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Problem: The customer complained about poor dissolved oxygen readings. This Deaerator (DA) went into service in 1987. It has only been inspected one time since its commissioning. Upon opening the DA, the customer found corrosion on the tray enclosure door.
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After removing what was left of the tray enclosure door, Kansas City Deaerator found the trays upset, explaining the high dissolved oxygen readings. Tray upset is usually caused from a sudden change in pressure causing the water from the storage tank to flash into the heater where few tray hold-downs can prevent the upset. |
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Upon closer inspection, most of the trays had corrosion along with the rest of the stainless steel internals. Parts of the trays and enclosure were sent off to a metallurgist and it was determined that the corrosion was from an attack of sulfide and oxygen. The carbon steel was not nearly as affected as the stainless steel. |
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Solution: Kansas City Deaerator recommended replacing all of the spray valves and trays. This alone would reduce the dissolved oxygen levels and stop one part of the sulfide and oxygen attack. The customer now has low dissolved oxygen readings and has reduced the amount of sulfides they put into their system. To prevent future problems, the customer has incorporated a yearly maintenance schedule to catch upsets and problems before they get out of hand. |
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Problem: Tray upsets are common in the deaerator industry if proper tray hold-downs and baffling are not provided. This is a sample of a tray that was dislodged and dropped to the bottom of the deaerator.
Solution: Proper tray hold-downs, baffling and trays eliminate most possibilities of tray upsets. Also, try to operate the deaerator with reduced turbine trips.
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Chicago Heater Replacement Vent |
Problem: The deaerator had water out the vent.
Solution: Replace the deaerator vent that travels throught the original water box to a separate vent that does not travel through the water box. This eliminates short circuiting of the water into the vent due to poor welds and/or load failures. |
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Chloride Attack Tray Enclosure |
Problem: The tray enclosure was ineffective due to chloride attacks on the deaerator.
Solution: Change the metallurgy of the tray box and/or modify the incoming water chemistry. |
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Problem: Corrosion pits were observed in the deaerator storage section. Corrosion pitting could be due to a number of items such as improper operation of the deaerator, a poor performing deaerator or mechanical failures within the deaerator. Corrosion pitting could be very serious due to the individual pit being the weakest link in the chain, regardless of shell thickness.
Solution: A full inspection and repairs in deaerator in accordance with NACE and ASME codes. |
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Problem: A deaerator was thinning out due to erosion underneath the tray stack.
Solution: A solution would be to distribute the water properly within the tray enclosure. Reduce the steam flow underneath the tray box to an acceptable velocity and/or change operating pressure reduce velocities. Originally the deaerator should have been properly built with adequate area underneath the steam tray box. |
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Problem: This deaerator had a catastrophic explosion and catastrophic failure due to non-inspection of the deaerator and/or inadequate inspections.
Solution: Inspect the deaerator periodically and perform repairs in accordance with NACE and ASME Codes as well as the insurance carrier. |
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Tray Enclosure Support Wear |
Problem: The tray enclosure supports were ruined due to relatively high steam velocities and water erosion.
Solution: Provide adequate distribution of the water and steam along with the proper velocities and materials. |
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Problem: Many deaerators in the past have been built with vent piping that protrudes through the water box to the external portion of the heater allowing the oxygen to be eliminated from the deaerator. Many manufacturers provided a dog leg type vent with a miter joint which typically fails due to inadequate welding where the vent pipe supports the valve plate.
Solution: This whole design can be solved by making typical structural modifications. |
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Problem: Spray valves were improperly located over the trays allowing for water carry over to the vent.
Solution: Relocate waterbox spray valves properly so the maximum distribution from a conical spray valve is obtained on the trays. Also, allow adequate distance so that water carry over is not in the vent. |
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Problem: Various locations of the deaerator and heating section can be eroded by steam due to inadequate downcomer sizing, inadequate baffling and inadequate water distribution.
Solution: Resolve the steam velocities water distribution baffle and downcomer sizing. |
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Problem: Tray holdowns on older units rely on sheer force to the tray box to compress the trays and hold the trays down.
Solution: Kansas City Deaerator Company has switched and now utilizes compression type tray holddowns to compact the trays as opposed to a sheer type solution. |
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Problem: Pitting was noticed on the supports of the tray box.
Solution: Verify that the deaerator is performing properly as oxygen should be removed prior to exiting the tray box. |
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Flow Accelerated Corrosion (FAC) |
Problem: Vessel had severely high corrosion due to FAC
Solution: Weld overlay vessel in accordance with the ASME Code. See Kansas City Deaerator FAC Paper. |
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Problem: The deaerator scrubber was inadequately sized and destructed due to high velocities.
Solution: Modify the scrubber by allowing the scrubber to be sized properly to accommodate the high velocities and high flow rates. |
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Spray Deaerator Corrosion |
Problem: The deaerator is open to oxygen degradation due to non containment of the oxygen in the vessel. The deaerator is susceptible to oxygen attack due to a lack of a liner.
Solution: Modify the deaerator to incorporate a tray box and contain all the oxygen within the tray box area. |
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Spray Deaerator Water Bypass |
Problem: Water is bypassed in this spray deaerator due to high flow rates and/or inadequate sizing of tray pans. Water overflowing to the vessels walls of the deaerator causes damage to the vessel walls and inadequate deaeration.
Solution: Properly size the spray pan and water spray valves so that flow is contained and directed properly to the scrubber. |
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Stress Crack in Vessel Shell |
Problem: Cracking in deaerators is most often attributed to corrosion fatigue, where a crack results from a cyclic stress (i.e. vibration and pressure fluctuations) in a corrosive environment.
Solution: Cracks in deaerators are normally not detectable with the naked eye. For this reason, wet fluorescent magnetic particle examination of all internal pressure welds is recommended periodically. Cracking can occur in the deaerator or the storage tank, and often initiates in a pressure weld or the associated heat-affected zone. When detected, cracks are normally ground out and repaired by qualified welders. |
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Weld Overlay on Deaerator |
Problem: Excessive pitting and/or erosion due to high velocity of steam and the deaerator will not perform properly.
Solution: Overlay the tray deaerator section and/or replace the vessel shell. |
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