Composites USAA Critical Process Systems Group Company

High Quality Exhaust StacksManufactured to yourSpecifications!

FiberglassStacks

Fiberglass Stacks

Fiberglass stacks can be manufactured and installed inmuch the same way as steel stacks. The advantages offiberglass stacks are similar to the advantages of otherfiberglass products: corrosion resistance, lightweight -leading to lower installation costs and ease of mainten-ance among the primary benefits.

Stack Design Considerations:

Proper stack design begins with an understanding ofthe process exhaust stream (volume, corrosiveness,temperature, flammability), local environmental re-quirements (height requirements, sampling require-ments), local and regional building code requirements(height requirements, wind, seismic, snow loads), anddesired geometry (gas entry, base configuration, lateralsupport).

Composites USA has manufactured and installed fiber-glass stacks for years. Some examples of our stacks areshown on this and the following pages.

Stacks can be designed either freestanding or laterallysupported. There are many possibilities available forbase mounting design. Similarly, there are many waysto laterally support stacks if desired. The most commonoptions for lateral support would include guying withwires, or bolting to adjacent structural steel. With adescription of the design criteria and constraints im-posed by the physical location of the stack, CompositesUSA can offer a P.E. stamped design solution.

In addition to designing the stack structural require-ments, Composites USA can also offer design guidanceand installation options for ladders, platforms, sampling,inspection, and access ports, grounding systems, and ifrequired, FAA markings and warning lights.

The strength to weight and corrosion resistance offiberglass reinforced plastic make it an ideal material ofconstruction up to the temperature limits of the resin(250 - 350°F depending on resin and service).

In general, the stack must first be designed for the pro-cess, and then designed mechanically and for accessoryrequirements. The process will dictate location andsize. The mechanical analysis will take into accountloads contributed by the stack dead weight, seismic andwind loads, buckling and bending moments, guy wirecompression and temperature gradients to name a few.A brief discussion follows:

The location and height of the stack is greatlyinfluenced by the location, size, and configuration ofthe surrounding buildings and topography.

Airflow over a building creates a positive pressurezone on the upstream side of the building and a nega-tive pressure zone or cavity on the roof and down-stream side. Although the contour of these zones willremain relatively stable as the wind velocity changes, the contour zone and cavity envelopes can be erratic in shape due to wind turbulence.

High stack discharge velocity and temperature increaseflume height and thus effective stack height. This inturn aids in dispersion of stack contaminants, reducingground level concentrations. The effective stack heightis the sum of the actual stack height, the plume rise dueto the exhaust velocity, and the buoyancy rise, which isa function of the temperature of the exit gases and theatmospheric conditions.

Wind flow over stacks creates negative zones andeddies behind the stack in the same manner as airflowaround buildings. Low stack discharge velocities permitthe effluent to be drawn into the stack eddy zone andmay cause down wash. This reduces the effective stackheight and may cause the effluent to enter the buildingcavity, even though the discharge may be well abovethis cavity.

The stack to wind velocity ratio should be at least 1.5to 1. Under these conditions, the effluent will breakcleanly from the stack, down wash will be eliminated,and the effective stack height will be maximized. Inmost cases, a stack discharge velocity of 3,000-4,000feet per minute will provide good performance.

The guidelines above are satisfactory for most cases.However, federal, state and local environmental andzoning agencies may dictate stack height and location.In some cases the Federal Aviation Agency (FAA) reg-ulations will govern some aspects of stack location orconfiguration. See the photograph on the first pagefor a view of FAA stack markings.

Wind loads are calculated by ASCE guidelines. Seismicloads are calculated in accordance with the applicablelocal building codes (BOCA, SBBA, or ICBO in the U.S.).These form the basis for the mechanical designs. Thedeflection at the top of the cantilever is limited to length(inches) / 180 per recommendations of the SPI. This isusually confirmed by finite element analysis of the stackso local reactions and bending moments can bedetermined.

Stacks may be designed either free standing, with alllateral support provided by the main stack support (ie.base flange or lugs) and stack shell, or with a combina-tion of base and side lateral support. Side lateral sup-port may consist of guy wires, hard connections tobuildings or structural steel, or some other similarmethod. Use of lateral supports can often simplify thedesign of the stack and stack base, but this is not alwaysthe case. An analysis of the design conditions and stackgeometry will often need to be done with and withoutlateral support to determine the most economical over-all approach.

External stiffeners may need to be added to the shell tohelp resist buckling. An analysis of all forces acting onthe stack will determine these requirements. Guy wires,when used are recommended as stainless steel aircraftcable, set at an angle of 45°.

Vortex shedding occurs at the sides of the stack andcauses force transverse to the wind flow direction. Thestack dimensions determine the critical wind speedconditions at which a stack may resonate. The stackmay require aerodynamic devices such as strakes or ashroud to disrupt the formation of vortices, or dampingdevices to absorb the dynamic energy.

Location:

Stack Lateral Support:

Stack Dynamic Analysis:

Wind & Seismic Loads:

Exhaust stacks are manufactured in a wide variety ofconfigurations and supported in just as many ways.Stacks may be open with bottom entry, or closedbottom with side inlet. The stack may be designed forsupport at the base or along the sidewall, dependingupon the stack size and physical constraints of theinstallation.

