A Different Perspective: Boiler Lagging on Hot Applications for the Steam and Power Industry

Gary Bases

Gary Bases is the President of BRIL Inc., an independent consulting firm specializing in brick, refractory, insulation, and lagging. He is also the author of The Bril Book (a complete guide to brick, refractory, insulation, and lagging systems); The Bril Book II (a technical manual that includes bril application drawings for the power-generating industry); The Bril Book III—the Book of Bril; and The Bril Book IV—Boiler Construction. He can be reached at brilinc@roadrunner.com.

May 1, 2015

A quality lagging installation requires an experienced work force that truly understands lagging design and applications. When properly stiffened in either the cold or hot position, correctly-installed lagging should last over 20 years. It can be difficult to find the expertise needed in lagging design and installations for high-temperature applications (above 350°F). As a result, there is a need for an industry-accepted standard for fabricating and installing outer lagging on steam-generating boilers and equipment.

While some may question the importance of lagging, it is a crucial component that is cost effective and plays a key role in thermal calculations (emissivity). Properly designed and installed outer lagging protects insulation, sheds water, and is aesthetically appealing. Outer lagging was first used on steam-generating boilers when the flat-studded boiler tube design developed between the late 1920s and early 1930s. The flat-studded tube design featured an inner steel casing welded over the top face of the tubes, thus separating the insulation from the refractory. This allowed outer lagging—galvanized steel at this time—to be used over the insulation instead of the commonly found welded or battened steel outer casing. Galvanized steel lagging was used predominately until aluminum rib lagging was developed around the early 1970s.

By the late 1960s boiler manufacturers (also known as original equipment manufacturers [OEMs]) started to developed their own lagging application standards. Past practices had allowed lagging supervisors or contractors a relatively free hand on construction techniques. This often resulted in inconsistency in design and appearance, as well as causing a natural disparity in labor cost that impacted profitability. Most of the boiler lagging standards were developed by Babcock & Wilcox, Combustion Engineering, D.B. Riley, and Foster Wheeler.

The lagging applications and standards varied between the different boiler manufacturers.

However, the one common thread of all the OEMs was the lack of detail provided on how to actually make or fabricate the flashing. All OEMs and construction companies assumed that the lagging installer or their own superintendents had the expertise to fabricate and install the lagging and flashing design requirements.

Lagging expertise could be found in most parts of the country from 1970 to the mid-1980s due to the sheer volume of boilers being built. For example, one boiler manufacturer alone built over 530 boilers during that 15-year period. The boilers sold by these OEM companies were mostly regional as the OEMs carved out their portion of the boiler industry. The work was plentiful and the lagging training was essentially handed down from one generation to the next. The expertise on how to measure, lay out, brake, bend, and fabricate was also handed down and not taught in a classroom or through an apprentice program. By region, the work force learned how to install lagging based on the standards issued by those OEM companies. In most cases that meant the labor craft had limited exposure to alternate ways of installing the lagging and flashing.

The standards issued by OEMs have created an inconsistency of design, appearance, and quality of workmanship around the country. Today, large lagging projects within the steam and power industry only exist where new air-pollution equipment is being installed on the existing power plant boilers. Between these projects, lagging work is sporadic at best. Most sheet-metal installers work in low-temperature applications such as heat ventilating and air-conditioning systems for most of the year. Working at power plants is a small portion of their install work. As a result, the work force is not familiar with hot applications (systems operating over 350°F) commonly found on steam-generating boilers. Again, this decrease of a knowledgeable work force along with shorter construction schedules creates a desperate need to establish industry-wide standards for proper lagging design and application.

The differences in the OEM standards also affected the insulation and its application but on a smaller scale. The changes between OEM standards for insulation were not just about the type of insulation being used, but also how it was to be attached. For example:

  • To go over external stiffeners, some specified 22-gage corrugated inner lagging versus road mesh with either 4″ x 4″ or 6″ x 6″, using J-hooks as the insulation pin.
  • How to insulate boiler walls—some required double layer while others required single-layer insulation.
  • The mineral wool board-type insulation density was different as some wanted nominal 8-pound density and others wanted minimum 8-pound density.
  • Insulation pin spacing—some required a 18″ x 21″ center spacing while others required 12″ x 12″ centers.
  • Some wanted all top surfaces to be designed for walking and others only required it on penthouse roof areas.
  • Some preferred 2 ½” square speed clips and others preferred 1 ½” round speed clips to hold the insulation in place.

The fundamentals for lagging that were clearly understood years ago are now forgotten or ignored. The following are some of the most fundamental standards rarely understood today:

  • Left to right is right;
  • Rib lagging first, flash behind;
  • Bottom to top;
  • Square to the world;
  • Do not let the metal be smarter than you; and
  • Use the tools of the trade.

Left to Right Is Right

It has been an industry practice to start rib-lagging sheets from the left and work to the right (left to right). This rule has been used throughout many industries including laying fire brick and installing insulation board (i.e., mineral wool board on boiler walls). Using the left-to-right method allows the rib lagging to be located at the same distance from the corner on different walls. Thus, different walls can be started at the same time with different work crews. Only the last rib-lagging piece may vary nearest the far corner.

