storage

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Operations

Training

Unit P-05-03

Gravity Separation, Distillation and Storage

Storage

Learner’s

Resource Material

Petrofac Training

Storage - Unit P-05-03

UNIT P-05-03

STORAGE

1.0. OBJECTIVES/ INTRODUCTION.........................................................4

2.0 CONE ROOF STORAGE TANK (FIXED ROOF STORAGE TANKS)...........5

2.1 CONE ROOF TANK AUXILIARY EQUIPMENT.....................................6

2.1.1 Vents..........................................................................................................6

2.1.2 Product Mixers............................................................................................9

2.1.3 Tank Heaters............................................................................................10

2.1.4 Level Measuring Devices..........................................................................10

3.0 FLOATING ROOF STORAGE TANKS...............................................11

3.1 PONTOON ROOF.........................................................................12

3.2 DOUBLE DECK ROOF...................................................................13

3.2.1 Floating Roof Tank Function.....................................................................13

3.2.2 Floating Roof Tank Auxiliary Equipment...................................................14

3.2.3 Roof Drains...............................................................................................14

3.2.4 Tank Drains..............................................................................................15

3.2.5 Mixers.......................................................................................................15

3.2.6 Level Measuring........................................................................................15

3.2.7 Sampling...................................................................................................15

4.0 DOME ROOF STORAGE TANKS.....................................................16

4.1 DOME ROOF FUNCTION...............................................................16

4.2 DOME ROOF TANK AUXILIARY EQUIPMENT...................................17

4.2.1 Mixing Pumps...........................................................................................17

4.2.2 Heaters.....................................................................................................17

4.2.3 Temperature, Pressure And Level Indicators............................................17

4.2.4 Pressure relief systems.............................................................................18

4.2.5 Insulation..................................................................................................19

5.0 SAFE TANK DESIGN AND OPERATION...........................................20

5.1 STATIC ELECTRICITY / LIGHTNING................................................20

5.2 CORROSION...............................................................................20

5.3 OVERFILLING.............................................................................21

5.4 LEAKS.......................................................................................21

5.5 FIRE..........................................................................................21

6.0 SPHEROIDS................................................................................22

7.0 SPHERES...................................................................................23

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8.0 PROCESS VESSELS (ACTING TEMPORARY STORAGE TANKS)..........24

8.1 KNOCKOUT DRUMS....................................................................24

8.2 SURGE DRUMS...........................................................................24

8.3 REFLUX DRUMS............................................................................................24


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1.0. OBJECTIVES/ INTRODUCTION

Objectives

On completion of this unit the trainee will be able to:

Identify the function of storage tanks with respect to volatile and non-volatile fluids storage

Identify types of storage tanks

Describe the basic construction of different types of storage tanks.

Describe storage tank auxiliary and safety equipment

Introduction

Tanks are used to store both volatile and non-volatile liquids. Different kinds of

tanks are used to store different liquids. Volatile liquids oil must be stored in

pressure tanks. Highly volatile liquids will evaporate in tanks that are not sealed.

This would be a waste of valuable products. It would also be a fire hazard.

Storage tanks are normally used at both ends of a process system. They hold the

feed stock supplied to plants for processing. When the processing is complete the

tanks hold the refined liquids for blending, final shipment and sales.

Fluids such as crude oil and water are stored in two different types of vertical tanks:

water in a "coned roof" and crude oil in "floating roof" tanks.


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2.0 CONE ROOF STORAGE TANK (FIXED ROOF STORAGE TANKS)

A cone roof storage tank is a metal cylinder with a cone shaped roof and a metal

floor, they operate at atmospheric pressure. (See Figure 2-1). A cone shaped roof

is used for two reasons. First, rain water will run off the sloping roof. The second

reason is that the shape of the cone makes the roof very strong. Cone roof storage

tanks can hold up to 300,000 barrels.

Figure 2-1

The cone shape allows thinner metal to be used for the roof because the shape of

the cone supports itself. However, large cone roof tanks need support beams for

the roof because a large roof is much heavier. (See Figure 2-2).

