Checklist: Preventive Maintenance Procedure for Closed Tank Wet Leg Level Measurement

This Procedure outlines the preventive maintenance procedures for a differential pressure (DP) type level transmitter used for closed tank wet leg level measurement. Regular preventive maintenance is essential to ensure the accuracy, reliability, and longevity of the instrumentation.

 Purpose of this Procedure

• The purpose of this SOP is to establish a standardized approach for performing preventive maintenance on DP type level transmitters in closed tank wet leg configurations. 
• This ensures the instrumentation operates within specified parameters, preventing unexpected failures and maintaining process integrity.

Scope of this Procedure

• This procedure applies to all DP type level transmitters used in closed tank wet leg level measurement systems in the process area. 
• It includes maintenance personnel, instrumentation engineers, and operators involved in the maintenance and calibration of these instruments.

Definitions

• Differential Pressure (DP) Transmitter: A device that measures the difference in pressure between two points and converts it into an electrical signal proportional to the level of the liquid in the tank.
• Closed Tank: A tank that is sealed to prevent the ingress or egress of gases or liquids.
• Wet Leg: A reference leg filled with liquid (usually the same as the process liquid) to provide a constant pressure reference to the DP transmitter.
• Preventive Maintenance: Scheduled maintenance activities aimed at preventing equipment failures and ensuring optimal performance.

Responsibilities

• Maintenance Engineer: Oversees the maintenance process, ensures adherence to the SOP, and documents the maintenance activities.
• Instrumentation Technician: Performs the maintenance tasks as per the SOP, including inspection, calibration, and troubleshooting.
• Safety Officer: Ensures all safety protocols are followed during maintenance activities.
• Maintenance Manager: Coordinates with the maintenance team to schedule maintenance activities without disrupting the production process.

Safety Precautions

• Ensure the tank and associated piping are depressurized and isolated before beginning maintenance.
• Follow Lockout-Tagout (LOTO) procedures to prevent accidental energization.
• Wear appropriate personal protective equipment (PPE), including safety glasses, gloves, and protective clothing.
• Use intrinsically safe tools and equipment in hazardous areas.
• Ensure proper ventilation when working with hazardous materials.

Tools and Equipments Required for PM

• Multimeter
• HART communicator
• Pressure calibrator
• Standard Hand tools (wrenches, screwdrivers, etc.)
• Cleaning supplies (brushes, solvents)
• PPE (safety glasses, gloves, protective clothing)
• Documentation materials (transmitter data sheet, checklists, schematic diagram)

Step by Step Procedure  for Preventive Maintenance

Step 1: Preliminary Preparation

Review Documentation:

• Gather the transmitter’s manuals, previous calibration certificates, maintenance logs, instrument data sheet and schematic diagram.
• Understand the operational parameters, normal operating ranges, and any known issues from past records.

Notify Operations:

• Communicate the maintenance schedule to the operations team.
• Ensure a smooth transition and minimal disruption to the process by coordinating with operations for a suitable maintenance window.

IsolateTransmitter:

• Implement LOTO procedures to isolate the DP transmitter from the process.
• Depressurize the tank and associated piping to ensure safety.
• Tag the isolation points to inform personnel of ongoing maintenance.

Step 2: Visual Inspection

Inspect Transmitter Housing:

• Look for physical damage, cracks, corrosion, or any other signs of wear on the transmitter housing.
• Ensure the transmitter enclosure is intact and the cover is securely fastened to maintain environmental protection.

Check Electrical Connections:

• Verify that all wiring is correctly terminated and secured.
• Look for signs of corrosion or wear on connectors and replace if necessary.
• Ensure that cable connections  are tight  and prevent Loose connections.

Examine Process Connections:

• Inspect the flanges, gaskets, and seals for leaks.
• Ensure the wet leg is filled with the correct reference liquid and there are no air pockets.
• Check for any obstructions in the impulse lines and ensure they are properly connected.

Step 3: Electrical Inspection

Verify Power Supply:

• Use a multimeter to check the voltage at the transmitter’s power supply terminals.
• Ensure the voltage is within the transmitter’s specified operating range.

Check Signal Output:

• Measure the transmitter’s 4-20 mA output signal at zero and full span conditions.
• Compare the measured output with the expected values to ensure accuracy.

Test Grounding:

• Verify the transmitter is properly grounded to prevent electrical noise interference.
• Check the continuity of the grounding connections and ensure they are secure.

Step 4: Mechanical Inspection

Inspect Diaphragms and Seals:

• Check the diaphragms for any signs of damage, deformation, or leakage.
• Inspect the seals for wear and replace if necessary to prevent leaks.

Examine Wet Leg:

• Ensure the wet leg is properly filled with the reference liquid and free from air pockets.
• Check for any blockages or leaks in the wet leg and associated piping.

