Figure 12-13. E.ectric heat tracing system.
a Bar) of atmospheric pressure. It may be either open to the atmosphere or enclosed. Minimum cost is usually obtained with a vertical cylindrical shape and a relatively flat bottom at ground level.
Elevated Tanks
These can supply a large flow when required， but pump capacities need beonly for average flow.Thus， they may save on pump and piping investment.They also provide flow after pumpfailure， an important consideration for fire systems.
Open Tanks
These may be used to store materials that will not be harmed bywater，weather， or atmospheric pollution. Otherwise， aroof， either fixed or float-ing， is required. Fixed roofs are usually either domed or coned. Large tanks have coned roofs with intermediate supports. Since negligible pressure isinvolved， snow and wind are the principal design loads. Local building codes often give required values.
Fixed .oof Tanks
Atmospheric tanks require vents to prevent pressurechanges， which would otherwise result from temperature changes and the withdrawal orthe addition of liquid. API Standard2000， venting atmospheric and Low Pressure StorageTanks， gives practical rules for vent design. The principals of this standard can be applied to fluids other than petroleum products.Excessive losses of volatileliquids， particularly those with flash points below 100 0F (38 0C)， may result from the use of open vents on fixed-roof tanks.Sometimes vents are manifolded and led to a vent tank， or the vapor may be extracted by a recovery system.
An effective way of preventing vent loss is to use one of the many types of variable-volume tanks. These are built under API Standard 650. They may have floating roofs of the double-deck or the single-deck type. There are lifter-roof types in which the roof either has a skirt moving up and down in an annular liquid seal or is connected to the tank shell by a flexible mem-brane. A fabric expansion chamber housed in a compartment on top of the tank roof also permits variation in volume.
Floating .oof Tanks
These tanks must have a seal between the roof and the tank shell. If notprotected by a fixed roof， they must have drains for the removal ofwater， and the tank shell must have a ""wind girder"" to avoid distortion. An industry has developed to retrofit existing tanks with floating roofs. Much detail on the various types of tank roofs is given in manufacturers" literature. Figure 12-14 shows types. These roofs cause less condensation build up and are highly recommended.
Pressure Tanks
Vertical cylindrical tanks constructed with domed or coned roofs， which operate at pressures above 15 psia (1 Bar) but are still relatively close toatmospheric pressure， can be built according to API Standard 650. The pressure force acting against the roof istransmitted to theshell， whichmay have sufficient weight to resist it. Ifnot， the uplift will act on the tank bottom.The strength of the bottom， however， is limited， and if it is not
Cone–roof tank – Dome–roof tank – Floating-roof tank Lifter-roof tank – Variable-vapour-space tank – supported roof self-supported roof dry-seal type dry-seal type
Figure 12-14. Some types of atmospheric storage tanks.
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