Figure 2-2


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Cone roof tanks are used to store non-volatile liquids like water and are normally

vented to atmosphere. They are also used for products which do not ignite easily

like bunker fuel which is used to refuel tanker ships. These tanks are normally

vented to a flare to prevent air contamination and fire hazards.

2.1 Cone Roof Tank Auxiliary Equipment

Cone roof tanks may have the following auxiliary equipment:

Vents with Flame Arresters or Pressure/Vacuum Relief Valves

Product Mixers

Tank Heaters

Level Measuring Devices

Some liquids must be heated. Some liquids must be mixed. Cone roof tanks must

be vented to prevent any pressure build-up. These requirements make certain

auxiliary equipment necessary.

2.1.1 Vents

Vent pipes on top of storage tanks are turned downward to prevent rain water

entering the tank. Turning the pipe down also reduces the chance of the wind

blowing sand or dust into the tank (see Figure 2-3).

Figure 2-3


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Cone roof storage tanks must never be completely filled. Vapours from the stored

liquid collect in the vapour space under the cone roof. The vent pipe installed on

the top of the roof allows any vapour under the roof to escape to the atmosphere.

Small amounts of vapour quickly disperse in the air so they are not dangerous.

To prevent fire and explosions, vent pipes are fitted with flame or spark arresters.

The hazard is the confined space under the roof. A lightening strike on the high

point vent could ignite the gas in the vapour space. To prevent flashback into the

tank vapour space a flame arrester is fitted to the end of the vent pipe. Mesh

screens or baffles inside the arrester allow gas to pass out but make it almost

impossible for a spark to pass in.

The flame arrestors illustrated in Figure 2-4 are designed to allow free venting (tank

breathing) in combination with flame protection. The arrestor prevents flame

propagation by absorbing and dissipating heat using a using a spiral wound flame

cell made of aluminium or stainless steel. The flame arrestor housing is constructed

from aluminium, carbon steel or stainless steel.

Figure 2-4


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Pressure Vacuum Safety Valves

Pressure vacuum safety valves are another type of tank breathing device installed

on cone roof tanks. The operating sequence is described below

Pressure Normal

When the outside pressure is equal to the pressure within the tank, both pressure vent

caps are closed. They are gravity sealed and prevent any direct opening between the

tank and the atmosphere

Figure 2-5

Negative Tank Pressure

When the pressure in the tank starts to lower, the pressure in the centre cavity becomes slightly negative (a slight vacuum is created). The pressure (0.01 PSI) exerted on the vacuum valve from the outside will force it to rise and allow outside air to bleed into the tank.


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Figure 2-6

Positive Tank Pressure

When the pressure in the tank starts to rise, the pressure in the centre cavity becomes

slightly positive. This positive pressure (0.01 psig) will force the pressure valve to rise

and allow tank air to bleed to the outside

Figure 2-7

2.1.2 Product Mixers

Most hydrocarbon sales products are blends or mixtures of different fractions.

When a blended product is stored for a long time, the different fractions may

separate and cause the product to go off specification.


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Figure 2-8

A mixer is built into the tank to prevent product separation. The mixer is used to

stir the product in the tank. This mixing ensures that the entire product is the right

blend. If the product in a storage tank is viscose the product is usually heated

before it is mixed.


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2.1.3 Tank Heaters

Storage tank heaters are used where thick heavy liquids are stored. The heaters

keep the liquid thin enough to be pumped. When the product is heated it is also

easier to mix.

A tank heater is a bundle of tubes installed near the bottom of the storage tank.

Steam is circulated through the pipes. This heats the liquid in the storage tank.

The tank mixer circulates the liquid to ensure that the entire product is evenly

heated.

2.1.4 Level Measuring Devices

It is important to know how much liquid is stored in a tank. Some cone roof tanks

use a float and tape system to show the level in the storage tank. (See Figure 2-9).

Figure 2-9

Any sales product must be accurately measured. The most accurate level

measuring device for a storage tank is a "Hand Reel Tape". There is a Gauging

Platform on the top of the tank surrounded by a safety railing. The platform

provides a place for an operator to use the hand reel tape. There is a "Gauging

Well" through the tank roof. The weighted tape is lowered into the tank through the

gauging well.


