Increase Manhole Sizes. Many older aboveground
vertical tanks have inadequately sized shell manholes. The
ventilation and equipment requirements for maintenance have
created a need for 36-inch (900 mm) diameter manholes. If
they do not exist, consider rehabilitation. Provide in
accordance with Section 8 of this handbook.
Replace Tank Floors. Replace existing tank floors
only when an API Std 653 inspection (including a magnetic flux
leakage test) indicates that the useful life of the bottom has
expired, if the tank is leaking, or if significant fuel
quality problems due to ponding water exist. Conduct an
analysis of the existing tank and local conditions to
determine the most desirable approach. The type of new bottom
to be installed depends upon a number of factors, including:
condition of the existing bottom, tank foundation, shell
condition, and amount of tank capacity which can be "lost."
Install double bottoms or replacement bottoms in accordance
with API Std 653. Install sloped bottoms (3 to 5 percent)
either above the existing bottom if the user can accept the
resulting loss in tank capacity or remove the existing bottom
and install a new sloped bottom in its place. If conditions
and cost make the recommended slope impractical, provide a
minimum slope of 2 percent. In either case, install an
impermeable liner in accordance with Standard Design AW 78-24-
27, as well as leak detection between the two floors. The
technology exists and it may be cost effective to raise the
tank off its foundation to accomplish under tank modifications
or repairs (e.g., adding containment liner/tell-tale system,
cathodic protection, ringwall, etc.).
18.104.22.168 Double Bottom With Washed Sand Layer and Liner.
Clean and repair the existing tank bottom, install a liner on
top of the existing tank bottom, add a 4- to 6-inch (100 to
150 mm) silica sand layer on top of the liner, and install the
new steel tank bottom on top of the sand layer. Refer to
Section 8 of this handbook for suitable sand criteria. Leak
detection consists of tell-tale slotted PVC pipes within the
sand layer at regularly spaced intervals and extending out
through the outer shell of the tank. Provide cathodic
protection by sacrificial anodes or impressed current close to
the liner to allow maximum clearance from the new tank bottom.
Install adequate numbers of a cathodic protection reference
cells between the two bottoms.