Rib Lagging, First Flash Behind

This is a relatively new standard and is due to short construction schedules. Prior to the 1980s, construction schedules were twice as long as they are today. The OEM standards today show flashing installed behind the rib lagging sheets. This is impractical due to the shorter construction schedules. Years ago it was a common practice to flash first then install the rib-lagging. With today’s shorter schedule it is essential to get as many rib-lagging sheets on as possible. By keeping a minimum of 4 inches away from all penetrations, doors, or wall boxes, the rib sheets can be installed quickly.* The detail work of measuring the flashing around these penetrations can then be done “behind” or while the rest of the rib lagging is still being installed. With the rib-lagging sheets in place, it is easier to get exact measurements for layout and fabrication of the flashing. The flashing will now be installed on the outside of the rib sheets from high rib to high rib and the tops are protected with finger-tab flashing (which can be pre-made).

*Keeping back away from the wall box, penetration, or door will also require minimizing or eliminating the insulation attachments in this area. By eliminating or minimizing the number of insulation attachments (i.e., 12-gage insulation pins with 2 ½” square speed clips) will allow the flat lagging to lay flat against the insulation and give a better fit around the penetration.

Bottom to Top and Square to the World

When fabricating rib lagging or flashing for wall boxes, doors, expansion joints, etc., it is important to always work from the bottom and then work to the top. Equally important is to keep the lagging level (square to the world) from the beginning. As long as you start square or level, the rest of the lagging (sides, front, and rear) will also be square.

Do Not Let the Metal Be Smarter Than You

Rib or flat lagging takes special small tools to help make the job easier and the finished product more professional. When cutting metal, for example, if the metal binds, the snips get stuck, or you are struggling to make the finished cut, the metal is probably telling you that you are using the wrong tool. Using all of the proper tools makes for a better finished product and makes the work more efficient.

Use the Tools of the Trade

There are a lot of small tools associated with the fabrication of sheet metal. That is because the majority of the lagging work is finishing. This is where the true measure of quality workmanship can be found. Finishing work for the fabrication of flashing around doors, wall boxes, expansion joints, buckstay flashing, or furnace corners are almost entirely done at elevation. For any finishing work the measurements are first taken at elevation, then pieces are pre-cut to length and broken in the metal shop using a break and slitter. Then those pre-cut pieces are taken to elevation and cut, tabbed, and bent to fit. Following is a list of small tools that are typically used to fabricate all of those items. An experienced sheet metal worker will utilize almost all of the tools listed below on any given work area:

  • 11R locking clamp and small locking pliers—to hold flashing pieces together;
  • Utility knife—to score and cut a rib sheet lengthwise;
  • De-burring tool—to de-burr the rib sheet where it was cut to length around the high and low ribs;
  • Mill file—for de-burring metal;
  • 12″ T-square—for ensuring the metal is square and for scoring metal;
  • 3′ level—for ensuring the lagging or flashing is level;
  • Plumb bob and string—for keeping rib sheets and attachments in place;
  • 12″ dividers to scribe circles—for measuring and scoring sheet metal;
  • Small 10′ ¼” wide tape measure—for measuring the circumference of a drum or tank;
  • Folding ruler—for measuring at elevation;
  • Chalk and line—to ensure screw lines are level and straight and to make cut lines for sizing lagging sheets;
  • Drills and nut drivers—for installing sheet metal screws; and
  • Battery operated or electric hand saw—for cutting rib sheets to length at elevation.

The following is a list of tools needed to do finish or flashing work such as penetration, doors, expansion joints, and wall boxes:

  • Regular-pattern snips with green handle, (cuts right), regular-pattern snips with red handle (cuts left), and regular-pattern snips with yellow handle (cuts straight);
  • Offset-type snips with green handle (cuts right), and offset-type snips with red handle (cuts left)—to keep your hands farther off of the metal surface;
  • Bulldog-type snips—to cut heavy gage metal (i.e., 18-gage sub-girt used for lagging supports);
  • 12″ snips with red handle—the longer handle makes it easier to cut metal and gives you a longer reach;
  • Bender (3″ wide)—to make tabs;
  • Bender (locking type, 3″ wide)—to make tabs and hold flashing pieces together;
  • 11R locking clamp and small locking pliers—to hold flashing pieces together;
  • Utility knife—to score and cut metal;
  • De-burring tool—to de-burr the rib sheet where it was cut to length around the high and low ribs;
  • Mill file—for de-burring metal (i.e., rib sheets cut outs for doors and wall boxes);
  • Scribe scratch (all-punch type with metal handle)—for punching and scribing metal;
  • Sheet metal hammer (20 ounce)—for fabricating flashing and as a finishing tool;
  • 3′ level—for ensuring the lagging or flashing is level;
  • 9″ level—for ensuring the lagging or flashing is level;
  • 12″ T-square—for ensuring the metal is square and for scoring metal;
  • 12″ dividers to scribe circles—for measuring and scoring lagging for locations of penetrations;
  • Folding ruler—for measuring at elevation and for determining angles;
  • Small 10′, ¼” wide tape measure—for measuring the around pipe penetrations;
  • Chalk and line—to ensure screw lines are level and straight; and
  • Drills and nut drivers—for installing sheet metal screws.

It is vital to have current, updated lagging designs in order to get a quality lagging project that is appealing to the eye, sheds water, and is properly stiffened. The industry as a whole needs to establish some acceptable standards to avoid inconsistencies that costs the end user money for repair and re-works. The industry also needs a trained and educated work force that understands how to install lagging on high-temperature flue, duct, and boiler applications. This will save the industry money and, more importantly, protect the insulation installed under the lagging, keeping the areas thermally and energy efficient.