Welding Fume Control

Close

The Welding Fume Problem Fume control in the welding and cutting industry is a continuing challenge for design engineers, plant engineers and environmental safety enaineers. The varietv of weldina " pr&esses and materhls, the mobility of fume generation points and the span of possible installations (from a school shop booth to a high bay open fabrication shop) make this challenge particularly difficult.

This brochure addresses the problems, principles and products used by AAFISnyderGeneral to successfully control welding fume.

- - - - --

Welding Fume Generation

MOLTEN METAL

FUME

Welding fume consists of particulate I I solids and gases generated by the

-

base metal, electrode. various fluxes and shielding gases. Metallic oxides produced by the condensation of constituents of the visible plume. melted metal are the prime These oxides consist of ultra-fine

particulate matter and normally are what is considered "weld fume."

Typical Welding Contaminants Sources of lnformation on Welding Exposure Limits lnformation on exposure to hazardous condittons IS

published by three primary organizations:

American Conference of Threshold Limit Values Governmental industrial Hygienists (ACGIH)

(TLV)

Occupational Safety and Permlssable Exposure Health Administration (OSHA) Limits (PEL)

National Institute of Recommended Exposure Occupational Safety Llmlts (REL) and Health (NIOSH)

ACGIH aublishes a wide selectton of manuals devoted to all phases of hazardous substances and conditions Recommended publications available from ACGIH include:

Industrial Ventilation: A Manual of Recommended Practice

A widely used source of information in the area of industrlal ventilation system design, construction, and evaluation. Basic ventilation principles and sample calculations are presented.

Threshold Limit Values and Biological Indices

Used worldwide in recognizing, evaluating and controlling exposures to hazardouscond~tions. ~ec~mmendations - or auidelines are Dresented for over 600 chemical suktances, phy&cal agents, and biological exposure determtnants.

Gu~de to Occupatmal Exposure Values

Annual reference guide that presents the current TLV's from ACGIH; PEL'S from OSHA; and REL's from NIOSH.

Health Aspects

The health aspects associated with welding and cutting are complex and the industry is continuing its research to evaluate the effects of the welder's exposure to typical con- stituents of welding fumes and gases. Regardless, it makes good sense to minimize exposure through effective weld fume control. The table on page 2 lists typical fumes and gases and their possible health hazards. Not every welding process will involve these materials and some, such as iron oxide, the main con- stituent of steel welding fume, are merely classified as nuisance dust. Other contaminants, such as zinc oxide from welding galvanized steel, produce metal fume fever. Nickel and chromium, by-products of stainless steel welding, are suspected carcinogens in certain forms.

The gaseous by-products vary wlth the welding process. in general, they are not considered a major environmental problem. Ozone, for example, is generated in arc weldlng but breaks down rapldly. Normally, it IS not a concern except in aluminum welding where ozone IS produced in considerable quantlty. Nitrogen oxide is released in gas metal arc welding and flame cutting but dilutes rapidly with normal ventilation.

Welding or cutting of coated metals can produce a variety of contaml- nants, some of which may be harmful. Painted metals can release pigment or organic by-products from lead to chromium, to carbon monoxide and carbon dioxlde. Galvanized metals produce zinc oxide, while plated metals may produce cadmium, chromium, nlckel or copper. Coatings such as anti-spatter sprays, oils, and degreaslng solvents can generate complex organic gases.

Particle Size

Fume partlcle size IS Important because of health and environmental alr quality control aspects. Generally, fume is agreed to be submicron in size with average diameters from 0.3

to 0.7 microns. Normally there is minlmal lung retention of particles in this size range. However, thermal effects can cause agglomeration resulting in partlcle chains and clus- ters exceeding one micron wh~ch can be deposited In the human respiratory tract.

Particle size is especially important to pollution control engineers since it affects the behavior of the contami- nant and the type of collector necessary to remove it. Submicron fume particles of metallic oxide have low mass and are easily influenced by air movement, so lower capture and carrying velocities can be used to control them. However, it is more difficult to filter fine particulate, especially submicron particulate. High efficiency air filters or dust collectors are required; but the demands of the application, higher particulate loading and rugged industrial environments further restrict the type of equipment that can be used. The most successful types of equipment are cartridge col- lectors, fabric collectors and elec- tronic air cleaners.

Electrode Classification

Fume Generation

Of the varlous weldlng processes (arc welding, resistance weldlng and gas welding), arc welding IS the most prollflc fume producer. Associated processes such as carbon arc gouglng and plasma arc cutting often produce heavier fume concentrations, while submerged arc welding and water table flame cutting produce very Ilttle.

