API 650- Annex I- Sec I5.5

[maneesha0702]

As per this section "If drain pipes are used around the tank perimeter, they shall be at least NPS 1 in diameter and have a minimum
wall thickness of Schedule 40. The pipes may be perforated in the undertank area to improve their leak detection
function. The inner ends and perforations of the drain pipes shall be protected from clogging by the use of gravel,
screening, geotextiles, or another method approved by the tank owner. The drain pipes shall exit through the
foundation and shall be visible to indicate any leakage. If specified by the owner, the undertank drains shall be fitted
with a valve or piped to a leak detection well as shown in Figure I.10. The maximum spacing of drain pipes shall be
15 m (50 ft), with a minimum of four drain pipes per tank; however, two drain pipes may be used for tanks 6 m (20 ft)
or less in diameter."

There is no commentary. If you want to have a leak detection and not drainage should you provide 4 pipes ( if the diameter of tank is >20 ft) or can a single pipe suffice.

 

[IFRs]

For leak detection where the two bottoms are separated by sand I always use one pipe for each 50 feet of tank circumference, rounded up, with a minimum of 4 unless the tank is less than 20 feet diameter in which case 2. I tend to interpret the code conservatively...my clients like that approach.