Introduction to Storage Tanks

1 Overview of the Article

This article aim is to present a general overview of petroleum products storage tanks. Only cylindrical vertical tanks are covered in this article.

Though storage tanks are used to store varieties of liquid including water, crude oil, refined petroleum products, the scope of this article is limited to liquid hydrocarbon storage tanks with emphasis on refined products such as Premium Motor Spirit (PMS), Automotive Gas oil (AGO), and Dual Purpose Kerosene (DPK). Tanks used to store Liquefied Natural Gas (LNG) are not covered in this article.

The storage tanks covered in this article are above ground atmospheric tanks.

The guidelines and recommendations discussed in this article are referenced to Author’s experience, the National Fire Protection Association (NFPA 30), American Petroleum Institute (API 650), EN 14015

2 Introduction

Refined petroleum products are some of the most vital energy sources used to power cars, motorcycles, electricity generators, pumps drivers, and other equipment. The fuel is usually stored in a compartment/reservoir from where it is drawn into the equipment. This compartment is generally referred to as a storage tank.

The storage tanks described above is not the focus of this article. Storage tanks in oil and gas facilities refer to large volume reservoir, mainly cylindrical but could be rectangular or any other shape. These tanks are mostly made of steel materials. However, the tanks may be horizontal or vertical for large volume storage tanks.

Storage tanks are an essential component of any oil and gas facility. They are found in Crude oil storage facilities, Refineries, refined products storage depots.

2.1 General Terminologies Related to Tanks

Below are some key terminologies that relate to petroleum products storage tanks.

2.1.1 Above Ground Storage Tanks

Above ground storage tanks are defined in NFPA 30, section 3.3.51.1, as tanks installed above grade, at grade, or below grade without backfill. This means any tank installed below grade elevation that is not backfilled is considered an above-ground storage tank.

2.1.2 Atmospheric Tanks

As defined in section 3.3.51.2* of NFPA 30, Atmospheric Tank is a storage tank that has been designed to operate at a pressure ranging from atmospheric through a gauge pressure of 1.0 psi (6.9 kPa) (i.e., 760 mmHg through 812 mm Hg) measured at the top of the tank. Note that Atmospheric tanks are not Low-Pressure storage tanks. As defined in section  3.3.51.3 of NFPA 30, a low-pressure storage tank is a storage tank designed to withstand an internal pressure above a gauge pressure of 1.0 psi (6.9 kPa) but not more than a gauge pressure 15 psi (103 kPa) measured at the top of the tank.

2.1.3 Storage Tank

As defined in section 3.3.51.6 of NFPA 30; A storage tank is a vessel having a liquid capacity that exceeds 60 gallons (230 L), is intended for fixed installation, and is not used for processing.

2.1.4 Vapour Pressure of a Liquid

As defined in NFPA 30 section 4.2.6, Vapor pressure is the pressure measured in pounds per square inch, absolute (psia), exerted by a liquid, as determined by ASTM D 323

3 Components of Storage A Tanks

A storage tank is an assemblage of various components and parts. Below is a short description of the components

3.1 Tank Foundation

The tank foundation is the load-carrying member external of the tank. The tank seats on the foundation.

Tank foundation should be designed with the tank bottom elevated minimum of 300mm above the surrounding grade.

Note that a Release Prevention Barrier such as synthetic materials, steel bottoms, clay liners, a combination of different barriers may be installed under the tank foundation.

The barriers functions include

• Prevent the escape of contaminated material into the soil
• Containing or channelling released products for leak detection

The most prominent types of tank foundations are:

3.1.1 Earth Foundation without Ring Wall

This type of foundation is applicable primarily for small diameter storage tanks. Used when the soil load-bearing capacity and settlement is adequate. Soil compacting is very crucial in this type of foundation to minimise settlement.

The foundation material may consist of compacted crushed stones, screenings, fine gravel, clean sand, or other materials placed directly on virgin soil.

3.1.2 Earth foundations with Crushed Stone and Gravel Ringwall

This type of foundation is also referred to as Gravel Packed Foundation because gravel is used instead of concrete ring wall.