The quantlty of fume generated by the arc welding process IS a function of the particular process, size and type of electrode, amperage and operator proficiency. Most of the fume particles come from melted electrode. The electrode and its flux melt at the t~p. They are displaced under the force of the plasma gas stream, leave the site and condense into fume, which IS seen as the weld plume. The larger the electrode and the higher the amperage, the greater the quantity of fume generated. The Amencan Welding Society has published fume generation rates for varylng conditions whlch can be helpful In designing fume removal systems.

Fume generahon rates

.Fumesand Gassss in zhe Weiflog Envimnmenl. Amenam Welding Sacsac~ly Mmm Fionda

Welding Fume Control

Fume control IS used to protect the welder, others in the vicinity and the physical plant and equipment from the by-products of the welding process. Source capture is the recommended technique for effective fume control. Weld fume should be collected at or near the weld zone to prevent it from entering the breathing zone of the welder, contaminating air breathed by others and from deposi- tion on plant equipment and building surfaces. This may be difficult because welding is often a mobile process precluding the use of f~xed hooding. In addition, it is often done in areas where hoods and ductwork are impossible because of overhead cranes and other obstructions. Regardless, close capture with well designed hood systems is always preferred.

Where hoodmg is not appropriate, other techniques have been used with varying degrees of success. If properly applied, low-volume, high- velocity suction nozzles and hose systems can provide good source capture. They require operator placement and control, however. Another close-capture technique is the use of a capture device directly on the welding gun. These suction devices will remove fume at the source if properly used and main- tained. However, they require opera- tor acceptance and cooperation.

In those cases where fixed, flexible or high-velocity suction devices are

not appropriate and the fume is allowed to disperse throughout the plant space, fume control is still possible through general ventilation techniques. In this situation, it is essential that the welder not breathe in the weld plume. In a well designed general ventilation system the plume will be displaced upward by design alr flow patterns and drawn into the air cleaner intake.

Design flow velocity across the weld zone should not exceed 100 ft. per minute to maintain the gas shield.

Direct Capture When welding in a confined location direct capture fume control should be used. The design of the pick-up device will depend on the details of the process. The flow pattern should be selected to avoid the welder's breathing zone. Fixed local control devices can be either flexible or stationary. Flexible hoses or duct sections with intake nozzles normally are positioned near the weld zone by the welder, and their effectiveness depends on the welder. Fixed control equipment covers a variety of hood arrange- ments from a simple slot side draft pick-up to a complete overhead canopy. In all cases, the behavior of the plume should be considered. In general, the plume rises under

thermal pressure directly from the weld site, is influenced easily by drafts and may vary as the welder changes position.

Design Criteria Hood design data presented in the Industrial Ventilation Manual* should be used. High-velocity slot hoods can provide good capture velocities at welding tables or welding bench- es. Rear or side locations will direct the plume from the welder. Lower inlet velocities in open, tapered or cone hoods may allow some fume loss, especially if the hood is above the welder. Large open welding tables where access is required on all four sides can be sewed by canopy hoods. This also applies to larger, but confined, automatic weld- ing production lines. Through the use of baffles and dampers or peripheral slots, canopies can be

designed to minimize air volumes and maximize inlet velocity. Regardless of design consideration, hood efficiency never will be 100 percent nor will the fume generation location stay at the design point. Some fugitive fume will escape into the general plant space. In situations where this is of concern, auxiliary general ventilation collectors should be positioned lo capture fugitive fume and maintain a cleaner environment. Direct source capture is the most effective means of controlling weld fume. With good pick up design, fume can be prevented from entering the worker's breathing zone or adja- cent areas. Air volumes also can be minimized to reduce the cost of col- lection equipment. In general, weld fume is easy to capture, easy to transport, and can be collected with appropriate air cleaners.

C

. , FWIBLE DUCT

L3' FLANGE

able Exhaus

Booth - Type Hoods (Same Principle Applies to Canopy Type

/?Am VELOCITY rCI 8WO FPM OR HIGHER

SldeDraft and Suspended Hoods

General Ventilation General ventilation or area control is often the only alternative in large fab- ricatlon bays where overhead cranes and large weldments preclude fixed or mobile local control devices. This means the entire volume of the space berng served must be considered in determining the capacity of the air cleanmg system. Overall volumetric flow rates will be much greater than those used for direct capture. It also means that good flow patterns must be established to induce the fume upward into the system.

