Deaerator Maintenance

Transcription

Deaerator Vessel
Inspection, Maintenance, and Repair
Presented by:
D a v i d B . S t e w a r t , P. E .
E n g i n e e r i n g M a n a ge r
Ka n s a s C i t y D e a e r a t o r C o . , I n c .
Authored by:
D a v i d B . S t e w a r t , P. E .
M i c h a e l K o o n t z , P. E .
Presentation Prepared by:
Ky l e W h i t t a k e r
Kansas City Deaerator Co., Inc.
6731 W. 121st St · Overland Park, KS 66209
913-338-2111 · www.deaerator.com
Why Inspect?
• Deaerators and associated storage tanks
should be inspected at planned intervals to
allow for early detection and repair.
• Early detection and repair of problems avoids
• Accidents
• Emergency Shutdowns
• Propagation of existing issues
• Design to meet ASME, HEI and NACE
standards will prevent problems.
Deaerator Configuration
Deaerator designs typically consist of one of the following formats:
Horizontal Tray
Deaerator
Vertical Tray
Deaerator
• Horizontal heater
• Trays contained in
protective stainless
steel enclosure
• Typically located above
a horizontal storage
tank
• Vertical heater
• Trays contained in
protective stainless
steel enclosure
• Typically located above
a horizontal storage
tank
Horizontal SpraySparger /Rake
Deaerator
• Single horizontal vessel
Heater/Storage
combined
• No trays, no enclosure
• Deaerator also serves
as storage tank
Horizontal Tray Deaerator
Vertical Tray Deaerator
Vertical Tray Deaerator
Horizontal Spray-Sparger/Rake Deaerator
Horizontal Spray-Sparger /Rake Deaerator
Internal Inspections
• Internal inspections are
recommended in all cases.
• Most problems begin inside
the vessel.
• Deaerator internals sometimes
obstruct internal inspection of
some pressure components.
• Inspection should be performed
where access is available.
Visual Examination (VT)
• All internal pressure and
non-pressure components
should be visually inspected
by personnel with expertise
in Deaerators and pressure
vessels.
• Deaerator internal
components such as trays
and spray valves should be
checked for functionality
and wear.
Magnetic Particle Examination (MT)
• Used to determine discontinuities both on and below
the metal surface.
• All pressure and attachment welds should be
inspected by the wet fluorescent magnetic particle
method.
• Inspection should be performed by qualified NDE
personnel (ASNT or equal).
Dye Penetrant Examination (PT)
• Used for non-magnetic components, such as 300 series
austenitic stainless steels.
• Also used where magnetic particle testing is not practical.
Ultrasonic Examination (UT)
• Vessel thickness should be monitored by means of ultrasonic
examination.
• Inspection should be performed by qualified NDE personnel (ASNT or
equal).
• Ultrasonic testing can also be used to detect defects under the
material surface.
Inspection Intervals
• Deaerators & BFW Storage Tanks should be inspected as
recommended by NACE:
• Visual Examination (VT) –
• Annual visual inspection is recommended.
• An internal visual inspection can be performed in a relatively short time, and
can be well worth the effort.
• Magnetic Particle Examination (MT) –
• Every 1-5 years, depending on the history of the vessel.
• If the vessel has never experienced any problems, every 3-5 years may be
adequate.
• If the vessel has a history of problems, MT inspection annually may be in
order.
Common Problems
in Deaerators
• Tray & Spray Valve Upsets
• Erosion
• FAC (Flow-Accelerated Corrosion)
• Oxygen Pitting
• Cracking of Pressure Parts or Welds
• Chloride Stress Corrosion Cracking in Tray
Enclosure
Tray & Spray Valve Upset
• Trays can become disrupted during upset
conditions.
• Spray valves can become damaged or clogged
over time.
Erosion
• When inlet streams impinge
directly on the pressure
envelope, vessel components
erode.
• Wall thinning can be severe.
• Some carbon steel internal
components such as tray
supports or vortex breakers
may also be affected by
erosion.
FAC (Flow-Accelerated Corrosion)
• FAC is a mechanism where the
oxide layer is continually
washed off by flowing water.
• FAC occurs mainly on the
vessel shell, under the tray
enclosure in the deaerator
vessel where water is flowing
toward the outlet
(downcomer).
Pitting
• Pitting occurs in deaerators when the
deaerator is not functioning properly,
and there is excess oxygen in the water.
• Pitting is often the worst near the
water level in the storage tank in
non-performing deaerators.
• Pitting may also occur if proper shutdown and layup procedures are not
followed.
• In horizontal spray-sparger/rake
deaerators, oxygen is released while
the water is in reservoir at the water
level. This makes carbon steel vessels
susceptible to oxygen pitting,
particularly at the water level and in the
vicinity of the vents.
Cracking of Pressure Parts
• Cracking of pressure parts occurs mainly along weld seams in the
heat affected zone ~2” from the weld.
• Cracks can develop due to faulty original construction, or due to
fatigue, exacerbated by stress risers and discontinuities.
Chloride Stress Corrosion Cracking
• Chloride damage affects austenitic stainless steels
(300 series) when the water contains excessive
chloride concentrations.
• The upper portion of stainless steel tray enclosures is
particularly affected, and severe damage may occur
in a short period of time.
• Damaged tray enclosures may allow corrosive vent gases to
contact the carbon steel vessel.
Maintenance
• Why should internal components
be maintained in good working
condition?
• Maintaining internals in good condition
keeps the deaerator working properly
(thus avoiding corrosion problems).
• Damaged or clogged spray valves can
cause a spike in dissolved oxygen.
Repair
• Common Repairs & Repair
Methods
• Damaged trays
• Replacement is recommended
• Spray valves
• Replacement is recommended
Common Repairs & Repair Methods
• Erosion & FAC
• Weld overlay
or, depending on extent of damage…
• Partial shell replacement
• 1 ¼ Chromium overlay/replacement
materials should be considered to avoid
recurrence of damage.
• If erosion is caused by direct
impingement of an inlet stream,
consider relocating the inlet connection
or installing an impingement baffle
plate to prevent direct impingement on
vessel wall.
• Cracking
• Cracks are repaired by standard weld repair methods.
• Repair method is defined by the depth of the crack.
• Does the crack go deeper than the corrosion allowance?
• Does the crack extend into the material required for minimum thickness?
• See details for NACE category ‘II’ & ‘III’ vessels below:
• Category II – Re-inspect no less than once every (3) years
• Category III – Re-inspect no less than once every year
(II)
(III)
• Determine the depth
of the crack
Crack Repair Methodology
Verify
Depth
Does the crack go
deeper than the Corrosion
Allowance?
(into the minimum thickness
required for pressure)
• Grind the crack to
sound base metal
No
Grind
PT Test
• Verify that the crack has
been completely ground
out with dye penetrant
testing
Yes
PT Test
• Grind the crack to
sound base metal
Grind
• Verify that the crack has
been completely ground
out with dye penetrant
testing
• Fill the groove
completely with
weld metal
Weld
Grind
• Grind the weld
smooth
• Verify that the repair
is successful with a dye
penetrant examination
PT Test
• Chloride damage
• Typically, complete or partial tray enclosure
replacement is the required.
• The use of alloys such as Duplex stainless
steel should be considered to better resist
the effects of Chlorides.
Questions?
This presentation is brought to you by:
Referenced Texts/Codes
•ASME Section I & Section VIII
•NACE SP0590
•H.E.I. Standards and Typical Specifications for Tray Type
Deaerators, Ninth Edition
•EPRI paper “Repair of Deaerators”
•ASTM Standards & Procedures

Deaerator Maintenance

Transcription

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