Contact Section - Gas-to-Gas Heat Exchangers - Washing
February 25, 2011

Introduction Safety Equipment and Supplies Tube Side Procedure Shell Side Procedure

Associated Links Materials of Construction Maintenance

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Introduction

Fouling of gas-to gas heat exchangers will occur when there are problems with the operation of the gas cleaning system or drying tower which allow excessive acid mist or moisture to enter the contact section of the acid plant.  The exchangers most susceptible to fouling are the Cold, Cold Reheat and SO3 Coolers.  These exchangers receive the improperly treated gas and operated at the coldest temperatures so acid condensation is always a problem.

Fouling of these heat exchangers is generally in the form of sulphate buildup on the tube side and/or shell side of the heat exchanger.  An increase in pressure drop across the unit will be the first sign that the unit is fouling.   At some point the fouling may become so bad that the plant production is affected because the blower has reached its maximum capacity.

Heat exchangers can be washed with an alkaline solution to remove the accumulated sulphate.  Washing of the shell side will involve flooding the unit with water.  The design of the heat exchanger and foundations should be checked to determine if the equipment has been designed with sufficient safety to allow the heat exchanger to be flooded.  Most heat exchangers have NOT been designed to flooded!!!

Safety

Personnel shall wear as a minimum dust respirators, protective goggles, gloves and acid/caustic resistant protective clothing. 

Standard vessel entry procedures should be followed at all times.  This should include as a minimum:

• Routine testing of oxygen levels inside the heat exchanger
• A person stationed outside the converter manway to monitor the workers inside the vessel.  This individual should be equipped with rescue gear and an independent air supply.

Equipment and Supplies

The following equipment will be required:

 

• Circulating pump
• Mixing/Surge tank
• Potable water
• Soda ash
• Hoses
• Tube plugs
• High pressure washer

Tube Side Procedure

1.       Isolate the heat exchanger prior to entering the unit.  A complete plant shutdown will generally be required to perform the work.

2.   Working from the upper vestibule, clean out the block tubes with a rod.  The tubes that cannot be cleaned out should be plugged prior to washing to prevent liquid from entering the blocked tube and getting trapped in the tube.

3.   Ensure that the drain in the lower vestibule is open and that the wash water is collected or disposed of properly.

4.   Systematically, begin washing each tube with fresh clean water.  A high pressure washer with a spray nozzle mounted on the end of a long hose is suitable for the job.  Work the spray nozzle up and down the tube until the tube is clean.

5.   Proceed to the next tube until all the tubes are cleaned.

6.   Air test the unit plug any leaking tubes.

7.   Dry out the heat exchanger prior to returning it to service.

 

Shell Side Procedure

1.       Isolate the heat exchanger prior to entering the unit.  A complete plant shutdown will generally be required to perform the work.

2.   Blank off the inlet and outlet gas ducts.

3.   Install a 1½" or 2" connection directly above the bottom tubesheet and near the gas nozzle for the wash solution inlet.

4.   Install a 2 or 2½" connection directly below the top tubesheet and near the gas nozzle for the wash solution outlet.

5.   Fill the surge tank with hot water and dissolved the soda ash into the water.  Use approximately 50 lbs of soda ash for every 100 US gallons of water.

6.   Begin filling the shell side of the heat exchanger with the wash solution.  Refill the surge tank with water as required and add soda ash to maintain the required alkalinity.

7.   Continue filling the exchanger with the wash solution until the solution overflows from the top outlet connection.  If the initial solution contains considerable amounts of sulphate sludge dispose of the solution.  When the solution is rlatively clean return it to the surge tank.

8.   Continue recirculating the wash solution in the exchanger for approximatelt six (6) hours.  Add soda ash as required to maintain the required alkalinity.

9.   As the solution is circulating inspect the tube side of the exchanger for leaking tubes and plug as required.

10.  After washing with the alkaline solution, discard the solution and drain the heat exchanger.

11.  Fill the unit with fresh clean water and circulate for one (1) hour and then drain the unit.

12.  Air test the unti and plug any leaking tubes.

13.  Dry out the heat exchanger prior to returning it to service.