2 replies to this topic

[Art Montemayor]

PED:

If I interpret your dilemma correctly, I believe you are correct in your engineering logic and reasoning. Allow me to show how I arrive at this conclusion:

I assume your spheres are stored with saturated Propylene and, at the worst spillage conditions, the storage pressure would be 135 psia at 60 ^oF .
The definition of this worst spillage scenario is to generate the amount of liquid C₃ hitting the ground under the spheres. If it can get colder, then make the adjustments below.

The % of vapor Propylene generated (V/F) =(h_F - h_L)/(H_v - h_L)

where,

V = C₃ vapor formed, mass/time
F = C₃ saturated liquid spilling out of sphere(s), mass/time
h_F = 330 Btu/lb @ 60 ^oF & 135 psia
h_L = 265 Btu/lb @ -50 ^oF & 14.7 psia
H_v = 453 Btu/lb @ -50 ^oF & 14.7 psia

Propylene vapor generated by leakage, % of leakage = 34.5%

Propylene liquid hitting grade, % of leakage = 65.5%

My source of information is the GPSA Engineering Data Book.

Although you have 2/3 of the total leakage hitting the deck below, this liquid Propylene is at -50 oF and will evaporate almost instantaneously at first. As the containment concrete floor cools, the liquid will remain liquid for a longer duration. However, this liquid is destined to be evaporated eventually and nothing can stop that process. In fact, firefighters will have to abandon the site because of the uncertaintly of when it will evaporate completely.

If you use a porous, gravel grade under your tanks (with a concrete containing wall, of course) the situation will change. You will have a greater (and faster) amount of Propylene evaporated in the same time frame. From a safety (& insurance) point of view, this is what you desire if you have to accept the scenario of a leak. It would seem far more expedient to accept a vaporization of the C3 (assuming you can simulate and predict the resultant cloud) rather than try to deal with a contained pool of liquid Propylene fuel emanating from the containment area over a longer time.

I would not, at this point of a disaster scenario, place importance on the ground protection. I am more interested in the human protection and, right after that, the protection to the tanks and the vital equipment around them that will facilitate controlling and fighting this potential fire/explosion. The resultant vapor cloud and its characteristics are very important.

I believe that you can challenge the insurance company on their ruling and negotiate for what you feel is the safest scheme. However, I know there are persons better founded and knowledgeable in the vapor cloud phenomena and scenarios such as the one you describe. Such a person fortunately visits this forum from time-to-time. His name is Milton Beychok and he is a person I would definitely call upon to give his valued and experienced comments (& possible recommendations). I think Milton would be a great help in aiding you as well as educating most of us who may, from time to time, have to confront similar disaster scenarios.

I'll cross my fingers and hope Milton is reading this and responding accordingly.

I hope I have helped, albeit in a small way.

Art Montemayor

[mbeychok]

Ped:

I agree with everything that Art Montemayor said in his response. As pointed out by Art, only about one-third of the leaked propylene will immediately flash into vapor. The residual liquid will pool on the ground beneath your spheres and will then evaporate as it absorbs heat from the atmosphere and from the ground. The propylene would very probably evaporate completely long before any of it reached an underground aquifer. But even if some small amount of the propylene did reach an underground aquifer, it would eventually evaporate out of the water ... without leaving behind any hazardous substances.

On the other hand, in my opinion, a concrete pad underneath the tank might be damaged by being subjected to a temperature of -50 degrees Fahrenheit. If the pad were to be badly damaged, that could lead to further damage of the spheres which would only aggravate the situation rather than help.

I agree with Art that you should try negotiating with your insurance company. However, if the insurance company should insist upon having a concrete pad, then I would suggest the following:

(1) Place a retainer wall around the pad of sufficient height to contain the entire propylene capacity of the spheres.

(2) Provide the pad with a drainage sump equipped with a pump to route the drained propylene into an alternate storage vessel.