Recommended Practices for Storage and Transport of Edible Oils and Fats

Introduction

The international trade in oils and fats increases every year in volume and diversity. It included some 18 million tonnes of shipped cargo in 1984. The system involved in bringing the products from the harvest field to the end user is complex and inevitably a number of independent management is involved in processing, storage, and transport.
It is clearly in the interest of all the parties involved that any deterioration in the products should be minimised. To this end, a number of trade associations have published advice for the benefit of their members. Individual sources available are however incomplete and it was therefore proposed by PORIM at the 1983 AOCS world conference on the processing of Oils and Fats at The Hague. That an attempt should be made to compile and bring up to date the material available into a single reference document.
The procedure adopted was to prepare a draft and circulate it to major trade associations and to individual companies which showed interest. Comments provided by these organizations were incorporated in subsequent drafts for re-circulation. The resulting document, therefore, incorporates the ‘collective wisdom’ and practical experience of the participants. It has received endorsements from many important organizations in the trade.

Endorsing Organizations

It is our pleasure to thank most sincerely all the individuals and organizations who have collaborated. Formal endorsement of the ‘Advisory Practices’ has been received from the organizations listed below:

1. O.S.F.A Federation of Oils, Seeds and Fats Associations, U.K
2. A.S.C International Association of Seed Crushers, U.K.
3. O.S.I.C Japan Oil Stuff Inspectors Corporation, Japan
4. E.O.M.A Malaysian Edible Oil Manufacturers Association, Malaysia
5. O.P.G.C Malaysian Oil Palm Growers Council, Malaysia
6. I.O.P National Institute of Oil Seed Products, U.S.A
7. O.R.A.M Palm Oil Refiners Association of Malaysia, Malaysia
8. E.R.N.O.F Dutch Association of Oils and Fats Manufacturers, Holland
9. Hyam Myers and Associates, Australia
10. International Paints, U.K.
11. Karlshamn Oljefabriker, Sweden
12. Margarinbolaget, Sweden
13. Mitsubishi Corporation, Philippines
14. Palmco Incorporated, U.S.A
15. Unilever International
16. United Plantations Berhad, Malaysia

Use of the Practices

The document is advisory in nature. Its value rests on the fact that all the practices proposed are in actual and successful use somewhere or the other and that it has received a wide endorsement.
The ‘Advisory Practices’ are applicable to all oils and fats
It is hoped that newcomers to the field will use as far as possible the principles given in the design of their facility
It is recognized that existing installations may not conform in some respects to the ‘Advisory Practices’. It is hoped that over a period of time as repairs, maintenance, enlargements are required; they can be gradually brought in line.
A large degree of consensus on facilities and operations has been obtained. It is clear however that there is widely varying practical situations where management has to depart from normal and correspondent’s favored alternative designs or procedures. As one example when pumping oil in a northern winter, a higher than normal oil temperature may be necessary to prevent pipeline blockage. In order to reflect such instances adequately, the Advisory Practices are accompanied by explanatory notes and comments, and the two sections are best studied together.
It is intended to carry out a formal review of the ‘advisory practices’ within two years of publication. This will enable additions and amendments to be incorporated as a result of experience. In the meantime, any comments by recipients of the document will be welcomed by the institute.

Storage Installations and Transport

Tanks

Storage Tanks

The Most suitable shape is the vertical circular cross-section tank with a self-supporting fixed roof, preferably convex in shape. Where possible, tall, narrow tanks are preferred, to minimize exposed surface areas. Tank bottoms should be conical or sloped to be self – draining.

Capacity

For each installation, the storage capacity needs to be related to the expected storage period, the rate of turnover and the number of different products to be handled. The following sizes are suggested as a guide:
For refineries or end users, the capacities of storage tanks should be small, and it is desirable to have a number of tanks ranging from 200 to 1000 tonnes.
For export and import tank farms, suitable capacities for the various products are:

• Crude liquid oils, 1000 to 5000 tonnes.
• Crude non-liquid oils and refined oils, 500 to 2000 tonnes
• High melting fats such as palm stearin, tallow, hydrogenated oils, 500 to 1000 tonnes
• Fatty acid distillate or acid oil generally 500 tonnes or larger where turnover is large

Ship’s tanks

Ship’s tanks of mild steel should be coated with a suitable inert coating suitable for contact with food. It is preferable to construct a number of smaller tanks with capacities ranging from 200 to 1000 tonnes.