This a prevalent type of foundation used for medium and large diameter storage tanks. The gravel pack ringwall carries the load resulting from the shell plates and the self-supporting roof.

The foundation should be well compacted to minimise settlement resulting from the tank’s weight and contents.

May install Asphalt/Bitumen on top of the compacted foundation and gravel pack to provide a compressive membrane between the foundation and the tank bottom plates.

Some of the advantages of this foundation include

• Provision of better distribution of the concentrated load of the shell plates to produce a more uniform soil loading under the tank
• It retains the fill material under the tank bottom and prevents loss of material resulting from erosion
• This type of foundation can smoothly accommodate differential settlement because of its flexibility.

3.1.3 Earth Foundation with Concrete Ringwall

This type of foundation is very prominent, usually applicable for medium to large diameter tanks. This foundation is used when substantial loads resulting from heavy plate thickness, tall shell and self-supporting roofs is transferred to the soil. The resulting load is transferred to the ring foundation under the shell.

Appropriate steel reinforcement is embedded into the ringwall to provide the required strength.

The ringwall thickness is usually a minimum of 300mm, and the depth depends on the condition of the soil.

The soil surrounded by the concrete ringwall must be appropriately compacted to minimise settlement. Also, may install Asphalt/Bitumen on top of the compacted foundation and ringwall to provide a compressive membrane between the foundation and the tank bottom plates.

The advantages of this type of foundation include:

• Provides a good distribution of the concentrated load of the shell plates to produce a more uniform soil loading under the tank
• Minimises moisture under the tank
• It retains the soil or material fill under the tank bottom and prevents loss of material that may result from erosion.
• When tank anchorage is required, this is a suitable foundation because the anchor bolts are installed on the ring foundation.

3.2 Tank Floor

The tank floor may be a combination of bottom plates and annular plates. There are different types of floor arrangement.

3.2.1 Tank Floor Arrangement

There are two types of floor arrangement described below. However, some tanks have flat floor arrangement.

3.2.1.1 Cone Down Floor Arrangement

This type of floor arrangement is applicable mainly for cone roof and other fixed roof tanks. In this type of arrangement, the centre of the tank is the lowest point. A drain pit is installed at the centre to collect all accumulated water in the tank bottom.

3.2.1.2 Cone Up Floor Arrangement

Generally, this type of design is applicable for floating roofs tanks. 3 to 4 collector or drain pits are installed close to the tank shell plate. Each of these drain pits is provided with a water draw-off line.

3.2.2 Tank Floor Plates

The floor plates of a storage tank could be a combination of the bottom plate and annular plates.

3.3.2.1 Bottom Plates

The bottom plate of a storage tank seats inside the shell plate. It is welded to the annular ring plates. The shell plate shall extend from underneath the tank shell outward for a tank that does not have annular ring plates. As stated in section 5.4.1 of API 650, the minimum bottom plate thickness shall be 6mm exclusive of any corrosion thickness specified by the purchaser. Refer to API 650 or other codes for dimensions and plate overlap requirements for tank bottom plates.

3.2.2.2 Annular Plates

The annular plate is a ring of plates that sit directly underneath the shell plates of a tank. They are usually thicker than the bottom plate because they carry the shell and/or self-supported roof weight.

As specified in section 5.5.2 of API 650, Annular bottom plates shall have a radial width that provides at least 600 mm (24 in.) between the inside of the shell and any lap-welded joint in the remainder of the bottom. Also, the minimum annular plate outward projection specified in section 5.4.2 of API 650 is 50mm.

3.3 Shell

The shell is the vertical product containing part of the tank welded to the floor plates. The shell is made of steel plates manufactured as per applicable specification, including ASTM CSA, ISO etc. The shell is made of rolled plates that are welded together to form the cylindrical shape of the tank. The cylindrical shape is formed on top of the previous plate until the tank height is achieved. Each plate height is called the tank course, i.e. the first plate welded to the bottom plate is called the first course or course one while the next is the second course or course two etc.

