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.
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.
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.
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.
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.
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.
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.
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.
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
Solution: This whole design can be solved by
making typical structural modifications.
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
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
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.