Rotary air valves used for material feed and explosion protection are required to meet the criteria of NFPA 69 (2014)

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2 Rotary air valves used for material feed and explosion protection are required to meet the criteria of NFPA 69 (2014) rotary valve design criteria (close clearance) THE DESIGN CRITERIA IN THROUGH SHALL BE APPLICABLE TO EITHER CONCEPT DEFINED IN DESIGNED VALVE BODY & ROTOR FOR Pred BY 3RD PARTY THE DESIGN BASIS INCLUDES THE SPECIFIC EXPLOSION CHARACTERISTICS (KST,AND PMAX) OF THE POWDER BEING HANDLED PRES. RESISTANCE CERTIFIED BY A KNOWLEDGEABLE TEST AUTHORITY AT LEAST 6 VANES ON ROTOR AT LEAST 2 VANES EA. SIDE IN MINIMUM CLEARANCE AT ALL TIMES METAL BODYAND ROTOR. VANES ROTARY VALVE BEARINGS EXTENALLY MOUNTED EXPLOSION DETECTION DEVICE OR INTERLOCK TO AUTO-STOP VALVE THE NEED FOR PERFORMANCE TESTING OF VALVE DETERMINED BY A.H.J CLOSE CLEARANCE ROTARY VALVES CLOSE-CLEARANCE VALVES 1MTH A CLEARANCE OF 0.2 MM (0.0079") THE CLEARANCE BETWEEN VALVE AND BODY SHALL BE SMALL ENOUGH TO PREVENT THE PASSAGE OF FLAME BETWEEN ROTOR AND VALVE HOUSING THE ACTUAL CLEARANCES ARE MEASURED AFTER ASSY, A MAINTENANCE PROGRAM TO ADDRESS WEAR IS THE RESPONSIBILITY OF THE END USER. - NFPA 70 GROUND STRAP REQUIRED (3) 1 M/S (200 FPS) RIM SPEED MAX OPERATING PRESSURE 4.5 PSIG EXPLOSION VENTING ABOVE AIRLOCK BY OTHERS NOMINAL BURST PRESSURE OF PRESSURE RELIEF VENTING 1.5 PSIAAT 100F Kst value 299 (bar m/sec) Pred = 14.5 PSIA MAX. ROTOR TO HOUSING TEMPERATURE DIFFERENTIAL 25F DESIGN TEMPERATURE 70F OPERATING TEMPERATURE RANGE-40F TO 90F DO NOTAPPLY STRESS, WELD OR CUT ON THIS UNIT STATIC LOAD 2500 LBS SERIAL NUMBER

3 1- Stress analysis Table 1 shows parameters used in the stress analysis Table 1. Stress analysis detail Material (including bolts) Yield stress Surface pressure exerted Mesh type Mesher used Jacobian points Max element size Min element size Mesh quality Total nodes Total elements Maximum aspect ratio Percentage of elements with aspect ratio < 3 Percentage of elements with aspect ratio > 10 The static stress analysis was performed using SOLIDWORKS Simulation. Housing, end bells, and knife door were analyzed in a single setup. Housing was fixed from the bottom flange. End bells and knife door were bolted to the housing. As per NFPA , the valve body should have sufficient strength to withstand reduced explosion pressure in the vented enclosure above airlock housing, Pred (bar g), which is determined by customer. As Pred of wood dust is not >14.9 psi therefore 15 psi was exerted to the inner surfaces of the housing, the end bells, and the knife door. Results show no significant deformation in the model under above mentioned pressure, hence the valve body has sufficient strength to withstand 15 psig of pressure. Figure 1 shows contour of von Mises stress on the model. The maximum stress was found at the endbell with the value of 8378 psi about 23% of the material's yield point. 3

4 2- Airlock explosion test 2-1 Introduction Figure 1. Von Mises stress for the housing, the end plate, and the knife door. This section is a summary of our research into the maximum safe clearance as per NFPA we can use to prevent the passage of flame between the rotor and housing for a rotary airlock knife feeder (isolator) that will perform as well as the (0.0079") 0.2mm x 3mm wide tip clearance specified in NFPA section