It may be possible to combine the air cleaning system with the general heating, ventilating and cooling system, thus combining fans and air distr~bution systems while ensurrng the cleanltness of the heating and cooling coils. Air cleaning equipment often is added to existing air handling systemstoaccomplishthesamething.

The general ventilation approach depends on air movement to con- stantly flush the space. The greater the volumetric flow rate, the greater the potential for complete removal of the weld fume. Because of the large air volumes involved, these systems should be designed to minimize pres- sure drop and maximize maintenance convenience and accessibility.

Sizing a general ventilation system involves an analysis of the space being served as well as the type and level of actwity within the space. For example, by defining the type and amount of welding fume, generation rates can be calculated. Along with other air calculations, these will determine the capacity of the requred air cleaning systems. Since these calculations can become complex, a simple air change approach often is used. The volume

of the building becomes the key variable with 'X' air changes per hour chosen on the basis of the level of contamination existing in the space. For example, 800,000 cubic feet at six air changes per hour (or one complete volume of the space every I0 minutes) would require 80,000 ACFM of air movement.

A more precise approach to deter- mining general recirculation capacity is to calculate the air volume using actual job data such as fume generation, exhaust or exfiltration air volume, and the target air cleanli- ness. Such an approach uses a simple mass balance equation where the equilibrium or steadv-state is achieved between the fume generated and the fume removed through air cleaners, exhaust and other factors. At this point, the concentration

of fume in the general plant air levels off. This simple modal implies constant fume generation, no fallout or deposition and perfedt air mixihg These factors can be induded if data is available.

The steady-state premise Is that fume-in isbalanced by fume-out, a relationship expressed by the equation:

F=QC Where F = fume generation rate;

Q- volume of air leaving the space; and

C =fume concentratlon.

Since the presumption is that the steady-state concentration is unacceptable and must be reduced to an acceptable level, called C', the steady-state equation must be expressed:

F=QC'+EQC' Where Ch= target concentration;

E = air cleaner efficiency and Q'= recirculation air volume.

Recirculation air volumes to lower the fume concentration level can then be determined by combining the two previous equations:

QC' + EQ'C' = QC Q7 = QC-QC'

EC'

,,I fffiers (Iefi) and elecimnic air cleaner (right) Y,-,r and recinrulaie air -., .,, we!dmg lines In an automot7ve plant.

With this equation, it is simple to calculate the volume of recirculation air at a given efficiency to reach a new steady-state concentration, if

1

Q = 50,000 CFM (seecurvebelow)

the exhaust volume and original The simplicity of these relationships steady-state concentration are is such that they can be expressed known. For example, given a plant graphically, and this is the kind of exhausting 30,000 CFM (849 intormation used by sales engineers ms/min) of air at a steady-state con- in sizing recirculation projects. centration of 5 grains per 1,000

feet (28.3 m~lmin), what ~ ~ 1 . Where it is impossible to determine ume of ~r is required at an air clean- steady-state concentrations, as ing efficiency of 90 percent to obtain would be the m e if a building were a new steadystate concentration of in the design stage, the approach 2 grains per 1,000 cubic feet? differs slightly, but the same basic

mass balance approach is used.

EXHAUST AIR FROM WELDING AREA (CFM)

AAF Air Cleaners for Welding Fume Control SnyderGeneral manufactures all three types of air cleaning equipment used for welding control:

Cartridge

The OptiFlo system is a completely modular design that allows an unlimited range of sizes Modules can be interconnected to accommodate the largest air cleaning task The compact modules consewe valuable space.

OptiFlo units have the lowest Bange- to-flange pressure drop, allowing up to 10% greater air flow with lower fan horsepower than competitive models. The OptiFlo design permits free-fall of dislodged weld fume particulate into the hopper without direct

-Cartridge Collectors -Fabric Collectors -Electronic Air Cleaners

impingement of contaminant on the cartridaes minimizina abrasion and - dust b;ild-up.

Raplacement of the oartrklges is from outside the collector so personnel do not have to enter the unit. No tools are needed. OptiFlo cartridges have only one gasket sealing surfm significantly reducing the potential for leakage. Continuously welded tube sheets ensure no fume bypass. Bag- InIBag-Out is a standard option.

A wide selection of cartridge types, options, and accessories enable the collector to be tailored to specific application requirements. Choose from top or side inletloutlet arrangements.

S~eBuNefin APC-1-102.

External access simplifies mrtridQe replment

/ I 55-gallon drum A c o ~ g 8 ermer:

to removelreplace -,

Design S PulsePakM

The PulsePak Design S is a compact, self-contained cartridge collector designed for lower air volume applications. The cartridges are automatically cleaned by reverse pulsing allowing continuous duty operation.

