Common Problems and Solutions for Level Meters (3)

Liquid Level Measurement Fault Diagnosis

1. Fault Phenomena

Abnormal liquid level indications (no change, high/low readings, or no indication).

Taking a dual-flange liquid level transmitter as an example:

First, understand process conditions, the measured medium, and the type of vessel (e.g., crude stabilization tower, storage tank, or reactor).

Dual-flange transmitters for liquid level measurement are often paired with local level gauges. Process engineers use local gauges (more intuitive) as a reference to judge if the DP transmitter’s readings are high or low.

Maintenance personnel should diagnose and inspect based on process conditions and instrument fault phenomena, following the troubleshooting flowchart below:

2. Common Faults and Analysis of Liquid Level Instrument System

(1) Sudden Increase in Liquid Level

Mainly check whether the pressure guiding system of the transmitter’s negative pressure chamber is clogged, leaking, air-locked, or lack of fluid. The specific liquid filling method is as follows:

• Stop the meter in accordance with the meter shutdown sequence first;

• Close the positive and negative pressure root valves;

• Open the positive and negative pressure blowdown valves to release pressure;

• Unscrew the liquid filling plug of the double-chamber balance container;

• Unscrew the blowdown plugs of the positive and negative pressure chambers;

• At this time, the liquid level indication will be at maximum.

• Close the blowdown valves;

• Retighten the blowdown plugs of the positive and negative pressure chambers;

• Slowly fill the double-chamber balance container with the same medium, and slightly open the blowdown plug to exhaust air during this process;

• Continue filling until the container is full—at this point, open the plug of the positive pressure chamber, and the transmitter indication should return to zero.

• Then put the transmitter into operation in accordance with the meter commissioning sequence.

(2) Sudden Decrease in Liquid Level

• Mainly check whether the pressure guiding system of the transmitter’s positive pressure chamber is clogged, leaking, air-locked, or lack of fluid, and whether the balance valve is fully closed.

• To check if the pressure guiding system is unobstructed, the specific method is: stop the transmitter, open the blowdown valve, and check the blowdown condition (except for media that cannot be discharged to the outside).

(3) Inconsistency Between the Indication in the Central Control Room and the On-site Liquid Level

• First, determine if the fault lies with the on-site liquid level gauge. You can manually increase or decrease the liquid level, then analyze the root cause based on the indication differences between the on-site gauge and the central control system. (Closing, clogging, or external leakage of the root valve of the on-site liquid level gauge may easily lead to inaccurate on-site indications.)

• The normal liquid level indication can be restored by checking the zero point, measuring range, and refilling the fluid.

• If the indication is still abnormal, notify the process personnel to conduct on-site supervision, remove the transmitter, and perform pressure calibration.

(4) Frequent Fluctuation of Liquid Level

First, coordinate with process personnel to check the feeding and discharging conditions. After confirming that the process conditions are normal, stabilize the liquid level by adjusting the PID parameters.
The specific method is as follows:

• Switch the control valve to manual mode;

• First adjust the set value to match the measured value to stabilize the liquid level fluctuation;
  

• Then slowly adjust the opening degree of the control valve to make the liquid level rise or fall gradually until it meets the process requirements;
  

• Adjust the set value to match the measured value again;
  

• After the parameters stabilize, switch the control valve to automatic mode.