If an open bottom design is chosen, the choiceusually comes down to supply with either a flange ora plain (raw) end. If a flange style is chosen, it couldbe possible to support the stack off the flange. Openbase stacks are generally chosen where short stacksections are required to mount directly off the top ofa piece of equipment, such as a fume scrubber, or offthe discharge of a vertical discharge fan.

Closed base stacks may be supplied with mountinglugs similar to those supplied on process tanks, orwith flanged bases. Flanged bases, in turn, may besupplied as stand alone drilled flanges, as gussetedflanges, or as double flanged bases with gussets.

In all cases, the stack base will be designed for theappropriate number and size of hold down bolts byfirst calculating the maximum overturning load.Using this value in combination with the stackdiameter, the load-carrying requirement for each boltcan be determined. Iterations may be performedwith varying numbers of hold down bolts to achievean optimum balance of size vs. number of bolts, andspacing on the concrete pad. In all calculations, novalue is assigned to the bolts in compression on thedownstream side of the neutral axis (opposite thewind). The entire load is divided among the boltsconsidered to be in tension.

In a similar manner, the base flange or hold downlugs are designed to satisfy the appropriate tensionor compression loads imposed by the wind loadsspecified. Calculations for base flange thickness aremodeled on relationships and equations specified inASME RTP-1, ASME Section X, and other relevantcodes.

Sampling ports are quite often installed on stacks toprovide measurements for mandated emission testing.Two ports, installed at 90° to each other, are locatedeight to ten stack diameters above bends or inlets,and at least two stack diameters below stack exits.Sample ports can be any size, but are typically 3" or4" diameter with flanged or threaded ends, with blindflanges or plugs as appropriate.

Access to sampling ports can be off adjacentbuilding steel, if available, or off a platform andladder supplied and installed with the stack.Platforms are designed per the requirements ofOSHA and building codes. Three rail (top, mid,toe) systems with a minimum 3'-0" wide gratingplatforms are typically supplied. Support clips offthe stack are designed and reinforced for loadsimposed. Access to the platform can be by aladder, caged per requirements, supported off thestack. Materials of construction for ladders andplatform systems can be fiberglass (as shown), steelor alloy.

Stack materials of construction need to beevaluated for long term performance with theanticipated corrosive contaminants at temperature.Generally speaking, most fiberglass resin materialused for tanks and piping will perform very well forstacks as well. Additional guidance in this area canbe obtained from any of several publishedcorrosion tables, including the one at the end ofthis design guide.

Stack Base:

Stack Sampling:

Access Platforms:

Corrosion Requirements:

All stacks should be manufactured using fireretardant materials. Many fiberglass materials havebeen tested and classified per the ASTM E-84 tunneltest. Many resins have successfully demonstrated aClass 1 flame resistance (measuring <25 with redoak = 100 and concrete = 0) by the measurementsof this test. Every effort should be made to use oneof these resins.

Consideration should be given to the installation ofa liquid flush or sprinkler system. This is particularlytrue if there is a tendency to build up large cake ordust deposits in the stack, if oxidizing substances suchas sodium chlorate or potassium permanganate canbe deposited on the wall, or if process upsets canproduce a gas stream that is potentially combustible.

Lightning bolts seek the path of least resistance. De-pending upon the stack size and location, a lightningprotection system might be considered. If a lightningprotection system is desired, the points, conductors,fasteners, air terminals, joints and ground can befactory supplied and installed by Composites USA.

For tall stacks, aircraft warning bands or lighting maybe required or advisable. These may be suppliedand installed at Composites USA in full accordancewith FAA requirements. Lighting systems, wheresupplied, are provided with manual or electriccabling and winch systems to allow servicing fromground level.

Stack gases are very often saturated vapors. It iscommon for stack gases to cool and condensatethese vapors into a liquid condensate whileoperating. In addition, driving rains can often allowwater into the stack from above. For both thesereasons, drain connections are usually installed inclosed bottom stacks. Some stacks incorporate asloped bottom, similar to those on some storagetanks, to facilitate full drainage.

Access to the base of the stack for periodicinspections and maintenance is recommended.Inspection ports can take the form of flanged andbolted manways, or flush mounted covers withhinged and lockable doors.

Rain caps of various styles are often incorporatedto minimize liquid entering the stack (rain, snow).In addition to rain caps, drip rings are sometimesincorporated to catch and channel any stackcondensation down to the base of the stack and outof the plume.

Finally, where considered necessary, stack back draftdampers may be installed to prevent unwanted airfrom being drawn from outside through non-operational fans or process equipment.

Call Composites USA for Design Assistance& Recommendations for Your Stack!

Flammability:

Aircraft Warning:

Electrical Conductivity & LightningProtection:

Condensate Handling & Maintenance:

Since 1982, Composites USA continues to execute on the demands of manyof the world’s most discerning and demanding users. From intricate and complex parts to multi-million dollardesign/fabricate/construct schemes, we invoke hundreds of years of combined experience and technologicalsuperiority in each and every initiative undertaken.

With industry recognized Approvals and Listings from Factory Mutual, Underwriters Laboratories, and the U.S.Government, Composites USA is well positioned to be your sole source provider for reinforced plastic needs.

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