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Maintenance and Inspection - Anodically Protected Shell and Tube Exchangers
September 12, 2003

Introduction

The vendors operating and maintenance instructions and procedures should be followed at all times and take precedence over the following maintenance instructions.

Maintenance and Inspection Schedule

Regular inspection of the acid cooler will ensure that the exchanger will provide trouble free operation throughout the life of the plant.

Shell Side - Acid

After the first three months of operation the cooler should be inspected internally via the inspection ports located on the shell.  The condition of the shell, the tube surfaces and all visible internals should be inspected and records made of there condition.  Thereafter, the shell side should be inspected on an annual basis.

Tube Side - Water

After the fist three (3) months of operation, the cooler should be inspected by removing the water box inspection port covers.  The condition of the water box, tubesheet welds and tube boxes should be inspected and records made of their condition.  If there is debris blocking any tubes, the water box end covers should be removed and every tube inspected to ensure that they are clear along their entire length.  In particular the tubes should be checked for sludge or scale accumulation.

The cooler should be inspected at least once a year following the initial three month inspection. 

The high water velocities within the tubes during normal operation is generally effective in keeping scale and sludge formation to a minimum.   If tube should require cleaning, hydroblasting using a lance is recommended.   If further cleaning is required, mechanical turbining may be required.

The manufacturer generally provides after sales service to perform acid cooler inspections and maintenance if required.

Cleaning

The manufacturer’s instructions for cleaning the acid cooler should be followed.  The following is a general guideline for cleaning the acid cooler on both the tube and shell side. 

Tube Side

Fouling or scaling on the tube side of the acid cooler can easily be removed by hydroblasting each tube.  If further cleaning is required mechanical turbining of each tube can be performed.

If heavy calcium of phosphate scaling has occurred the tube side may need to be chemically cleaned.  A hot solution of formic acid (< 12 wt% @ 65 to 95^oC) can be re-circulated through the tubes to remove scaling.  As the unit is cleaned the acid concentration will decrease.  Additional formic acid should be added to maintain the original acid concentration.  When the acid concentration remains constant for a period of time, the unit is clean.

Shell Side

Cleaning of the shell side can only be done by chemical means.  A 2 wt% solution of sodium hydroxide is re-circulated through the shell side of the cooler at a temperature of 80 to 95^oC.  Concentrated sodium hydroxide is added to maintain the pH of the cleaning solution above pH 8.   The cleaning solution should be circulated for a minimum of 2 hours.

When the chemical cleaning is completed, the caustic solution must be thoroughly drained from the cooler and the unit flushed completely with fresh water.  When all water is drained from the cooler it is ready to be returned to service.

In all cases, water with low chlorides must be used for the chemical cleaning solutions and for flushing the equipment afterwards.

Tube Plugging

If a leaking tube is found, the tube must be plugged before the cooler can be returned to service.  If a lot of tubes have been plugged, the thermal performance of the cooler may no longer be adequate and the cooler must be re-tubed or replaced.

Tube plugging kits are usually available from the manufacturer.  Plugs are either a piece tapered plug or a two piece tapered plug and ring constructed of Alloy 20.  The kit may also include tools such as a reamer, wire brush, socket drive, and cutting fluid for stainless steel.

In general, the procedure for plugging a tube is as follows:

• Using a stainless steel wire brush clean the area around the tube.

• Wash the area with water and dry completely

• Ream out the tube.  The reaming tool will also cut into the face of the tube-to-tubesheet weld to create a flat surface for welding.

• Insert the ring piece so that it sit flush against the face of the tube-to-tubesheet weld.

• Insert the tapered plug and set with several sharp blows with a hammer.

• Weld the plug to the ring and the ring to the face of the tubesheet using a continuous bead.

Reference Electrode

The most common problems with the reference electrodes usually relate to acid leakage and mechanical damage.  After each year of operation the electrodes should be removed and checked for cleanliness and soundness of the PTFE insulator.  Care should be taken not to bend or break the platinum element.   PTFE tape must be used on the 1/2" NPT thread.  The seal nut should be tightened securely to ensure against leakage.

CAUTION: Do not pull on the platinum element as it could come out of the PTFE insulator and release a jet of acid.

Main and Pin Cathode

It is imperative that all electrical connections be sound.   A ground or short will not burn the fuses and therefore it will not be easily located.

The main and pin cathode rods must be removed for inspection after the first year of service.  The condition of the cathodes will indicate the frequency for future inspections.

CAUTION: Do not adjust pin cathodes with acid in the cooler.

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