NFPA Requirements for Structural Ductwork Design in Power Plants

[tdunaway]

I have heard several times that ductwork in power plants needs to be designed for a minimum pressure of +/- 35" H20 per NFPA. Does anyone know where I can find that requirement? Which NFPA Standard? Which section, page, or paragraph?

I am specifically looking for interpretation as to whether or not the 35" requirement is in addition to other design loads (i.e. dead load, live load, wind load, etc...). Or is it just a minimum total design pressure (cumulative of all design loads)?

Thanks,

 

[JedClampett]

That seems like a lot of pressure. Thirty five inches is like 180 psf. Are you sure there's not a decimal point missing there?

 

[tdunaway]

Update -

I think I found the standard I was looking for: NFPA 85 (2011 edition), section 6.5 - Furnace Implosion Protection. My interpretation is that the furnace and duct should at a minimum be designed for the test block capability of the fan, except that design pressure greater than 35" is not required (positive or negative). This is called the transient pressure, and is not the same as operating pressure of the duct. The transient pressure does not need to be combined with other transient loads, such as wind or seismic. So the 35" requirement is more like a maximum than a minimum.

This is not stated in NFPA, but I assume the operating pressure should be combined with other loads such as dead, live, wind, seismic, etc... as applicable to determine the overall design pressure of the duct. The determined design pressure should be compared to the transient pressure and the duct should be designed for the worst case of the two.

If anyone has any experience and can provide any feedback on this topic, it would be welcome.

Thanks,

 

[tdunaway]

JedClampett -

I checked, and NFPA does indeed say 35" of water pressure. I agree, its a lot of load, but I think the standard is saying the duct need not be designed for more than 35". Not the other way around like I had previously thought.

 

[tdunaway]

If anyone is interested, here is a pdf of the NFPA Section I am referring to.