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3.0 FLOATING ROOF STORAGE TANKS

Floating roof tanks are generally used to store volatile liquids such as crude oil,

naphtha and gasoline. The floating roof rests on the top of the stored liquid so

there is no space for vapours to collect under the roof.

The floating roof rises and falls with the liquid inside the tank. A flexible seal

connects the floating roof to the tank shell (the walls of the tank). This prevents

any evaporation of the product. (See Figure 3-1).

Figure 3-1

There are two main types of floating roof tanks:

Pontoon Roof

Double Deck Roof


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3.1 Pontoon Roof

The outer edge of the pontoon roof is a float called a pontoon. This is a sealed metal

air chamber that provides flotation for the roof. (See Figure 3-2). The under side of

the roof is flat.

Figure 3-2

The pontoon around the roof also provides some air space insulation for the liquid

surface which is under the pontoon.


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3.2 Double Deck Roof

The double deck floating roof is the most efficient type of floating roof. It is like a

large hollow disc. There is air space between the top and bottom of the roof (the

upper and lower deck). This gives good insulation from the heat of the sun over the

entire surface of the stored liquid. The top deck slopes in to the centre so rain

water flows to the centre of the roof.

Figure 3-3

3.2.1 Floating Roof Tank Function

Floating roof tanks are used to store hydrocarbon liquids such as crude oil and

naphtha. These liquids are not very volatile therefore before crude oil is placed in a

floating roof storage tank the liquid must be stabilised. This means removing all the

easily separated gases. This is done in the production plants. Stable oil will not

release a large amount of vapour when it is stored.


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3.2.2 Floating Roof Tank Auxiliary Equipment

Floating roof tanks require the following auxiliary equipment to function properly:

Drains.

Mixers.

Level measuring and sampling stations.

Figure 3-4

3.2.3 Roof Drains

It is important to prevent water from collecting on a floating roof. Water will cause

problems with pontoon and double deck floating roofs.

A common type of drain is the hinged pipe roof drain. This drains water from the

centre of the roof. One end of the hinged pipe is connected to the roof drain. The

other end is connected to a drain sewer.

The hinge is flexible so the drain pipe can move up and down with the roof. (See

Figure 3-5).


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Figure 3-5

3.2.4 Tank Drains

Crude oil usually contains a small percentage of water. When the crude oil is placed

in a storage tank some of the water will settle to the bottom of the tank. Drain

valves are located at the bottom of the tank. They are used to drain off any water

that collects there.

Figure 3-6

The floor of a crude oil storage tank is higher in the centre than at the edges. The

water collects in the low part of the floor. A drain is located in the lowest part of the

floor to drain off any collected water. The operator uses a manual drain valve to

drain the water into a sewer as necessary.

3.2.5 Mixers

Tanks which store refined products usually have mixers. The tank mixer prevents

separation of liquid products during storage. Tank mixers are installed in floating

roof storage tanks. When more product comes into the tank the mixers mix it with


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the product already in the tank. Mixing ensures that all the liquid in the tank is of

uniform quality.

3.2.6 Level Measuring

On a floating roof tank the gauge’s platform moves up and down with the roof.

There is a "rolling ladder" from the tank shell to the gauge’s platform. An operator

must follow all the company safety rules when he works on a floating roof tank.

3.2.7 Sampling

Samples are taken from the storage tanks before loading to ensure uniform product

quality. Oil products must be tested before they are delivered to customers.

Sampling is an operator task. Samples are often drawn from the gauge well.


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4.0 DOME ROOF STORAGE TANKS

Dome roof storage tanks are used to store highly volatile liquid petroleum products.

These products are usually stored under pressure.

Figure 4-1

A dome roof tank is a closed metal cylinder. It has a rounded top (a dome). Dome

roof storage tanks are large, low-pressure tanks. They are used in gas terminals

and their capacity can be as high as 600,000 barrels. Some tanks are constructed

(made) of a single layer of cold resistant steel. Some tanks are double walled to

provide insulation from the heat of the sun. Some are buried underground.

