A BOVEG R OU ND

Introduction

Classification of tanks

Tank Foundation

Tank Structure

Floating Roof Tanks

Tank Accessories

Tank Erection

CONTENTS

Tanks have been around since the beginning of hydrocarbon production.

Tanks vary considerably, in the type and size based on the type of products to be stored and the volume involved.

The failure of a tank can have several undesirable effects such as endangering personnel, affecting the environment and interrupting the Operator’s business.

Companies therefore, require a consistent approach for assessing tank integrity and maintaining compliance with industry and regulatory standards, (that is community requirements). Such an approach must;

Ensure tanks are not leaking and will not leak before next inspection.

Reduce the potential for releases.

Maintain tanks in safe operating conditions, and

Make repairs and determine when replacement is necessary.

BACKGROUND

The primary function of a storage tank is to store liquid substance.

This liquid substance may be:

Feedstock

Finished products prior to shipping out to customers

Unfinished petroleum components awaiting for further

- processing (intermediate)

- blending

While in the storage tanks, these products may settle out undesirable substances such as;

Water

Emulsions

Dirt etc.

This undesirable substances can then be removed through draw-off devices. Products may also be mixed, blended and treated in storage tanks effectively, using the large capacity available in these tanks.

INTRODUCTION

Storage tanks are broadly classified into the following categories

Atmospheric tanks

Low-pressure tanks

High pressure tanks / Pressure vessels

Atmospheric Tanks: (Vertical)

These tanks operate at or near atmospheric pressure.

They are use to store / hold liquids that do not vaporize at ambient temperature.

These tanks are further classified into different categories based on the roof type.

Low-pressure Tanks: (Vertical)

These tanks are designed to operate from atmospheric pressure upto 15 psig.

These tanks are used to hold hydrocarbons having relatively high vapor pressure.

High pressure tanks / Pressure vessels: (Horizontal)

These are vessels (mounted bullets / spherical) operating above 15 psig and are treated separately from tanks by all codes, standards, and regulations.

CLASSIFICATION

Vertical storage tanks are further classified into the following types

based on the roof configuration,

Open top tank

Fixed roof tank

Floating roof tank External Floating roof

Internal Floating roof

Fixed roof tanks are further divided into, Cone roof tank

Dome roof tank

External floating roofs are further categorized as, Single Deck type

Double Deck type

CLASSIFICATION BASED ON ROOF CONFIGURATION

This type of tank have no roof and are used to store,

City Water

Fire Water

Cooling Water

OPEN TOP TANK

FIXED CONE ROOF TANK

Fixed cone roof tanks are well known type of storage tanks, constructed over 100 years ago and provided mostly with self supporting roof structures.

For large diameter tanks column supported roofs are used.

FIXED DOME ROOF TANK

These tanks have roof shape similar to dome.

These tank roofs can be self supporting structure hence no column supports are necessary.

FLOATING ROOF - SIGNIFICANCE

Emission of oil vapour represents:

1. Considerable monetary loss termed as “wastages”.

2. Harmful long-term consequences to environment.

In-order to reduce these vapour losses Floating roofs are used.

In these types of roofs there is no vapor space which greatly reduces the emission arising due to breathing loss and evaporative loss

Surface, subsurface and climatic conditions vary from place to place, so it is

not practical to establish design data to cover all situations. The allowable soil loading and exact type of subsurface construction to be used must be decided for each individual case after careful consideration.

Some of the many conditions that require special engineering consideration are as follows:

site on hillsides – undisturbed / land filled

sites at swampy or filled ground

sites underlain by soils, such as organic clays that will settle over long period and can cause lateral ground stability problem site adjacent to water courses or deep excavation

site exposed to flood water

site in regions of high seismicity

TANK FOUNDATION

SAND PAD FOUNDATION When subsurface conditions shows

adequate bearing capacity and that settlements will be acceptable, satisfactory foundations may be constructed from earth material.

For small tanks, foundations can consist of compacted crushed stone, screenings, fine gravel, clean sand, or similar material placed directly on virgin soil.

SAND PAD FOUNDATION

RINGWALL FOUNDATION

Earth foundations with a concrete ring wall large tanks, with heavy or tall shell and/or self-supported roofs impose a substantial load on the foundation under the shell. When there is some doubt whether a foundation will be able to carry the load directly, a concrete ringwall foundation should be used.

Advantages of concrete ring wall are:

It provides better distribution of the concentrated load of the shell to produce a more uniform soil loading under the tank.

It provides a level, solid starting plane for concentration of the shell.

It is capable of preserving its contour during construction.

It retains the fill under the tank bottom and prevents loss of material as a result of erosion.

It minimizes moisture under the tank.

