NATIONAL BOARD OF BOILER AND PRESSURE VESSEL INSPECTORS
This article is part two in a two-part series about inspecting watertube power boilers. Part one is available at https://shorturl.at/vDGQ8.
To ensure boilers remain reliable and well-maintained, consistent inspections are necessary. This two-part article is based on the National Board Inspection Code (NBIC) Part 2, which outlines the requirements for inspecting watertube power boilers. Some of the requirements are applicable to firetube and other types of boilers. The following information outlines key inspection requirements for watertube power boilers.
Tube Exterior Surfaces
Various fuels have different effects on boiler tubes. Fuels burned in watertube boilers may contain ash, which can form an abrasive grit in the flue gas stream. The abrasive action of the ash in high-velocity flue gas can quickly erode them.
Each part of the boiler that contains tubes, such as the generating bank, economizer, superheater, and soot blowers, should be carefully and thoroughly inspected. Tube exterior surfaces should be inspected for corrosion, deterioration, warping/sagging, cracking, pitting, and scaling. Tube welds should be examined for cracks and complete fusion.
Tubes lose thickness by high-velocity impingement of fuel and ash particles or by the improper installation or use of soot blowers. A leak from a tube can cause serious corrosion or erosion on adjacent tubes.
Watertube boilers use a variety of fuels for combustion, including black liquor, coal, gas, oil, trash, or wood. The soot and fly ash deposits, as well as sulfate and hydrocarbon particulates, are associated with generating tubes that are closest to the flames of combustion but may be found on other boiler tubes, such as the economizer and downcomers (see Figure 1).
Other effects on boiler tubes consist of the following:
- Blisters and general enlargement: result of overheating, such as waterside deposits producing an insulation effect; and
- Tube marriage: result of overheating and warping to the point two tubes come in contact with each other. The superheater support structure should be inspected to ensure clips, clamps, support bars, and/or fasteners are not eroded to the extent they no longer function as intended.
Boiler Drums and Headers
Drums and headers should be inspected internally and externally for signs of leakage, corrosion, overheating, and erosion. Be sure to check header seals for gasket leakage. To conduct a thorough internal inspection of the steam drum (see Figure 2), it is recommended the internal components, such as the baffles, shrouds, separators, and dryers, be removed for the inspector or ensure that the nondestructive evaluation (NDE) technician has full access to them.
The area where the boiler tubes are joined with the drum or header, including the ligaments, should be carefully checked for cracks, evidence of leaks, corrosion, and pitting.
The areas where cracking and crack indications are most common include:
- Longitudinal and structural welds;
- Bored openings (radial cracks);
- Ligament cracks between tube holes;
- Circumferential cracks in curvature of formed heads;
- Manway reinforcing ring welds;
- Structural support welds for internals; and
- In superheater headers, craze cracks near drain or vent openings.
The steam drum surface blow pipe and chemical feed pipes, if equipped, should be checked to ensure proper location, orientation of the holes, and clogged holes.
Centrifugal steam separators and dryer drains should be free of clogging and damage; the fasteners should not be loose, stripped, or corroded; and any welds should be intact with no visible cracks.
Steam baffles and plates should be free of excessive corrosion, pitting, and warping. The joints and apron/end plates should be inspected for leaks and plugged holes on perforated plate. Fasteners should be intact and have complete thread engagement, and stripped fasteners should be replaced.
Soot blower mechanical gears, chains, and pulleys should be checked for broken or worn parts.
Be sure to inspect supply piping to the soot blowers for faulty supports, leakage, and expansion and contraction provisions. Checking the design for proper installation that allows for complete drainage of condensate can prevent erosion.
Fireside Components and Surfaces
Watertube boilers may contain dead air spaces between the boiler casing and the fireside cavity. These dead air spaces include the penthouse, upper arch dead air space, and lower throat dead air spaces that frequently house the drums and headers. There is a tendency for unburned solid fuel and ash to collect in these dead air spaces that may limit the ability to inspect these spaces for corrosion, tube bulges, service-induced cracking, or defective welds. These dead air spaces should be thoroughly cleaned and examined.
Dead Air Space Support Steel and Hangers
The boiler refractory (see Figure 3) and insulation are key components within a boiler. This interior boiler component serves two functions: It allows the boiler to operate efficiently by keeping the flames and products of combustion flowing through the boiler tubes and out the stack, and it keeps boiler operators and personnel safe. Boiler refractory and insulation starts at the furnace floor and may extend up the sides and to the roof of the boiler. It should be in good condition, not broken, unfastened, or otherwise damaged. Fasteners, such as clips, and wire used to secure insulation, including blankets, should be intact and functional. A variety of materials are used to insulate power boilers, including:
- Calcium silicate – temperature limit of 1,200°F (649°C);
- Mineral wood board – temperature limit of 450°F (232°C).
