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By John Lojek, PE, Principal Engineer, Analysis and Testing, Westinghouse Electric Company, LLC and Kevin Ramsden, Nuclear Services Chief Engineer, Fauske & Associates, LLC

At the top of the containment vessel of pressurized water reactors, hundreds of spray nozzles are positioned to provide a mechanism for cooling during a postulated event. Since operation of the containment spray system is not feasible during normal operations, confirmation of the flow path is typically performed using gas flow. At our test facility, we were able to replicate the flow rate of one of these spray nozzles.

This high-speed video shows a sudden inrush of water with debris to the nozzle and an eventual steady-state nozzle spray flow. The design of the nozzle allows for a droplet size and flow rate to adequately cool containment to prevent over pressurization. The flow rate is about equal to 14 gpm or 5 - 6x the flow rate of a standard shower head. In this experiment, upstream pressure transmitters and flow meters provided information relevant to match the design conditions. A custom developed National Instruments LabVIEW program was used to monitor and acquire data. After baselining the facility, sensitivity studies were conducted to determine adequate nozzle performance. Utilizing a significant foundation of debris transport design and testing experience, the performance of a single spray nozzle with different debris conditions of varying size distributions was investigated.

Fauske & Associates, LLC and Westinghouse have long histories of thermal hydraulic testing and analysis and state-of-the-art laboratories that can be used for full-scale industrial testing services of power plant components and systems. This is one example of a type of test that can be conducted.

Solving and testing complex fluid problems are part of unique full service lab and engineering service capability.

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A Workshop on Safety Technology by Dr. Jim Burelbach, Director of Process Safety Consulting, Fauske & Associates, LLC Thermal Runaway
  • A thermal runaway is the progressive production of heat from a chemical process and occurs when the rate of heat production exceeds the rate of heat removal
  • The batch temperature rises because there is insufficient cooling available to remove heat from the system to maintain isothermal conditions

Heat Generation > Heat Loss = Thermal Runaway


Hazards Arise from Pressure
  • When considering reaction hazards, temperature is rarely a hazard on its own. The impact of temperature rise on the system pressure is much more important.
  • There are three potential sources of overpressure:
      -Gas generation from the normal process
      -Vapor pressure effects (as a consequence of heat from the normal process)
       -Heat from the normal process leading to secondary reactions at elevated temperature (causing gas and/or vapor 
        pressure effects)
  • Emergency Relief System (ERS) must be designed to safely vent possible sources of overpressure

Upset Scenario Selection

Determine plausible upset scenarios from Process Hazard Analysis (PHA)

What leads to or triggers runaway reaction?

  • Incorrect reagents or wrong order of addition
  • Reactant accumulation
  • Contamination
  • Corrosion→unwanted catalytic effects
  • Overcharge / undercharge catalyst
  • Addition at wrong temperature
  • Loss-of-cooling
  • Loss-of-mixing
  • Inadvertent heating
  • Fire exposure
  • Material compatibility

Adiabatic Calorimetry
  • Low phi-factor calorimetry allows for direct application of data to process scale
  • Directly simulate upset scenarios of interest

ARSST Methodology
  • Low thermal inertia (phi-factor φ = 1.05)
  • Thermal scan to identify moderate to high exothermic activity
  • Open system
    - Impose backpressure to suppress boiling
    - Initial pressure depends on goal of test
  • Direct measurement of sample temperature 

VSP2 Methodology
  • Low thermal inertia (phi-factor φ = 1.05-1.15)
  • Simulate normal process or upset conditions
  • Identify mild to high exothermic activity
  • Open or closed cell
  • Uses pressure-balancing technique

Injection Piston


Syringe Pump


Setup for Gas Addition

Source Term Determination for Relief System Design
  • System classification
    -Vapor (Tempered)
    -Gassy (Non-tempered)
    -Hybrid, tempered
    -Hybrid, non-tempered
  • “Source term” determined based on system classification

Vapor System
  • Pressure generation is due to increased vapor pressure
  • Latent heat of vaporization (tempering)
  • Temperature rise rate is used for vent sizing
  • Reaction temperature rise can be controlled by venting

             


Gassy System
  • Generates non-condensable gas
  • Latent heat of cooling not available
  • Typical of a decomposition reaction yielding gassy products
  • Reaction temperature rise cannot be controlled by venting

           


Hybrid System
  • Latent heat of cooling is available at the relief pressure and temperature (tempered)
  • Reaction temperature rise can be controlled by venting
  • Generates non-condensable gas

             



Flow Regime Considerations
  • Entrained liquid reduces the flow area available for venting

Two-phase flow (foamy)    All gas or vapor flow (non-foamy)

Flow Regime Detector (FRED) for ARSST

Flow Regime Detector (FRED) for ARSST

If reactants foam up, the sensor temperature cools down to the reactant temperature

Blowdown Testing in VSP2
  • Depressurize VSP2 test cell and determine how much material remains
  • Used to mimic superficial velocity to determine expected flow regime

       

Simple Vent Sizing Formula
  • Use with low phi-factor calorimetry data
  • Limited material properties required
  • Vent size is based on all vapor or gas venting
    -This does NOT mean there is no two-phase flow
    -Two-phase flow can still occur but uncertainties are accommodated by allowing sufficient overpressure above the relief set pressure
  • Equation presented here is applicable to critical flow
    • “Relief System Sizing for Runaway Chemical Reactions: A Simple Comprehensive Approach,” 11th Global Congress on Process Safety, 2015, K. Kurko
    • “Vent Sizing Applications for Reactive Systems”, AIChE 2001, 5th Process Plant Safety Symposium, J. Burelbach

A  =  required vent area (m2)

