Inspect boiler drums effectively

Figure 1. Mark the drum’s normal water level.

Then examine the piping and piping supports — feedwater, chemical addition, and surface blowdown — for serviceability. Inspect the outer surfaces of the piping for deep pitting — a classic indication of past water chemistry problems; also, inspect connections for broken or loose pipe fittings and loose or missing support hardware. The supports for the interior piping should have enough clearance to allow for thermal expansion during normal drum operation.

Scrutinize the head’s interior surface for cracks, any indications of corrosion (oxygen pitting and iron oxide caps), and evidence of wall thinning (gouges or appreciable loss of wall thickness). Identify the normal water level (NWL) — the red or brown line on the interior surface about half way up the side of the drum — with a paint marker (Figure 1).

Clean and buff to bright metal an area of about 1 sq ft centered on the NWL. This cleaned area helps quantify the amount of material deposited on the drum wall during normal operation. During subsequent maintenance crawl-through inspections, the accumulated water deposits on the cleaned area will give a good visual indication of the quantity of suspended solids present in the drum water and the effectiveness of the water treatment system.

 Figure 2. Look for cracks in the heat-affected zone at the head-to-shell weld.

The last and probably the most important inspection in the drum head is a visual examination of the head-to-shell attachment welds. This examination requires that any standing water and debris be removed and the circumferential weld area including the heat affected zone (HAZ) be cleaned on the lower half of the boiler drum (Figure 2).

After cleaning, inspect this weld area closely for circumferential cracking at the edges of the heat affected zone, transverse cracking across the weld, and any porosity (both linear or cluster porosity). If the weld has been subjected to long periods of acidic boiler water, then the weld’s profile will be either indistinct or completely absent.

Interior inspection — shell

The interior arrangement of shell components (steam separators, shields, piping, scrubbers, or dryers) will differ greatly among boiler manufacturers. In most cases, the inspector will find that access can be extremely restrictive or almost impossible between the interior components.

Figure 3. Before starting the interior inspection, remove one or more shell or belly plates from both ends of the drum.

Before beginning this part of the inspection, it is recommended that maintenance personnel remove one or more shell or belly plates from both ends of the drum (Figure 3). This allows for the examination of the space beneath the interior shield plates where boiler water enters the drum and is a collection point for magnetite produced in the waterwall circuits and any other debris trapped in the water circuits.

If possible, the inspector should collect about 1 lb of this deposit for later chemical analysis. The metals found in the drum deposits indicate the component wear in the condensate, feedwater, and boiler water circuits. Furthermore, shield plate removal allows for a visual examination of the tube sheet bore holes. Examine the horizontal and vertical areas between the drum holes (ligaments) for any linear cracks originating inside the holes and running to an adjacent hole; additionally, examine the interior of these holes and tubing for corrosion, wall thinning, pitting, debris, blockage, and tube plugs. Please note that any tube plugs found during the inspection should be located and referenced for later replacement of the failed tube. 

Before beginning the shell inspection, look for cracks, porosity, and undercutting at the welds where shields, liners, separators, and scrubbers are attached to the drum wall. 

During the shell crawl-through, the inspector should examine the following:

• steam separators (screen, baffle, turbo, and cyclone) for mounting hardware, tightness, and overall serviceability (often, tightness is determined by rapping on each separator — a loose separator makes a low-pitched “thud” noise, and a tight separator has a high-pitched “ding” sound).
• downcomer nozzles and inlet screens for cracked, broken attachments, porosity and undercutting at the attachment welds
• shield retainers (dogs or fillet welds) for looseness, cracks, and missing or loose cap nuts on the retainers
• shield plate boundaries (there should be no openings or separation between floor and side plates; openings allow water to bypass the separators, causing solid carryover into the superheat steam circuits)
• scrubber and dryer tightness (use same method as used on separators)
• feedwater, chemical addition, and surface blowdown piping connections for serviceability (broken welds, loose, and missing hardware)
• any drum circumferential or longitudinal welds in the drum’s shell and any attachment welds to the drum’s interior surface.

Also, the inspector should number each separator or dryer while performing the crawl-through to specify the physical location to identify discrepancies within the shell.

Inspection report

The final segment of the drum inspection is the inspection report. Its format should be concise, and the observations and recommendation should be easy to comprehend.

The inspector should assign a priority system for qualify the severity of each observation. This gives the boiler’s owner and maintenance staff a way to establish priorities for later maintenance work. The following is an example:

• Priority 1 — Observation must to be addressed now. It should be considered as an emergency repair. Failure can result in lost generation or endanger plant personnel.
• Priority 2 — Observation should be addressed during this outage or can be deferred until next scheduled outage. Failure might adversely affect generation or might endanger personnel as the condition deteriorates over time. Priority 2 observations often deteriorate to Priority 1 conditions.
• Priority 3 — Observation should be monitored during future inspections. Failure won’t affect generation adversely or endanger personnel. Often used for engineering studies.

The prioritized observations need to be matched with a corresponding picture. This provides a visual aid for later finding the discrepancy. The report should follow the same sequence as the inspection — drum head, shell, and other drum head; in addition, the inspector should develop a list at the end of the report of all Priority 1, 2, and 3 observations.

Final observation

The average design life for a subcritical power boiler drum is about 40 years. Depending on the mode of operation (cyclic vs. base-loaded), maintenance history, and current boiler drum condition, the remaining safe operational life and serviceability of the boiler drum will require at least one extensive assessment of the drum’s shell and its interior components during its service life. This assessment requires the following maintenance, inspection, and non-destructive examination activities:

• removal of interior components, including shields, separators, scrubbers, screens, and baffle plates
• extensive cleaning of interior surfaces, including nozzle penetration and pipe openings, manways, and circumferential and longitudinal shell welds
• nondestructive examination of wall and head welds using phase array or RT; wet fluorescent magnetic particle (wet mag) for nozzles, drum door, bore holes, tube sheet, and attachment welds
• mapping of the remaining drum wall thickness in selected areas using an ultrasonic thickness gauge at tube sheets ligaments, heat-affected zones at circumferential and longitudinal welds, and locations of previous thinning.

This extensive assessment should be performed at least once in the operational life of the boiler and most definitely after reaching the boiler drum’s design life of 40 years.