storage of liquids and gases

Intro, Storage of Liquids & Gases

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Storage of Liquids and Gases

Materials are held in storage as a buffer between supply and demand. This lesson

explains some of the methods used in industry for the safe storage of liquids and

gases.

On completion of this lesson you should:

understand the problems associated with the storage of large quantities of

materials in industry

appreciate the main safety hazards

know the main methods used in the safe storage of liquids and gases.

There are many different types of equipment used for the storage of liquids and gases

in industry. In a course such as this it would not be possible to cover all the equipment

used. However, we have included sufficient examples for you to understand the

principles of storing liquids and gases and the major problems involved.

As you work through this lesson, keep in mind that the method of storage chosen

depends upon:

the quantity of fluid being stored

the chemical nature of the material, i.e. is it toxic, flammable, corrosive?

the physical state of the material, i.e. its temperature and pressure.

For a full understanding of this lesson you will need to know the meaning of the

following terms:

[I] corrosion

[ii] density

[iii] pressure


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[iv] temperature

[vi] volatile.

An explanation of the meaning of each of these terms is given in the Appendix which

you will find at the end of this lesson.

Storage of Liquids

Before process engineers can choose suitable storage methods, they must have

answers to the following questions:

• What is the maximum quantity of liquid I shall have to store?

This is an important consideration because if the storage capacity is not sufficient

then the manufacturing process may have to stop.

• How toxic is the liquid?

This is important because special precautions may have to be taken and there may

be legal requirements.

• How flammable is the liquid?

Again, there may be legal requirements regarding the size and location of storage

vessels.

• How corrosive is the liquid?

A highly corrosive liquid would require a container made of special materials.

• How volatile is the liquid?

A volatile liquid is one which evaporates quickly. Special precautions need to be

taken to avoid losses of product by evaporation, particularly if the liquid is

flammable or toxic.

• Does the liquid need to be stored in a hot state?


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If it is necessary to store the liquid hot, and keep it hot, then special insulation

techniques will be required.

• Is the liquid under pressure, other than that due to its own weight?

If the liquid is under pressure, then the storage vessel would have to be

constructed to withstand such pressure.

Keep these points in mind as you study the methods used to store materials.

STORING LIQUIDS

Small quantities of liquid are usually stored in carboys, large quantities in drums and

‘bulk’ quantities in tanks. We can examine each one in turn.

Carboys

For many years carboys have been spherical in shape, with a flat base for stability,

and made from glass, which will resist the corrosion from most chemicals.

This type of carboy is now being replaced by others, cylindrical or rectangular in

shape, of the same capacity but made from plastic material. Carboys usually hold 20

litres, or more, of liquid. From a safety point of view, carboys must not be pressurized,

rolled or exposed to heat sources.

Drums

Larger quantities of liquids are stored in drums, which are cylindrical and usually hold

200 litres of fluid [approximately 45 UK gallons]. They can be made from a wide range

of materials to suit the fluid being stored; for example, stainless steel, aluminium, or

mild steel with a rubber or plastic lining. The material that a drum is made from can

therefore be chosen so that the chances of corrosion are reduced.

Drums, like carboys, must be handled with care. They must not be pressurized or

stored near sources of heat, e.g. steam pipes.


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All containers should carry labels showing their contents and any safety hazards

associated with the liquid they contain.

Unlabelled containers should not be used under any circumstances. Similarly

containers labelled as containing one type of liquid should not be reused by

filling with another type of liquid.

Storage Tanks

Very large quantities of liquid are usually stored in the open, in squat cylindrical

storage tanks fitted with conical roofs. Several hundred of these storage vessels may

be grouped together in an area called a tank farm.

Tank farms are located in remote areas to reduce the safety hazards involved when

potentially flammable and toxic materials are stored in very large quantities.

Tanks of up to 100 000 tonnes capacity [about 80 metres in diameter and 25 metres

high] are in common use in the oil industry. The bigger the tank the lower the capital

cost per tonne of liquid stored.

Let us now look at the main features of this type of storage tank. It should be noted

that not all storage tanks will have all these features. For example, some storage tanks

will be open and therefore not require a relief valve, some will be shallow and not

require an access manhole. What we have included here, for completeness, are all the

main features that you could find on a storage tank. Refer to

Figure 1 as you read on.


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Storage tanks may be fitted with the following ancillary equipment: • roof access ladder with safety rail • roof or ground level access manholes • water drain • filling and emptying pumps • bund areas [often called firewalls] • relief valves • inert gas blanket inlet pipes and valves • foam injection box • vapour vent • hatch for sampling or gauging the tank contents • earth connections.


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ROOF ACCESS LADDER

This is obviously required for safe access to the roof area, for maintenance and

repairs.

ACCESS MANHOLES

Usually fitted either at ground level or on the roof, the manholes allow access to the

interior of the tank for cleaning, maintenance and repair.

WATER DRAIN

Some substances contain very small quantities of water which eventually settle to the

base of the tank. Over several months this can accumulate and quite large quantities

can settle at the base of the vessel. This water needs to be drained away since it will

present a corrosion problem if not removed. The drain is also a useful facility during

internal tank cleaning.

FILLING AND EMPTYING PUMPS

Pumps are usually of the centrifugal type [note the symbol on

Figure 1]. Centrifugal pumps are a common design used in industry for pumping many

different types of liquid. They consist of an impeller which rotates inside a cylindrical

housing.

Liquid is drawn into the housing by the impeller through the inlet pipe. The liquid is

then pushed out of the housing by the impeller through the outlet pipe.

BUND AREAS [FIREWALL]

These can be of two types:

• A wall surrounding the tank, high enough to hold all the liquid in the tank should

the tank burst or leak. In this way any leaked flammable, toxic or corrosive liquid

will be contained in a small, safe, isolated area.

• Alternatively the tank is situated in a cylindrical hole in the ground, deep enough

to contain all the contents of the tank should it burst open or leak. [This type is

shown in Figure 1.]


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RELIEF VALUES

These are necessary in case the pressure inside the tank exceeds the tank’s safe limit.

INERT GAS BLANKET INLET

Some liquids produce vapours which become flammable, or explode, when mixed with

air. This hazard can be reduced by replacing the vapour above the liquid at the top of

a tank with an inert gas, that is a gas which will neither react chemically nor produce

explosive mixtures with the tank’s contents. Nitrogen gas is often used for this

purpose. The inert gas would be fed in as shown in Figure 1.

FOAM INJECTION BOX

In an emergency the foam injection box allows foam to be sprayed into the tank and

over the surface of the liquid. Thus, hazards arising from toxic or flammable vapours

can be reduced.

VAPOUR VENT

The vent allows air to enter the tank when it is emptying, and to leave when the tank is

being filled. A tank without a vent would burst open during filling, and collapse in on

itself when being emptied.

HATCH FOR SAMPLING AND GAUGING

It is often necessary to take samples from tanks for laboratory analysis. These can be

taken safely and conveniently through this hatch.

A dip-stick, or a dip-tape, can also be passed down through the hatch, either to

measure the level of liquid in the tank or as a check on automatic liquid level

measuring devices.

EARTH CONNECTIONS

The possibility of a build-up of static electricity, and its associated hazards, can be

eliminated by the use of earth connections. Such connections are essential, as static

electricity does tend to build up during filling and emptying operations.

If a build-up of static electricity was to occur where the liquid being stored was

flammable, then a fire hazard would arise.


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Venting Tanks

We have already touched on venting problems in the ‘Vapour Vent’ section above.

When a tank is being filled, then the air above the rising liquid must be allowed to

escape, otherwise very high pressures would occur due to the compression of the air

in the tank as the liquid is pumped in. This excess pressure is released through a

specially designed valve called a vent valve.

When the tank is being emptied, air must be drawn in through the vent to take the

place of the liquid, otherwise the tank would collapse in on itself due to the vacuum-like

conditions being created inside the tank. In the event of a safety hazard, then the

vapour vent [as well as the inert gas blanket valve] can be used to create an inert gas

safety shield.

The storage of large volumes of liquid in tanks can lead to problems arising from the

daily and seasonal variations in the weather. The most important factor to vary is the

air temperature.

Such variations lead to:

• Expansion of the stored liquid inside the tank

• Evaporation of liquid [particularly volatile liquid] in the tank.

Expansion is allowed for by leaving a space above the liquid.

Evaporation can take place through the vent valve before the pressure of the vapour

becomes higher than the safe working level. Such evaporated material disperses

safely into the atmosphere. Vent valves are sometimes called ‘breather’ valves.

On larger vessels a flexible roof is fitted beneath the fixed roof, and floats on the

surface of the stored liquid as the level rises and falls; this is called an internal

floating roof tank.


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FLOATING ROOF STORAGE

Look at Figure 2[a] as you work through the following description. The roof floats on

the surface of the liquid. [The structure shown in Figure 2[a] is the ‘floating’ roof

contained within the tank. The ‘support legs’ actually stand in the stored material —

see Figure 2[b].] Since the roof moves up and down with the level of the liquid, there

is never a large empty space above the liquid. This is true even when the tank is

nearly empty. Since there is no large empty space for the liquid to evaporate into then

vapour losses are reduced. The expansion and contraction of the liquid due to

temperature changes is also accommodated by the roof moving up and down with

changes in volume.

A flexible seal is fitted around the edge of, and moves with, the roof. This eliminates

leakage and does not interfere with the roof’s movement.

Legs [see Figure 2[b]] are fitted to the underside of the roof to allow access into the

empty tank. These support legs are approximately 2 — 3 metres long. When the tank

is empty the legs rest on the base of the tank and support the roof in such a way as to

allow access, and to provide a space at the base of the tank for maintenance and

repair, etc.


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The roof will thus move up and down, but will not rest on the base of the tank whilst the

support legs are in position.

Roof drainage is essential and the roof must be strong enough to support the weight of

snow. Floating roof storage is usually used for the storage of large quantities of petrol

and other very volatile liquids.

Materials of Construction for Storage Tanks

Storage tanks are constructed from a variety of materials. The material chosen

depends upon both the size of the tank and the chemical nature of the liquid being

stored.


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Materials commonly used are as follows.

Mild Steel [Low Carbon Steel]

This is an inexpensive material, but does not offer much resistance to atmospheric

corrosion unless specially protected, e.g. by painting with bitumen-type paints. It is

also subject to attack [to a greater or lesser degree] by most liquids. However, mild

steel tanks are easy to make and repair, and they can be lined with rubber or plastic

material as protection against corrosion.

Low Alloy Steel

Low alloy steels are a mixture of iron and small quantities of other metals such as

nickel and chromium. The presence of these metals whilst giving added strength to the

main metal [in this case iron], also improves the resistance to corrosion and other

damage from process fluids.

Stainless Steel

Stainless steel is a very common grade of alloy steel. There are many types of

stainless steel but a typical grade contains 18% chromium and 8% nickel, the

remaining 74% being mostly iron. It has a very good mechanical strength and, as its

name suggests, resists corrosion. Stainless steel, however, is very expensive.

Aluminium

Aluminium is a light metal and resists corrosion better than mild steel, but it is more

expensive than mild steel.

Storage of Gases at High Pressure

Gases are usually stored at high pressure, often in liquid form. This results in a saving

in the provision of storage tanks and storage area requirements. A gas may occupy

over 1000 times the volume it would occupy as a liquid. However, storage pressures of

as much as 1500 kPa [15 atmospheres or 15 bar] might be required to keep the gas in

liquid form.

Storage vessels must be specially constructed in order to withstand these high

pressures. There are several designs, as we will now see.


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SPERICAL STORAGE

Spherical storage vessels are preferred for storage of gases or liquids at high

pressure. A sphere is a very strong structure, since there is an even distribution of

stresses on the surface of the sphere. Hence, there are no weak points. However, they

are very much more difficult to manufacture and insulate than rectangular or cylindrical

storage vessels of similar capacity.

Storage is usually at atmospheric temperature.

This type of storage vessel must be fitted with a pressure relief valve which, in an emergency, is able to vent off any vapour formed in the vessel without the pressure rising above the relief valve setting.

Figure 3 shows a spherical storage vessel. Ancillary equipment required is similar to

tank storage, e.g. access manholes, safety valves, access ladders, earthing points etc.


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Table 1 shows the typical range of storage pressures used for different spherical storage vessel capacities.

CAPACITY (TONNES)

NORMAL PRESSURE RANGE

200

2000

4000

2—12bar

2—6bar

2—4bar

Table 1

Another major advantage of spherical storage vessels is that they have a lower

surface area per unit volume than any other shape of vessel.

This means that the quantity of heat transferred from warmer surroundings to the liquid

in the vessel, will be less than that for rectangular or cylindrical storage vessels.

Thus, liquid stored in spherical vessels will not ‘warm up’ as quickly as liquid stored in

other types of vessel. The ‘heat leakage’ into the cold liquid will [say on a hot

summer’s day!] be kept to a minimum.

For this reason, liquids such as liquified petroleum gases [LPG] and ammonia [which

require storage at low temperatures] are usually stored in spherical insulated vessels.

Substances such as these, which have to be cooled down to low temperatures before

they become liquids, are often referred to as cryogenic liquids.

CYLINDRICAL

Cylindrical storage vessels are less expensive to make than spherical types. They are, however, also less strong, since they have a weak point at each end. This mechanical weakness is reduced by providing the cylinder with rounded end sections, as indicated in Figure 4. The whole vessel is made from thicker metal than a comparable spherical type of similar capacity. Using the thicker material, storage pressures can be similar to spherical storage.


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ULLAGE

When filling vessels with volatile liquids under pressure it is always necessary to leave

adequate space above the liquid. This space is called ullage and is there to allow for

expansion arising from climate temperature changes.

The amount of space left depends upon the density of the volatile liquid being stored, and usually increases with increase in fluid density.

GAS CYLINDERS

To store very small quantities of gases at very high pressures [up to 15 bar] gas

cylinders are used. You probably see them every day on various plants. They are

convenient in size, but have thick walls and, as a result, are very heavy. When a

cylinder is in use a special head is attached which contains a needle valve for fine

pressure adjustment. The outsides of cylinders are painted in a BSI [British Standards

Institution] colour code. In addition, toxic or hazardous gases have BSI colour bands

around the neck of the cylinders as given in Table 2. However, BS 349 ‘Identification

of the contents of industrial gas containers’ places greater emphasis on marking the

cylinders with the name of the gas and its chemical formula or symbol at the valve end.

This naming of the gas is an essential requirement and is, therefore, in addition to the

colour bands.


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Table 2 Colour bands and example gases from BS 349

Storage of Gases at Low Pressure VARIABLE SIZE STORAGE The traditional way of storing gases at low pressure is in water-sealed gasholders [or

gasometers].

Though not very common these days, gasholders can still be seen in use for storing

domestic gas. There are two main types, both of which are shown in Figure 5 below.

NATURE OF GAS EXAMPLE GASES AND FORMULAE

COLOUR OF BANDS ON CYLINDER NECK TO

DENOTE HAZARD PROPERTIES

Nonflammable and nonpoisonous Nonflammable and poisonous Flammable and non-poisonous Flammable and poisonous

Oxygen [021 Chlorine [Cl2] Hydrogen [H2] Ammonia [NH3]

None

Golden yellow

Signal red

Signal red and golden yellow [red band next to valve at the top with yellow band between

red and colour of cylinder]


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The water sealed gasholder [Figure 5[a]], is constructed in sections that telescope either up or down according to the quantity of gas the holder contains. Seals fitted at the base and between section joints are filled with oil or water to facilitate the up and down movement.

The piston type, shown in Figure 5[b], has a sealed piston which moves up and down

inside the gas holder as the gas is either fed into or withdrawn from the equipment.

Materials of Construction for Gas Storage

As already mentioned the materials used in the construction of a vessel depend upon

its duty. Some materials used are:

• stainless steel

• aluminium

• mild steel [often lined with protective material].

Since gases are usually stored as liquids at high pressure, then materials used in

vessel construction need to be made thicker in order to give them greater mechanical


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strength than would be the case for low pressure storage.

Storage tanks are usually externally insulated against overheating by the sun on hot

days. The outside of the insulating material is either painted with bright reflecting paint

or covered with aluminium foil to further reduce overheating by absorption of the sun’s

rays.

This technique works because bright or shiny surfaces reflect heat energy away from

themselves.

You have now completed this lesson which has given you all the information that you

require at this level on the storage of liquids and gases.

SUMMARY

Liquids are usually stored out of doors in safe, remote areas called tank farms.

Storage tanks are fitted with a variety of safety devices [which are shown in Figure 1].

Floating roofs are often used to facilitate the filling and emptying of tanks, and to

reduce problems due to liquid expansion, contraction and evaporation. Gases are

usually converted to liquids and stored in spherical vessels at high pressure to reduce

costs of providing storage space.

APPENDIX


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CORROSION

Corrosion can be defined as the deterioration of a substance [usually a metal] because

of a reaction with its environment. For example, a motor car body will eventually

corrode due to the action of water and oxygen in the air. The original steel in the car

body will eventually turn to rust [which is a form of iron oxide].

The example of a rusting car is typical of corrosion caused by the oxidation of a metal.

In corrosion due to the oxidation of a metal, a chemical reaction occurs between the

metal and the oxygen from the air to form rust.

Corrosion by oxidation is the most common type of corrosion. Corrosion can occur by

solvents such as liquid metals or melted salts. If these materials can dissolve the metal

of the containers or pipes in which they are held then corrosion will occur. Unlike the

first type of corrosion where a chemical reaction takes place, corrosion by solvents

only involves the direct physical removal of material.

DENSITY

Density is a physical property of a material defined as the mass [often measured in

kilograms [kg]] of one cubic metre of the substance. For example, one cubic metre of

water has a mass of 1000 kg [at 20 °C]. This means that its density is 1000 kilograms

per cubic metre. This is written as 1000 kg/m3 or alternatively as 1000 kg m-3.

E.g. The density of aluminium is 2700 kg m-3

The density of lead is 11 340 kg m-3

The density of alcohol is 800 kg m-3. In the SI system of units the unit of density is kg m-3.

Density is calculated from the formula:

Density = Mass/Volume

This can be written using symbols as:

ρ = m/V


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Where ρ = density in kg/m3

m = mass in kg

V = volume in m3

Most density figures are given as relative density, that is, the ratio of the actual

density of a material to the actual density of water at a given temperature [usually 20

°C]. As the figures are ratios, there are no units. Therefore, the relative densities for

the three substances above are aluminium 2.7, lead 11.34, and alcohol 0.8 [i.e. lighter

than water].

PRESSURE

This is defined as the force per unit area and is calculated from the formula:

P = F/A Where P = pressure in pascals [Pa] F = force in Newtons [N] A = area in metres squared [m2]

In the SI system the unit of pressure is the pascal with the symbol Pa. This is a very

small unit and usually the kilopascal, kPa, is used. Atmospheric pressure is 101 325

Pa or approximately 101 kPa. This is approximately equal to 14.7 psi where psi stands

for pounds per square inch, a unit of pressure that is still commonly used especially in

Great Britain and the USA.

TEMPERATURE


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Temperature can be defined as the condition of a substance which decides whether

heat energy flows to or from the substance. Temperature is caused by the vibration of

molecules within a substance, the faster the molecules vibrate, the higher the

temperature of the substance.

The following diagrams illustrate the way in which heat flows from a substance at a higher temperature than the surroundings [Figure 6[a]], and to a substance at a lower temperature than the surroundings [Figure [6b]]

Temperature can be measured using a temperature scale. The customary unit of

temperature is the degree Celsius [often referred to as Centigrade]. This is defined as

1/100 of the difference between the temperature of melting ice and that of water

boiling under standard atmospheric pressure. Other temperature scales exist, such as

the Fahrenheit scale [180 degrees for the same difference in temperature].


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VOLATILE

A volatile liquid is one which easily evaporates into a gas above the liquid surface. If

the temperature of a liquid increases then this usually results in an increase in the

amount of the liquid which evaporates into the gas. If a volatile liquid becomes warmer

then a substantial increase in the amount of liquid evaporating into the gas will occur.

The following diagram [Figure 7] illustrates the increased amount of evaporation that

takes place for a volatile liquid compared to a less volatile one at the same

temperature.

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