CD  =  discharge coefficient (-)

m  =  mass of vessel contents (kg)

cp  =  heat capacity of vessel contents   (J/kg·K)

 =  temperature rise rate from   calorimetry test (K/s)

=  latent heat of vessel contents  (J/kg)

T  =  venting pressure (Pa)

R  =  universal gas constant   (8,314.47 J/kmol·K)

T  =  venting temperature (K)

MWv  =  molecular weight of vapor   (kg/kmol) 

v  =  freeboard volume of test (m3)

     =  pressure rise rate from   calorimetry test (Pa/s)

mt  =  mass of test sample (kg)

MWg  =  molecular weight of gas   (kg/kmol)

Guidelines for Use of Simplified Formula
  • Vapor systems
    -Evaluate material properties at set pressure of relief device
    -Use set pressure as venting pressure
    -Use temperature rise rate at set pressure (from calorimetry test)
    -Use of equation requires 40% overpressure (on an absolute basis)
    -For foamy systems, multiply vent area by 2
  • Gassy systems
    -Evaluate material properties at peak gas generation rate
    -Use maximum allowable accumulation pressure (1.1×MAWP) as venting pressure
    -Use peak pressure rise rate (from calorimetry test)
  • Hybrid systems
    -Evaluate material properties at set pressure of relief device
    -Use set pressure as venting pressure
    -Use temperature and pressure rise rates at set pressure (from calorimetry test)

Vapor System Example
  • 3500 kg of a phenol-formaldehyde resin is produced in a 5 m3 reactor
  • Reactor MAWP is 30 psig
  • Reaction is run at 50°C
  • Sodium hydroxide is added to reaction mixture of phenol, water, and formaldehyde
  • Results of PHA indicate fast addition of sodium hydroxide would overwhelm cooling capacity
  • Desired set pressure of rupture disk is 10 psig
  • Closed test cell VSP2 test run
Vapor System Example (Closed Cell VSP2 Test Data)

   

 

Vapor System Example Calculation


Gassy System Example (Open Cell ARSST Test Data, P0 = 88 psig)
  • 210 kg of 40% dicumyl peroxide in 2,2,4-trimethyl-1,3-pentanediol diisobutyrate stored in 340 L tank
  • Tank MAWP is 80 psig
  • Results of PHA indicate a fire in surrounding area would elevate the temperature of the tank to cause decomposition of dicumyl peroxide
  • Fire exposure rate determined to be 0.5°C/min
  • Desired set pressure of rupture disk is 50 psig
  • Open test cell ARSST test run at 88 psig

 

 

Gassy System Example Calculation

Hybrid System Example (Open Cell VSP2 Test Data, P0 = 110 psig)
  • 2000 kg of 25% hydrogen peroxide is stored in a 700 gallon tank
  • Tank MAWP is 100 psig
  • Results of PHA indicate iron contamination could cause a runaway reaction due to accelerated decomposition of hydrogen peroxide
  • Desired set pressure of rupture disk is 20 psig
  • Open test cell VSP2 tests run at 110 psig and 20 psig

    

 

Hybrid System Example – Calculation

Summary – Relief Device Sizing
  • Determine all credible upset scenarios
  • Perform calorimetric tests
    - Use low thermal inertia adiabatic calorimetry
  • - Simulate actual upset scenarios
  • Apply experimental data to vent sizing formula
    - Minimal physical property data required
    - Results compare well with large scale experimental data
Learn more about FAI University's Relief System Design Course. 

“Without data, all you have is an opinion”

Dr. Burelbach received his PhD in Chemical Engineering from Northwestern University in 1989.  Since then he has been a senior staff member at Fauske & Associates, LLC, holding a variety of leadership roles in process safety for the chemical and nuclear industries. This workshop was presented to the Safety Technology for Pharmaceutical can Chemical Processes (STPCP). 

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By: Sung Jin Lee, Senior Consulting Engineer, Fauske & Associates, LLC and AnnMarie Fauske, Outreach, Fauske & Associates, LLC

FATETM 2.0 software is a general facility model developed by Fauske & Associates, LLC (FAI), a wholly owned subsidiary of Westinghouse Electric Company, LLC. FATE is used for process analyses, which considers engineering process components as well as an entire facility analyzing facility response, including transport and distribution of evolved gas and contamination, fire and smoke migration and room heat-up. It calculates temperatures, pressures, flow rates and compositions for fluids, gases and structures associated with a nuclear or chemical process and its surrounding facility. The FATE predecessor won U.S. DOE award for metallic spent nuclear fuel applications.

F - Flow of gases, vapor and particulates in a compartmentalized facility
A - Aerosol generation and transport
T - Thermal response of rooms and equipment
E - Explosion hazards from flammable gases and entrained particulates (dust)

Here is a list of FATE applications, which should give some idea about the code’s capabilities - such as a current application involving flammable concentration evolution during the operation of a major manufacturer's sand blender, where sand molds are broken up after being used and are fed into the blender to break up the remaining sand chunks. Flammable gas is released as the resin in the sand decomposes in the blender. In another industrial application, FATE is used to show that pressure transient in a newly designed flange guard device, which is clamped around a flanged pipe connection and has an open vent pipe, remains below the design pressure in the event of “pinhole” leak in the pipe gasket. In both applications, FAI will identify the pertinent phenomena, set up a simple FATE model, make parametric runs, and report the results with one or two weeks of effort. 

