Classification: General Business Use

Atmospheric Storage Tanks are an integral part of any refinery or process plant as they are
used to store fluids for various purposes for short or long duration. Feedstock tanks and product
tanks provide buffer capacity between process plants, which are generally in continuous
operation, and the supply/dispatch of feedstock/products, which are batch operations in general.
Intermediate tankage is provided where there is a requirement to buffer the flow of an
intermediate from one process unit to another. Component tanks are the intermediate tanks
between the process units and the blenders. These tanks are necessary to de-couple blender
operation from process unit operation, in order to give the blender sufficient degrees of freedom
to blend the product optimally.

Swing Tanks
In determining the requirements for feedstock/product, intermediate and component tankage, it
may become apparent that some of the tanks will only be in service for a relatively small
proportion of the time. In these circumstances, there is potential for specifying/designing tank(s)
for more than one duty. Reducing the number of tanks can have a significant impact on capital
costs. Such multi-purpose tanks are often referred to as “swing tanks”.

Tanks are frequently used at the start and finish of a process and it is, therefore, necessary to
know the quantity and quality of the contents. Most fixed roof tanks are calibrated after
construction, and a dip hole is frequently fitted to the top of the tank so that the depth of the
liquid within can be measured accurately with dip tape. This dip hole is often used to collect
samples using a specialist container that can be opened once it has reached the required depth.

Modern-level instrumentation can be very accurate and has in many cases replaced the need for
regular tank dipping. However, it is good practice to retain the dip pipe both as a means of
checking the level instruments and as a means of getting samples. Storage tanks can be of the
following types:

Roofless Tanks
This type of tank (Fig. 1) is normally used for water services including fire water, cooling water
and service water. Potable water will normally be stored in a tank with a roof. Although open-
top tanks are sometimes made of steel, it is common to construct them from concrete, or glass-
reinforced plastic (GRP). The main advantage of a roofless tank is its low cost.

Fixed Roof Tanks

Fixed roof tanks (Fig. 1) usually make up the majority of tanks used in a typical tank farm. The
tanks are generally used for low vapour pressure materials rather than higher vapour pressure
materials, as these would tend to give high VOC emissions during tank breathing and could
generate an explosive mixture in the vapour space.

Fixed roof tanks are used typically for gas oil, fuel oil, and sometimes for kerosene in cold
locations.
Classification: General Business Use

Tanks are very good settling vessels and any water or similar liquid that is not completely
removed by the processing unit is likely to settle in downstream tanks. This can affect the
material selection for the tank, the provision of drains, or the provision of mixers if it is
necessary to keep the two phases in suspension.

Floating Roof Tanks

Floating roof tanks are used for higher vapour pressure materials as they reduce both evaporative
losses and the concentration of explosive mixtures. However, it is essential that the vapour
pressure can never reach atmospheric pressure.

The design of the floating roof can be pan, pontoon, or double skin. However, regardless of
type, all floating roofs need to have a seal. These are to minimise the losses from the tanks and
to prevent the ingress of rainwater into the tank. Most modern tanks are fitted with double seals
to reduce VOC emissions. A scraper seal is fitted if the tank contents are prone to adhering to
the tank walls. Floating roof tanks are used typically for crude oil, naphtha, gasoline, and
kerosene in tropical locations.

Fig. 1: Various Types of Tanks

Internal Floating Roof Tanks

Internal floating roof tanks (Fig. 1) are becoming of increased importance as a result of tighter
environmental requirements. They are more expensive than conventional fixed or floating roof
tanks, but they do offer a number of advantages.
Classification: General Business Use

 They can be used in place of a conventional floating roof tank where there is a need to
exclude the possibility of air contamination. In this case, an internal floating roof tank can be
nitrogen blanketed.
 They can be used for floating roof applications where there are heavy snowfalls. Snow can
sink a conventional floating roof. Tanks built for this reason may be provided with ample
ventilation apertures in the sidewalls just below the roof.
 They can be used where the exclusion of water is very important. Rainwater has been known
to find its way around roof seals and leak rainwater into the tank. This type of tank is used for
Jet Fuel where water should be eliminated from the product.
 They can be used to reduce VOC emissions since the vapour space above the internal roof can
be vented via emission control equipment.
There are some disadvantages to IFRTs in addition to increased cost.

