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Sulphuric Acid on the WebTM Technical Manual DKL Engineering, Inc.

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Maintenance and Inspection - Electrostatic Precipitators
December 12, 2008

Introduction
Maintenance and Inspection Schedule
Insulator Compartment Ventilating System
Insulators
Air Load Test
Problems
        Tubesheet Leakage
        Collection Tube Burn Through
        Broken Discharge Electrode
        Bulging Tubesheet
Associated Links

Operation
Materials of Construction
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Erection
Testing


Introduction

A properly maintained electrostatic precipitator is important to ensure clean gas enters the contact section of the acid plant.  Failure of the WESP to perform its function will shorthen the life of downstream equipment because the acid mist that carries forward causes accelerated corrosion.  The following describes some typical problems with WESP's.

Maintenance and Inspection Schedule

Daily
  • Note and record switch board readings at least once per shift
  • Check that all flushing system components are working properly
  • Ensure insulator compartment ventilating system is operating
Weekly
  • Remove dust and foreign matter from electrical equipment
  • Check control system and note any alarm conditions
  • Clean or replace insulator compartment ventilation system filters
Shutdown
  • Clean the insulators in the insulator compartments
  • Thoroughly inspect the interior of the unit.  Particular attention should be given to the high voltage electrodes.
  • Check FRP components for damage

Insulator Compartment Ventilating System

The following maintenance is required:

Insulators

All insulators in the system must be cleaned at regular intervals to remove accumulation of dust, acid or moisture.

Wipe the insulators with a dry clean cloth to remove all foreign matter.  If acid is to be removed, clean with ammonia and polish with a clean dry cloth.   Never use wire brushes as they may damage the surface finish of the insulators.  Remove any accumulation of dust and dirt inside the insulator compartments.

Any cracked or damaged insulators must be replaced immediately.

Air Load Test

An air load test of the precipitator provides a baseline for the comparison of precipitator performance.  An air load test should be performed on every unit at initial start-up and after any maintenance, repair or inspection is done on the unit.

The test is performed with air and not process gas.  The unit is energized and operated in manual.  The current is increased in increments of 10% of the transformer/rectifier nameplate rating.  All voltage and current readings are recorded at each increment until the rated current is reached or until sparking/arcing occurs.

Problems

esptubes.GIF (33913 bytes)

Tubesheet Leakage

In the old style WESP's the lead collection tubes were welded to a carbon steel tubesheet overlaid with lead.  As the WESP ages the weld between the tubesheet and tubes may become damaged and crack.  The result is a leak path for weak acid to pass through the protective lead covering and begin attacking the carbon steel underneath.  The easiest way to see when this is occurring is to look for signs of liquid running down from the tubesheet corroding and staining the outside of the tubes.   Usually when this occurs the WESP unit is approaching the end of its useful life.   There will be other problems in addition to the tubesheet leaks which the plant maintenance staff will need to contend with.  The tubesheet leaks can be fixed by fixing the tube-to-tubesheet weld.

esp1.GIF (25341 bytes)

Collection Tube Burn Through

The discharge electrode should be centered in each of the collecting tubes to ensure optimum operation of the unit and maximum collection efficiency.  If one of the discharge electrodes is off-centre then it will affect the over all operation of the unit.  Spark-overs will be more frequent and in the worse case, the electrical system will short out.  Off-centreing may be caused by gas turbulence causing the lower support frame to swing, loss of the weight that holds the discharge electrode taught, movement in the collecting tube, etc.  When a discharge electrode is off-centre, excessive sparking will occur and the sparking may cause the collecting electrode to burn through as illustrated in the photo.  When WESP's are inspected, each tube should be checked to ensure that the discharge electrode is exactly centred in the collecting tube.  If it isn't, either the discharge or collecting electrode will need to be re-aligned.

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Broken Discharge Electrode

Broken discharge electrodes may cause the high voltage system to electrically short and become ineffective if the broken electrode remains lodge in the collecting tube.  Broken discharge electrodes are a problem with lead star wires but is less of a problem with the rigid discharge electrode systems.  The broken discharge electrode must be removed and replaced or the tube blanked off until repairs can be made.  The broken wire is indicated by the fallen weight in the back of the photo.

tubesheetbuldge.JPG (14818 bytes) Bulging Tubesheet

The top support tubesheet of lead type precipitators are lead lined to protect the carbon steel from corrosion.  To reduce costs, the lead was often spot bonded to the carbon steel tubesheet.  In most cases weak acid eventually penetrates through the lead lining through cracks or pin hole and corrodes the steel.  The resulting iron sulphate occupies a greater volume and causes the lead lining to bulge upwards as shown in the photo.  The lead lining may crack further leading to even more weak acid penetrating the tubesheet.  A homogeneously lead lined tubesheet would have prevented this type of damage.