PERMAX PANELS

Recovery Boiler tubes engineered to last

Under normal operating conditions, the skin temperature of the tubes is low enough to freeze the smelt and insulate the tube from direct contact with smelt. Even boilers running at 1500 psi have this frozen layer. The problem is that this layer is highly unstable and flakes off on and on and each time it spalls off some minute portions of the tube material are lost. Gradually the tube thins and after a certain time it will call for replacement.

Prevent direct contact between smelt and the tube itself will grant the tube a life as long as the expected life of the whole boiler. This is within reach today with our approach.

The studs engineered by Sage not only last much longer than normal studs but they wear in a very peculiar way, where they never lose their cylinder shape. That enhances the cooling effect, slows down the smelt and has the self preparatory ability that allows the studs to be restored to their original condition when repair is needed.

Difference in wear pattern

1.1 Wall thinning mechanism

As smelt touches the tubes a frozen layer is immediately formed. Under its own weight, portions of the frozen layer will detach and fall, removing a small amounts of iron sulfide or iron oxide. Throughout the years, this licking effect results in wall thinning and the need for panel replacement.

Studs with adequate density and design will promote a thick and stable frozen layer that will protect the tubes for very long cycles between stud restoration.

1.2 Benefits of a stable and thicker frozen layer

A thick and stable frozen layer will

As shown on the graph produced by Babcock & Wilcox the temperature of the interface tube-smelt-studs is much lower as the stud density increases. Permax panels have 80,90 or 100 studs/ft depending on the boiler operating pressure and maintenance free warranty required by the user.

ORIGINAL DESIGN VERSUS PERMAX DESIGN

A frozen layer will be stable if studs are present in adequate proportion to the amount of smelt AND if their geometry is maintained. Problems related to the welding procedure and to the stud design could not prevent the studs to become conical as shown below.

Several other issues added to the problem as for instance,

• Lack of standards to qualify contractors and operators plus the fact that stud welding has been perfected to a semiautomatic process spawned a large number of "specialists" lacking the necessary understanding of the whole operation. Allowing non specialized personnel to apply studs on pressure parts with equipment delivering 1000 A is extremely risky and has often resulted in ESPs.

• Lack of an "official" guideline defining when studs should be repaired. Premature restudding and lack of adequate repair procedures accelerates studs degradation and does not provide the needed frozen layer.

As shown above the edges of the stud collapse because they operate at temperatures that carbon steel cannot withstand. Modifying the boundary conditions of the studs resulted in a completely different wear pattern allowing the material to withstand the furnace conditions. This is achieved by controlled diffusion of chromium in the stud material. Below at left the chromium skin is seen after the stud is sectioned and etched. At right is shown one of the first field test results. These studs were simply cleaned, no grinding was done. That is the normal wear of a chromized stud, where geometry is maintained and the stud does not lose its ability to promote and hold a solid frozen layer.

The flat tip presented by the enhanced studs not only expedites its repair but grants a quality level never achieved before. In fact, instead of using the term restudding we prefer to refer to this type of repair as restoring.

Below we show key difference between the stud design and also between the welding procedures to apply them

Real Recovery Boiler tube with original studs and stud welded by Sage

Right side Sage studs in a 5 x 5 pattern Left original 4x4 Both "look good" but when scanned reality takes place. Gray means perfect bond

Resistance weld does not promote actual fusion between stud and tube as Sage's stud welding with drawn arc does. The results are seen in cross section and ... in the field.

The importance of a perfect metal bond between stud and tube is demonstrated in these pictures. Lack of perfect heat flow at the stud base will promote localized corrosion at the interface. Any restudding under these conditions causes more harm than benefit.

FURTHER ENHANCEMENTS

Issues related to microhardness at the weld zone led us to eliminate the chromium diffusion at the weld tip.

We also noticed that the stud remains intact up to the moment its "cap" is consumed. To maximize the cap's life the stud was redesigned to allow deeper diffusion at the top and also to create its own "frozen layer". The final design is shown below.

Ribbed area increases the heat absorption. The non ribbed zone tells the user that restudding is overdue. Spikes at top (not shown) grant the cap its own frozen layer. Weld tip is chromium free thus avoiding microhardness at the welded zone.

Higher densities of 90 and 100 studs/ft are feasible but they render inspection with ultrasonic probes very difficult. To accomodate this situation we have modified the studs distribution to create inspection lines already built into the panels. This forces the inspector to use the same reference line year after year and ensures that realistic wear rates are determined.

FIELD TESTED

Permax panels are gradually becoming a new standard in this "hard to compete" market. So far (2/06) seven boilers have Permax Panels applied and the field results speak for themselves. Pressures vary from 600 to 1200 psi. All deliver to their promise.