January 25, 2005 at 8:34 pm #4387
With a top head open, there is a potential of H2S exposure due to the possibility of a reverse flow through coke drum relief valves. This issue has been previously discussed earlier in this forum but I thought that I would bring it up again because we also have contemplated installing a MOV on a downstream isolation valve on the relief line from each coke drum with an interlock tied to the drum vent MOV. We would like to do this but we see a draw back. What if the MOV actuator on the isolation valve goes bad and does not open the valve or the gate breaks loose from the stem? The relief header from that drum would still be isolated during the next coking cycle. A coke drum without a relief capacity could be just as dangerous as H2S exposure. We have purge steam on the downstream side of the relief valves to mitigate a reverse flow problem. We also have auto top head removal on one coker but not on the other. We have considered having the Operators wear personal H2S monitors for an early warning indication but we don’t want them to think that this is a protection devise so we are reluctant to do that because this may give them a false sense of security. What other solutions or alternatives are there?
February 3, 2005 at 11:17 am #7859
We have had many discussions about this:
The PSV is designed for out flow
The PSV is robust by design and routinely inspected
The PSV has a steam tattletale if the bellows has been blown
In 33 years, we have never had gasses from the flare over come the steam purge and the integrity of the valve.
No history of a PSV failure in reverse flow
We have downstream block valves they are car sealed open and not used for LOTO unless the PSV is removed for maintenance.
Our operators all have H2S montiors on while in the unit.
February 3, 2005 at 8:01 pm #7856
We are riding a Low Line system (gas recovery system) on one coker unit and are riding the Fractiontor OH Accumulator on the other coker unit so we do not flare unless we overpressure the Low Line or Fractionator OH. The main exposure we have for a reverse flow on a bad relief valve is when another Drum overhead vapors are switched to the blowdown system. It is a rare event when we get a reverse flow but never-the-less, it is an exposure. In 36 years, we have only had just a few of these events happen.
February 9, 2005 at 10:12 am #7854
I will be against MOV installation on the relief line. As it seems, your drum cover opening is not remotely operated. Nonetheless, to check on H2S exposure to the crew, I would suggest some kind of drive in the Coke drum vent line that would drive off the steam/gases in the coke drum through the vent line to a safe location.
February 9, 2005 at 9:11 pm #7852
Great suggestion! I like the idea of installing an H2S monitor at the vent line.
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