If there is no change, it can be largely concluded that there is an issue with the instrument system; if it changes normally, it can be generally determined that there are no major problems with the instrument system.
• If the recorded curve shows a sudden jump to its maximum or minimum value when process parameters are adjusted, the fault at this moment also often lies within the instrument system.
• If the DCS display shows abnormal instrument readings, you can check the indicator value of the corresponding local instrument on-site. If there is a significant discrepancy between the two, it is likely that a fault has occurred in the instrument system.
• If the instrument’s recording curve had been normal before the fault occurred but became irregular or made the system difficult to control—or even uncontrollable manually—after the fluctuation appeared, the cause of the fault may lie in the process operating system.

Flow Instrument Failure

When the flowmeter reading reaches the maximum, the monitoring instrument usually also indicates the maximum value.

• At this point, you can manually adjust the remote control valve to open or close. If the flow value decreases, it is typically caused by process operations.
• If the flow value does not decrease, it indicates an issue with the instrument system. In this case, it is necessary to check whether the instrument signal transmission system, the measurement pressure piping system, and the control valve of the flow control instrument system are functioning properly.
• When the flow instrument reading fluctuates frequently, you can switch from automatic control to manual mode.
• If the fluctuations decrease, it is likely due to inappropriate PID parameters or a fault in the instrument itself.

Reference for 25 Instrument Maintenance Methods

1. Crystallization Issues
   In existing units, instruments in the hydrogenation desulfurization tower section and the sulfur-containing ammonia acid gas section are prone to ammonium salt crystallization.
   Treatment Method: Use steam heating to melt the ammonium salts, restoring normal instrument function. However, the fundamental solution involves optimizing the process to minimize ammonium salt crystallization.
2. Instrument Power Failure
   When a field instrument loses power and fails to operate normally, check the following aspects:
   (1) Water ingress in the field instrument junction box, meter head, or conduit, causing instrument malfunction;
   (2) Poor wiring connections—inspect all wiring from the control room cabinet to the field;
   (3) Faulty safety barriers or isolators;
   (4) Issues with card or card channel in the control system;
   (5) Short circuit between signal wire and shield wire, leading to voltage drop and instrument power loss.
   Solution: Temporarily disconnect the shield wire from the ground.

3. Thermocouple Malfunction

Common faults in thermocouples (displaying low, unstable readings) can be identified based on the following experience:

Display reading too low (thermoelectric potential too small):
(1) Short circuit in the thermoelectrodes;
(2) Short circuit in the compensation leads;
(3) Dust accumulation on thermocouple terminals causing a short circuit;
(4) Reversed polarity connection between compensation leads and thermocouple;
(5) Deterioration of thermocouple electrodes;
(6) Mismatch between compensation leads and thermocouple type;
(7) Incorrect installation position or insertion depth of the thermocouple;
(8) Improper temperature compensation for the thermocouple;
(9) Mismatch between thermocouple and display instrument.

Display reading too high:
(1) Mismatch between thermocouple and display instrument;
(2) Mismatch between compensation leads and thermocouple;
(3) Interference from DC signals.

Unstable output from thermocouple:
(1) Poor contact between thermocouple terminals and electrodes;
(2) Damaged insulation in the thermocouple measurement circuit, causing intermittent short circuits or grounding;
(3) Loose installation of the thermocouple or external vibration;
(4) Thermoelectrodes partially broken or on the verge of breaking.

4. Electric-to-Pneumatic Converter

Common faults of electric-to-pneumatic converters include:

(1) Poor Linearity:

· Improper alignment between nozzle and baffle
· Baffle not sealing tightly
· Damage to the baffle or nozzle

(2) Excessive Hysteresis:

· Mechanical friction
· Slight sticking of the moving coil

(3) Failure to Achieve Full Range:

· After repeated adjustments, this is often due to demagnetization of the permanent magnet

(4) Maximum Output or Failure to Return to Zero Without Input Signal:

· Nozzle blockage
· Damage to the orifice sealing ring
· Amplifier malfunction

(5) Output Deviation Exceeds Allowable Range with Air Supply Variations Within 10%:

· Orifice size too large
· Poor positioning between baffle and nozzle

5. Four-Way Valve
   An analysis of common faults and handling measures for four-way valves:

(1) Loss of 380V power supply to the four-way valve, motor fails to operate

· Handling: Check wiring connections and restore power.

(2) 9V battery depleted

· Handling: Replace the battery and recalibrate the four-way valve.

(3) Valve fails to fully close or open

· Handling: Power off the valve and reset the open and closed limit positions.

(4) Abnormal tower switching of the four-way valve

· Handling: Check whether the switching mechanism between the four-way valve and the isolation valves of Tower A/B is set to remote mode and verify the proper functioning of the position switches.

6. Level Instruments

For liquid level measurement instruments:

During the startup phase, it is often impractical to isolate a displacer (float-type level transmitter) due to high system pressure. In such cases, a 375 field communicator can be used to directly calibrate the on-site displacer (including high-pressure displacers). The priority is to ensure the accuracy of the glass level gauge and the stability of the process operation.

For interface or liquid level calibration, access the PVTRIM ZERO function on the 375 to directly calibrate the displacer. If the displacer reading is too high, simply adjust PVTRIM ZERO to match the level indicated by the on-site glass gauge, and vice versa. Generally, a two-point calibration is not required.

If the process frequently reports that the on-site glass level gauge is blocked, first check whether the upper and lower ball valves are in the correct position. Typically, they should be opened 3-4 turns after being fully closed—not fully opened—to avoid false assumptions of blockage at the pressure taps, which might lead to unnecessary disassembly. If the gauge is clogged due to dirty media, attempt to clear it by testing the flow through the upper and lower valves: close the hand valves sequentially and purge with air or steam. If it remains blocked, disassembly may be necessary.

For level transmitters, especially those with capillary-type pressure measurement, if they have been operating normally for a long time and suddenly show abnormalities, do not arbitrarily modify the instrument range. First, verify the transmitter’s zero point. In winter, if tracing heat is insufficient, the pressure taps may be frozen. If the issue persists after thorough steam heating, accumulated debris may be causing unstable pressure differentials. In this case, disassemble the flange and check for foreign material. Ensure proper isolation, depressurization, and cooling before handling.

7. Interlock-Protected Quick-Closing Valves
   For solenoid-operated valves that fail to actuate when energized, first check whether the relay in the cabinet outputs 24V power, then verify on-site if voltage is present. If voltage exists, the electrical circuit is likely intact.

If the valve is equipped with a handwheel, diagnosis is simpler: switch to manual mode. If the valve can be operated manually, the issue likely lies with the solenoid valve’s pneumatic circuit. Check for smooth air passage and any blockages; disassemble and clean the solenoid valve if necessary.

For valves without handwheels, there are two methods to determine if the valve is stuck:

1. De-energize the solenoid valve and manually operate the solenoid to open the valve (some actuators, like those in coking, hydrogenation, or flue gas systems, require power loss before manual operation). Check if air output is present. If not, the valve may be stuck and require disassembly to remove obstructions.

Internal valve leakage may occur in ball or butterfly valves due to incomplete closure or misalignment. Adjust the valve accordingly. If leakage persists, the ball/seat may be severely worn or the sealing gasket damaged. For control valves, high zero-point settings or foreign material between the plug and seat may prevent tight closure.

8.Temperature Instruments
If a thermocouple (or RTD) shows abnormally low or fluctuating temperatures:

Ensure wiring in the cabinet is secure. Loose connections, low-current interference, or short circuits in the card can cause unstable voltage/resistance, affecting readings.

Use a thermocouple/RTD signal simulator to compare values: first, verify if the reading matches the DCS display; then, send a simulated signal to the DCS and check consistency.

If values differ, suspect card issues and replace the card after confirming with operations whether the point is interlocked (e.g., sudden high-temperature readings may trigger shutdowns).

If values match, inspect on-site issues: loose connections, moisture ingress, or grounding at terminals causing high resistance. Secure or clean terminals as needed.

For wiring shorts, check junction boxes or conduit bends for wear or moisture damage. Insulate with tape or re-terminate corroded connections.

9.Pressure Instruments
If field and control room transmitter readings differ:

Verify DCS range configuration (improper database initialization during updates may cause mismatches).

Check for grounded wiring (e.g., prolonged rain may ground cables, affecting differential pressure transmitters with intermediate field indicators).

10. Rotameter (Variable Area Flowmeter)
    If the float sticks at a fixed value, debris (e.g., stones, welding slag) may be lodged. Isolate the rotameter, remove, clean, and reinstall.

11.Coriolis Mass Flowmeter
Empty pipes or two-phase reverse flow can cause inaccurate or no readings. Partially close downstream valves to ensure full-pipe flow for stable operation.

