Duct and cover
Blame can be found in duct-fabrication problems, joint fractures, poor maintenance, system design and more. Contractors are getting some new options, however, as industry acceptance of newer systems and technology puts better alternatives within contractors' reach - possibly at lower cost.
Consider this situation: A general contractor hires a mechanical contractor to make grease ducts above a restaurant's grilling stations. Ten years later, a fire ravages the building during the dinner hour. Inspectors look for the culprit. Was the duct welded correctly throughout? Were clearances adequate? Were codes met? Did the duct fail at a weld that the inspector could not see? Was the system leak-tested?
‘Ingredients for disaster'Answering such questions could take years and may involve litigation. Contractors are left with the potential of a long, expensive battle.
Grease duct is a compact fire chamber with all the ingredients for disaster. A steel tube often lined with fuel - the grease - on the inside, with plenty of access to oxygen and an ignition source.
Cooking fires can harm or kill employees, restaurant patrons, destroy buildings and consume neighboring businesses. It is fast, ravenous and not picky about fuels - where grease is limited it will happily move on to wood and other combustibles.
Every vaporized or coagulated drop of grease is tremendously flammable. Fires can reach temperatures of 1,600 degrees F to 2,000 degrees F within minutes. They can melt aluminum and ignite surrounding materials within seconds.
"The duct system is but one component of the safety intervention associated with grease risk factor. But it's a crucial factor - it must be hermetic and designed in a fashion that it can be cleaned and doesn't leak," said Tom Johnson, owner of Johnson Diversified Products, a Minnesota-based consulting firm. Johnson serves on NSF International's Joint Committee on Food Equipment, as well as a user advisory council for Underwriters Laboratories.
Grease-duct systemsThere are three main types of grease duct. For years, a site-built sytem with fire-rated shaft enclosure was the only type allowed. This system uses rectangular, carbon-steel duct, welded on site, enclosed by a constructed, fire-rated shaft. Various building codes specify the construction. Building materials are easily accessible and the system can be customized on site. Warranties generally last one year.
Another type is the site-built, duct-wrapped system. First hitting the market in the early 1990s, this system is typically site-welded rectangular duct. The system avoids a constructed shaft in favor of insulation layers. This reduces code-prescribed clearances to combustibles, in some cases to zero. Seams in the wrap are taped or clamped. Warranties generally last one year.
The third type of grease duct commonly used is factory-built, insulated and integral-chased. Manufacturers of factory-built insulated chimneys soon realized the product they had been offering could easily replace the need for making and welding duct on site, wrapping it and constructing shafts. Insulated chimneys would also cost less and offer consistent quality.
With the chimneys, components are factory-made and inspected, and can be assembled liquid-tight on site generally without welding. The cylinder-shaped, stainless steel duct is wrapped with insulation and enclosed in a stainless steel casing.
Saving money"A big portion of job costs can be found in constructing shafts, welding duct and wrapping the duct," said Shaun Ray, director of engineering for Metal-Fab Inc., a Wichita, Kan.-based manufacturer of factory-built grease-duct products. "Factory-built products skip those steps. Contractors can assemble the duct, seal it and move on to the next job."
Factory-built products also save money, Ray adds.
"The cost-per-labor-hour can be reduced as well ... you don't need a welder to assemble most of these systems. You can focus your welders on other projects where their skill is crucial and bid a project more competitively," he said.
The systems are also reliable: Most warranties on factory-built systems last at least one year. One manufacturer offers a 12-year warranty. Most manufacturers offer one-hour, two-hour and three-hour fire-rated systems, as well as single-wall duct.
Despite these qualities, factory-built duct manufacturers faced problems early on. No building codes approved them and testing standards had not yet been developed. Contractors could not install the systems, since inspectors could not approve them.
Expanding metalThe problems caused by hot, expanding metal is significant even in the smallest of fires. Inconsistent welds in duct expand at different rates and can fracture or contort the duct system in the event of a fire. If the twisting tension or "torsion" is too great, the duct can separate itself from supporting braces. At best, the system must be repaired; at worst, a shaft is damaged or duct moves too close to surrounding combustibles.
"If a welder spends a half-second too long on one weld than another, it puts a metallurgical stress that isn't uniform on the carbon steel," said Johnson. "Pre-engineered systems are made by machines, no variability. More homogeneous mix of materials, concentric. They have a different structural integrity than a torsion box."