More:

  • The transient and severe accident analysis capability of the SAS4A/SASSYS-1 code developed by Argonne National Laboratory is coupled with the radionuclide transport analysis capability of the FATE code to predict radionuclide release from a broad spectrum of accidents that can be postulated to occur at liquid metal cooled reactor facilities.
  • Hydrogen combustion consequences in a 3 cubic meter Box and Skip including the effect of heat transfer to walls
  • Diesel generator room heat up during the loss of ventilation event was analyzed. A simple FATE model was built to validate the GOTHIC calculation
  • Hydrogen risk during the steam generator wet layup additive process was analyzed. The secondary side of the steam generator and piping leading up to the automatic relief valves were modeled using FATE.
  • Hydrogen accumulation in the AP1000 auxiliary building and primary containment building was analyzed in the event of a break in the chemical and volume control system (CVS) hydrogen injection line
  • Transient thermal response of concrete walls within the facility was analyzed during postulated upset scenarios where pool cooling is unavailable and the fuel storage halls are cooled either by active ventilation or natural circulation for an extended duration
  • The drying process of damaged fuel chunks in an nuclear  reactor was simulated
  • Transient model of HCl gas release in a vessel and piping system
  • Transient thermal and gas generation analyses for planned loading of sludge basins was analyzed. In particular, this simulation addresses the off-normal scenario for the loss of active ventilation after sludge loading prior to its transport.
  • Transient thermal and gas generation relevant to the vent/purge process step following receipt of sludge in a Sludge Transport and Storage Container (STSC) was analyzed
  • Used fuel drying in a special container was simulated
  • Evaluated the risk from flammable gas accumulation in the buildings attached external to the containment by modeling the transport and distribution of leaked flammable gas (hydrogen and carbon monoxide) in the penetration buildings
  • Gas generation and flow were analyzed for an ingot casting facility being constructed in the US. During the ingot producing process, occasionally a break in the mold will develop, allowing the molten metal to spill out of the hardened ingot into the casting pit, producing hydrogen through a reaction of water with lithium and aluminum.
  • Transient thermal and gas generation analyses for planned transport of sludge in STSCs
  • Transient thermal and gas generation analyses for planned transport and interim storage of sludge in STSC
  • The dry cask storage (DCS) of spent nuclear fuel assemblies was analyzed for steady-state thermal behavior in an isolated loss-of-flow condition. The thermal analysis must assure that the peak cladding temperature remains below the regulatory limit for the dehumidification process.
  • Transient thermal and gas generation analyses for transportation of settler sludge in the STSC
  • Predictions for the nominal performance of cold vacuum drying (CVD) of knock out pot (KOP) material, as well as behavior of the system in off-normal and accident conditions. Results of the calculations can form a basis for process design, system design, operating procedures, and safety documentation including technical safety requirements.
  • Transient thermal and gas analyses of candidate conceptual designs for shipping from the basins and storage of sludge
  • Analysis of settler sludge thermal, gas generation, and gas storage behavior during retrieval and interim storage. There are two specific system areas for analysis: Transfer piping, which is bounded by a filter, and a storage container.
  • Scoping thermal and gas calculations for a container sludge retrieval system. This is a scoping evaluation to assist in design and hazard evaluation as part of integration of safety into design.
  • A fire was simulated to quantify the potential for equipment damage and fire propagation for reactor building rooms and corridors in Fire Zone 1
  • Analysis of heat transfer, chemical reactions, and gas generation for retrieval and transfer of sludge from the vacuum drying facility via a double-contained line, an essential phase prior to processing of sludge to remove reactive metal.
  • Thermal and chemical process simulation of the aqueous sludge corrosion process. The scope includes normal operation and recovery from selected off-normal and accident conditions. The model considers the corrosion vessel, quench vessel, process enclosure, process off-gas piping, and  vacuum drying bays.
  • The leak path factor (LPF), the fraction of material released in the facility that is transported to the environment, is determined by modeling the facility including active systems
  • Thermal response and hydrogen gas generation are examined in the Fuel Transfer System (FTS) for fuel transfer
  • Analysis of heat transfer, chemical reactions and gas generation for transfer of sludge via a double-contained line and sludge loading and staging in consolidation containers
  • Independent calculations for thermal and chemical reaction response of Spent Nuclear Fuel (SNF) sludge in a large diameter container (LDC) for select transportation and storage scenarios
  • A simulation of the behavior of the contents of a multi-canister overpack container (MCO) from the inception of cold vacuum drying through interim storage. The simulations include prediction of fuel temperatures, gas temperature, gas composition, and pressure, and address the potential for ignition of fuel.
  • Consequences of air ingress into an MCO during vacuum drying. A separate detailed assessment of the effect of hydride inclusions and the potential for ignition.
  • A mechanistic model of organic-nitrate reactions initiated in hypothetically reactive waste in underground storage tanks. A thermal-hydraulic assessment of tank transient pressure and temperature to yield flows of gases to the environment, a release model to predict vaporization of volatile materials from reacted waste, and an aerosol transport and deposition model to provide the source term to the environment.

We thought this information was relevant to share and discuss as FATE's code capabilities are vast. Your facility may have an innovative idea or need to address similar issues for which FATE can be tailored to suit. If you'd like to learn more about FATE, check out FAI's work on the Development of the Source Term Analysis Tool SAS4A-FATE for Lead-and Sodium-Cooled Fast Reactors by clicking below. 

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 By Zachary Hachmeister, Chief Operating Officer, Fauske & Associates, LLC

20 Liter Chamber

NFPA 652 provides a prescriptive approach and a performance based approach as compliance options to mitigate dust explosion hazards. The prescriptive approach requires that air-material separators with a dirty side volume greater than 8 ft3 are equipped with explosion protection. Air-material separators with dirty side volumes less than 8 ft3 do not require explosion protection to achieve compliance with NFPA 652. This volume exemption also applies to other process equipment with small volumes (< 8 ft3).
 

