Writing a Lagging Specification
The primary cause of insulation system failure is improper installation and/or design of the outer lagging. If insufficient consideration is given to the lagging in the design stage, the result may be a lagging system that is improperly installed. This is why the power industry will spend more than forty million dollars in the next five years reworking existing insulation and lagging systems on new air pollution systems. Good lagging practices must be enforced from the design level to the finished product, and it all begins with a good lagging specification.
Some readers may be familiar with “Writing an Insulation Specification” (published in this magazine in July 2005). The procedures for developing a lagging specification are very similar to those procedures. Similar to an insulation specification, a lagging specification should do the following:
- Convey to the lagging installer, clearly and without exception, the intent of the specification.
- Have enough information necessary for the installation to proceed without stating what is not required or necessary.
- List only the things required for the proper installation of the lagging system to the scope, as defined in the specification.
- Work hand in hand with the insulation system being utilized.
- Be broken down into four major divisions: General Conditions, Scope Definition, Material, and Application.
- The General Information section states what the job conditions are, such as storage, warehousing, and specific responsibilities.
- The Scope section sets the boundaries within the specification.
- The Material section states clearly what materials can be used.
- The Application section states clearly, either by written specification and/or application drawings, the minimum requirements and accepted standards for applying the material.
In an insulation specification, the amount of information required for a proper install can be somewhat minimal. Insulation specification contents tend to provide just enough to give the installers the flexibility to make some choices on the external stiffeners (hump, bury, or use an inner support). However, in regard to the physical application of the material (attachment spacing, gaps between the insulation, double layer versus single layer), the insulation specification must state clearly what is required for a proper application, with no room for deviation.
When it comes to a lagging specification, more detailed information must be included, with much less flexibility given to the installer. The industry has lost a lot of expertise in installing lagging on hot applications (over 350°F). Lagging installation requires the installer to have imagination and skill, very much like the carpenter who trims out a house. You would not want an inexperience person trimming out your new home. Similarly, it takes an experienced craftsman to design and install lagging on an air pollution system operating at 700°F. The individual must consider aesthetics, watershed, weather protection for the insulation under the lagging, and the expansion that sometimes occurs in two directions. Also, due to the short erection schedules, many air pollution projects require the lagging to be installed while the unit is in operation.
Unfortunately, the current lack of experience appears to be industry-wide, due to the prevalent use of nonconventional labor in the application of sheet metal, lack of training related to good lagging practices within the industry, lack of accepted industry standards for installing lagging, and lack of major capital improvements or new boiler work in the power industry. It is only in recent years that the power industry has had major projects that involve hot flue, duct, air pollution equipment, or new boilers. Since major projects are few and far between and involve shorter outages, the number of people experienced with the hot systems associated with a power plant is diminishing. When a major capital improvement project takeplace, the workforce pool must include those who have little or no experience working at power plants on equipment that will be operating at elevated temperatures.
These issues potentially cost the power industry millions of dollars, which is one reason why the American Society of Mechanical Engineers (ASME) just announced a new Continuing Education Institute short course on lagging. It is recommended that all plant managers, plant engineers, plant operators, maintenance personnel, professional and service engineers, and installing contractors take this course.
Like the insulation specification, the lagging specification—when properly prepared—will be used as a guide for the installer. It should include a quantitative takeoff of both the lagging and the insulation being installed. Lagging may be the last thing installed, but it must be the first thing considered when designing an insulation system. The lagging specification always must take into account the insulation system being utilized and the operating conditions of the equipment being insulated and lagged.
In years past, it was common practice for original equipment manufacturers (OEMs) to include only a few general lagging application drawings with their specifications. The OEM companies assumed—and rightly so—that those installing the lagging would be able to do so properly taking into account expansion, watershed, and aesthetics. Unfortunately, that was then, and this is now. Today, a good lagging specification must include application drawings that clearly show how to flash around doors and penetrations, install corner flashing, deal with watershed, account for expansion in long vertical lagging surfaces, or install lagging when the unit is in operation.
Sample Lagging Specification
Good lagging installation begins with the specification. Here is one example of a complete lagging specification without the drawings.
These specifications cover the lagging material and application for the hot selective catalytic reduction system (operating at 700°F), including new flue work from the boiler gas outlet to the air heater gas inlet. All materials and their application must be in accordance with the following pages and in accordance with any and all reference drawings.
The installer shall perform all phases of work and supply all materials required in an acceptable manner, as shown on the drawings and documents submitted with these specifications. The quantitative takeoff provided with these specifications is intended as a guide solely for the purpose of defining the scope of work. It is the installer’s responsibility to verify material types and quantities. Nothing in these specifications relieves the installer of the responsibility to verify quantities or types of materials being provided.