 The vapour space above the internal roof, unless specially ventilated, will be prone to the
production of explosive mixtures. This often has to be blanketed.
 In practice, there are likely to be limitations in tank size (maximum diameter approximately
40 m ) resulting in an increase in the number of tanks.
 In order to work in association with other facilities like vapour removal units, the tanks may
need to be designed for slightly higher pressures

Dome Roof Tanks

Sometimes there is a need to store materials, such as very light naphthas, that have vapour
pressures in excess of atmospheric pressure but are not sufficiently high to justify a sphere or a
bullet.

API 620 allows for such instances by permitting designs up to a design pressure of 15
PSIG. Such vessels can have a variety of shapes, but the most commonly employed is the dome-
roofed tank. A pressurised dome roof is always more costly to fabricate than a coned one. This
is usually due to the major additional cost of the foundations. The additional pressure is likely to
push out the flat floor, and a special foundation is usually required to stop the tank from
deforming. Over/under pressure protection for such tanks is also more like those facilities
provided for pressure vessels.

There are instances when tanks such as these are needed, but most installations avoid their use by
ensuring that the light hydrocarbon streams involved are safely disposed of without storing them.

Design Pressure
Although atmospheric storage tanks are intended to operate at about atmospheric pressure, in
practice they do have a design pressure, even if it is only a few millibar gauges.
Classification: General Business Use

Tank Breathing
When the gas enters or leaves a vapour space in a liquid storage tank, the tank is said to be
breathing. Breathing can occur from a number of causes.

 A rise or fall in the liquid level.

 A rise or fall in the tank temperature (consider the effect of a rain shower on a tank that has
been exposed to the sun all day).
 The escape of gases coming out of solution, or even the ingress of gases being absorbed such
as vapour breakthrough.
 The escape of gases entering the tank from other sources.
The vents on the tank have to be large enough to pass all coincident vapour flow, both in-
breathing and out. API RP 2000 forms the basis for establishing tank breathing rates.

The type of vent to be used depends on the pressure to be held in the tank. If the tank is to be
held as close as possible to atmospheric pressure, and air ingress is acceptable, then a covered
vent (to keep out birds etc.) will be used. The only pressure generated will be from the frictional
resistance of the gas entering or leaving.

If the tank is to be blanketed, then it will probably need to operate at a slightly higher pressure so
that sufficient margins can be allowed between in-breathing and out-breathing for the blanketing
controls to work. It would be normal to expect a blanketed tank to be fitted with a
pressure/vacuum valve (PVV) and have a controlled supply of blanket gas. Under normal
operation, in-breathing by the tank will cause the blanket gas to be introduced. Should it fail,
then the vacuum facility will permit air ingress on the grounds that it is better to take in the air
than to collapse the tank. Out-breathing causes the pressure section of the PVV to operate. The
control signal for the letdown control of the blanket gas must come directly from the tank, as the
frictional pressure drop from the controller to the tank is likely to be significant in comparison
with the set pressure.

When out-breathing is required to pass to some kind of treatment facility, then clearly higher
pressures are required as a driving force. In these circumstances, it is normal to provide PVVs
with flanged outlets.

Inerting
Fixed roof tanks and internal floating roof tanks normally have a vapour space that can
accumulate volatile gases and/or contain air. If explosive mixtures can be produced in the
vapour space, or if it is necessary to exclude air for reasons of contamination, then these tanks
will require a gas blanket. In most cases, the most appropriate blanketing gas will be
nitrogen. However, occasionally fuel gas may be used. When fuel gas is to be used, it is
essential to ensure there are no inadvertent releases to the atmosphere, and that any PVVs
discharge to an appropriate disposal system