12. Loading Operation Alarm Halts Loading
    If an alarm stops the loading process: first, confirm on-site conditions and acknowledge the alarm to resume loading. After completion, address the cause. For static alarms, ensure grounding clamps have solid contact and clean the contact surfaces. For overfill alarms, clean the overflow sensor probe thoroughly.
13. Batch Controller Failure During Truck Loading/Unloading
    If a batch controller freezes, notify operations to avoid the affected bays. Power-cycle the controller by turning it off and then on to restore normal operation.
14. Incorrect Feedback from Programmed Control Valves
    Causes of incorrect feedback from PSA valves include damaged feedback probes, broken cables or poor connections, improperly installed probes, blown fuses, or burnt-out cards.
    Valves failing to open/close fully or not moving may result from insufficient air pressure, leaks in air lines or diaphragm heads, burnt solenoid coils, clogged air distribution manifolds, burnt cards, blown fuses, or mechanical jamming.
15. DCS Display Mismatch
    If the DCS value consistently differs from the field instrument by a fixed multiple, likely due to range mismatch, log in with engineer privileges to correct the DCS range or adjust the field instrument.
16. Servo Level Gauge Malfunction
    If the servo level gauge disagrees with manual measurements, check for normal operation. Issues like “reset failure” or “motor timeout” may stem from electrostatic interference. Check the debugging interface, reset the gauge, and re-measure. If deviation persists, perform a “level adjustment.”
17. Radar Level Gauge Fluctuations or Deviations
    For radar level fluctuations, missing readings, or deviations:

· Ensure the radar model matches design specifications (e.g., guided-wave radar for crude oil tanks).
· Verify proper installation; a horizontal reflector plate can stabilize fluctuating levels.
· If manual measurements consistently differ, adjust the reference height via command-line settings.

18. Tank Farm Parameters Not Transmitting to Control Room
    If tank level or temperature data fails to reach the control room:

· Verify the level transmitter output signal (normally around 2.5V).
· Check the tank preprocessor for normal display; replace the circuit board if abnormal.
· Inspect wiring from the junction box to the control room.
· Ensure the MOXA card in the server is functioning (typically provides a 5V output).

19. Controller and Card Replacement During System Startup
    During normal startup, if controllers or cards need replacement:

· Card failure is usually due to faulty channels. Check if the card is redundant (only 148R and 152 cards in DCS are redundant).
· For redundant cards: confirm the other card is running (green light). If functional, directly replace the faulty card.
· For non‑redundant cards: verify if any interlocked points exist. Coordinate with operations to deactivate interlocks before replacing.
· If a controller fails during startup, the system will switch to the backup. Before replacement, set the DIP switch to ON. After replacement, also set it to ON to ensure automatic failover.

20. Operator Station Soft Keyboard Malfunction
    If the soft keyboard is unresponsive or some keys do not work:

· Do not restart the machine or disconnect the keyboard interface.
· Simply exit and re‑enter the operator station.

21. System Shows Interlocked Valve or Pump Not Open/Started

· First, attempt to log into the controller. If there is output, check the corresponding channel’s fuse or relay for the solenoid valve.
· If the valve output exists, inspect the field device. If the relay has output, the issue is on the electrical side. If no relay output, check if the fuse is blown.
· If the fuse is intact, the relay may be faulty.
· For a pump that starts but feedback is missing: short the corresponding channel in the cabinet with a jumper wire. If feedback is normal, the issue lies in the electrical circuit or wiring between the electrical system and DCS.

22. Modifying Operation from System or Cabinet
    If safety requires changing a solenoid valve from energized‑close to de‑energized‑close:

· In configuration: invert the DO point before the interlock output (does not affect system operation).
· In the cabinet: swap the normally‑open and normally‑closed contacts of the relay.

23. ESD System Errors
    If actuators (e.g., quick‑open dampers, flue gas baffles) remain in an incorrect state when interlock conditions are met or cleared:

· The point may be forced in the engineering station (ELOP II). Open ELOP II, locate the program (blue triangle), right‑click, select ONLINE‑TEST, log in, find the DO/DI point (not an intermediate variable), drag out the yellow box, and double‑click to move the black block upward to remove the force.
· The fuse for the corresponding point in the cabinet may be blown—replace it.
· The corresponding relay in the cabinet may be faulty—replace it with the same model.
· The corresponding mini‑switch on the terminal block may be open.

Note: When handling instruments related to ESD systems (e.g., coking instrument cabinets), always notify operations personnel to deactivate relevant interlocks before maintenance to avoid unnecessary trips.

24. SNETB Network Card Issues
    In Macsv systems, servers and operator stations normally communicate via the A network (segment 128 or 130). Over time, the system may automatically disable the B network card. The status diagram will show the B network as red on one or all machines, but it remains pingable.

Resolution:
Desktop → My Computer → Right‑click → Manage → Device Manager → Expand “Network Adapters” → Intel Pro100 (or Dlink) → Right‑click Properties → Power Management → Uncheck “Allow the computer to turn off this device to save power” → Restart the computer.

25. Incorrect Valve Output After PID Loop is Switched to Auto
    If operators report incorrect valve position after a PID loop is placed in auto, the cause is often reversed direct/reverse action in the PID parameters.

Resolution: Log into the operator station as an engineer, open the PID panel, and correct the direct/reverse action setting.