API 650 has specified a minimum 1800m nominal plate width unless otherwise agreed by the purchaser. A plate width of 1800m represents a course height of 1800mm.

For details of Shell thickness design and calculation, refer to:

API 650 Section 5.6.3 (1-Foot Method)   tank diameter up to 60m.

API 650 Section 5.6.4 (Variable-Design-Point Method) for tank diameter exceeding 60m.

3.4 Inlet Nozzle

The product is introduced into the storage tank via the inlet nozzle. Small diameter tanks may have their inlet nozzle located on the tank top course, while a large diameter tank with large inlet pipes has its inlet nozzle located at the bottom (First course).

In setting the nozzle elevation for floating roof storage tanks, the minimum roof elevation should be considered to ensure appropriate clearance from the nozzle.

In some tanks, the inlet nozzle is utilised as the discharge nozzle.

3.5 Discharge Nozzle

Products are drawn out of the tank via the discharge nozzle. As previously stated, one nozzle may be utilised as suction and discharge. Also, Nozzle elevation for floating roof tanks should be such that the roof at its lowest elevation does not obstruct the nozzle.

3.6 Tank Roof

All tanks have a roof that shields the stored product from the surrounding. The roof also limits the emission of vapour into the atmosphere. The selection of the roof type is based on various factors such as

• Tank Diameter
• Local regulations
• Environmental considerations such as Emission Control
• True Vapour Pressure (TVP) of the stored product
• Flashpoint of the stored product
• Company requirements

Below is a brief description of the types of roofs.

3.6.1 Fixed Roofs

These are the tanks with covered roofs. The shape of the roof could be flat, umbrella, conical or dome. Fixed roof is used when the stored product True Vapour Pressure (TVP) is lesser than or equal to 1.5 Psia

3.6.1.1 Cone Roof Tanks

The shape of the roof is conical made of steel plates. These tanks are used to store liquids that do not rapidly vaporise. The roof is supported on structural members such as beams and rafter evenly distributed around the tank.

3.6.1.2 Dome Roofs

This type of roof is made of aluminium materials. They are self-supporting on aluminium structural members that are assembled to form the dome shape. They are generally referred to as Aluminium geodesic dome roof tanks. These tanks are becoming very prominent in the storage of premium motor spirit and Dual Purpose Kerosene, especially when the tank is designed to have an internal floating roof.

3.6.2 Floating Roof

These tanks are used to store very volatile liquid, and they are used when the Product True Vapour Pressure measured at ambient temperature is greater than 1.5Psia but lesser than or equal to 11.5Psia.

The tank roof floats on top of the stored products. The floating roof design must consider the buoyancy of the roof based on the stored liquid density.

The roofs usually have evenly distributed support legs. The tank floor where the support lands during maintenance should have a striker plate. The striker plates ensure that the roof support does not damage the bottom plate.

Usually, the roof supports should not touch the tank’s floor during regular operation except during maintenance.

Below are the types of floating roofs

3.6.2.1 Single Deck Floating Roof

Single deck roof is also referred to as pontoon roof. The Pontoon provide the required buoyancy for the roof. The deck of the roof should be designed to be in contact with the liquid.

A Manhole, Vents and other instruments should be provided on the roof.

A seal is installed between the tank shell and the Pontoon.

3.6.2.2 Double Deck Floating Roof

A double-deck floating roof consists of lower and upper membrane separated by bulkheads.

Radial bulkheads further subdivide the bulkheads. The subdivision ensures that if one of the bulkheads is punctured, the entire roof does not collapse. The double-deck roofs are more rigid compared to a single deck. The separation space between the upper deck and bottom deck plates serves as an insulation that reduces the heat reaching the stored product during hot weather.

Manhole, Vents and other instruments should be provided on the roof.

A seal is installed between the tank shell and the Pontoon.

3.6.2.3 Internal Floating Roof

These types of tanks have the floating roof installed inside a fixed roof tank. Internal floating roofs are mostly single deck type floating roof. The tank diameter is usually smaller than external floating roofs. They are mainly used when the tank diameter is lesser than 40m.

The internal roof may be made of aluminium material or steel; however, the design must consider the floating of the roof on the stored liquid.