The 8 ft3 exemption does not mean that an explosion hazard does not exist. In fact, the majority of data used for addressing dust explosion hazards is generated in a test apparatus with a volume of  20 L  (~0.7 ft3). The exemption exists because it can be difficult to provide these small vessels with explosion protection equipment and that the hazard risk is considered to be modest. As an illustration, let us assume an 8 to 1 volumetric expansion from a typical dust deflagration, an 8 ft3 enclosure (sphere with a radius of 1 ¼ feet) can produce a fireball with a volume of 64 ft3.  If the fireball is spherical then the radius would be ~2.5 ft.  Therefore, the decision to strike a balance between the economic impact and potential consequence justified the exemption.

The important take away is; per NFPA, you’re not required to protect these small volume units. However, we would strongly encourage to take a risk based approach when making this decision. This approach should include (but not limited to) consideration of consequences to personnel, property, and operations and probability of ignition give the ignition sensitivity of the material and how the equipment is being used.

Hopefully, this provides some clarification to a frequently asked question. If you'd like to learn more about dust hazard analysis and mitigation, check out FAI's 3 Step Approach by clicking below.

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By Elizabeth Raines, Chemical Engineer, Fauske & Associates, LLC

Introduction

Ensuring that the VSP2 heater assembly is setup properly is crucial to performing a successful test. While the heater and heater glands are reusable parts, testing their integrity prior to beginning a test and replacing any questionable parts will help to ensure success. Further, care should be taken to insulate the test cell and heater assembly inside of the containment vessel as well as possible. The heat loss of the assembly during the test will be a determining factor in the overall quality of the test data. This article and video discusses the proper electrical testing and insulating procedures for the heater assembly and provides some setup tips and tricks.

VSP2 – Everything You Need to Know about Setting up Your Heater Assembly - YouTube

Heat Assembly Background

The VSP2 heater assembly is comprised of two separate heaters; the "Main" or "Auxiliary" heater (the "driver" heater used to raise the test cell temperature), and the "Guard" heater (used to maintain the adiabatic environment). The Auxiliary (test cell) heater has approximately three turns of heater wire over the lower quarter and base of the test cell. The nominal resistance of this heater element is 18 ohms. After inserting the test cell in the Auxiliary (Main) heater, a layer of insulation is wrapped around the cell. The Auxiliary heater has wires with male prongs that are shrink wrapped in yellow tape. This insulated cell assembly in turn fits inside the Guard heater assembly. The Guard heater consists of two separate, connected circuits; the larger comprising the bottom and walls, and the top comprising the lid. These are connected using the smaller male and female prongs (these are wrapped with red shrink tape while the Guard heater's other connectors are larger male prongs and the standard assembly uses green shrink tape along these fittings). The nominal resistance of the Guard heater assembly (both elements connected) is 58 ohms. A dual zone “Multizone” Guard heater divides the Guard heater into two zones. The lower zone (controlled by TC2, green tape) comprises the base and lower half of the Guard heater while the top zone (controlled by TC3, and now includes additional blue wrapped male prongs) comprises the upper half. The nominal resistance of the base zone is 29 ohms and of the top zone is 33 ohms. This video and article focuses on the standard heater assembly, but the procedure and principles apply to the Multizone heater assembly as well.

Electrical Checks

Note, in the following pictures, the locations where the multimeter leads should make contact are identified by blue and red dots, respectively.

Heater

There are four primary checks to be performed to ensure your heater is in good working condition:

A. Verify that the resistance across the Auxiliary heater is approximately 18 ohms

- Turn your multimeter on and set to read resistance (might be shown with the Greek letter omega, Ω, or the word “ohms”)

- Connect each of the leads from the multimeter onto each male prong on the Auxiliary heater:

- The resistance across these two locations should read approximately 18 ohms

B. Verify that the Auxiliary heater is not shorted out and that the resistance is > 10 Mega ohms

- Turn your multimeter on and set to read resistance

- Connect one of the leads from the multimeter to one of the Auxiliary heater male prongs and the other lead to a       part of the Auxiliary heater coil:

- The resistance across these two locations should read a very large resistance indicating that the heater is not shorted to the foil (>10 Mega ohms)

C. Verify that the resistance across the Guard heater is approximately 58 ohms

- Turn your multimeter on and set to read resistance

- Connect the lid heater to the base and lower half heater by connecting the two red smaller female and male prong (the green dots in the photo below)

- Connect each of the leads from the multimeter onto each of the green Guard heater prongs:

- The resistance across these two locations should read approximately 58 ohms

D. Verify that the Guard heater is not shorted out and that the resistance is > 10 Mega ohms

- Turn your multimeter on and set to read resistance

- Connect the lid heater to the base and lower half heater by connecting the two red smaller female and male prong

- Connect one of the leads from the multimeter to one of the Guard heater male prongs and the other lead to a part of the Guard heater coil:

-  The resistance across these two locations should read a very large resistance indicating that the heater is not shorted out (>10 Mega ohms)

Heater Gland

There are four primary checks to be performed to ensure your heater gland is in good working condition:

A. Verify the heater gland continuity through the vessel

- Turn your multimeter on and set to read continuity (it might look this this symbol, otherwise set to resistance)

- Connect one of the multimeter leads into one of the female heater gland locations (on the interior of the vessel) and connect the other lead to the male prong on the exterior side of the vessel (touch the metal part on each side):

- When a complete path is reached, the multimeter will beep (and show a very small resistance of ~0.3-0.5 ohms). Only one pair of male/female parts should be detected as a complete path

- Swap pairs and test that the other wire is intact

- Repeat this procedure on the other set of wires

B. Verify the heater gland wires are not shorted to the vessel and that it is > 10 Mega ohms

 - Turn your multimeter on and set to read resistance

 - Connect one of the multimeter leads onto one of the male heater gland prongs and connect the other lead to the vessel:

- The resistance across these two locations should read a very large resistance indicating that the heater is not shorted to the vessel

- Repeat this procedure on the other set of wires

C. Plug the Auxiliary and Guard heaters into the heater gland and verify that the resistance across the Auxiliary heater is 18 ohms and the resistance across the Guard Heater is 58 ohms

- Turn your multimeter on and set to read resistance

- Connect each of the leads from the multimeter onto each male prong on the exterior heater gland that is connected to the Auxiliary heater:

- The resistance across these two locations should read approximately 18 ohms

Note, the heater glands consist of two black wires and two white wires. The two Auxiliary heater wires (yellow) should be connected to either two white or two black. The two Guard heater wires (green) should be connected to the other two wires not used for the Auxiliary heater. Then, on the exterior of the vessel, the resistance across the two male prongs should match the resistance of the heater that is connected to it.

- Connect each of the leads from the multimeter onto each male prong on the exterior heater gland that is connected to the Guard heater:

- The resistance across these two locations should read approximately 58 ohms

D. Plug the Auxiliary and Guard heaters into the heater gland and verify that the heater assembly is not shorted out to the vessel and that the resistance is > 10 Mega ohms

- Turn your multimeter on and set to read resistance

- Connect one of the leads from the multimeter onto one of male prongs on the exterior heater gland and connect the other lead to the vessel:

- The resistance across these two locations should read a very large resistance (>10 Mega ohms)

- Repeat with the other prongs

Test Cell Installation

The recommended method of insulating and installing the VSP2 test cell is as follows:
Tip—lay out all the required materials before we begin wrapping the test cell to make the process as quick as possible.

  1. Place the test cell into the Auxiliary heater.

    Note, it is good practice to record the weight of the test cell before (for phi-factor calculations) and after (for measuring mass loss during the test) installing the required fittings.

  2. Wrap the test cell and test cell heater with 2 3/4" wide paper insulation. Use one full-length strip (24") followed by 1/4 of a strip (6"). Use masking tape to hold the insulation in place.

    Note, when wrapping the test cell with the paper insulation, it is helpful to tuck the starting end of the insulation strip underneath the vertical leads of the test cell heater. Subsequent wraps are on top of the test cell heater leads. This provides a snugger fit of the test cell in the insulation, minimizing the air gap between the test cell and the insulation. Elimination of the air gap prevents possible convective heat transfer and thereby tends to improve insulation performance particularly at high temperatures and pressures. Normally there is no problem seating the new test cell into the test cell heater, but a twisting motion is necessary for all-welded test cells that have a weld bead on the bottom lip.

    Tip—begin by using 3 ~2-3" pieces of masking tape to seal the seams of the paper insulation. Then use 5~6" pieces of tape to wrap the bottom of the test cell. This creates a smoother barrier that makes it easier to slide the test cell into the Guard heater.

    Note that the test cell should be insulated from the Guard heater on all sides – along the wall, top, and bottom with the same thickness insulation layer.

  3. Place a 1/2-thickness disk of Fiberfrax insulation (about 1/4" thick) inside the Guard heater can.

    Note that the insulation can irritate the respiratory tract or skin if it comes in contact with it so use caution and the proper PPE.

  4. Slide the wrapped test cell down into the Guard heater can.

    Tip—mark the thermocouples 1 for the sample thermocouple and 2 for the Guard heater (If using Multizone Multizone heater, mark 3 for the upper zone heater). This makes it clear on the proper connections when the test cell is installed in the containment vessel.

  5. Place a full thickness Fiberfrax disk on top of the test cell. Note, for the thermocouple or if a test cell with a vent is used, the disk on top of the test cell should be cut in half to make room for the different parts.

    Tip—it is helpful to have a separate pair of scissors for cutting tape and for cutting the Fiberfrax disks. Cutting the Fiberfrax disks dull the scissors making it difficult to cut tape overtime.

  6. Install the lid heater on top of the test cell assembly, crimping over the top edge of the Guard can if desired to hold the lid heater in a fixed position. A strip or two of white glass-cloth insulation tape can be used to help hold the lid heater securely on top of the assembly.

    Tip—it is helpful to align the test cell, Auxiliary heater, and Guard heater assembly so that the heater wires remain close to each other to make it easier to make the required connections.

  7. Place a large Fiberfrax disk on the containment vessel floor. Note, this current video shows the heater gland checks and the test cell insulation. A future video will walk through installing the test cell into the containment vessel and making the required connections.

  8. Install several large doughnuts (have a whole cut out from the middle for the test cell) of insulation in the vessel to just below where the fill lines connect.

  9. Slide the Guard heater/test cell assembly down into this nest.

  10. Connect the fill line(s) and Guard heater ground strap(s).
    Note, for the standard heater assembly, this can be accomplished by using a three way ground strap and connecting one of the lines to the fill line, and connecting to each of the red heater wires

  11. Tighten the nuts to the fill line and the pressure line.

    Note, if used, support, the 1/16" fill line when bending it to avoid a sharp bend in the tube. Also, ensure that the tube are aligned with the 1/16" or 1/8" bulkhead fitting by first rotating the nut counterclockwise until a click is heard. Then tighten the nut finger tight. Using a wrench tighten the nut an additional 1/8 of a turn.

  12. Continue installing doughnut segments (usually cut in two or three pieces so they can be fitted around obstacles).

  13. Use small Fiberfrax discs on top of the test cell assembly, cutting them in half or notching as needed to fit around thermocouples and fittings. Insulate up to the upper containment vessel wall penetration. The overall assembly is illustrated below.
 

