Scenarios 1 and 2: Using physical chemical models to ...



Scenarios 1 and 2: Using physical chemical models to estimate respirable dust and silica in an iron foundry4762502056765Figure 1. Operator moves cope and drag to conveyor, aided by a pulley system00Figure 1. Operator moves cope and drag to conveyor, aided by a pulley system52387456515Moving onto Shaker00Moving onto Shaker 32861252540Figure 2. Parts unloaded from cope and drag are conveyed to the back end00Figure 2. Parts unloaded from cope and drag are conveyed to the back endFoundry molds containing iron parts are lifted by crane pulley and placed on a vibrating platform called a shaker, where the iron part and sand are knocked onto the platform. The sand mixture is removed by the shake-out conveyor. The iron parts move to the back end of the shake-out area, where they are manually broken into individual pieces. Mold and parts from the Continumatic area (a casting area with an auto conveyor, following the “BP machine”) move under the walkway to the conveyor, and are moved to the back end, as well. Operators work in the front and back end of the shake-out area. The operator in the front end of shake-out moves the mold (cope and drag) to the shaker using a crane pulley. After transferring the mold onto the conveyor, the vibrating shaker removes and separates the sand mixture and casting parts from the cope and drag by shaking the mold apart. The sand gradually falls off the conveyor, where it is removed by a second (open) conveyor system. A second operator works at a station on the back end of shake-out, breaking the molded parts into individual pieces by picking and dropping them onto table, tossing them into a bin. Generation rate, G: Respirable Dust:Four sources contributed to the respirable dust and silica exposures in this area. These wereFront end:unloading the cope and drag using a pulley moving them onto the shaker which vibrated, causing the sand mold and metal part to separateTransport of parts from another area onto a conveyor located under the walkway.Back end: Operator breaks molded iron pieces into individual parts. Pieces are picked up and dropped onto table, then tossed into bins as parts moved up the line onto the shaker tableThe generation rate was estimated by solving for G, using the average respirable dust concentrations measured using direct-reading instruments at 4 locations. The relationship between C and G is:G= C×Q WhereG is the Generation rate (mg/min)CNF is the concentration (mg/m3)Q is the ventilation rate (m3/min)Generation rateG1 (round)G2 (round)G3 (rectangular)G4 (round)Sample 123123123123Dimensions1.041.041.041.041.041.042.24 *1.65 2.24 *1.65 2.24 *1.65 1.271.271.27Area (m2)0.850.850.850.850.850.853.703.703.701.271.271.27Vs, velocity (m/min)172448231919171115442819Q, flowrate (m3/min) 14.820.440.819,415.816.364.342.155.456.234.923.6C_Respirable (mg/m3)0.783.310.321.121.711.482.156.162.290.671.751.39G_Respirable (mg/min)11.5267.3712.9621.6826.9724.13138.40259.63126.9037.5061.2932.66Table II. Calculating an average G for each source from C measured at each source. An overall average G is calculated from these average values.Average G (mg/min)G1G2G3G4G_Respirable30.624.3174.943.8Table III. An average G is calculated for each of the four sources, as well as an overall average G.Average Respirable Dust G = 68.4 mg/minTo facilitate Monte Carlo analysis, the generation rate is characterized using a lognormal distribution with a geometric mean (GM) and a geometric standard deviation (GSD): G = LN(43.7, 2.64)Respirable Silica (Quartz) Generation rateSilica is a component in the respirable dust, present at 0.5 to 15% according to the msds.The generation rate is estimated by multiplying the Respirable Dust G by this fraction Gmin mg/min= 68.4 ×.05 =3.42mg/minGmaxmg/min= 68.4×.15 =12.3 mg/minFor Monte Carlo analysis, G is described as a lognormal distribution:LN(43.7,2.64)*U(min =0.05, max=0.15)Ventilation rate, Q:The ventilation rate was estimated using professional judgment, Accounting for the combined mechanical and natural ventilation sources Large doors open at frequent intervals to allow forklifts into the building allowing fresh air to enter the Shake Out areaQ was estimated at 4 to 5 air changes per hour Q = 1200 m3 x (4 to 5)/60 = 80 to 100 m3/minQ = U(80, 100)Room volume, V:Comprised of Shake Out front and back endDimensions Volume=2 ×5m ×12m ×10m=1200m3V = 1200 m3The Near Field Volume, VNFVNF is calculated from the volume of a hemisphere,WhereVolume of a hemisphere: 2/3πr3Radius, r = 0.8 m, representing arms’ length between the operator and the respective source. VNF = 1.1 m3The Far Field Volume is the total room volume – NF volumeVFF = 1200 m3 – 1.1 m3 = 1199 m3Interzonal airflow rate, ?? is estimated from the free surface area and local random air speedFor the four sources, measurements of these inputs were taken to estimate ? and an average value across the four locations calculatedWhere ?= 1/2 free surface area ×sFSA = 2πr2 for a hemispherical geometry, r = 0.8m = 4.02 Median random airspeed, based on measured values = 18.9 m/min? = LN(34, 1.5)TimeThis was a full shift activity. Time = 480 minutes.Exposure Assessment:Occupational Exposure LimitThe ACGIH TLV for Respirable Dust is 3 mg/m3. The ACGIH TLV for Quartz is 0.05 mg/m3. Predicted ExposureApplying a Monte Carlo approach, the 95th percentile CNF Respirable Dust exposure = 5.7 mg/m3. Using the AIHA Exposure Control Categories, the predicted NF concentration falls into Category 4.The 95th percentile CNF for Silica (Quartz) exposure based on Monte Carlo analysis = 0.96 mg/m3. Using the AIHA Exposure Control Categories, the predicted NF concentration falls into Category 4.Reference Exposure CategoryThe 95th percentile Respirable Dust exposure based on 8 personal samples was 3.27 mg/m3. The reference exposure category is Category 4. The 95th percentile Quartz exposure based on 8 personal samples was 0.38 mg/m3. The reference exposure category is Category 4. ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download