3.6.2.4 External Floating

This type of floating roof is used mainly in medium to large diameter tanks. They are widely used in crude oil storage tanks because of their large diameters. Usually applicable for tanks with a diameter greater than 40m.

3.7 Tank Vents (PV Vents)

Storage tanks may fail due to excessive internal pressure arising from vapour build-up or filling the tank. Vacuum is created in the tank during the withdrawal of products from the tank.

A vent is installed on top of the tank to prevent failure resulting from pressure build-up and vacuum. One such standard vent installed is the PV Vent (Pressure/Vacuum Vent or pressure vacuum relief valve). It is an automatic safety device that allows air to be drawn into the tank during product withdrawal and open when there is excessive pressure in the tank, hence Breather Valve’s name.

These valves also minimise emission losses and, at the same time protecting the environment.

The size of a PV relief valve may range from 2″ to 12″ depending on the envisaged pressure build-up or vacuum during product withdrawal. Also, the number of Breather valves on a tank may be more than one.

3.8 Instrument Connection

The primary instrument connections on a tank are the level measurement instruments. These instruments may be a transmitter or field display instrument connected to nozzles attached to the roof or shell of the tank. Usually, the transmitters work with the control system to display the liquid level and regulate product flow into and out of the tank.

3.9 Manhole/Manway

Manhole Manways are used to access the inside of the storage tank during maintenance. Medium to large diameter storage tanks should have more than one Manhole. Also, the roof of a tank should have a Manhole to access the inside of the tank.

3.10 Tank Cleanout Connections

This opening is for maintenance and cleaning of the tank. It is not very common in refined petroleum products storage tanks but found in tanks prone to debris accumulation.

3.11 Top Curb Angle

The top curb angle is located on the top of the shell. It serves as an attachment or seat for the tank roof and provides a stiffening function for the tank shell.

3.12 Wind Girder

The wind girders provide stability for the tank, most especially when the tank is empty. If the shell is not stiffened, there is a possibility of shell buckling.

The wind girders, in addition to the top curb angle, provide stiffening for the shell.

Although the primary wind girder or roof provide considerable stability for the tank over its full height, local buckling may occur in empty tall tanks. This buckling may occur between the top of the tank and the base. To prevent buckling, intermediate wind girders are provided at intervals in the tank height.

The outer periphery of a wind girder may be polygonal or circular. Drain holes should be provided for trapped rainwater.

3.13 Tank Spiral Stairways

Tanks shall have spiral stairways with an intermediate landing. The stairways are provided with handrails. The stairways are designed for the maximum envisaged load it will carry with a factor of safety applied.

3.14 Earthing and Cathodic Protection Connection

All tanks shall be provided with earthing connection and cathodic protection system. Some tanks are protected using impressed current, while some have sacrificial anodes.

3.15 Tank Gauging System Hatch

The gauging system is a combination of a gauging pole and hatch. Gauging hatch provides access to the tank when manual level gauging or sampling is desired. The gauging hatch is connected to a gauge pole that is installed through the roof into the tank.

3.16 Tank Fire Fighting System

Tanks storing petroleum products are provided with a firefighting system. The firefighting system combines foam and water piped around the shell at a calculated elevation and around the tank on top.

Spray nozzles are connected to the piping to discharge the foam and water mixture.

3.17 Plate Material Selection

Plate material selection is a function of the fluid stored in the tank, design process temperature etc. Details of materials and requirements are specified in section 4 of API 650.  As stipulated in API 650, tank material may be manufactured to any of the following specifications:

• ASTM Specifications
• CSA specification
• ISO specification
• National specifications.

Refer to section 4.2.3, 4.2.4 and 4.2.5 of API 650 for details

4 References

API 650: Welded Steel Tanks for Oil Storage

EN 140155: Specification for the Design and Manufacture of Site Built, Vertical, Cylindrical, Flat-Bottomed, Above Ground, Welded, Steel Tanks for the Storage of Liquids at Ambient Temperature and Above

NFPA 30: Flammable and Combustible Liquids Code