In industry, there are many different types of equipment used for the storage of liquids and gases. The many and varied types of storage systems are too many for all to be covered in this lesson. However, sufficient examples have been included in order to convey the principles of storing liquids and gases and the major problems involved.
The method of storage chosen depends on the following :
- The Quantity of Fluid being stored.
- The Nature of the fluid - is it Toxic, Flammable, Corrosive.
- The Physical State of the Fluid - Gas or Liquid, the Temperature and Pressure
Small quantities of liquids are often stored in 'CARBOYS' ; Larger quantities in 'DRUMS' and 'Bulk' quantities in 'TANKS' or 'RECEIVERS'
A carboy is a large, fat, glass, bottle-type container with a flat base for stability. They are used mainly for storing corrosive chemicals. Modern carboys may be made of a plastic material and may be cylindrical or rectangular in shape. Carboys generally contain 20 litres or more of liquid. They should NOT be stored near heat sources, should NOT be rolled NOR contain pressure liquids.
2. DRUMS (OR BARRELS)
Drums (barrels), are used to store larger volumes of liquid. They are cylindrical in shape and can be made from a variety of materials depending on the liquid (or solid) to be stored. Stainless steel, Aluminium or Mild steel with rubber or plastic lining are examples of materials used. Like carboys, drums must be handled with care. They CAN be rolled but should NOT be pressurised and NOT stored near heat sources. All containers should carry labels showing their contents and any hazards associated with them.
See Figure : 33
'Unlabelled containers should not be used under any circumstances.
Similarly, a container labelled as containing one material should not be re-used by refilling with another type of material'.
Figure : 33
3. STORAGE TANKS
Large volumes of harmless liquids involving little or no pressure are usually stored in simple, mild steel, rectangular or cylindrical tanks fitted with a flat or conical roof - known as a 'Fixed Roof Tank'.
Flammable or toxic liquids can also be stored in similar vessels but generally are located well away from residential and process areas due to the safety hazards involved.
When a number of tanks are installed, they are usually referred to as 'Tank Farms' and each tank may contain many thousands of tons (or tonnes) of liquid. The larger the tank, the lower the capital cost per ton (tonne), of liquid stored.
Figure : 34 shows such a tank and the possible combination of features built in. ( Note that not all tanks will have all of the features shown - some types of fittings depend upon the type of liquids the tanks contain ).
The diagram shows all the main features that MAY be found on a storage tank. For example, some tanks may be open and will not need a relief valve. Some will be shallow and will not need an access manhole …etc.
- Roof Access Ladder - As its name implies, is used for safe access to the tank roof.
- Access Manholes -Usually fitted at ground level or on the roof for access to the inside of the tank for cleaning, maintenance and repair.
- Water Drain - For use where water separation occurs in the tank and is to be drained off. Also useful during internal cleaning operations.
- Transfer Pumps - These are used to transfer some or all of the tank contents to a process or to other storage tanks, ships, vehicles .. etc. Depending on their piping arrangement, the pumps may be used for re-circulation (mixing) of the tank contents or pumping liquid into the tank from another source.
- Bund Walls (or Firewalls) - These can be of two types :
- A wall surrounding the tank, high enough to contain the entire tank contents in the event of a burst or severe leak. In this way, flammable, toxic or corrosive substances can be contained safely until recovered.
- An alternative to the above, is having the tank situated in a cylindrical hole in the ground, again large enough to contain all of the tank contents in the event of burst or leakage.
- Relief Valves - Installed where a tank contains pressure and set to relieve excess pressure if it rises to the safe operating limit.
- Inert Gas Blanket - This facility is installed when a tank contains volatile liquid which may produce vapours that become flammable or explosive when mixed with air. The inert gas injection (often Nitrogen), replaces the vapour above the liquid and will not react chemically nor produce an explosive mixture with the tank's contents.
- Foam Injection -In emergency, foam can be sprayed into the tank and over the surface of the liquid. Hazards arising from toxic or flammable vapours can be minimised.
- Vapour Vent - (Often called the 'Breather Valve') - This allows the tank to 'breathe' when emptying or filling. If a vent is not fitted, the tank could over-pressure when filling and cause a rupture in the tank and, when emptying, a vacuum would be pulled possibly causing the tank to collapse (implode). (The vapour vent can consist of simply an open hatch where the liquid being stored has no hazards).
For hazardous materials, the vapour vent may consist of a two-way safety valve. This will operate at a rising, pre-set pressure to vent excess gas from the tank to atmosphere or flare system to prevent rupture of the tank. At a falling pre-set pressure, the valve will operate to admit air, gas or inert gas (called 'Blanket Gas'), into the tank to prevent collapse of the tank if the pressure falls to a vacuum. (Gas or inert gas would be used where ingress of air is undesirable).
