Deaerator Pressure Vessel Inspections

Deaerators are used to remove harmful oxygen and other non-condensable gases from boiler feedwater, in addition to heating the feedwater.  The pressure vessel is exposed to fluctuation in temperature and pressure, vibration and chemicals introduced into the condensate supply.

Deaerator pressure vessels are required to be periodically inspected by a Qualified Professional Inspector, internally and externally.  Cracking is a common issue in Deaerators.  According to the National Board of Boiler and Pressure Vessel Inspections, 30% – 50% of all inspected industrial Deaerators have been recorded the most failure point is corrosion fatigue induced cracking.  Proactive routine inspection and maintenance is imperative to prevent personnel injuries in a catastrophic failure and ensure boiler system reliability. 

Testing

Today OSHA, NACE, DOD, and the National Board Inspection Code [NBIC] recommend periodic Deaerator inspection using WFMT method. 

The Wet Fluorescence Magnetic Particle Test (WFMT) has been proven to best detect tight cracks. This test must be conducted by an ASNT SNT-TC-1A certified Level II Inspector. This internal inspection will begin with cleaning all major pressure boundary welds. 

Planning and Preparation for Inspection

Annual internal inspection is best with a maximum of 3 years in between inspections, given there are no major issues with the deaerator vessel. To prepare:

• Gather all Operating and Maintenance documentation 
• Gather and review all past weld repair or inspection (NDE) reports
• Gather the Manufacturers Data Report which indicates the material construction, thickness, dimensions and heat treatments performed
• Coordinate the inspection to include time for shutdown, cool time and a qualified authorized inspector. 

Prepare the Deaerator 

The inspection of the internal tank and weld seams should include circumferential, longitudinal, nozzle, and other welds affected by operating stresses. The actual nondestructive examination should be in accordance with ASME requirements and NACE guidelines and must be conducted from inside the vessels. Each weld should be prepared and treated by taking the following steps:

1. Clean the weld to the base metal (brush, wipe down, etc.). 
2. Inspect visually, especially at the intersections of circumferential and longitudinal seams
   
3. Test by the wet fluorescent magnetic particle (WFMP) procedure over the entire weld, in “as welded” condition.
   
4. If cracks are found, grind approximately 20 percent of each weld having cracks flush to the parent metal and retest with WFMP. 
5. If cracks are found after retesting with WFMP, they must be ground out. Cracks then should be classified as (a) Class I, in which the depth of the crack does not exceed the corrosion allowance, (b) Class II, in which the depth exceeds the corrosion allowance but not the minimum design thickness, or (c) Class III, in which the depth exceeds the minimum design thickness
   
6. It is mandatory to repair Class II cracks. Repairs must conform to the original design, and field repairs must be acceptable to the authorized (insurance company) inspector.b The repair paperwork should be filed with the owner’s records of the vessel
   
7. Summarize the inspection and repair results in the data file for each deaerator.

The National Board has established and recommended a regular program of inspection and testing of deaerators to prevent and detect failure conditions. 

Internal Inspection & Non-Destructive Examination (NDE)

Wet Fluorescent Magnetic Particle Technique (WFMT) is a method of finding surface and slightly subsurface discontinuities in magnetic materials.  Small magnetic particles are suspended in a liquid which is then coated on the weld to be tested.  When the weld is magnetized, the arrangement of the particles shows any weld discontinuities, such as cracks.  Surface preparation, such as cleaning and wire brushing is necessary prior to the test.

The tests must be conducted under the overall supervision of Qualified Professional Inspectors (QPE) by qualified Level II non-destructive examination technicians.  Prior to the WFMT test, the welds must be visually inspected by the QPE.

After that, the following frequency of inspections was suggested at the NACE 1986 conference:

• Repaired cracks, 1 year or less
• Nonrepaired cracks, 1 – 2 years
• No cracks/no repairs, 3 – 5 years

Repaired vessels, at the same operating conditions, tend to re-crack faster than the original vessel. Inspections therefore should be more frequent after repairs.

External Inspection

Some of the factors to consider when evaluating the condition of your Deaerator:

Age – Older deaerator vessels do not have the design updates and code approval requirements.  Today, vessels are designed with thicker metal accounting for resistance against corrosion. In addition, all vessels must meet National Board or an ASME stamped nameplate. 

Maintenance Log – Temperatures, pressure, safety valves and trends in chemical usage should all be monitored through an equipment log to flag potential issues. Stress can be indicated by varied water levels or temperature fluctuations.  Noises can indicate water hammer and leaks should be remedied immediately. If you are trending high or low on chemical usage, your boiler should be checked or your DA may need oxygen testing. 

Deaerator cracking can be controlled with proper operation, consistent maintenance, inspection and acting on repairs when they are required. Keep your boiler plant and personnel safe by following the recommended procedures. 

Resources:

The National Board of Boiler and Pressure Vessel Inspectors, www.nationalboard.org Technical Articles, System Design, Specifications, Operation, and Inspection of Deaerators, April 1988.