Re: Firewall Capacity VolumeFrom: CNT 
Date: Wednesday, February 22, 2012
Time: 8:24:26 AM
Remote Name: 188.8.131.52
CommentsI have seen full shell capacity used and I have seen fill capacity used. I think you can easily argue that fill capacity is acceptable if there are overflow vents cut in the shell or there is some other physical restriction. I think you can successfully argue that it would be acceptable if there is a sufficient alarm system / procedure that wouldnt let the tank get that full. I think this is a case where the owner could make the call on which to use and you just need to document why you used what you did it.
Most places you have to account for rain water as well. 110% of the tank capacity has been an industry rule of thumb, but some places actually call out a rain event in their fire codes. Typically a 25-yr 24-hr event. I have gone to using the greater of the rain event or the 110% just to cover my bases. I have seen several requirements for adequate capacity or some such terminology, which lets you guess what adequate means and wait for them to fine you if you are wrong. That is another reason I go with the greater of the two above.
If you are building new, I think it is best to build the dike walls up to get the most capacity now. We all know they can get erroded down and lose capacity and it is costly to build them up later. If you are working with an existing installation you might need to do more thinking and it could be that your dike capacity becomes a limiting factor on your tank fill height.