- Periodic inspections help ensure that a subcritical power boiler drum achieves its typical 40-year design life.
- A structured, rigorous inspection and documentation procedure captures the details of the drum’s current condition.
- The inspection report forms the basis for decisions about the boiler maintenance program.
As an integral component of a viable boiler maintenance program, the internal visual inspection of the boiler drum, or crawl-through, is a mandatory part of the annual boiler condition assessment program.
In the past, typically plant engineering, maintenance, or the boiler manufacturer’s field representative performed this inspection. However, because of operating and maintenance costs and fewer trained in-house inspectors, many boiler owners elect to allow the annual boiler operating permit inspection to be conducted by the authorized inspector or jurisdictional inspector as a substitute for their assessment inspection.
Because the authorized or jurisdictional inspectors typically are given a limited amount of time to perform a drum crawl-through, a comprehensive assessment inspection is normally not performed during the life of the boiler. To correct this situation, the following approach provides in-house inspection personnel with a detailed strategy for subcritical boiler drum assessment inspections.
The boiler drum is considered a confined space. As a result, any interior inspection must meet OSHA 1910.146 requirements for entry permits, lock out/tag out, atmospheric monitoring, and an outside attendant.
Boiler drum clearance verification requires that the inspector make a visual inspection to confirm that any system moving liquids or gases into and out of the drum has been isolated (closed), secured, and tagged to prevent operation while the inspector is inside the drum. Only after experienced personnel have checked and verified that clearances are hung and the interior atmosphere is safe can the inspector enter the drum.
Before entering the drum, the inspector should determine the drum’s physical orientation for referencing observations made during the inspection. Usually, the drum doors are referenced to a compass direction (north, south, east, or west), and the drum is divided into five sections:
- drum head at entrance
- end of shell
- end of shell
- drum head at exit.
Additionally, all observations should also be referenced to the drum’s top, bottom, left, or right side. Interior components — steam separators, piping, and chevron scrubbers — should be referenced by sequential numbers starting at the drum’s entrance and continuing toward the drum’s exit.
The following equipment should be available for the crawl-through inspection:
- large flashlight (50 lumens or more)
- paint marker or pen (white or black)
- 3-by-5-in. spiral-bound notebook
- digital camera or 35-mm disposable camera with flash
- pens or pencils (minimum of two each)
- small, high-intensity flashlight for backup.
The observations written in the notebook should correspond to the sequence of photographs taken during the inspection. The inspector also should take a picture of the drum’s manway before entering the drum. This signifies that all subsequent pictures will be unique to that particular drum inspection. This becomes extremely important if the inspector uses the same camera to document crawl-through inspections of other boiler drums.
Interior drum head inspection
Before entering the drum, the inspector should examine the entry door for indications of over-temperature (black or dark blue metal color), water stains at the bottom of the door (gasket leak), and loose, unserviceable, or missing door hardware. Then inspect the door gasket face both by sight and touch for any gouges, cuts, and rough spots.
After entering the drum, investigate the gasket face on the door opening (in the head) for gouges, cuts, and rough spots, again, by using both sight and touch. Pull the door away from the drum’s side wall to examine the area behind the door for cracks, gouges, and metal delamination. In addition, inspect the door’s support mechanism and its attachment welds at the door and at the drum’s side wall for cracks, porosity, and undercutting. Mark observations with the paint pen and record it in the notebook. Take corresponding photos of each observation before moving to the next area of interest.
The next item to examine is the drum wall penetrations. Check each penetration — bore holes or pipe sleeves for any cracks at the edges of openings, cracks in the attachment welds and any porosity (cluster or linear). The visual inspection should include upper and lower bore holes at the drum’s instrumentation bridle, upper safety valves, and at the drum vents. These openings should be free of debris, cracks, gouges, and appreciable loss of material thickness in the head and the attached piping.
Some boiler drums have an opening for a downcomer pipe in the bottom of the head. Check the anti-vortex screen and attachment welds to the head at the downcomer entrance for serviceability and tightness.