Materials

1. Copper, brass or bronze should be absolutely avoided for use in any part of the storage installation and means of transport that has contact with the oils, such as piping, pipe connections, valves, heating coils, temperature gauges for oil, strainers, pumps, etc. Or in sampling apparatus. Gauges containing mercury should not be used.
2. Any other material used for the construction of the tank should be inert to oils and fats.
3. Mild steel is acceptable for oil products but the tank wall, floor, and roof should preferably be coated. A number of different coating products are available, and specific assurance as to suitability for contact with foodstuff should be obtained from the manufacturers. Before applying the coating, the surface must be sand or shot blasted to bright metal (Swedish Standard SA3). There is usually a temperature limitation to the coating, which must be observed, and cleaning with live steam is not recommended.
4. Mild steel is not suitable for acid oil or fatty acid. Fiberglass or 316 stainless steel can be used; for many grades of product, aluminum is also satisfactory.

Heating Installation – Tanks

All tanks for solid or semi-solid products should be installed with heating facilities in order to obtain homogenous products when they are transferred or unloaded. Heating coil should be of mild steel for mild steel tanks and of stainless steel for coated and stainless steel tankage. The following means are suitable:

1. Bare hot water pipes: Heating by hot water (controlled at 80°C) circulated through the coils is the best procedure because it is least likely to cause local overheating.
2. Bare steam pipes: Heating by steam with pressure up to 1.5 Kg cm^-2 gauge (temperature of 127ºC). The heating coils are normally mild steel 5 cm (2”) bare pipes and should rest on supporting legs about 7.5 cm (3”) above the base of the tank. Some operators prefer supporting legs 6” to 12” high. Vertical hairpin coils or side heating coils installed on the tank walls also should be provided. Where no provision exists for mixing the oil, a maximum heating rate of 5ºC/24hrs should be maintained to avoid local overheating at the coil surface. Where mixing is provided with a higher heating rate is permissible. As a guide, a coil area of about 0.1m² per tonne of tank capacity is required if the fat has to be melted, but 0.05m² per tonne suffices for heating up purposes. For lagged tanks a smaller coil area is adequate. The total coil length is normally divided into two or more separate coils, of a length to avoid excessive accumulation of steam condensate.
3. Mixing: When the product is being kept liquid, local overheating is avoided if mixing facilities are provided, preferably by side entry agitators. Alternatively, oil may be re-circulated by pumping from the bottom and returning from the top through a line reaching below the oil surface. Aeration must be avoided. Mixing also reduces sampling problems.

Heating Installations – Road and Rail Tankers

For solid or semi-solid fats tankers should be fitted with stainless steel or mild steel steam coils which can be coupled to a source of hot water or low-pressure steam (steam pressure up to 1.5 kg cm^-2 gauge). In temperate and cold climates tankers should be insulated.

Tank Insulation

Storage tanks for solid and semi-solid fats should preferably be insulated, particularly in temperate and cold climates. Insulation is usually fitted externally to the wall of the tanks and must be designed to avoid the absorption of oil or water. Insulation has been proved of benefit even in tropical climates. Oil quality is preserved and energy is saved.

Control of Temperature

All ships and storage tanks with heating installations should be equipped with temperature sensors and automatic control devices to prevent over – heating of oil in the tank. Thermometers must be carefully sited and away from heating coils. It is useful to have automatic recording type thermometers to provide records of temperature control. The recorder should be installed in a conspicuous location such as supervisor’s office or the ship’s bridge.

Protection from Aeration

Pipeline connections should be designed so that admixture of air is avoided. Filling and emptying should be done from the bottom of the rank to avoid aeration.

Inert Gas Protection

Ships and storage tanks for high-quality products or for long storage should have facilities for sparring and blanket with inert gas of appropriate purity.

Pipelines

At a storage installation, a sufficient number of separate pipelines should be provided to avoid cross contamination of different products.At a ship loading pipelines should reach the bottom of the ship tanks; there should be a proper drain put pipeline at the base of each tank so that it can be completely drained.

