(Tank %V Displacement)
and its associated upward
buoyant force is a function
of the % volume in water,
where Zones 1, 2, 3 & 4
approximate flood heights
to 25%, 50%, 75% & 100%
of the tank height, with
corresponding “obround”
volumes of 20%, 50%, 80%
& 100%.
(Oil W + Tank W) combines
the downward forces of the
oil and tank weight, which is
consistent, regardless of the
flood level, which only affects
the buoyant force. Oil weight
is calculated @ 40% fill,
and the tank weight is from
manufacturer data.
(Pad Size & Weight) of the
concrete pad is calculated,
after determining the Net
Buoyancy Force with a 15%
safety factor, by using an
area extending 6" beyond
the tank footprint, and
adjusting for the depth.
*Note 6" is a NFPA 31
minimum pad thickness.
The (+) or (-) of Net
Buoyancy Force indicates
an upward or downward
force.
National Oilheat Research Alliance 65
Appendix - Vertical Obround Steel Tanks
Tank capacity & Size
(gal) (L" x W" x H")
Flood
Zone #
(Tank %V Disp) – (Oil W + Tank W)
= Net Buoyant Force (lbs)
Pad Size & Weight
(L" x W" x H") (lbs)
120 30 x 23 x 46 0 (0%V) – (345 + 154) = -499 48 x 35 x 6.00 NA*
1 (20%V = 206) – (499) = -293 x 6.00 NA*
2 (50%V = 515) – (499) = +17 x 6.00 NA*
3 (80%V = 825) – (499) = +326 x 6.00 NA*
4 (100%V = 1031) – (499) = +532 x 8.89 612
138 30 x 27 x 44 0 (0%V) – (396 + 158) = -554 48 x 39 x 6.00 NA*
1 (20%V = 237) – (554) = -317 x 6.00 NA*
2 (50%V = 593) – (554) = +38 x 6.00 NA*
3 (80%V = 948) – (554) = +394 x 6.00 NA*
4 (100%V = 1185) – (544) = +631 x 9.46 726
220 49 x 27 x 44 0 (0%V) – (632 + 218) = -850 61 x 39 x 6.00 NA*
1 (20%V = 378) – (850) = -472 x 6.00 NA*
2 (50%V = 945) – (850) = +95 x 6.00 NA*
3 (80%V = 1512) – (850) = +662 x 6.83 761
4 (100%V = 1890) – (850) = +1041 x10.74 1197
230 60 x 22 x 44 0 (0%V) – (660 + 236) = -896 72 x 34 x 6.00 NA*
1 (20%V = 395) – (896) = -501 x 6.00 NA*
2 (50%V = 988) – (896) = +92 x 6.00 NA*
3 (80%V = 1581) – (896) = +685 x 6.86 788
4 (100%V = 1976) – (896) = +1080 x10.83 1242
240 60 x 23 x 46 0 (0%V) – (689 + 246) = –935 72 x 35 x 6.00 NA*
1 (20%V = 412) – (935) = -523 x 6.00 NA*
2 (50%V = 1031) – (935) = +96 x 6.00 NA*
3 (80%V = 1650) – (935) = +715 x 6.96 822
4 (100%V = 2062) – (935) = +1127 x10.97 1296
275 60 x 27 x 44 0 (0%V) – (790 + 252) = -1042 72 x 39 x 6.00 NA*
1 (20% V = 472) – (1042) = -568 x 6.00 NA*
2 (50% V = 1180) – (1042) = +138 x 6.00 NA*
3 (80% V = 1890) – (1042) = +848 x 7.41 975
4 (100%V = 2362) – (1042) = +1320 x11.53 1518
330 72 x 27 x 44 0 (0%V) – (948 + 288) = -1236 84 x 39 x 6.00 NA*
1 (20%V = 567) – (1236) = -669 x 6.00 NA*
2 (50%V = 1418) – (1236) = +182 x 6.00 NA*
3 (80%V = 2268) – (1236) = +1032 x 7.74 1187
4 (100% = 2835) – (1236) = +1599 x11.89 1839
Special Notes - Calculations for vertical obround tanks are based on, or assumes:
a) 12 ga. steel and typical dimensions (most common types are similar),
b) tank weight & oil weight based on manufacturer averages & volume charts,
c) tank is 40% filled with #2 heating oil @ 0.86 spg (7.18 lb.gal),
d) flooding is in salt water (worst case vs fresh water),
e) concrete pad L&W extend 6.0" beyond the tank footprint,
f) concrete density is 145 lb/cuft (represents common types),
g) short legs provide minimal tank/pad clearance (worst case), and
h) safety factor is +15%, applied to the net buoyancy force