Adiabatic calorimeter testing provides data for relief system design, safe scale-up of chemical processes, and changes to process recipes.  Safe process design requires knowledge of chemical reaction rates, character and energy release - all of which can be obtained from a low phi-factor adiabatic calorimeter such as the VSP2TM (Vent Sizing Package 2) or ARSSTTM (Advanced Reactive System Screening Tool). If you are interested in purchasing either the VSP2TM  or ARSSTTM check out our store by clicking below.

 

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As a principal investigator with the DIERS research project and founder of Fauske & Associates, LLC (FAI), Dr. Hans K. Fauske, has established FAI as the unmatched industry leader when it comes to relief system design. Learn about relief system design directly from our experts when you join us for our upcoming Relief System Design Course, October 16-18.Unlike other emergency vent sizing courses, this curriculum highlights simplifi­ed calculation methods capable of giving safe - but not overly conservative - relief system designs, with an emphasis on reactive chemistries and the role of two-phase flow.

FAI is accredited through International Association for Continuing Education and Training (IACET)and FAI University course attendees are awarded CEUs upon meeting all the necessary requirements.  FAI University offers a wide array of training courses globally to augment our customized testing, engineering and consulting services. 

 

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By: Richard Kwasny, Ph.D., Senior Consulting Engineer, Fauske & Associates, LLC (FAI)

We will be publishing four articles on the topic of safer scale-up for batch and semi-batch reactions. This initial article is on desktop reviews and preliminary hazard analysis.

Thermal runaway incidents continue to occur in batch production facilities in the chemical and pharmaceutical industries. Serious incidents can result in death, injury, capital loss, and business interruptions. Despite the best efforts of the chemical/pharmaceutical industries to be responsible, major incidents cast a negative light on this industry as a whole. In order to prevent incidents from occurring there is a need for all R&D, process development, and batch production facilities to have an effective process safety strategy in place including sound safety-management systems. Prior to scale-up, it is critical to have a clear understanding of the reactivity of all process chemicals as well as the energetics of both desired reaction(s) and undesired reactions, defining worst-case scenarios, characterizing the resulting adverse reaction, and understanding how to mitigate the process safety impact. A partial flowchart detailing these steps is shown in Figure 1. Processes that cannot be adequately controlled must be redesigned if possible or utilize less hazardous material.

 Figure 1 Flowchart of a Preliminary Hazard Assessment

This article attempts to provide guidelines that can be used as a basis for developing and designing safer new processes. It can also be used to identify process safety information gaps when existing processes undergo periodic reviews, as required in part by OSHA Process Safety Management 1910.119, Hazard Communication 1910.1200, and the General Duty Clause.

Causes of Thermal Runaway Reactions

Studies have determined that thermal runaway reactions occur due to the following four reasons:

1. Insufficient understanding of the process chemistry and the energy/kinetics for the desired reactions

2. Improper design of the heat transfer capacity required at the plant level

3. Insufficient understanding of the adverse reaction and controls including plant-safety back-up systems, as well as adequate emergency venting

4. Inadequate written batch procedures and poor operator training.

Never assume a chemical is not hazardous because of a low-hazard rating. Many incidents involve materials that have NFPA hazard ratings of 0 and 1. It is best to develop a proper testing program to identify and characterize all reactive materials and reaction mixtures under a variety of process conditions. If your company does not have a testing facility, FAI will be pleased to work with you to identify and conduct appropriate tests. Subsequently, a process hazard analysis can then be used to assign appropriate controls and safeguards to reduce risk of an adverse event. It is important to remember to update the process safety information, as a process undergoes changes during its lifecycle. The interim process-safety information reports can then serve as a reference for technology-transfer purposes as the process scales from R&D, kilolab, pilot plant to commercial-production stage. Once the process has been set, the final process safety report can then be used by a variety of end users either in-house or by outsource facilities. When developing safety documentation, it is important to keep in mind that it must comply with company policies and procedures as well as country and local regulations.

Desktop Reviews and Screening Tests
The following items should be considered in relation to a process safety hazard evaluation.

  • Decision to Scale-Up
    When management wants to scale-up a chemical reaction in an existing facility, the amount of information available can vary significantly. Therefore, it is essential to review the desired process and inform the organization if there are any issues that need to be addressed. Therefore, there is a need for a preliminary hazard assessment based on a balanced equation of the desired chemistry.

Preliminary Hazard Assessment:
• Develop an inventory of all process materials including but not limited to:
    o Starting and product substrates
    o Reagents
    o Catalyst
    o Solvents
    o By-products
    o Off-gasses
• Identification of material properties, hazards, and other potential problematic issues:
    o Physical properties
    o Health hazards
    o Flammability and static properties
    o Thermal stability of materials including the potential for shock sensitivity and explosion propagation
    o Review the molecular structure of the reaction materials for highly reactive functional groups
    o Conduct preliminary screening testing using differential scanning calorimetry (DSC) to identify thermal instability in the          starting and final substrates
    o Vapor phase reactivity
    o Material of construction issues (catalytic, corrosion, compatibility, and so forth)
    o Special hazards (oxidizers, pyrophoric, water-reactive, and so forth)
• Methodologies:
    o Conduct a literature search for the above mentioned information and work with production/ process engineers to better
        understand process limitations
    o Estimate the heat of reaction using estimation techniques
    o Quantitate the non-condensable off-gases to estimate volume and rate
    o Interpret the potential hazards with respect to the process temperature and pressure including other critical issues

Initial Evaluation of the Reaction
Once we have all of the above mentioned information, we are in a better position to determine if there are any potential issues that would prevent scale-up.