Leaking grease duct invites instant disaster. Leaks are most common at joints where welds may have been inconsistent or a fracture has occurred. They can also occur when adjoining components are not assembled and sealed according to manufacturer specifications.
More grease inside ductwork means hotter and longer-lasting fires.
"The closer we get to zero for a deposition rate, the safer the assembly," Johnson said. "The safest assemblies are totally insulated; where there is no temperature change in the sidewall. When you have a stream of air that is hotter than the duct wall, you end up with condensation and precipitation. It is basic physics. Just like stepping out of the shower in the morning and seeing condensation on the windows."
Cylindrical, stainless steel duct is easy to clean by crews equipped with rotary cleaning systems. Grease tends to naturally run down the inside walls and is pushed out.
Crews are easily equipped to clean welded rectangular ducts with a pressure washer or by scraping deposits where they pool in corners. Cleaning can also create or uncover leaks as welds are scraped or pressure-washed. A consistent cleaning schedule is essential for grease ducts.
ClearanceDuct-system installers must consider surrounding combustibles, especially when penetrating fire-rated walls, floors or ceilings. Codes and test standards specify these clearances. Contractors must deal with them daily; other mechanical systems, architectural obstacles and other space considerations can make an installer's day challenging. Standard duct-and-chase systems often are the most difficult installations due to meeting increased clearances as well as constructing chases in cramped places.
The structural effects alone of a grease fire must also be considered. After the fire is out, inspectors must investigate the failure.
"Any time we have fires in (rectangular duct), what happens is the fire is so hot it starts to implode the shaft itself," said Mike Baker, a senior plans examiner for the city of Peoria, Ill. "A lot of times we have problems with the hangers connecting to the shaft pulling away, causing holes and some of the walls to fail."
The costs incurred to replace a system are significant. However, nothing compares to the cost of fire that escapes confinement.
Factory-built manufacturers worked with various testing agencies in an effort to establish criteria for what they saw as better product. Perhaps the most widely accepted standard came in the form of UL's Standard No. 1978, "Standard for Grease Ducts."
Duct-and-chase systems need not meet any of these testing minimums. Rather, they conform to accepted code requirements that are largely unchanged from a century ago.
Local authorities have the ability to accept all or none of the contents of any testing standard, though they generally reference the common building codes. No single product is accepted everywhere.
The complexities of the issue make it expensive and labor-intensive for manufacturers to market products, which limit the choices installers have. Manufacturers must test to multiple standards and adhere to code requirements that vary by area of the country, some overlapping or conflicting, with various levels of protection.
"The lack of conformity or harmonization from one region to another gets confusing for the industry. The industry has been looking for one type of test for all products, a unified standard for a safe system and that simplifies the approval process," said Metal-Fab's Shaun Ray.
A new standardRecognizing that the existing standard does not test for systems penetrating a fire-rated floor, wall or ceiling, industry experts sought a solution. They wanted a new test standard that when combined with UL-1978, evaluates an entire grease duct system from hood to roof, regardless of fire-rated structure penetrations. A standard that code bodies could easily accept and contractors could reference when comparing systems. Underwriters Laboratories Standard No. 2221, "Tests of Fire Resistive Duct Enclosure Assemblies," was the result.
"UL-2221 is a test to determine the fire resistance of grease duct enclosures and is used to evaluate classified grease duct enclosures as required in NFPA (the National Fire Protection Association's) Standard No. 96 and the IMC (International Mechanical Code), " said Roy Meacham, a principal engineer for Underwriters Laboratories who deals with solid fuel, chimneys and venting standards.
The additional test standard is a rigorous one, so much so that few factory-built systems have earned the classification. It subjects a product to a more severe internal fire test, an external fire-engulfment test, and a through-penetration test. It requires that the tested sample retain 75 percent structural integrity.
The standard is under review for acceptance by the fire association and the International Mechanical Code. Sources are confident the standard will be accepted in some form.
"Uniform standards can only help the industry," Ray said. "Providing that systems are designed to meet the standards, better safety and more contractor choice will follow."
Contractors will not be limited to installing standard duct-and-chase systems. They can also install factory-built systems that are assembled and sealed on-site without welding. When comparing product quality and safety, contractors can look for UL listings. When installing, they need only follow manufacturer installation instructions and consider code requirements. If those requirements are met, an inspector can easily approve the job.
(This article was supplied by Metal-Fab Inc.)