4.1 Dome Roof Function

Dome roof tanks store LPG products at specific pressures and temperatures. This

prevents too much product from vaporising. Dome roof tanks are built to store

highly volatile liquefied petroleum gases (LPG) such as ethane, propane and butane.

The temperature of LPG must be kept below its boiling point. The boiling point is

approximately 30o F at atmospheric pressure. At that temperature the liquid will

change back to its gaseous state.

LPG is pressurised to slightly above atmospheric pressure. The pressure in a dome

roof tank must be maintained between 0.2 psig and 0.9 psig. LPG storage tanks are

never filled completely. The domed roof provides a space for some of the liquid to

change back to gas.


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4.2 Dome Roof Tank Auxiliary Equipment

Dome roof storage tanks require the following auxiliary equipment to function

properly:

Mixing Pumps.

Heaters.

Temperature Indicator.

Pressure Indicator

Level Indicator.

Pressure Relief System.

Insulation.

4.2.1 Mixing Pumps

Most dome roof tanks have mixing pumps. These pumps circulate the liquid from

the bottom to the top of the tank. The mixing pumps help to keep the product in

the tank at a uniform temperature. Mixing pumps are also used to move LPG

through pipelines from one tank to another.

4.2.2 Heaters

The heater is not located inside the storage tank. It is located under the floor of the

tank. The heater is usually made up of electric elements which are embedded in

the foundation (the concrete base under the tank). The heater keeps the

foundation temperature between 350 F and 400 F.

The purpose of the heater is to prevent ice forming in the foundation. The metal

floor of the dome tank is as cold as the liquid in the tank. Any moisture under the

tank would freeze. Water expands when it turns to ice, so ice formation would

damage the concrete foundation.

4.2.3 Temperature, Pressure And Level Indicators

Temperature, pressure and level are critical variables in LPG storage. They must be

carefully measured and controlled. Dome roof tanks cannot be opened to check the

liquid level in the tank. They are gauged by an automatic gauging system.

Temperature sensors are attached to the shell of dome roof tanks. These sensors

measure the temperature of the liquid in the tank. These automatic gauging

systems and fixed temperature sensors are normally used as "official gauges". For

this reason these instruments must be tested regularly.

The gauges are called "official" because they are used to show how much LPG is

being delivered to the customers.


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4.2.4 Pressure relief systems

In an earlier unit we discussed pressure relief systems used to protect equipment

from high pressure. LPG products are very volatile (they vaporise easily).

Therefore, some vaporisation is happening all the time in dome roof tanks.

Therefore, the tanks have pressure relief valves.

Figure 4-2

The relief valves prevent the pressure of the LPG vapours from getting too high.

The relief valves are normally set to open at approximately 1.1 psig.

Vacuum relief valves are also used on dome roof tanks. They prevent a vacuum

forming inside a tank when product is taken out. The valves will open automatically

if the pressure in the tank falls to 0.2 psig. They allow blanket gas into the dome of

the tank. Blanket gas (usually nitrogen) is a gas that does not mix with LPG to

cause an explosion.


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Figure 4-3

4.2.5 Insulation

Liquid propane and liquid butane are stored at low temperatures. Therefore storage

tanks must be well insulated. Insulation helps to reduce heat absorption from the

hot ground and the air. The insulation keeps the LPG products cool and reduces the

amount of product vaporisation.

Figure 4-4


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5.0 SAFE TANK DESIGN AND OPERATION

During normal operation, storage tanks must be protected against the following:

5.1 Static Electricity / Lightning

Static electricity is an imbalance of electrical charges between one piece of

equipment and another, e.g. a tank and the surrounding pipework.

A storage tank may have a positive charge. A vehicle (a tanker) which needs to be

filled from the storage tank may have a negative charge. When you connect the

loading hose from the vehicle to the storage tank there will be a movement of

electricity or a spark. This can cause an explosion if there are any hydrocarbon

vapours present.

