# Overpressure valves



## partimehero (Dec 24, 2010)

Hi, 

Does anybody know if these will be essential in a purpose built concrete shelter?

I am making my own ventilation system using a centrifugal blower, carbon / HEPA filter box and external blast valves of some kind. 

The only thing I'm not sure about is the overpressure valve inside the shelter. 

I am having an external blast valve over the air intake to protect the system. There will be a 'hole' in the wall opposite to expel stale air. On the external side of this, there will be another blast valve and on the inside of this would I need an overpressure valve of some kind???

Pumping fresh air into a shelter constantly should create a positive pressure right? So this should force out the stale air right? BUT whilst being positive pressure, it should stop any nasty air from the outside coming in....... right?

Im a little confused.......

Any help would be appreciated.

Thanks


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## priest75 (May 1, 2011)

*Yes and no...*

Good question. In theory and on paper it looks and sounds just dandy. As long as you are pumping and filtering air coming in at a higher rate than your outlet going out is letting airflow back out you have essentially created a positive pressure containment. HOWEVER... it is really hard to overstate that you can't really overdo this (within reason.) Unless you are building your bunker, wig out room, fallout shelter, whatever it is under several feet of soil and have near 12 inches of water membrane SEALED concrete, rebar, mesh (steel i-beams depending on dimensions with continuous bar running the length) electrical (if generated or ran in) water pumps (unless tanked out or inside) sealed blast doors, emergency escape hatch (if so designed) and ALL other considerations taken into account, odds are you are still not air tight besides that exhaust outlet. Hey let's face it, if you're going to put all your time and money into preparing for you and your family's survival WSHTF, you best be doing it 100% and no slacking half-ass work. My best suggestion on that would be this: before you bury that bunker with your 3+ feet of soil, close her all up and either run your smoker or get a bunch of them cheap smoke bombs (if legal in your area.) Go about the entire outside perimeter of the unit and make sure you see NO smoke getting out - conduit, piping, etc is all very vunlerable to this released air. If that air is getting in, if for some reason your air filtration faulters for a short time, then them same leaks are where air will be coming in from as well - as well as unwanted moiture. As far as the rest of your question, blast valves are to be placed on both incoming and outflowing exhaust pipes. An overpressure relief valve is critical to creating an overpressure in the protected space by adding the proper, metered resistance to the outgoing airflow. The valve is normally closed and will not open unless the air pressure inside the shelter is slightly greater than the outside air pressure.
Hope this helps ya out there and good luck!


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## partimehero (Dec 24, 2010)

*Thank you*

All of this definitely makes sense. The structure design will be very similar to how you have described and will be made to (possibly) 3 bar resistance.

I appreciate you taking the time to write all that stuff.

Thanks!!!!


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