PILE SUPPORTED FOUNDATION

The pile-supported foundation is used where the soil bearing pressures are very low. They are also used where high foundation uplift forces are encountered resulting from internal pressure or seismic loading.

PILE SUPPORTED FOUNDATION

RELEASE PREVENTION BARRIER

API supports a general position of installing a Release Prevention barrier (RPB) under new tanks during initial construction. An RPB includes steel bottoms, synthetic materials, clay liners, and all other barriers or combination of barriers placed in the bottom of or under an aboveground storage tank, which have the following functions:

Preventing the escape of contaminated material and

Containing or channeling released material for leak detection

TANK STRUCTURE

Major tanks components are as follows, Tank Bottom

Tank Shell

Wind Girders

Roof

Stairway

Bottom / Floor design – Designed to, permit complete draw-off,

minimize product contact and to utilize maximum tank capacity and prevention of corrosion of bottom plate.

Two types of tank flooring are:

Cone down bottom (Bottom down)

Generally, bottom down is design for cone roof tanks. Centre of the flooring is installed with drain pit. Water in the tank is accumulated in the pit (lowest point of the bottom plate / floor).

Cone up bottom (bottom up)

Generally, this type of design is used for floating-roof tanks, 3 to 4 collector pits are installed, close to the shell plate. Each of the pit is provided with a water draw-off line. However, only one is connected to the closed water draw system in PPMSB.

TANK - BOTTOM

LAYING OF BOTTOM PLATE

The floor plating consists of two layers of arrangement,

The inner layer – a series of flat, rectangular plates with lapped joints, fillet-welded on the top side.

The outer ring of annular plates – which have a circular outside circumference and usually a regular polygonal shape inside the tank.

TANK - SHELL

Typical Tank Shell – General Arrangement

Tank Under Fabrication

WIND GIRDERS

Primary Wind Girder

Open Top tanks & Floating Roof tanks are provided with primary wind girders in order to maintain roundness of tank when tank is subjected to wind load.

The wind girder shall be in the form of a ring located on the outside of the tank shell, approximately 1 m below the top of the uppermost shell course. The top of the uppermost shell course shall be provided with a top curb angle.

Wind girders may be constructed from formed plate sections, by welding. The outer periphery of the wind girder may be circular or polygonal.

Secondary Wind Girder

Tank may require secondary rings to maintain roundness over the full height of the tank shell under wind and/or vacuum conditions.

There are basically, additional stiffening rings. Continuous welding (full penetration butt welds) shall be used for all connections of the secondary wind girders.

WIND GIRDERS

FLOATING ROOF

Floating roof greatly reduces vapour losses due to changes in climatic conditions and during tank filling operations.

These losses are particularly significant where volatile organic compounds are stored in tanks which are subject to high filing and emptying cycles.

Floating roof works based on (buoyancy) Archimedes’ principle, the object having lower density floats on liquid having higher density.

As per API 650 floating roofs are designed to float on liquids having minimum specific gravity of 0.7

FLOATING ROOFS - TYPES

SINGLE DECK - EXTERNAL

This type of roofs are used in tanks upto 65 – meters in diameters.

The buoyancy required by roof to float is provided from outer annular pontoon which is radially divided into liquid tight compartments.

DOUBLE DECK - EXTERNAL

This type of roof consists of an upper and lower steel membrane separated by a series of circumferential bulkheads which are subdivided by radial bulkheads.

This type of roof is favored for tanks having diameter < 12 meter and large tanks > 65 meter.

INTERNAL FLOATING ROOF

An internal floating roof tank has both a permanent fixed roof and a floating desk inside.

The term "deck" or "floating roof" is used in reference to the structure floating on the liquid stored within the tank.

The deck of an internal floating roof tank rises and falls with the liquid level whilst in full contact on the underside thus achieving no vapor zone.

ALUMINIUM INTERNAL FLOATING ROOF

Seals

Roof Drain System

Pontoon Leg Support

Emergency Drain

Auto Bleeder Vent

Rim Venting (Breather Valves)

FLOATING ROOF ACCESSORIES

FLOATING ROOF ACCESSORIES

The space between the outer periphery of the roof

and the tank shell shall be sealed by a flexible device that provides a reasonable close fit to shell surface

Steel shoes with fabric or nonmetallic material used as seal or seal components.

Material shall be durable and shall not discolor or contaminate the product stored.

The main purpose of the seal is to prevent emission of the stored liquid into surrounding atmosphere.

SEALS

PRIMARY SEALS

A rim seal, or in the case of a two-seal system, the lower (primary) rim seal, can be made from various materials suitable for organic liquid service.

The basic designs available for external floating roof rim seals are,

Mechanical (metallic) shoe seals,

Liquid-filled seals, and

Resilient foam-filled seals.