Lightning is another form of static electricity. Its like a very big spark of static

electricity from the air to the ground.

Static electricity is caused by:

The flow of liquids into or out of a tank.

The mixing of fluids in a tank.

The movement of vapour into or out of a tank.

To protect against static electricity and lightning the following equipment is used:

All tanks are connected by wires to a special grid fitted into the ground around the tank. This special grid transmits any unbalanced electrical charge to the ground.

Any loading equipment is grounded to the tank ground system before the hose is connected.

Metal rods are put on the high points of some tanks. These will take the charge from lightning and discharge it into the ground.

5.2 Corrosion

Water and air in storage tanks cause corrosion of the metal in two main areas; the

floor and the roof. To minimise roof corrosion the best method is to prevent air

from entering the tank, i.e. by using a blanket gas (an inert gas such as nitrogen).

The floor of the tank may corrode because of the water which collects there. This

must be drained off as often as possible. "ANODES" are installed in the floor of the

tank. These will attract the corrosive action and prevent the corrosion of the tank

floor. These devices are called "sacrificial anodes".


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5.3 Overfilling

Many different types of level gauges are used on storage tanks to show the level

inside the tank. These are differential pressure meters, sight glasses and floating

level indicators. Also the operator should dip the tank manually to prove the level,

if this is possible. When taking these readings the operator should compare the

amount of product which goes in to the tank with the amount of product which

leaves the tank. This should match the level gauge readings.

5.4 Leaks

Dikes (strong walls) are built around each storage tank. They are large enough to

hold all the liquid contents of the tank if the tank wall leaks or breaks. There are

also ditches around the tank which contain any spill and prevent the liquid from

contaminating the environment.

5.5 Fire

Foam systems, water systems and halon systems are all types of fire extinguishing

systems that are installed around storage tanks. Ultra Violet (UV) and Infra Red (IR)

fire detectors are usually installed around the storage tanks. These are linked to

the control room and an automatic fire detection system.

If a fire is detected the fire detectors will send a signal to the control room. This

signal sets off a fire alarm in the control room and starts the fire extinguishing

system automatically.


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6.0 SPHEROIDS

Figure 6-1

Spheroids are low pressure processing vessels. They are used in the last stage of

oil / gas separation before the crude oil is sent to the refinery. A spheroid is not

actually round. It is the shape of a soft football when it is resting on a flat surface.

This shape is very strong but does not need a lot of metal to make it.


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7.0 spheres

Spheres look like large balls. They are used to store volatile liquids that are being

processed. The round shape is very strong. It is the best shape for use with high

pressure fluids.

Figure 7-1

A sphere uses thinner metal than other vessels. However, spheres and spheroids

are more expensive to build than cylinders.


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8.0 PROCESS VESSELS (ACTING TEMPORARY STORAGE TANKS)

8.1 Knockout Drums

Knockout drums are usually vertically installed metal cylinders. Knockout drums

are used to remove liquid from gas. A knockout drum is used to remove the liquid

droplets before the gas goes to a compressor. Drops of liquid in the gas stream will

damage a compressor. Knockout drums act as a temporary gas storage vessel.

8.2 Surge Drums

Surge drums are metal cylinders with inlet and outlet pipes. Their purpose is to

smooth out sudden increases and decreases in process pressures and flows. In this

way, surge drums balance supply and demand. They are a temporary storage tank.

For example, suppose a process is using 100 gallons per minute of a liquid. If the

supply suddenly increases to 120 gallons per minute, the surge drum will absorb

the extra 20 gallons, while the process adjusts to the change. If the supply reduces

to 90 gallons per minute, the surge drum will supply the extra 10 gallons per minute

needed.

8.3 Reflux Drums

Reflux drums are used in distillation and fractionation processes. Reflux drums are

used with fractionation columns in refineries and gas plants. We will study

distillation and fractionation in another unit. Figure 8-1 shows where a reflux drum

fits into a process.

Figure 8-1


storage

Documents

cone roof storage tanks

floating roof tanks

floating roof storage

dome roof storage tanks

large cone roof tanks

roof drains

large roof

pontoon roof