- Sampling & Gauging Hatch - As the name implies, this is installed in order to obtain samples of the tank's contents for analysis and is also used for dropping a ' dip-tape ' or ' dip-stick ' into the tank to check the liquid level and also check the automatic level measuring instruments.
- Earth Connections -An ' Earthing-strip ' is connected to tanks (and other equipment) in order to carry away and prevent build up of 'Static Electricity' which tends to form during filling and emptying operations. If static was allowed to build up where flammable liquids are being stored, then we would have a potential fire or explosion hazard. The storage of large volumes of liquid can lead to problems arising from the daily and seasonal variations in the weather. The most important variable, is that of the ambient temperature.
Variations in ambient temperature can lead to :
- Expansion (volume increase), and contraction (volume decrease), of the stored liquid. This is allowed for by leaving sufficient space above the liquid.
- Evaporation of liquid, particularly volatile liquid, in the tank on temperature increase. This will be taken care of by the 'breather' valve.
TYPICAL STORAGE TANK
Figure : 34
The picture on the following page shows a 'Tank Farm' having Fixed and Floating roof tanks.
FLOATING ROOF STORAGE TANKS
A 'Floating Roof', as its name implies, actually floats on the surface of the liquid in the tank. As the liquid level changes the roof is designed to move up and down with the liquid level - i.e. Filling, Emptying, Expansion and Contraction due to temperature changes. This type of tank roof minimises the vapour space between it and the liquid surface. Since there is no large vapour space for the liquid to evaporate into, vapour losses are also minimised.
In its simplest form, the floating roof is merely a large, flat pan (or disc), slightly smaller in diameter than the tank shell. It is fitted with a system of flexible 'shoes' to close the space between the edge of the roof and the tank shell in order to minimise vapour loss.
In modern usage, there are two main designs of floating roof :
- The ' Pontoon ' roof
- The ' Double-Deck ' roof
1. The ' Pontoon ' Roof:
This consists of a series of pontoons - (closed compartments) around the outer edge of a central plate. This construction increases the floating stability. The roof is also sloped towards the centre to allow rain water to flow into a water drainage system.
2. The 'Double-Deck' Roof:
This is a development which employs two separate decks of steel plate over the entire tank area. The space between the upper and lower plates is divided into compartments. With this type, the liquid is never in contact with the underside of a plate whose top surface is directly exposed to the sun's rays, as is the centre section area of the pontoon roof and the full area of the pan type roof.
The double-deck roof therefore, has much lower evaporation losses than any other type of floating roof. Because of the very small vapour volume, ' breathing ' due to temperature change and vapour formation when pumping into the tank, is greatly reduced. Also, because the actual vapour space is so small, danger of fire is also minimised due to the liquid surface being entirely covered.
Floating Roof Seals, Legs, Water Drain & Access Ladder
Between the edge of the floating roof and the inside of the tank shell, special seals are fitted to minimise leakage of vapour. The seal used is generally of a continuous strip of flexible, special rubber material which is attached to the roof and to the seal ring around the inside circumference of the tank shell.
The seal ring is held against the tank wall by a counter-weighted 'Pantograph Hanger'.
The complete seal unit moves with the roof maintaining a virtually vapour tight seal.
2. ROOF LEGS:
The underside of the roof is fitted with support legs in order to leave space between the roof and the tank bottom when the tank is empty. These legs allow access to the tank for cleaning and / or maintenance.
3. ROOF WATER DRAIN:
Rainwater drains to the centre of the roof into a well or water trap. The well is fitted with a flexible pipe which runs through the inside of the tank with a check valve in the well which will open as the well fills up with water. The bottom end of the drain line is connected to an outside valve which is normally left open. The check valve also prevents tank liquid from leaking on to the tank rooftop if the internal drain pipe should leak.
4. ROOF ACCESS LADDER:
A ladder is installed up the side of the tank and over on to the rooftop. This is used for access for sampling, gauging or maintenance ..etc. As the roof moves up and down, the internal section of the ladder will slide back and forth on wheels running along a track.
STORAGE OF HIGH PRESSURE GASES
Gases are usually stored under high pressure, often in liquid form. Liquefied gases take up very much less volume than in the gaseous state. The decrease in volume also decreases the size of the storage vessel. Different gases need different pressures in order to condense them at atmospheric pressure. It is often necessary to use refrigeration together with pressure to change a gas to liquid. In addition to liquefaction decreasing the gaseous volume, liquids are also easier to transfer from place to place.