Based on the configuration / position of the primary seal it is classified as follows,

Liquid Mounted Seal Vapor Mounted Seal

MECHANICAL SHOE SEAL

MECHANICAL SHOE SEAL

LIQUID / FOAM FILLED SEALS

TUBE SEAL – LIQUID FILLED SEAL

RESILIENT FOAM FILLED SEAL

LIQUID MOUNTED SEAL

VAPOUR MOUNTED SEALS

To further reduce the loss of stored liquid / comply

with strict emission laws a second / secondary seal may be used.

SECONDARY SEALS

vapour barrier

WIPER SEAL

WIPER SEAL -WITH VAPOUR

BARRIER

WIPER SEAL WITH ROLLER ARRANGEMENT

Automatic bleeder vents are provided to vent the air

from under the floating roof when the tank is being filled initially.

They shall also open automatically just before the roof lands on its supports, thereby preventing the development of a vacuum under the roof.

The capacity of the vents are based on the maximum pumping rates.

AUTOMATIC BLEEDER VENT

AUTOMATIC BLEEDER VENT

RIM VENTING:

Rim vent shall be provided to prevent any excess pressure in the rim space, as this might press the shoe ring too tightly against the tank shell. Settling shall be plus and minus 2.5 mbar.

FOAM DAM:

Foam dams collect debris and therefore retain water causing excessive corrosion of dam / deck joint, roof annular and seal connections. The dam drain holes should always unplugged.

RIM VENTING & FOAM DAM

ROOF DRAIN SYSTEM This system is used to

remove rain water accumulated above the floating roof to the drain near dyke wall.

It is of the following configuration, Flexible Hose Type

Articulated Swivel Type

Flexible Hose Type

Articulated Swivel Type

PONTOON LEGS

Pontoon legs are roof supports with which the floating roof lands when it reaches the lowest level (while emptying the tank).

The length of these supports varies based on the elevation of the floor (since the floor may have been coned up or coned down during design).

MIXERS

HEATING COIL

FOAM SYSTEM

COOLING WATER SYSTEM

CATHODIC PROTECTION SYSTEM

EARTHING AND LIGHTINING ARRESTOR

FLAME ARRESTOR

LEVEL INDICATORS

TANK VENTING

TANK ACCESSORIES

MIXERS

Mixing equipment can be installed in vertical tanks in order to: Blend or homogenise

components and products, including bitumen.

Control tank bottom products - usually for crude oils. In this case it is not necessary to mix the whole tank contents but to create a flow pattern in the lower part of the tank so that the solids are kept in suspension.

MIXERS

Flow Patterns for Sediment Removal in Crude Oil Tanks

Flow Patterns for Blending, Homogenising

and Temperature Uniformity Duties

Side Entry Mixer

Products which require to be heated in order to facilitate

movement or loading into transport units need to be heated to the minimum temperature that will ensure economic pumping and loading.

This may be achieved by providing:

heating coils inside the storage tanks,

electrical heating elements,

suction and/or line heaters, or

a combination of these heating methods.

TANK HEATING

TYPICAL HEATING COIL ARRANGEMENT

Thermal insulation is applied to storage tanks for the

following reasons: Heat conservation where heated products are being

handled. In such cases it is usual to insulate both the shell and roof plates.

To minimise evaporation losses as a result of solar heat. In these circumstances it is usual that only the roof is insulated. As an alternative to roof insulation, floating covers may be considered for Class I products.

For personnel protection - to prevent injury that may result by coming into contact with hot surfaces.

THERMAL INSULATION

The foam system are used for fire prevention, control or direct

extinguishment of any flammable or combustible liquid fire within the tank.

There are two types of systems employed in storage tank fire protection, viz.

A Fixed System is a complete installation piped from a central foam station, discharging through fixed discharge devices on the hazard being protected. Foam proportioning components are permanently installed.

A Semi Fixed System is an installation where the hazard is equipped with fixed discharge device(s) which connect to piping that terminates a safe distance from the hazard. Foam producing materials are transported to the scene after the fire starts and are connected to the piping.

In accordance with NFPA Standard No. 11, there are three accepted methods of protecting cone roof tanks:

Surface (Foam Chamber) Method

Subsurface Method

Portable Foam Nozzle and Monitor Method

FOAM SYSTEM

FOAM SYSTEM – FIXED ROOF

FOAM SYSTEM – FIXED ROOF

FOAM SYSTEM – FLOATING ROOF

FOAM SYSTEM – INTERNAL FLOATING ROOF

PORTABLE FOAM NOZZLE AND MONITOR

FOAM MAKER / FOAM CHAMBER

INLINE FOAM INDUCTOR

COOLING WATER SYSTEM

Despite taking all reasonable precautions as demanded by governing standards, a fire in an individual storage tank will generate significant radiant heat, which can damage and / or ignite adjacent tanks which would not otherwise be directly involved.