Check Mounting Hardware:

• Ensure all bolts, nuts, and mounting brackets are secure and free from corrosion.
• Verify the transmitter is properly aligned and mounted to avoid mechanical stress.

Step 5: Cleaning and Maintenance of Impulse Lines and Fittings

Isolation of Root Valve

• Close the root valve on both the high-pressure (HP) and low-pressure (LP) sides to isolate the transmitter from the process.
• Ensure that the isolation is complete and there is no pressure in the impulse lines.

Cleaning Impulse Lines

• Disconnect the impulse lines from the transmitter and the process connections.
• Use appropriate cleaning tools and solvents to clean the inside of the impulse lines. Ensure all blockages and deposits are removed.
• Check for any signs of corrosion or damage and replace the impulse lines if necessary.
• Reconnect the cleaned impulse lines securely.

Cleaning Fittings

• Inspect all fittings for wear, corrosion, or damage.
• Clean the fittings thoroughly to remove any deposits or debris.
• Replace any damaged or worn fittings to ensure a secure and leak-free connection.

Reassembly and Leak Testing

• Reconnect the cleaned impulse lines and fittings to the transmitter and process connections.
• Open the root valves slowly to reintroduce pressure into the impulse lines.
• Check for any leaks at the connections and ensure the system is secure.

Step 6: Liquid Filling for Wet Leg

Prepare Filling Liquid:

• Use the liquid specified in the datasheet for filling the wet leg.
• Ensure the liquid is clean and free from contaminants.

Fill Low-Pressure Side (LP):

• Slowly fill the LP side of the wet leg with the specified liquid.
• Ensure there are no air pockets in the wet leg. Air pockets can lead to inaccurate level measurements.
• Verify the liquid level in the wet leg matches the required reference level.

Step 7: Calibration of Transmitter If required

• Prepare Calibration Equipment: Set up the pressure calibrator and HART communicator or field calibrator.
• Zero Calibration: Isolate the transmitter from the process, apply zero pressure, and use the HART communicator to adjust the zero setting.
• Span Calibration: Apply a known pressure corresponding to the upper range limit and adjust the span setting using the HART communicator.
• Verify Calibration: Apply multiple pressure points within the range and verify that the transmitter’s output matches the applied pressures.

Click on the below link for detailed calibration procedure for DP type level transmitter in field.

Step 8: Inspection of Cables, Cable Glands, and Cable Trays

Inspect Cables:

• Check all cables for signs of wear, damage, or degradation.
• Ensure cable insulation is intact and there are no exposed wires.
• Replace any damaged cables to maintain signal integrity.

Check Cable Glands:

• Inspect cable glands for proper sealing and secure fitting.
• Ensure the cable glands are tight and provide a proper seal to prevent moisture ingress.
• Replace any damaged or worn cable glands.

Examine Cable Trays:

• Inspect cable trays for physical damage, corrosion, or obstructions.
• Ensure cables are properly supported and organized within the trays.
• Clean the cable trays to remove any dust or debris that may affect cable performance.

Step 9: Documentation

• Record Findings: Document all inspection findings, calibration results, and any maintenance actions taken.
• Update Calibration Records: Ensure calibration records are up to date and reflect the current status of the transmitter.
• Report Issues: Report any issues identified during maintenance to the maintenance engineer for further action.

Step 10: Troubleshooting if Required 

• No Signal Output: Check power supply, wiring connections, and fuses. Verify the transmitter is receiving power.
• Incorrect Level Reading: Verify wet leg is properly filled and free from air pockets. Check for process leaks or blockages.
• Erratic Signal: Inspect grounding and shielding of signal cables. Check for electrical noise or interference.
• Calibration Drift: Recalibrate the transmitter and verify stability. Check for temperature effects on the transmitter.

Click on below link for the detailed Troubleshooting of DP Type Level Transmitter

Step 11: Post-Maintenance Activities

1. Repressurize and Reconnect: Slowly repressurize the system and reconnect the transmitter to the process.
2. Functional Test: Perform a functional test to ensure the transmitter is operating correctly and providing accurate readings.
3. Notify Operations: Inform the operations team that maintenance is complete and the instrument is back in service.
4. Clean Work Area: Ensure the work area is clean and free from any maintenance debris or tools.

Compliance with Standards and Regulations

Ensure compliance with ISA, ANSI, IEC, and API standards, and OSHA, EPA, and ATEX regulations for DP type level transmitters. Maintain accurate records, provide ongoing training, and follow safety protocols. For detailed standards and regulations, visit ISA

Preventive Maintenance Checklist for Wet Leg Level Transmitter

This downloadable Excel checklist provides a comprehensive guide for performing preventive maintenance on DP type level transmitters in closed tank wet leg configurations. It ensures all critical inspection, cleaning, calibration, and documentation steps are thoroughly checked and verified to maintain optimal performance and compliance with industry standards and regulations.

Download the checklist to streamline your maintenance process and ensure consistent, reliable operation of your instrumentation.