For example, if the reaction involved a simple crystallization for the formation of a substrate salt with no off-gassing and a calculated adiabatic temperature rise that could be easily controlled through available agitation/heating/cooling of the reaction mass, then probably no additional testing is needed. However, for quality purposes we may need a more quantitative heat balance if there is crash crystallization. Then we could perform reaction calorimetry for this purpose.

There are times when the desired and quench reactions involve reactive functional groups that may become unstable. Therefore, the use of a preliminary hazard analysis will facilitate identification of problematic reactions that under
certain circumstances can be a potential hazard or become one if we lose control of the reaction. There are several ways in which this can occur; one is through a thermal runaway reaction, a fire, or process deviations due to misoperations such as mischarging, and so forth.

Quantification of the Desired Reaction
If we have a potentially problematic reaction then, the next step is to quantify the amount and rate at which heat is generated. Similarly, if there is off-gassing, we would require quantification of the evolved gas rate to ensure the process vent
capacity is adequate.

Therefore, the second article in this series will deal with how to characterize the desired reaction, as needed, based on issues encountered in the preliminary hazard assessment. Subsequent articles will include quantification of the adverse
reaction and case studies.

In the meantime, consider signing up for FAI University's Relief System Design Course. Unlike other emergency vent sizing courses, this curriculum highlights simplifed calculation methods capable of giving safe - but not overly conservative - relief system designs, with an emphasis on reactive chemistries and the role of two-phase flow. Attendees will participate in group workshops and complete an independent quiz at the end of the course in order to ensure comprehension of the material. 



References
    1. Hendershot, D. C., “A Checkl ist for Inherently Safer Chemical Reaction
        Process Design and Operation,” Center for Chemical Process Safety
        International Conference and Workshop on Risk and Reliability, 2002.
    2. Kwasny, R. S., “Hazard Assessment Strategies for Reduction Reactions,”
        London Southbank University, UK, 1999.
    3. Barton, J. and Rogers, R., “Chemical Reaction Hazards,” Second edition,
        Gulf Publ ishing, 1997.
    4. Bretherick, L., “Bretherick’s Handbook of Reactive Chemical Hazards,”
        Seventh edition, Butterworth Heinemann, 2008.
    5. Stoessel, F., “ Thermal Safety of Chemical Processes: Risk Assessment and
        Process Design,” Wiley-VCH , 2008.
    6. Merritt, C. W., 2004. “Chemical Process Safety at a Crossroads,”
        Environmental Health Perspectives, 112:a332-a333. doi:10.
        1289/ ehp.112-a332, 2004.

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By Rachelle Andreasen, Manager, Dust Testing Operations, Earl Johnson, Lab Technician and Ashok Ghose Dastidar, Ph.D. MBA, Vice President, Dust & Flammability Testing and Consulting Services, Fauske & Associates, LLC

In an effort to help our customers understand the importance of evaluating the “dust” hazards within their facility, we have taken a current ASTM method and modified the purpose to answer the “is my material a dust” question.

ASTM E2316 Standard Test Method for Determination of Particles Resulting from the Attrition of Granular Pesticides was originally authored to provide information on health hazards such as inhalation risks based on the amount of pesticide dust present within a working area. The method looks at the original size of the dust and simulates breakage from normal manufacturing and handling processes. The fines generated from this testing procedure are labeled as the “fines from attrition”.

As the amount of dust increases, the greater the risk for not only inhalation concerns, but for dust explosion hazards as well. As previously mentioned, this test procedure was modified to simulate the amount of dust that could be generated by transport via pneumatic and mechanical means within a facility or in containers on the road (i.e. sea or air).

Most recent revisions of the NFPA standards related to dust define a “dust” as a particulate with a particle size of 500 μm or smaller. For this reason, this analysis was performed by taking a sample of material and sieving the material to less than 500 μm to remove the inherent fines (i.e. the powder/dust at the bottom of the bag of cereal). Once the inherent fines were removed, the material was placed within a glass jar with an equal weight of glass beads. The material was tumbled, with the glass beads, for approximately 4500 rotations thus creating an attrition scenario. Once again, the material was sieved to less than 500 μm to remove the fines from attrition. The total quantity of fines then becomes an estimate of the amount of powder/dust that can be present in your material after transport.

As you can see from the histogram below, the commercial particle size of granulated sugar was 54% less than 500 μm (see Figure 1). After the tumbling process, the material was determined to have a particle size of 62% less than 500 μm (see figure 2). The table below also details the data generated from the analysis. The fines % was increased by approximately 8%, which is nearly a 15% increase in fines.

 Table 1 Average weight % of particles with a 500 μm diameter or smaller

Figure 1: “as received” particle size distribution

Figure 2: Particle Size distribution after attrition

The data generated in this analysis clearly shows that even though your material may be in granular form
(or larger), the potential for particle attrition based on your or the end users’ process should be evaluated.

There is not a definitive particle size that governs whether or not a material is explosible in dust cloud
form. The explosion characteristics can be altered based on a materials particle size distribution, moisture
content and even particle morphology. Care should be taken when operating within a facility with
explosible dust.

If you are interested in learning more about dust hazard mitigation, take a look at FAI's three step approach to dust hazard analysis below.

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By Himanshu Chichra, Principal Process Safety Engineer and host of ‘Process Safety and Risk Blog’ www.staub-ex.blogspot.com [staub-ex.blogspot.com]

First, of all, I would like to thank Fauske & Associates LLC (FAI) for their continuous support and for publishing my last blog post "Should Set Point For Pressure Relieving Device Be Equivalent to Design Pressure?" on their website.

