User Tag List

Results 1 to 7 of 7

Thread: NFPA 497----Open Transfer System Vs Closed Transfer System

  1. #1
    Join Date
    Mar 2013
    Location
    USA
    Posts
    1,146
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    NFPA 497----Open Transfer System Vs Closed Transfer System

    I am studying NFPA 497. I am looking at figure 5.10.4(c) and 5.10.4(e). (c) uss the terminology "Unloading via a closed transfer system", and (e) uses the terminology "unloading via an open transfer system". What do the terms open/closed transfer system mean in this context? Also, (e) refers to a "dome". What does "dome" mean in this context?
    Time is of the essence, and I am low on essence. ~ Graham Hill

  2. #2
    Join Date
    Apr 2004
    Location
    Mission Viejo, CA
    Posts
    5,253
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    I'm going to answer backwards. The "dome" is the auxiliary vessel on the top of the tank car. It houses the tank car's part of the vapor recovery system.

    "Open" and "closed" refer to the method the loading, unloading, and vapor recovery lines are connected to the tank car before loading or unloading occurs. A "closed" system makes all sealed connections before the external processes begin. An "open" system may have a brief moment where the connections aren't completed before the external processes begin.

    In modern nations, only closed connections are permitted.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  3. #3
    Join Date
    Mar 2013
    Location
    USA
    Posts
    1,146
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Thank you Rbalex, as always.
    Time is of the essence, and I am low on essence. ~ Graham Hill

  4. #4
    Join Date
    Mar 2013
    Location
    USA
    Posts
    1,146
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Looking at figure 5.10.4(b), which is a Tank Car Loading and Unloading via a Closed Transfer System, material transferred only through a dome, the material being transferred is a flammable liquid.

    This figure only has the hazardous area defined around the leak point, and I am surprised to see that the hazardous area does not exist at grade level. Is this because of the presence of the vapor hood? Any leaking or fugitive emissions are sucked up through the hood and do not make it to grade level?

    I am trying to think of other methods that could potentially limit the hazardous area to the radius defined around the leak point, and eliminate the classified area at grade level... For example, if I have a charging station for a mildly flammable heavier than air liquefied gas, would placing the vessel being charged at grade level on a grated platform still require a hazardous area (up to) 3 feet above the grade level extending 15-25 feet even through the grade level is a grated structure above a pit which is below grade?

    I know the pit below the grate would need to be considered a classified area, and would need to be monitored and ventilated, but would the hazardous area exist also above the grating above the pit or trench, or just in the trench itself? Would it be reasonable to say the area above the grating is not classified, but the area below the grating, ie the pit itself, would be the classified area?
    Time is of the essence, and I am low on essence. ~ Graham Hill

  5. #5
    Join Date
    Apr 2004
    Location
    Mission Viejo, CA
    Posts
    5,253
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Basically, you're dealing with a situation similar to a gasoline fuel pump nozzle. These are sealed connections and no emissions should occur in the first place. For the most part, the vapor recovery system should eliminate potential leaks; this is also why the hazard radius is only 3'.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  6. #6
    Join Date
    Mar 2013
    Location
    USA
    Posts
    1,146
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    That is interesting. What about the situation where the grade level flooring is grated above a pit? Would the hazardous area still potentially exist 3 feet (for example) above the grated grade level? Or would the grating potentially eliminate the hazardous area at grade? For example, if the floor was grated could an electrically powered truck of just Type E (according to NFPA 505) be driven across the grating?
    Time is of the essence, and I am low on essence. ~ Graham Hill

  7. #7
    Join Date
    Apr 2004
    Location
    Mission Viejo, CA
    Posts
    5,253
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Yes, it’s interesting, what about it? You do realize this is a virtually impossible scenario since the NFPA 497 Figure is based on a stationary railroad loading/unloading rack and the 505 example you fantasized about would not be a realistic application for a railroad tank car? In fact, look over the NFPA 505 Table 4.2(a) very carefully for examples of trucks likely to be approved for your scenario. Reading the NFPA 505 Scope carefully wouldn’t hurt either.

    If this doesn’t clarify the issue for you, contact David Wechsler. He is a member of CMP14 and both NFPA 497 and 505 Technical Committees. He has retired from Union Carbide and I no longer have his phone number so finding it is an excercise left for the student.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •