Process Safety Tools of the Trade

Just like different repairs around the house require different tools, process safety requires specific tools for specific applications. Engineering safety labs like ours work with our clients to determine the best and most cost effective way of meeting their process safety needs. Many of the jobs that we see involve vague chemistries with very few known physical properties. For these types of chemistries the VSP2^TM or ARSST^TM teamed with one of the simplified analytical relief sizing methods (Fauske vapor/gas or Omega methods) is typically the preferred choice for obtaining a viable, cost effective relief system design.

However, in cases where well defined chemistries/physical properties are considered, simulation programs can be a very useful tool. Runaway reaction simulators are particularly useful for projects where a large number of vessels with similar chemistries/reactants are being studied. An example of one such application would be a runaway reaction assessment/inhibitor effectiveness and relief system design for a complete unit. Simulation tools such as RRERSP simulation program (developed by Harold Fisher, Chair of the AIChE, DIERS group and Principal Consultant to FAI) can provide the necessary information to size relief systems and produce a variety of time dependant data for each runway scenario including temperature, pressure, venting rate and venting composition. Simulation can be used to explore a variety of mitigation strategies when necessary, including lowering set pressure, installation of insulation of various types and thickness, water spray, or other means of overpressure protection. The RRERSP simulation program can also be used to calculate heat losses due to radiation during a cooling period after a fire or inadvertent heating scenario to confirm continued stability of the material. An example of a design case using the RRERSP simulation program for a fire exposure scenario is outlined below.

Using the RRERSP simulation program, the fire exposure scenario was explored for the existing relief system and is shown in Figure 1. Simulation shows that the vessel exceeds the maximum allowable accumulated pressure (MAAP) for a reactive system of 1.1*MAWP. As a result, different mitigation strategies were considered and are summarized below:

•  Lower set pressure to see if existing valve can be used with a 1 barg set  pressure (see Figure 2)
•  Rupture disk (4 inch) to fit on vessel nozzle (see Figure 3)
•  Insulate vessel with 1 inch of cellular foam glass insulation (see Figure 4)

Using this methodology, mitigation strategies can be easily explored for large numbers of vessels with similar chemistries. It was determined that for this specific application, simply lowering the set pressure to 1 barg would not suffice, however utilizing a 4 inch rupture disk would provide adequate protection for the vessel. It was also determined that a relief scenario could be avoided by applying at least 1 inch of fire resistant foam glass insulation.

Figure 1                                    Simulation output for existing relief installation

Figure 2                                    Simulation output for 3K4 SRV with 1 barg Set Pressure

 Figure 3                                    Simulation output for 4 inch Rupture Disk 3.5 barg Set Pressure

Figure 4                                    Simulation for vessel with 1 inch foam glass insulation (does not vent)

Providing a comprehensive package of tools and design alternatives allows our customers to make better, more cost effective decisions when designing or verifying relief systems. To discuss a full list of our testing and simulation capabilities and which methodology is best for your application, please contact Mr. Chuck Kozlowski at (630) 887-5216 or kozlowski@fauske.com, www.fauske.com