As per the research conducted by Dr Phil Nolan (South Bank University, UK) and Dr. John Barton (UK Health and Safety Executive) and graduate students based on data analysis, the following four gaps have contributed equally i.e. 25% each to thermal runaway reactions leading to multiple incidents in past:

1. Lack of proper understanding of the thermochemistry (heat of reaction) and chemistry (balanced chemical equation)

2. Insufficient engineering design for reactor heat transfer system

3. Inadequate control and safety back-up systems including emergency relief systems, process vent, and other engineering controls

4. Poorly written batch procedures and insufficient operator training

Concept Sciences Inc. Explosion
 

Hence, it is imperative to develop a process safety strategy to address these four gaps. A process safety strategy should include the following:

1. Preliminary Hazard Analysis (Screening)

  • Review the molecular structure of known reactive groups and the balanced chemical equations
  • Conduct literature searches (SDS, NFPA, Bretherick's, Heat of Reaction Theoretical Calculations, CHETAH calculations, chemical compatibility, etc.)
  • Perform small-scale thermal stability tests (Example: DSC)

2. Assess the Desired Reaction

  • Conduct Reaction Calorimetry testing on the reaction of interest and the subsequent quench reaction
  • How much heat (or gas) is generated? Is cooling adequate?
  • If a scale-up assessment suggests a much longer addition is needed to allow the available cooling capacity to maintain temperature control, does the desired reaction still perform to standard (i.e. produce the same quality product as was made in the lab)?

3. Assess the Undesired or Decomposition Reaction

  • Conduct Adiabatic Calorimetry testing (Example: VSP2) to obtain data on temperature and pressure vs. time
  • From the Adiabatic data evaluate the rates of temperature and pressure rise.
  • How fast is heat (or gas) generated?
  • Is the pressure relief system adequate, both in terms of the relief vent area and set pressure?

Dr. James P. Burelbach, Director Process Safety at Fauske & Associates LLC says, "Reaction Calorimetry and Adiabatic Calorimetry are two distinctly different things that both have their place in a process safety strategy. Sometimes, it is a challenge to help people understand the difference and sometimes people think that by getting an RC1 or similar they are covered as far as process safety calorimetry goes. I think this can be a very dangerous approach. I try to simplify by saying that reaction calorimetry helps you understand the desired chemical reactions (for example to design process heat removal systems) while adiabatic calorimetry helps you understand (and prepare for) undesired chemical reactions (for example to ensure there is an adequate pressure relief system if process cooling is lost)."

4. Review Batch Directions and Operator Training

  • Batch review meetings to ensure procedures are correct
  • Operator training to ensure a clear understanding of the procedures

5. Reduce risk or redesign process

  • Mitigation by having an adequately sized pressure relief system (might include dump tank)
  • Redesigning to make it inherently safer using elimination or substitution

Process Safety Strategy: CRH (Ref: FAI)

A strategy such as outlined here helps to formalize the process for easy identification of hazards and enables companies to opt for appropriate measures to mitigate the risk to an acceptable level.

Hope you enjoyed reading this post and that you can relate to understand the steps one should follow while identifying chemical reaction hazards. If you would like to share on the topic, your experience, your questions or future blog topics, you can write to me on himanshuchichra@gmail.com.

Mr. Chichra is a guest blogger with whom Fauske & Associates, LLC (FAI) has recently worked to support customers in India. Read more of his posts related to process safety at www.staub-ex.blogspot.com [staub-ex.blogspot.com] and subscribe to FAI's blog to never miss out on any new process safety content.

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By Hans K. Fauske, D.Sc., Emeritus President, ANS Fellow, AIChE Fellow, NAE Member

As often claimed, the classic hydrodynamic instability theory by Zuber (1958) does not provide the upper external hydrodynamic limitation to onset of the heat transfer crisis for well-wetted horizontal surface,

     (1)

where = pool boiling critical heat flux,  = latent heat of evaporation, ρv (kg m-3) = vapor density, g (9.8 m s-2) = gravitational constant, (kg s-2) = liquid surface tension, and ρl (kg m-3) = liquid density. Here, I propose that the upper limiting value of the heat flux (independent of surface conditions such as porous, polished, or nanoscopically smooth surfaces) is determined by the onset of fluidization, i.e., change in flow regime from liquid to vapor continuous condition.

The superficial vapor velocity jv corresponding to fluidization can be estimated from (Wallis, 1969), 

     (2)

where α is the volume fraction of liquid droplets, and  is the terminal droplet velocity given by (Levich, 1962),

     (3)

Combining Eqs. (2) and (3) and setting CD = 1 and α = 0.6 (corresponding to a state when spherical liquid droplets no longer are touching each other) results in the minimum fluidization velocity,

     (4)

and the peak critical heat flux      (5)

It follows that  . Here we note that the highest measured deviations from Zuber’s instability theory is a factor of 1.78 obtained on microporous surfaces with the highly-wetting FC-72 fluid (Rainey et al., 2003).

In summary, considering an appropriate hydrodynamic limit based upon a flow regime change from liquid to vapor continuous condition due to incipient fluidization (Eq. 5), this limit is clearly substantiated based upon the highest reported heat flux values obtained with well-wetting surfaces at different pressures. As such, the microporous surfaces used by Rainey et al., provide the maximum possible heat removal rates.

References

Levich, V. G., 1962, “Physiochemical Hydrodynamics,” Prentice Hall .

Rainey, K. N., et al., 2003, “Pool Boiling Heat Transfer Microporous Surfaces in Surfaces in FL-72,” Journal of Heat

Transfer, Vol. 125/75 (February).

Wall is, G. B., 1969, “One-Dimensional Two-Phase Flow,” McGraw-Hill .

Zuber, N., 1958, “On the Stability of Boiling Heat Transfer,” ASME J. Heat Transfer 80(2), pp . 711-720.

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