CDOIF Guidance

Safety and environmental standards for fuel storage sites

Final report

223 Figure 2 in Part 2 of this report ‘Overfilling protection: Tank levels’ (based on API 2350) gives

three levels:

Normal fill level;

■

Tank rated capacity;

■

Overfill level.

■

224 When determining the bund size required, three modes of loss of containment have to be

addressed:

Overfills;

■

Leak;

■

Catastrophic failure.

■

225 The bund should be sized for 110% of the ‘tank rated capacity’ (TRC) as a minimum. This

assumes that the minimum standards for overfill protection systems

of control

are in place relating to:

tank levels and capacities are determined in accordance with Appendix 3;

■

position and type of level gauges and high level detectors;

■

how are these monitored and the required response;

■

response times to shutdown inflow.

■

226 If – for example, the TRC level is alarmed and the overfill protection system setting is at TRC –

it is reasonable to take this as tank capacity.

227 If – for example the TRC level is alarmed and interlocked at – it is reasonable to take this as

tank capacity (subject to failure rate of alarm and interlock).

228 Operators should also record overfill volumes to establish the difference in risk between TRC

and overfill levels – which may involve significant volumes for larger tanks. This is to be reported

for information only.

229 Unless multiple tanks sharing the same bund are hydraulically linked, simultaneous overfill of

independent tanks can be discounted as a realistic hazard. Therefore, the 25% criteria would not

apply to the Overfill level. For the bund capacity calculation based on 25% of the total capacity of

all the tanks, the normal fill levels of all the tanks within the bund should be used.

230 The 25% criterion applies to the risk of loss of containment of more than one tank and

provision for firewater management. This provides a buffer to deal with the incident and informs

risk assessment as to the degree of tertiary containment that may be required to deal with

subsequent failure of secondary containment in a severe and prolonged event. The actual sizing

for multi-tank bunds will be determined by the hazard and the risk – including the modifying

factors stated above. Where increased bund area leads to larger dispersion distances to a safe

vapour concentration, operators may consider providing remote secondary storage.

Bund strength

231 A bund should be capable of withstanding the full hydrostatic head of liquid that may arise

from the loss of primary containment.

232 Following catastrophic failure of a tank

– overtopping of a bund to some extent is usually

inevitable. In the absence of practical guidance on assessment of bunds for likely levels of

overtopping or hydrodynamic loads – emphasis should be placed on mitigation and control of the

effects of overtopping through tertiary containment.