A stored liquefied gas will remain as liquid at a specific temperature. For example, liquid Propane at 100 °F has a vapour pressure of 190 psia. This vapour pressure changes with temperature change. In winter when ambient temperature is low, the storage pressure of liquid Propane will be lower.
As a further example, LPG (Liquid Petroleum Gas), used for domestic purposes, is stored in small cylinders for ease of carrying. LPG may be pure Butane which has a vapour pressure of 51.6 psia at 100 °F. For winter use in cold countries, domestic LPG will consist of a high percentage of Propane or pure Propane because Butane may not give sufficient pressure.
As LPG gas is being used for heating or cooking, the pressure above the liquid tends to decrease. This causes the LPG to boil and produce gas to maintain its vapour pressure. In the case of Propane, if gas is used at a faster rate than the boiling liquid can produce, then, as the pressure decreases, the temperature also decreases. When the pressure above Propane liquid reaches atmospheric pressure, the temperature of the Propane will be at – 44 °F, this is the boiling point of Propane at 14.7 psia.
The pressure storage of liquids is therefore dependent on the vapour pressure of the liquid at a specific temperature. The type of storage vessel and its construction materials therefore depends upon the volatility of the liquid.
Following are some designs of storage vessels for high pressure fluids :
1. SPHERICAL STORAGE (STORAGE SPHERES) ( See Figure 35 )
This type of storage vessel is preferred for storage of high pressure fluids. A sphere is a very strong structure. The even distribution of stresses on the sphere's surfaces, both internally and externally, generally means that there are no weak points. Spheres however, are much more costly to manufacture than cylindrical or rectangular vessels.
Storage Spheres need ancillary equipment similar to tank storage - e.g. Access manholes, Safety valves, Access ladders, Earthing points .. etc.
An advantage of spherical storage vessels is, that they have a smaller 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 sphere, will be less than that for cylindrical or rectangular storage vessels.
2. CYLINDRICAL STORAGE VESSELS
Cylinders are widely used for storage due to their being less expensive to produce than spheres. However, cylinders are not as strong as spheres due to the weak point at each end.
This weakness is reduced by hemispherical or rounded ends being fitted. If the whole cylinder is manufactured from thicker material than a comparable spherical vessel of similar capacity, storage pressure can be similar to that of a sphere.
ULLAGE -The 'Ullage' space is the vapour space above a liquid in a storage vessel and is necessary to allow for expansion of the liquid during hot weather. The amount of ullage space depends on the density of the liquid, particularly volatile liquids and is usually increased with increase in liquid density.
3. GAS CYLINDERS
To store small quantities of gas at very high pressure, we use gas cylinders. These vessels however, are very heavy due to the thickness of the metal of construction.
When a gas cylinder is in use, a special head is connected which incorporates a regulator with a needle valve for fine adjustment and pressure gauges indicating the pressure upstream and downstream of the regulator. Such cylinders are painted with a colour code and, if the gas is toxic or hazardous, a coloured band is painted around the vessel neck which identifies the contents.
This naming of the gas is an essential requirement and is therefore in addition to the colour bands.
STORAGE OF LOW PRESSURE GASES
VARIABLE SIZE STORAGE
The traditional method of storing low-pressure gases is in a water-sealed Gas Holder (or Gasometer). Another LP storage vessel is the 'Piston Type' gas-holder.
These methods of gas storage are now almost obsolete due to modern methods of domestic gas distribution.
1. THE WATER-SEALED, TELESCOPING GASOMETER
This vessel is constructed in sections that 'Telescope' up and down as the quantity of contained gas changes. Water (or oil) seals are fitted between the telescoping sections with sufficient head pressure to prevent gas blowing out of the seals. The gas pressure is governed by the weight of the telescoping sections.
(See Figure 37)
2. THE PISTON TYPE GASOMETER
This gas holder contains a weighted piston with seals fitted between it and the vessel shell. Again, the quantity of gas governs the position of the piston. The pressure of the gas depends on the weight of the piston.
(See Figure 38)
The construction materials of storage vessels are selected by the Design Engineers and depend on the properties of the fluid to be stored. Mild steel, Stainless steel, Carbon steel, Aluminium, Special plastics and various other materials are used. Pressure and corrosive properties of the stored fluids are two of the main factors which govern the type and thickness of the construction materials. In many cases, vessels are coated internally with special substances to prevent corrosion. In other cases, a vessel may be externally ' lagged ' with an insulating material to minimise heat loss to, or heat absorption from, the surrounding atmosphere and other equipment.
PISTON TYPE GASOMETER