- Ceramic fiber blanket – temperature limit of 450°F (232°C).
- Fiberglass board – temperature limit of 850°F (454°C).
Boiler External Piping and Other Connections
Boiler external piping, such as a steam outlet and feedwater inlet, is not within the scope of this information, but the requirements are located in NBIC Part 2, paragraph 2.4.
All openings leading to external attachments, such as water column connections, low-water fuel cutoff devices, steam outlet pipes, and openings to safety valves, should be examined to ensure they are free from obstruction.
Expansion joints, whether in the boiler piping, exhaust ducting, or casing/foundation, should not have evidence of leakage, distortion, corrosion, or damage. Larger boilers are equipped with expansion joints in the steam drum supports to allow for expansion and contraction, whether the drum is supported from the base, by hanger supports, or bottom supported through downcomers. These components should be inspected to be in good working order and well-lubricated per the manufacturer’s recommendations.
The manhole and reinforcing plates, as well as nozzles or other connections flanged or bolted to the boiler, should be examined for evidence of defects both internally and externally to determine whether connections, welds, and threads are properly made to the boiler.
Manholes (see Figure 4) and handhole inspection opening seating areas should be inspected for cleanliness and to ensure the gasket surfaces do not have any steam cuts.
Along with the design and integrity of the boiler pressure-retaining components, establishing and maintaining safety controls and safety devices in good working order is essential to the proper operation of the boiler. Repairs to these devices must only be made by qualified individuals or organizations and should be documented as part of the boiler maintenance history.
The following are guidelines that may be required by the jurisdiction to aid in the evaluation of operating control devices. Controls should be evaluated at both the external inspection, preferably while the boiler is in operation, and the internal inspection, if applicable.
- Verify the burner (see Figure 5) is labeled and listed by a recognized testing agency, that piping and wiring diagrams exist, that commissioning tests have been conducted, and that a contractor or manufacturer’s installation report has been completed and is available for review.
- Verify function tests, inspection requirements, maintenance, and testing of all controls and safety devices are in accordance with the manufacturer’s recommendations.
- Verify these activities are conducted at assigned intervals in accordance with a written procedure, that non-conformances impacting continued safe operation of the boiler are corrected, and that the results are properly documented.
- Verify combustion air is supplied to the boiler room as required by the jurisdiction. If no jurisdictional requirements exist, see NBIC, Part 1, paragraph 2.4.5 and paragraph 3.5.4 for additional guidance.
- Verify that a manually operated remote boiler emergency stop button exists at each boiler room exit door, when required by the jurisdiction.
- Verify operation of low-water protection devices by observing the blowdown of these controls or the actual lowering of boiler water level under carefully controlled conditions with the burner operating. This test should shut off the heat source to the boiler. The return to normal condition, such as the restart of the burner, the silencing of an alarm, or stopping of a feed pump, should be noted. A sluggish response could indicate an obstruction in the connections to the boiler.
- On forced circulation boilers (ASME Section I), the flow-sensing device should be tested to verify that the burner will shut down the boiler on a loss of flow. Construction of high-temperature water boilers does not facilitate an internal inspection of the water tubes.
- In the event controls are inoperative or the correct water level is not indicated, the boiler should be taken out of service until the unsafe condition has been corrected.
- For the internal inspection, float, chamber, and probe-type low-water fuel cutoff devices (see Figure 6) should be disassembled and inspected, and linkage connections should be examined for wear. The float chamber should be examined to ensure it is free of sludge or other accumulation. Any necessary corrective action should be taken before the device is placed back into service. The operating instructions for the device should be made available to the inspector.
- In addition, the attached piping should be inspected to ensure it is free of scale buildup that might impede device operation. Removing inspection plugs typically allow this piping to be inspected and cleaned without disassembly of the system.
Boiler logs, records of maintenance, and feedwater treatment should be maintained to ensure continuity of regular and adequate tests have been made on the boiler and its controls. These should be readily available to present to the inspector.
Boiler inspections of watertube power boilers are important for their continued safe operation and to ensure the best possible efficiency. These two factors save both time and money by reducing injuries and costly boiler repairs. Inspections should be an integral part of any plant maintenance program, even if state or local government does not enforce a boiler statute.
By Louis Ponce, manager of technical services, NBBI