What can go wrong

In a kraft pulp mill, the recovery boiler is both the heart of the chemical cycle and its biggest hazard. It burns concentrated black liquor, the spent cooking chemicals and dissolved wood, to recover those chemicals and to raise steam. The chemicals collect as a pool of molten smelt on the furnace floor. If a tube in the furnace leaks and water reaches the smelt, the result can be a smelt-water explosion, which can destroy the boiler and endanger everyone nearby. Everything about recovery boiler inspection starts from that fact.

The other major equipment has its own problems:

  • Digesters, the pressure vessels that cook wood chips in hot caustic liquor, suffer general corrosion and caustic stress corrosion cracking, especially at welds.
  • Yankee dryers and dryer cylinders, large steam-heated pressure vessels that dry the paper sheet, can suffer corrosion, wear and cracking, and a failure releases a great deal of stored energy.
  • Tanks, piping and bleach plant equipment face corrosion from liquors and bleaching chemicals, and many are made of stainless steel, special alloys or fiber-reinforced plastic.
  • Power boilers and their tubes face the same fire-side and water-side damage as any industrial boiler.

Where NDT is used

Recovery boilers get the most attention. During outages, inspectors measure tube wall thickness across the lower furnace, floor, walls and at air ports, where the environment is most aggressive, and track the results from one outage to the next. Manual ultrasonic readings on a grid are the traditional approach; EMAT and other automated ultrasonic systems can cover large wall areas faster without couplant. Many boilers use composite tubes, carbon steel with a corrosion-resistant outer layer, which are also examined for cracking in that layer. Smelt spout openings, air port tubes and the boiler's welded attachments are checked visually and with surface methods such as penetrant testing. Superheater, boiler bank and economizer tubes are inspected for thinning, erosion from sootblowers, and cracking.

Digesters are inspected with ultrasonic thickness mapping to track corrosion and with wet fluorescent magnetic particle testing (WFMT) of welds to find stress corrosion cracks, often after the vessel has been cleaned by blasting. Corrosion-resistant weld overlays and thermal spray coatings applied to protect digester walls are inspected too.

Yankee and dryer cylinders are examined for shell thickness, often with ultrasonics, and for cracking at heads, journals and other stressed areas, following the manufacturer's recommendations and industry guidance; acoustic emission is sometimes used to monitor for active flaws.

Tanks and piping are covered by thickness surveys, weld inspection and, for fiber-reinforced plastic equipment, careful visual inspection. Heat exchanger and evaporator tubes are inspected with the techniques in our guide to tubing analysis.

How inspection fits into mill operations

Mills schedule major maintenance outages, commonly annual, and most internal inspection of recovery boilers, digesters and dryers is concentrated there. The outage window is short and the scope is large, so inspection is planned from the previous outage's findings, with priorities set by risk: recovery boiler pressure parts come first. Good programs keep a thickness history for every critical tube area, so the remaining life of each section is known and replacements can be planned before they are urgent.

Recovery boilers also depend on operating discipline. Industry practice calls for an emergency shutdown procedure that drains the boiler quickly when a leak is suspected in the furnace, to keep water away from the smelt. Inspection and operating practice work together: finding thin tubes before they leak is the first line of defense, and a fast, rehearsed response to a leak is the second.

Regulations and standards

  • ASME Section I covers the construction of power boilers, including recovery boilers, and Section VIII covers pressure vessels such as digesters and dryers.
  • The National Board Inspection Code (NBIC) covers inspection, repair and alteration of boilers and pressure vessels in service, and state and local jurisdictions require periodic inspection of boilers and pressure vessels under their own laws, generally by commissioned inspectors.
  • BLRBAC, the Black Liquor Recovery Boiler Advisory Committee, publishes recommended practices for recovery boiler safety, including emergency shutdown procedures.
  • TAPPI publishes Technical Information Papers with guidance on inspecting recovery boilers, digesters, dryers and other mill equipment. Check the current editions, since they are revised over time.

What this means for mill owners

A mill's inspection program has to deliver in a short outage and leave a record the next outage can build on. Are recovery boiler thickness readings taken at the same locations each time, with calibrated equipment, so that wastage rates are real? Are inspectors and NDT technicians certified in the specific technique they are using, including automated systems? Are digester crack findings sized and evaluated before the vessel returns to service? And is the data stored where it can be trended, not in a binder from three outages ago?

Frequently asked questions

What causes a recovery boiler smelt-water explosion?

It happens when water, usually from a leaking tube, contacts the molten smelt that collects on the recovery boiler floor. The water flashes to steam with explosive force. Finding thin and cracked tubes before they leak, and draining the boiler quickly under an emergency shutdown procedure when a leak is suspected, are the main defenses.

How are recovery boiler tubes inspected?

Mainly by ultrasonic thickness measurement across the floor, lower furnace, walls and air ports during outages, increasingly with automated or EMAT systems, plus visual inspection and surface crack detection at openings and attachments. Composite tubes are also checked for cracking in their outer layer.

How are digesters inspected?

With ultrasonic thickness mapping to track corrosion and wet fluorescent magnetic particle testing of welds to find stress corrosion cracking, usually after the vessel has been cleaned. Protective weld overlays and coatings are inspected as well.

How NDT LLC can help

NDT LLC helps mills and their inspection contractors with NDT procedures and written practices, technician qualification, outage inspection planning support and independent review of inspection data. Contact us before your next outage.

This article is general information for a business audience, not engineering, legal or regulatory advice. Always work to the current edition of the codes, standards, regulations and manufacturer data that apply to your equipment and contracts.

NDT LLC is an independent consulting firm. It is not affiliated with, endorsed by or sponsored by ASME, the National Board of Boiler and Pressure Vessel Inspectors, the Black Liquor Recovery Boiler Advisory Committee (BLRBAC), TAPPI, or any other organization named in this article; names are used only to identify the organizations and documents referred to.