4 replies to this topic

[Yajnavalkya]

Is there a restriction on the cumulative capacity that can be stored under one dike?
I know OISD 118 has. But does NFPA 30 have one? Please help.

[Steve Hall]

My understanding is that a dike must contain 1.5 times the volume of the largest tank within it. Other sections (of NFPA) address spacing of tanks. Since the tank spacing is intended to prevent a fire or leak from propagating from one tank to another, the dike capacity is unrelated to the number of tanks contained, assuming that only one tank will rupture. Any volume within the dike (tanks, pumps, etc.) must be subtracted from the volume of the empty dike to determine the containment volume.

[breizh]

These documents may support your query:


http://www.ene.gov.o...td01_079136.pdf


http://www.cheresour...adjacent-tanks/

Hope this helps
Breizh

Edited by breizh, 29 December 2012 - 03:47 AM.

[kkala]

I am not specifically familiar with NFPA, yet local fire fighting legislation for refineries (see 2nd link in post No 3 by breizh) is influenced by it. Some requirements of mentioned legislation are briefly mentioned below, just to give an orientation for search in NFPA, which may contain similar restrictions (probably not identical).
1. A cone roof crude oil tank shall have dedicated dike. A pair of floating roof crude oil tanks can be in one dike (with dedicated internal walls for 30% of each capacity); exceptionally 3 such tanks in the last dike are permitted. A crude oil tank dike shall not contain other tanks.
2. Floating roof and cone roof tanks shall not be placed in same dike.
3. If a group of floating roof tanks are in one dike, their total capacity is recommended to be less than 120 000 m3. This is limited to 60 000 m3 for cone roof tanks.
4. Dike shall be able to contain max capacity of biggest tank plus 10% of total capacity of rest tanks. This differs to relevant NFPA requirement (post No 2 by Steve Hall).
5. Tanks storing fuels (of flash point > 21 oc) at temperatures below their flash point can have dikes of more tolerant specs under certain conditions, but I have not seen such relaxations applied in refineries.

Apart from regulations, I think a dike cannot be separated into two distinct fire zones. In case of a tank collapse due to fire, the fuel will cover the whole dike area, so all tanks within the dike have to be water-cooled for protection against the pool fire. This would result in excessive fire water in case of a large, "crowded" dike. See a similar case in http://www.cheresources.com/invision/topic/16277-fire-zone-consideration-for-finger-type-slug-catcher/. Colleagues having some experience in refinery fire fighting design clarified that they try to enclose up to 3 tanks in a single dike (not more), depending on their size. Advice on this point is welcomed.
A local design case included four tanks (7500 - 15000 m3 each) in a single dike (each tank had a secondary wall around it to contain ~15% of its capacity).

Edited by kkala, 29 December 2012 - 03:36 PM.

[ankur2061]

As per a reputed company standard:

"Any dike enclosure shall be capable of holding the greater of 110 % of the greatest amount of liquid that can be released, or the greatest amount of liquid that can be released plus the volume of stormwater."

An example calculation is provided for adequacy of diked area for multiple tanks and which is referenced from "Flammable & Combustible Liquids Code Handbook" by Rober P. Benedetti

There are four tanks in a diked area. The largest is 30.5 m (100 ft) in diameter and 10.5 m (35 ft) high. Two other tanks are each 15 m (50 ft) in diameter and 6 m (20 ft) high, while the fourth tank is 12 m (40 ft) in diameter and 4.5 m (15 ft) high. The diked area is 6503 m² (70,000 ft²) and has a 1.52 m (5 ft) high dike wall. Is the capacity of the dike sufficient for the largest tank?

Begin by determining the total volume of the diked area, which is equal to the area times the height of the dike wall: 6503 x 1.52 = 9910 m³ (70,000 x 5 = 350,000 ft³) of capacity. Refer the attached sketch
Deduct the volume occupied by the three smaller tanks from the capacity of the dike enclosure. The tanks occupy available dike capacity only as high as the dike wall or 1.52 m (5 ft). The value in the volume formula (V= πd²h /4), therefore, shall be 1.52 m (5 ft).

The total volume of the smaller tanks, up to the 1.52 m (5 ft) level, is 710 m³ = 269 + 269 + 172 (25,074 ft³ = 9500 + 9500 + 6074). This volume, subtracted from the overall dike capacity, leaves 9200 m³ (324,926 ft³) of usable volume within the dike.

Determine the volume of the largest tank. Calculations give the tank capacity of 7671 m³ (270,900 ft³). The dike volume is adequate.

Hope this helps.

Regards,
Ankur

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