unit 4 storage tanks

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UNITS IN THIS COURSE

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UNIT 1 SEPARATORS

UNIT 2 TOWERS AND COLUMNS

UNIT 3 FRACTIONATION AND DISTILLATION EQUIPMENT

UNIT 4 STORAGE TANKS

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TABLE OF CONTENTS

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4.0 COURSE OBJECTIVE 3

4.1 TANKS AND THEIR FUNCTION 4

4.2 CONE ROOF STORAGE TANK (FIXED ROOF STORAGE TANKS) 4

4.3 CONE ROOF TANK AUXILIARY EQUIPMENT 5

4.4 FLOATING ROOF STORAGE TANKS 9

4.5 FLOATING ROOF TANK FUNCTION 11

4.6 FLOATING ROOF TANK AUXILIARY EQUIPMENT 13

4.7 DOME ROOF STORAGE TANKS 15

4.8 PRESSURE RELIEF SYSTEMS 17

4.9 SAFE TANK DESIGN AND OPERATION 19

4.10 SPHEROIDS 21

4..11 SPHERES 22

4. 12 PROCESS VESSELS THAT ACT AS TEMPORARY 22

STORAGE TANKS

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4.0 COURSE OBJECTIVE

On completion of this unit the trainee will be able to discuss and describe thefunctions and operations of the different types of storage tanks.

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4.1 TANKS AND THEIR FUNCTION

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Tanks are used to store liquids. Different kinds of tanks are used to store differentliquids. Some liquids such as crude oil can be stored in plain covered tanks. Someliquids must be stored in pressure tanks. Very light and highly volatile liquids willevaporate 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 thecrude product supplied to plants for processing. When the processing is completethe tanks hold the refined liquids for blending, final shipment and sales.

Fluids such as oil and water are stored in two types of vertical tanks: "coned roof"and "floating roof" tanks.

4.2 CONE ROOF STORAGE TANK (FIXED ROOF STORAGE TANKS)

Figure 4-1 Cone Roof Storage Tank

A cone roof storage tank is a metal cylinder with a cone shaped roof and a metalfloor. (See Figure 4-1). A cone shaped roof is used for two reasons. First, rain waterwill run off the sloping roof. The second reason is that the shape of the cone makesthe roof very strong. Cone roof storage tanks can hold up to.300,000 barrels.

The cone shape allows thinner metal to be used for the roof because the shape ofthe cone supports itself. However, large cone roof tanks need support beams for

the roof because a large roof is much heavier. (See Figure 4-2)

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Figure 4-2 Supported Cone Roof Tank

Cone roof tanks are used to store non-volatile liquids like water. The productsstored in cone roof tanks are normally vented to atmosphere. Cone roof tanks arealso used for products which do not ignite easily like bunker fuel which is used torefuel tanker ships. These tanks are normally vented to a flare to prevent aircontamination and fire hazards.

4.3 CONE ROOF TANK AUXILIARY EQUIPMENT

Cone roof tanks may have the following auxiliary equipment:

Vents.

Product Mixers.

Tank Heaters

Level Measuring Devices.

Flame Arresters.

Some liquids must be heated. Some liquids must be mixed. Cone roof tanks mustbe vented to prevent any pressure build-up. These requirements make certainauxiliary equipment necessary.

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VENTS

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Vent pipes on top of storage tanks are turned downward to prevent rain waterentering the tank. Turning the pipe down also reduces the chance of the windblowing sand or dust into the tank. See Figure 4-3

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Figure 4-3 Cone Roof Tank Auxiliary Equipment

Cone roof storage tanks must never be completely filled. Vapours from the storedliquid collect in the vapour space under the cone roof. The vent pipe installed at the

top of the roof allows any vapour under the roof to escape to the atmosphere. Smallamounts 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 spark from maintenance worksuch as welding could ignite the gas in the vapour space. A flame arrester is fittedto the end of the vent pipe. Mesh screens or baffles inside the arrester allow gas topass out but make it almost impossible for a spark to pass in.

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PRODUCT MIXERS

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Most hydrocarbon sales products are blends or mixtures of different fractions. Whena blended product is stored for a long time, the different fractions may separate andcause the product to go off specification. A mixer is built into the tank. The mixer isused to stir the product in the tank. This mixing ensures that all the product is theright blend. If the product in a storage tank is very thick the product is usuallyheated before it is mixed.

TANK HEATERS

Storage tank heaters are used where thick heavy liquids are stored. The heaterskeep the liquid thin enough to be pumped. When the product is heated it is alsoeasier 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. Thetank mixer circulates the liquid so all of the product is evenly heated.

Figure 4-4 Storage Tank Mixer

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LEVEL MEASURING DEVICES

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Figure 4-5 Storage Tank Measuring

It is important to know how much liquid is stored in a tank. Some cone roof tanksuse a float and tape system to show the level in the storage tank. See Figure 4-5.

Any sales product must be accurately measured. The most accurate levelmeasuring device for a storage tank is a "Hand Reel Tape". There is a GaugingPlatform on the top of the tank surrounded by a safety railing. The platform providesa place for an operator to use the hand reel tape. There is a "Gauging Well" throughthe tank roof. The weighted tape is lowered into the tank through the gauging well.

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4.4 FLOATING ROOF STORAGE TANKS

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Figure 4-6 Floating Roof Seal

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 sealconnects the floating roof to the tank shell (the walls of the tank). This prevents anyevaporation of the product. See Figure 4-6.

There are two main types of floating roof.

Pontoon roof.

Double Deck Roof.

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PONTOON ROOF

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Figure 4-7 Pontoon Type Floating Roof

The outer edge of the pontoon roof is a float called a pontoon. This a sealed metalair chamber that provides flotation for the roof. See Figure 4-7. The under side of

the roof is flat.

The pontoon around the roof also provides some air space insulation for the liquidsurface which is under the pontoon.

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DOUBLE DECK ROOF

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Figure 4-8 Double Deck Type Floating Roof

The double deck floating roof is the most efficient type of floating roof. It is like alarge hollow disc. There is air space between the top and bottom of the roof (theupper and lower deck). This gives good insulation from the heat of the sun over theentire surface of the stored liquid. The top deck slopes in to the centre so rain waterflows to the centre of the roof.

4.5 FLOATING ROOF TANK FUNCTION

Floating roof tanks are used to store hydrocarbon liquids such as crude oil andnaphtha. These liquids are not very volatile Before crude oil is placed in a floating-roof storage tank the liquid must be stabilised. This means removing all the easilyseparated gases. This is done in the production plants. Stable oil will not release alarge amount of vapour when it is stored.

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Figure 4-9 Hinged Roof Drain

TANK DRAINS

Figure 4-10 Water Drain

Crude oil usually contains a small percentage of water. When the crude oil is placedin a storage tank some of the water will settle to the bottom of the tank. Drain valvesare located at the bottom of the tank. They are used to drain off any water thatcollects there.

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4.6 FLOATING ROOF TANK AUXILIARY EQUIPMENT

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Figure 4-11 Floating Roof Auxiliary Equipment

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

Drains.

Mixers.

Level measuring and sampling stations.

ROOF DRAINS

It is important to prevent water from collecting on a floating roof. Water will causeproblems 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. Theother end is connected to a drain sewer.

The hinge is flexible so the drain pipe can move up and down with the roof. (Seefigure 4-11)

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

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water collects in the low part of the floor. A drain is located in the lowest part ofthe floor to drain off any collected water. The operator uses a manual drain valveto drain the water into a sewer as necessary.

MIXERS

Tanks which store refined products usually have mixers. The tank mixer preventsseparation of liquid products during storage. Tank mixers are installed in floatingroof storage tanks. When more product comes into the tank the mixers mix it withthe product already in the tank. Mixing ensures that all the liquid in the tank is ofuniform quality.

LEVEL MEASURING

On a floating roof tank the gauger's platform moves up and down with the roof.There is a "rolling ladder" from the tank shell to the gauger's platform. Anoperator must follow all the company safety rules when he works on a floatingroof tank.

SAMPLING

Samples are taken from the storage tanks before loading to ensure uniformproduct quality. Oil products must be tested before they are delivered tocustomers. Sampling is an operator task. Samples are often drawn from thegauge well.

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4.7 DOME ROOF STORAGE TANKS

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Figure 4-12 Dome Roof Storage Tank

Dome roof storage tanks are used to store highly volatile liquid petroleum products.These products are usually stored under pressure.

A dome roof tank is a closed metal cylinder. It has a rounded top (a dome). Domeroof 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 toprovide insulation from the heat of the sun. Some are buried underground.

DOME ROOF FUNCTION

Dome roof tanks store LPG products at specific pressures and temperatures. Thisprevents too much product from vaporising. Dome roof tanks are built to store highlyvolatile liquefied petroleum gases (LPG) such as ethane, propane and butane. Thetemperature of LPG must be kept below its boiling point. The boiling point is

approximately 30 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 domeroof tank must be maintained between 0.2 psig and 0.9 psig. LPG storage tanks arenever filled completely. The domed roof provides a space for some of the liquid tochange back to gas.

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DOME ROOF TANK AUXILIARY EQUIPMENT

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Dome roof storage tanks require the following auxiliary equipment to functionproperly:

Mixing Pumps.

Heaters.

Temperature Indicator.

Pressure Indicator

Level Indicator.

Pressure Relief System.

Insulation.

MIXING PUMPS

Most dome roof tanks have mixing pumps. These pumps circulate the liquid fromthe bottom to the top of the tank. The mixing pumps help to keep the product inthe tank at a uniform temperature. Mixing pumps are also used to move LPGthrough pipelines from one tank to another.

HEATERS

The heater is not located inside the storage tank. It is located under the floor ofthe tank. The heater is usually made up of electric elements which areembedded in the foundation (the concrete base under the tank). The heater

keeps the foundation temperature between 35 F and 40 F.

The purpose of the heater is to prevent ice forming in the foundation. The metalfloor of the dome tank is as cold as the liquid in the tank. Any moisture under thetank would freeze., Water expands when it turns to ice, so ice formation woulddamage the concrete foundation.

TEMPERATURE, PRESSURE AND LEVEL INDICATORS

Temperature, pressure and level are critical variables in LPG storage. Theymust be carefully measured and controlled. Dome roof tanks cannot be openedto check the liquid level in the tank. They are gauged by an automatic gaugingsystem.

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

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measure the temperature of the liquid in the tank. These automatic gaugingsystems and fixed temperature sensors are normally used as "official gauges". Forthis reason these instruments must be tested regularly.

The gauges are called "official" because they are used to show how much LPG isbeing delivered to the customers.

4.8 PRESSURE RELIEF SYSTEMS

In an earlier unit we discussed pressure relief systems used to protect equipmentfrom high pressure. LPG products are very volatile (they vaporise easily). Therefore,some vaporisation is happening all the time in dome roof tanks. Therefore, the tankshave pressure relief valves.

Figure 4-13 Dome Roof Pressure Relief Valves

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.

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Figure 4-14 Vacuum Relief Valves

Vacuum relief valves are also used on dome roof-tanks. They prevent a vacuumforming inside a tank when product is taken out. The valves will open automaticallyif the pressure in the tank falls to 0.2 psig. They allow blanket gas into the dome ofthe tank. Blanket gas (usually nitrogen) is a gas that does not mix with LPG tocause an explosion.

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INSULATION

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Figure 4-15 Dome Roof Tank Insulation

Liquid propane and liquid butane are stored at low temperatures. Therefore storagetanks must be well insulated. Insulation helps to reduce heat absorbtion from thehot ground and the air. The insulation keeps the LPG products cool and reduces the

amount of product vaporisation.

4.9 SAFE TANK DESIGN AND OPERATION

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

Static Electricity / Lightning

Static electricity is an imbalance of electrical charges between one piece ofequipment 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 befilled from the storage tank may have a negative charge. When you connect theloading hose from the vehicle to the storage tank there will be a movement ofelectricity or a spark. This can cause an explosion if there are any hydrocarbonvapours present.

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Lightning is another form of static electricity. Its like a very big spark of staticelectricity from the air to the ground.

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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 thetank. This special grid transmits any unbalanced electrical charge to the ground.

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

Metal rods are put on the, high points of some tanks. These will take the charge

from lightning and discharge it into the ground.

Corrosion

Water and air in storage tanks cause corrosion of the metal in two main areas. Thefloor and the roof. To minimise roof corrosion the best method is to prevent air fromentering 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. Thismust be drained off as often as possible. "ANODES" are installed in the floor of thetank. These will attract the corrosive action and prevent the corrosion of the tank

floor. These devices are called "sacrificial anodes".

OVERFILLING

Many different types of level gauges are used on storage tanks to show the level inside thetank. 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. Whentaking these readings the operator should compare the amount of product which goes in tothe tank with the amount of product which leaves the tank. This should match the levelgauge readings

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LEAKS

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Dikes (strong walls) are built around each storage tank. They are large enough tohold all the liquid contents of the tank if the tank wall leaks or breaks. There are alsoditches around the tank which contain any spill and prevent the liquid fromcontaminating the environment.

FIRE

Foam systems, water systems and halon systems are all types of fire extinguishingsystems 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 thecontrol room and an automatic fire detection system.

If a fire is detected the fire detectors will send a signal to the control room. Thissignal sets off a fire alarm in the control room and starts the fire extinguishingsystem automatically.

4.10 SPHEROIDS

Figure 4-16 Spheroid

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

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oil / gas separation before the crude oil is sent to the refinery. A spheroid is notactually 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.

4.11 SPHERES

Figure 4-17 Sphere

Spheres look like large balls. They are used to store volatile liquids that are beingprocessed. The round shape is very strong. It is the best shape for use with highpressure fluids.

A sphere uses thinner metal than other vessels. However, spheres and spheroidsare more expensive to build than cylinders.

4.12 PROCESS VESSELS THAT ACT AS TEMPORARY STORAGE TANKS

Knockout Drums

Knockout drums are usually vertically installed metal cylinders. Knockout drums areused to remove liquid from gas. A knockout drum is used to remove the liquiddroplets before the gas goes to a compressor. Drops of liquid in the gas stream willdamage a compressor. Knockout drums act as a temporary gas storage vessel.

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SURGE DRUMS

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Surge drums are metal cylinders with inlet and outlet pipes. Their purpose is tosmooth out sudden increases and decreases in process pressures and flows. In thisway, 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 reducesto 90 gallons per minute, the surge drum will supply the extra 10 gallons per minuteneeded.

REFLUX DRUMS

Figure 4-18 Operation of a Reflux Drum

Reflux drums are used in distillation and fractionation processes. Reflux drums areused with fractionation columns in refineries and gas plants. We will studydistillation and fractionation in another unit. Figure 4-18 shows where a reflux drumfits into a process.

unit 4 storage tanks

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