Pressurized DCS Room in Classified Area

[controlnovice]

We are re-locating a room with DCS equipment to an area surrounded by Zone 1 classification.

The DCS room will be pressurized and also has an 'air-lock' (the entry-way will have two doors before entering the DCS room). The design is to have pressure alarms in the room and tied to the DCS for alarming. I believe this to be Type X pressurization per NFPA 496.

Now the problem is, Type X requires that electrical equipment be disconnected on loss of pressurization. Power to the circuits may be continued for a period of time if the immediate loss of power would result in a more hazardous condition. This would shut down all control of the process....not a good idea.

How have others handles this situation?

I know the correct answer is don't build the room in a hazardous area, and I've fought this for 3 months now, but to no avail.



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[ScottyUK]

Is the building inhabited / occupied? If so then a more pertinent question might whether the building is designed to resist an explosion in the hazardous area. Blast-resistant bunkers are expensive - significantly more so than locating the DCS equipment out of the area.


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Sometimes I only open my mouth to swap feet...

 

[waross]

I would hope that the period of time allowed is not indefinite, but only suficient to allow an orderly shutdown of equipment.
I would emphasize the implications of an orderly shutdown when the pressurization fails. (Not IF but WHEN)
I would be asking the process folk for suggestions as to the best way to implement an AUTOMATIC, IMMEDIATE orderly shutdown WHEN the pressurization fails. Try to have the scheme submitted to the insureres for acceptance.
If I implemented an automatic, immediate orderly shutdown on loss of pressurization, and had the scheme signed off by both the operations department and the insureres, I would feel that I had achieved an acceptable compromise.
respectfully

 

[stephenw22]

Redundancy is your friend. Backup ventilation that either constantly runs, or is tied to an undercurrent relay on the main fan, so it kicks in when the main turns off. The more critical, the more redundancy. Separate power feeds for fans. UPS or generator backup power for fans. Pressure warnings whenever the pressure starts to drop below the required setpoint, not just if it hits a minimum value.

There are lots of ways to keep the building pressurized.

 

[controlnovice]

Okay:

The room is unoccupied.

We do have redundant pressurizing systems.

We do have an emergency generator that starts within 15 seconds.

The plant can be taken down orderly, using the DCS, if pressurization fails by reading the pressure in the DCS.

So, my question is: If we loose pressure in the DCS room, do we have to shut down the power feed to the room?

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[itsmoked]

I cannot imagine why you wouldn't! It is a non classified room ONLY on the basis of the pressurization.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[controlnovice]

Thank you. I've been trying to get that point accross to the PM and I just needed a sanity check.

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[LionelHutz]

Waross and Keith together give the answer;

On loss of ventilation;
1. Orderly but immediate shut down of the plant.
2. Remove all power feeds to the room