5 2-2 Dust explosion computational model development Dust cloud explosion is a complex problem from a computational point of view and is a function of different parameters such as dust thermophysical properties, water content of the mixture, particle size, etc. Hence any computational model needs to be evaluated using standard explosibility tests in order to yield accurate results. The numerical model in this work utilizes a single step global chemistry model to account for the conversion of combustible part of dust particles into and. Equation 1 shows the single step global reaction: This reaction is extracted from Ansys material database and is recommended for wood volatiles reactions. Arrhenius Reaction rate (Equation 2) with parameters reported in Table 2 was used. The forward rate constant is written as follows: / 2 Table 2. Arrhenius reaction rate parameters. A E (J/Kgmol) Fuel rate exponent, m Oxidizer rate exponent, n Temperature exponent R (J/mol-k) Particle size distribution Size of combustible particles play a significant role in the modeling process. Rosin-Ramler method was used to mimic the particle size distribution reported by Fike. Table 3 shows wood particle parameters used in the simulations. Table 3. Wood particle parameters. Particle Min Diameter (m) Particle Max Diameter (m) Particle Mean Diameter (m) Density (Kg/m3) Cp (J/Kg-K) Vaporization temp (K) Volatile component fraction (%) Binary diffusivity (m2/s) Validation To calibrate and validate the reaction presented in Equation 1 the results from experimental explosibility test with dried, all fine SPF wood dust as per ASTM E1226 grinding specifications and a Kst of by 5

6 Fike Canada Incorporated (FIKE REPORT NO. CTL Rev0) was used. Table 4 shows the experimental parameters and the results of the explosibility test, and Figure 2 shows the 20_L Siwek apparatus used for explosion testing. Table 4. Fike explosibility test parameters and results Supplied Material Classification Moisture Content Was the sample dried? Moisture Content (As Tested) Was the sample ground? Pmax (barg) Pmax Corrected (barg) (dp/dt)max KSt (bar m/sec) Ignition Source Figure 2. Siwek 20-L Apparatus Figure 3 shows the numerical domain generated to replicate the dust explosion test results under ASTM E1226 standard. 6

7 Figure 3. Axisymmetric model of Siwek 20-L apparatus Figure 4 shows particles in the container and the reaction zone at beginning of explosion. Figure 4. 20L container with wood particle and reaction area Table 5 shows the details numerical Siwek 20_L apparatus. Table 5. Detail of Axisymmetric model of Siwek 20-L apparatus. Grid size (m) Radius (m) Mesh type Mesher used 7

8 10 9 Absolute Pressure (bar) del P del t Sim results Experiment (FIKE REPORT NO. CTL Rev0) Time (ms) Figure 5. Pressure versus time curve for dust explosion Figure 5 shows computational results for pressure versus time curve in dust explosion. Pmax value matches the experimental values presented in Table (note the 1 bar initial pressure in the chamber). The other important explosibility parameter is the Kst. To compare the experimental values with the computational results Equation 3 was used: Hence: K dp dt V / K dp dt V / delp delt V / e / bar m /sec 3 Calculated Kst value is in good agreement with the experimental data. Table summarized the comparison between computational model results and experiment. Table 6. Simulation results vs experimental data from FIKE REPORT NO. CTL Rev0 Simulation FIKE REPORT NO. CTL Rev0 Pex (bar) Kst (bar m/sec) Cylindrical container In order to reduce computational cost a 2D Axisymmetric model was used for the simulations consisting of a cylindrical shape with a blade separating two volumes. Figure 6 shows the geometry at the beginning of the simulation. Wood particle are added at the start of simulation to match the concentration reported in Fike report. A spark increases the temperature of the particles in the right hand side of the cylinder. A blade separates the heated charge from the left hand side which is connected to a pressure outlet boundary 8

9 condition with 1 atm pressure. To reduce computational cost the middle part of the left hand side of the blade is removed. Blade dimensions are variable for each simulation. For this set of calculations X0.25, X0.5, and X0.5 was used. The computational time for each case was 200, 164, and 172 CPU hour respectively. Walls are assumed to be made of Steel and with a constant temperature of 300 K throughout the sims. The code calculates the heat transfer rate based on the gas temperature at any given location. Figure 6. Distribution of wood particles using Rosin-Ramler Figure 6 shows distribution of wood particles at the begining of the simulation. Figure 7, Figure 8, and Figure 9 show the results for tested clearances. 9

10 Figure 7. Temperature contour for X0.25 clearance 10

11 Figure 8. Temperature contour for X0.5 clearance 11

12 Figure 9. Temperature contour for X0.5 clearance 2-6 Conclusion These simulations use a Kst of from dried, all fine SPF wood dust as per ASTM E1226 grinding specifications, which we know is not the case in "as found" shavings within the sawmill industry, providing us with a much greater safety margin, when using 0.012" as a maximum clearance for a shavings only feeder. Shavings were tested by Fike "as found" at a Kst of