A deep-seated fire in the smallest diameter tank can create major problems unless cooling water is applied to its close neighbours.

REVOLVING NOZZLE

MEDIUM VELOCITY

WATER SPRAY NOZZLE

COOLING WATER SYSTEM

Corrosion of Steel Storage Tanks

Corrosion is the deterioration of metal due to reaction with the environment. Corrosion occurs when;

Areas with different electrical potentials exist on metal surface

These areas are electrically connected

Areas are in contact with electrolyte like moist soil in contact with tank bottom. Water and sludge are the electrolytes for internal bottom surface.

Forms of Corrosion:

General Corrosion leading to general metal loss and thinning

Pitting due to localized actions ( metal loss may be concentrated within relatively small area and other near area may be unaffected)

CATHODIC PROTECTION SYSTEM

Cathodic Protection Methods

Cathodic protection is the technique for preventing corrosion by making entire surface of the metal to act as cathode of an electrolyte cell. There are two methods commonly used;

Sacrificial Anode

Impressed current

Sacrificial Anode System (Galvanic System):

This system involves an anode buried in soil, but electrically connected to the structure (cathode). The anode is thus corroded (sacrificed) and metal surface is protected. Metals commonly used as anodes are magnesium and zinc ( cast or ribbon type). They are either buried beneath the bottom are distributed around the perimeter of the tank.

Impressed Current System

This system uses Direct Current (DC) usually provided by a rectifier. DC flows from the rectifier to the buried impressed current anode.

Power source include a step down transformer (reduces AC supply voltage) and rectifying element to provide DC output.

Impressed current anodes are of graphite, steel, high silicon cast iron or mixed metal oxide on titanium.

CATHODIC PROTECTION SYSTEM

EARTHING & LIGHTINING PROTECTION

VENTING

The Need for Venting Equipment

Protect Tank Integrity when Pumping In and Out

Minimize Evaporation Losses (Cost Savings)

Accommodate Thermal Effects

Overpressure Due to Fire Near Tank (Emergency)

Prevent Product Contamination

Reduce Internal Tank Corrosion

Comply with Clean Air Act Mandates

Proper Vapor Transfer Control for Low-Pressure Systems

Failure of tank due to Inadequate Venting

Flame arrestor is designed to stop the propagation of

flame from ignited flammable liquid vapours with low flash points.

They prevent flame propagation by absorbing and dispatching heat thereby reducing the temperature of the flame front preventing ignition behind the cell element.

Flame Arrestor can be installed either vertically or horizontally and is available in aluminium, carbon steel or stainless steel.

The cell element is available in stainless steel and special materials are available on request.

FLAME ARRESTOR

TANK GAUGING – MECHANICAL LEVEL INDICATOR

TANK GAUGING – RADAR TYPE

RIM SEAL FIRE PROTECTION

SYSTEM Rim Seal Fire Protection

System is a fully automatic detection cum extinguishing system. The system is designed and manufactured for extremely fast detection and extinction of rim seal fires as per the International Safety Standards.

To understand the complete working philosophy of the system, the system

can be divided in the following sub-systems:-

Linear Heat Detection System.

The Foam Based Extinguishing System.

Fire Alarm Panel & Automation.

The working of the detection unit is based on the principle of rate of rise in temperature and maximum temperature beyond the pre-configured parameters, which is sensed by means of a microprocessor based intelligent evaluation unit. The Sensor element, i.e. the Stainless Steel / Copper Tube, which is in non-pressurized state (ambient pressure only) senses the rate of rise in temperature (pressure) and triggers an audio visual alarm simultaneously activating the extinguishing systems. The Advance Detection System is Decentralize and has Pre Alarm facility for early warning.

RIM SEAL FIRE PROTECTION SYSTEM

RIM SEAL FIRE

PROTECTION SYSTEM

TANK ERECTION - PROGRESSIVE ASSEMBLY AND WELDING

TANK ERECTION – HYDRAULIC JACKING

Some contractors employ a system of erection in which the bottom plates are completed, the top course is erected on the bottom plates, the roof framing and sheeting are completed and a number of jacks are then assembled around the structure. By means of these jacks, the completed top course together with the roof framing and sheeting is lifted to a height sufficient to insert the next lower course. The jacking method and the supporting of the partly erected shell should have no adverse effect on the roundness of the shell.

The welding is completed at each stage of lift until all courses of the shell plates have been inserted and the finished height is reached.

The final operation is the welding

of the bottom course to the bottom

plates.

HYDRAULIC JACKING METHOD