Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

atmospheric AST without emergency vent

atmospheric AST without emergency vent

atmospheric AST without emergency vent

i recently saw some excellent condition atmospheric pressure tanks which were fabricated between 10-15 yrs ago.  The tanks range from 5000 to 12000 gallons.  One thing that caught my attention is that they do not have e-venting provided.  

The product has a "1" NFPA signage  (flash point over 200 degrees F).  apart from that, i don't really know much else about it.  i know the tanks would require e-venting if heptane were assumed for unknown fuel products per API 2000. the tanks have a 3" pipe serving as the operational vent which would be adequate for normal venting.

is there a good reference, link, or tip for emergency venting beyond the UL 142 fabrication standard and API 2000 venting document, or some advice? thanks in advance.

RE: atmospheric AST without emergency vent

Emergency vents that I currently see with new tanks for such product as fuel oil are manhole covers fitted with long threaded bolts allowing about 1/2" travel for the cover to lift.

RE: atmospheric AST without emergency vent

Depending on the roof construction and the code of construction, they may have had frangible roofs rather than emergency vents.  It's also possible they were built for products not requiring emergency venting.

RE: atmospheric AST without emergency vent

the tanks are horizontal tanks with no blow-off roof and no emergency vent manways or other forms of emergency vents.  the only thing that might count for e-vent is if the 3" operational vent were to be considered.  This 3" vent has it's own complications in that they have a floating level gauge in it as well.

i guess the big question is how stable does a product have to be to not require emergency venting and does that limit live within the "0" or the "1" NFPA signage rating.

thanks for the quick response you two... i've more or less been on vacation since the post and haven't thought much about it.

RE: atmospheric AST without emergency vent


Study up on NFPA-30 and understand the differences between the various categories of flammable liquids. Emeregency vents are required for certain types of liquids.

Additionally, consider the method of offloading the liquids you intend to store in the tanks. If the tanks are being filled by tanker trucks with an "air pad" (trucks that use air pressure to offload).

In that case, an emergency vent is POSITIVELY required. FRP/plastic tanks have been known to fail due to the pressure surge that occurs at the end of delivery.


RE: atmospheric AST without emergency vent

Why take a chance? Do the API 2000 calcs. based on hepthane and install the emergency vent (for these small tanks, the EV's won't have to be very large). If you consider the alternate circumstances, it's not worth risk.

PS - tanks this small can not be considered having frangible roofs.

RE: atmospheric AST without emergency vent

okay... so i managed to stay away from purchasing NFPA codes over the years (always too busy with the API and STI standards which are more applicable for existing tanks.  also, since all the other times i've seen ASTs without E-vents they were always holding very flammable product and were no-brainers that they would have to install) but i finally broke down and got one like MJCronin suggested... i've copy pasted an interesting portion that addresses this question.

appears to me that the tanks that are greater than 12000 gallons didn't need e-venting.(the big ones actually were 15000 not the 12000 that i threw out there in a previous post.) also worth noting, all the tanks in the containment area hold the same product.

Class IIIB liquids are flash point over 200F... which translates to 1 on the NFPA 704 hazard sign.   Side note....I bet i'm not the 1st person to think it's foolish for the NFPA to classify stable materials as "1" according to one standard and highly volatile materials as I (aka 1) according to another standard.

"22.7 Emergency Relief Venting for Fire Exposure for Aboveground
Storage Tanks.
22.7.1 General. Every aboveground storage tank shall have emergency
relief venting in the form of construction or a device or
devices that will relieve excessive internal pressure caused by
an exposure fire. This requirement shall apply to each compartment
of a compartmented tank, the interstitial space (annulus) of a
secondary containment–type tank, and the enclosed space of
tanks of closed-top dike construction. This requirement shall also apply to spaces or enclosed
volumes, such as those intended for insulation, membranes,
or weather shields, that are capable of containing liquid
because of a leak from the primary vessel. The insulation,
membrane, or weather shield shall not interfere with emergency
venting. Tanks storing Class IIIB liquids that are larger than
12,000 gal (45,400 L) capacity and are not within the diked
area or the drainage path of tanks storing Class I or Class II
liquids shall not be required to meet the requirements of"

There are some overfill prevention devices currently installed on some large flange connections which could be switched out for e-vents or possibly long-bolted for the smaller tanks.  I'm thinking i'll recommend they consider configuring those sensors to have a lifting top-plate for a base for the sub-12000 gallon tanks.

RE: atmospheric AST without emergency vent


I think that your post touches on the issues/problems related to the multiple usage of tanks in the US.

In other parts of the world, a tank has the flamablity/usage "rating" noted on the design drawings.(And on the tank) If you want to use it for something else, you must change the fire rated class AND CHANGE THE DRAWINGS!!!

(A watertank is a watertank is a ....apologies to Gertrude Stein)

But, in the USA, we have these generic containers that can be used for multiple services.......just get some PE to take responsibility and sign things of when the Fire Inspector comes by......and every thing will be OK


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close