Materials furnished by the installer shall be unloaded, stored, and weather-protected by the installer until applied. On coastal jobs, even with inside storage, contamination and discoloration can occur from atmospheric condensation between sheets when laid flat. Routine inspections should be carried out; if any condensation is found to be forming, sheets should be separated and ventilated.
The installer shall furnish all supervision, labor, tools, and equipment—including, but not limited to compressed air, scaffolding, dry storage, office, and change room facilities—necessary to install all materials furnished under these specifications.
All materials being furnished under this contract must be asbestos-free. The company shall reject any and all asbestos-containing materials.
The customer’s premises must be kept reasonably clean during construction and broom clean prior to leaving the job at completion.
The successful installer, afterward, shall submit a final as-built bill of materials listing the quantities and sizes of the materials used/required to complete the lagging requirements. This bill of materials should include the material manufacturer’s name and extra quantities included for wastage.
All materials required for proper support and fastening of the lagging shall be supplied and installed by the lagging installer. This will include, but not be limited to clips, bands, studs, angle iron, screws, etc. All attachments must be installed on proper or specified centers for the support of the lagging materials. See win loading chart below.
On systems operating above 350°F, the lagging must be installed in direct contact with the insulation wherever possible. When the lagging is not installed in direct contact with the insulation, expanded metal 1½” x 18ga c.s. must be installed over the face of the insulation.
Rib lagging should always be used wherever possible in lieu of flat sheet because it helps in expansion and contraction of the lagging system. Flat sheet, in most cases, should be used for flashing, round surfaces, elliptical or irregular surfaces such as drumheads, and areas where space prohibits the use of rib lagging.
All flashing shall be cut to fit and fabricated in the field from flat sheet or coils. No prefabricated corner flashing should be used.
Expansion must be incorporated into the lagging design. All lagging should be applied over insulated areas so as to present a true plane, stiffened and fastened on adequate centers to prevent excessive deflection or “oil canning” when hot or cold. Necessary provisions for expansion and contraction must be provided to maintain a neat and proper design when in service.
All lagging supports (sub-girt, Z or H type support, angle iron) should be installed utilizing short spans, with a maximum length of ten feet wherever possible. A small gap must be left between support assemblies equal to the amount of expansion expected in that direction. In no case should the short sections of the support system be welded or screwed together.
When caulking seals the lagging system, it must not restrict or hinder the contraction or expansion of the lagging. The materials used for the caulking must be consistent with temperature limitations and expected expansion and contraction to be encountered.
All the lagging attachments used should always be spaced in set patterns, both vertically and horizontally, to present a uniform appearance. They also should be located on adequate centers to prevent rattling or “oil canning” due to expansion or vibration, Insert MISSING TEXT HERE
Expansion joints located in the flues will take up most of the expansion or contraction of the equipment being lagged. The lagging over the expansion joints should be designed so as to provide movement equal to the expansion of the joint covered.
For lagging support spacing on any given area, the maximum allowable spacing should always be utilized. (See typical wind loading and support spacing charts)
Ribbed type lagging material shall be clad 3004 alloy. The cladding is to be 7072 alloy. The core material shall conform to ASTM B-209. The depth of the ribs shall not exceed 2½ inches overall. Lagging material may be furnished in precut custom lengths or in standard mill lengths.
Flat lagging material shall be clad 3003 alloy. The cladding is to be 7072 alloy. The core material shall conform to ASTM B-209.
All exposed surfaces of the lagging material shall have a stucco-embossed finish unless otherwise specified in these specifications.
All lagging materials shall be of such temper so that the lagging material shall bend flat on itself without cracking.
- Temper for the rib type lagging shall be H-174.
- Temper for the flat lagging shall be H-154.
- The mill finish flat sheet shall have a temper of H-14.
- The thickness of the rib lagging shall be a minimum of .032 inches.
- The thickness of the flat lagging shall be a minimum of .040 inches.
The lagging system must be of a design and thickness so as to withstand a minimum of thirty pounds per square foot section or pressure wind loading over a single span. Fasteners shall be spaced with a maximum spacing of three-foot by two-foot centers. Stitch screws should be installed on twelve-inch maximum centers.
All attachments (such as clips, bands, studs, screws, etc.) shall be supplied by the installing contractor. The screws shall be zinc plated, stainless steel #14, with hex head, type A-B, or tek type.
A lagging specification must convey to the installer, clearly and without exception, the intent of the specifications and must include detailed lagging application drawings. These detailed drawings will help an inexperienced workforce understand the level of quality required when installing lagging on flues, ducts, steam-generating boilers, and air pollution equipment. The right lagging specification is the first step to a good installation. If little consideration is given to the lagging in the beginning of the insulation system or equipment design, the result may be a lagging system that is improperly installed. Good lagging practices must be enforced from the design level to the finished product.
Lagging may be the last thing installed, but it must be the first thing you think about when designing flues, ducts, air pollution equipment, or insulation system—and it begins with a good lagging specification.