A wide variety of arrangements and sizes are available with capacities up to 4000 CFM. The units handle up to 35% more air flow capacity and use up to 113 less horsepower than comparable models. The Design S PulsePak is designed to serve a single source or a ducted system of multiple sources.

The high efficiency pleated filter cartridges are made with a blend of non-woven media. Optional medias are available for specialized appbcations. Cartridge replacement is an easy task with full size doors allowing complete access to the cartridge compartment. High efficiency final filters are also available for recirculating processed air back into the plant environment. See Bulletin APC-1414.

Fabric Collectors Design M FabriPulsea

The Design M FabriPulse is a compact, pulse jet fabric collector designed for continuous duty operation with low air volumes.

This collector can handle the heavi- est loads using "mini tubes" of fine denier Scafitex polyester fabric. Scafitex bags operate with low pressure drop and have excellent dust cake release.

The pulse jet cleaning operation is fully automatic with either a solid state timer or pressure differenfial switch.

With the optional integral fan and final filter, the Design M FabriPulse is a completely packaged clean air system allowing recirculation back into the plant area. See Bullsfin APC-1-411.

Electronic Air Cleaners Electronic air cleaners are especially well suited for general ventilation applications where air volumes are relatively high and fume concentra- tions are relatively low, resulting in longer periods between cleaning. With larger air volumes, pressure drop becomes an increasingly important operating cost variable. The wtde open plate configuration of electronic air cleaners produces the lowest static pressure of any collector type.

The basic orinci~le of ooeration of ~ ,~ ~

electronic air cleaners is electrostatic precipitation. Contaminated air is drawn into the air cleaner and first passes through a high intensity electric field, created by inposing a high voltage positive charge on the ionizer wires. Dust and fume parti- cles are positively charged in this intensive field and then enter a second electric field where they are attracted to and collected on grounded plates.

Environmental Control Unit (ECU)

The Environmental Control Un~t (ECU) collector is a completely self-contained, side access air handling un~t featuring a built-in mobile washer, collector cell section and fan. It is ava~lable for capacities from 5,000 through 30,000 CFM

( making this system suttable for relatively large general control or local control direct capture applications.

The fan is capable of handling external static pressure to accommo. date ductwork for proper contaminant pick up or supply side air control.

Options include additional cell section, charge neutralizer, detergent system and fully automatic control. See Bullef~n AFPS- 1-27 1

Dust and fume pafilculate are given aposrtrve eleotnc charge so that they we atfracted to negat~vely charged cailec8on plates The clean en can then be recirculatedhrrokinto the workarea.

The collection ~lates are easily SnyderGeneral offers three electronic cleaned, an imbortant factor - product types: the AmerTron for considering the quantity of fume small air volume, self-contained generated in thewelding environment. applications; the Environmental The aluminum plates are insensitive Control Unit (ECU) for medium size to the broadspectrum of contaminants air volumes; and the PerformAir for generated in welding applications larger volumes. from dry to oily.

- Environmental Control Unil (shown without fan)

The AmerTron collector is a small, self-contained unit for capacities up to 6000 CFM. It is available in fhree horizontal flow models for g e n h l ventilation or direct capture service. It is also available in two portable models for mobile capture service. See Bvllebns AFPS- 1-305,306,307 - eerling wall or frame mounted.

See Bulletm AFPS-1-3a8-portable.

AmerTron

I I

PerformAirTM The PerformAir collector is well desian _ flexibility of PerformAir units suited for general ventilation systems allows them to fit almost any where large air volumes (up to dimensional requirement. These 124,000 CFM) are required at mini- front access units are designed for mum static pressure. The modular built-up air systems such as rooftop

air housings or large plenum systems.

PerformAir -- ----6--

PerformAir collectors consist of a mobile washer and cell sections with optional additional cells and detergent system. See BUllehn AFPS- 1 - 183

Washing Action

Product Selection Guide

For more information on AAF Air Pollution Control Products

Call 1-800-678-4356 Ask for Department 1820 for the name of your nearest AAF Sales Representative

C b . . . . . 1 P. 0. BOX 35690 . LOUISVILLE, KENTUCKY 40232-5690 In Canada, write: SnyderGeneral Canada, Inc. 6969 TranoCanada Highway Suite 142 B St. Laurent, Quebec H4T 1VB APC 1905BSEP93 Pnmed 8n USA 01993 Snydero~nem! m w m w