Materials

Mild steel is acceptable for all crude and semi-refined oils and fats; for refined products and distilled fatty acids 316 stainless steel should be used.

Cleaning

A pipeline pigging system should be provided.

Flexible Hoses

All flexible hoses used to connect pipelines during loading and unloading must be inert material and be suitably reinforced.

Insulation and Heating

In temperate and cold climates pipelines should be lagged and also provided with heating, for example by steam tracing lines or electrical heating tape. Heating must be shut off when lines are empty.

Operations

Loading and Unloading

Heat Up

Solid and semi-solid products in refinery storage tanks, shore tanks, and ship tanks should be heated up slowly so that they are liquid and completely homogenous before transfer. Heating up should start without ever exceeding the maximum rate of 5ºC/24 hrs. If steam is used, the steam pressure should not exceed 1.5 kg cm^-2 gauge to prevent localized over – heating.

Temperatures

The various oil products should be heated up to the temperature shown in table 1 before transfer.

Table 1:

Unloading Temperature

Temperatures during storage and transport^aThe lower temperatures apply to soft grades, while the higher temperatures are necessary for hard grades. The temperatures apply to both crude and processed oils in each grade.
To prevent excessive crystallisation during short – term storage and shipping, oil in bulk tanks should be maintained within the temperature ranges given in table 2.

Table 2

Temperature during Storage and Transit

Loading and Unloading Sequence

The temperatures are chosen to minimise damage to the oil. Some crystallisation will occur, but not so much as to require excessively long heating before delivery. Thus palm oil stored at 32ºC – 40ºC will require about three days heating at 5ºC/ day to bring it to discharge temperature. Long term storage of all oils should be at ambient temperature and heating should be completely turned off.

Where a number of products are unloaded through a common pipeline system, the system must be cleared between different products or grades. The order of loading or discharge must be carefully chosen to minimise the consequence of contamination.

The following principles must be observed:

• Fully refined oils before partly refined oils
• Partly refined oils before crude oils
• Edible oils before technical grades
• Fatty acids or acid oils should be pumped last
• Special care should be taken to prevent contamination between lauric and non–lauric oils

If possible the first 3 – 5 tonnes of each grade should be collected in separate tanks for quality checks.When empty tanks, pipelines and pumps should be drained and all heating turned off.

Cleaning

Pipelines and valves should be immediately cleared after each pumping for which pigging systems are recommended. After clearing and/or emptying pipelines and tanks should be cleaned when no longer used for the same grade or product or if the inspection shows the presence of residues. Where tanks have been used for non-edible materials, the greatest care must be taken by cleaning and inspection that all residues have been totally removed.

Land tanks and pipelines should be constructed to drain by gravity and suitable drainage cocks etc. Should be provided. This is especially important where pipeline pigging facilities are not available. All common user pipelines and valves should be immediately cleared and cleaned after every pumping. If steam or water is used for cleaning, the system must be drained and completely dried before oil is handled. When not in use hoses and pipelines should be capped or blanked off to prevent contamination.

Ease of cleaning of mild steel tanks is greatly facilitated and costs of cleaning reduced by a suitable inert food grade coating.

Maintenance

Regular maintenance checks should be made. They should include functioning of steam pressure regulation valves; all steam supply valves and steam traps for leakage; thermometers and recording thermometers, weighing equipment and any gauge meters for function and accuracy; all oil pumps for leakage; condition of tank coatings; hoses (internal and external), and condition of tanks and ancillary equipment.

Others

There should be a suitable marking or identification system for the pipelines and storage tanks.The condition such as cleanliness of storage tanks, road tankers, ship’s tanks and pipelines should be inspected by suitably qualified personnel for every loading or unloading of oil and written reports provided.

All openings of tankers, storage tanks, etc. Such as manholes, inlets, outlets, draining out points, etc. Should be made such that they can be locked and/or effectively sealed.

Before heat up of oil, the heating coils should be covered completely

The temperature of loading or unloading should refer to the average of top, middle and bottom temperature readings. Bottom readings should be taken 12” away from the heating coils.

Where there are doubts about the cleanliness of pipes/manifold before discharge of oil from ship tank to shore tank. Then if possible, first running’s(3 – 10 tonnes) from the delivery line should be drawn into a separate container for inspection. Tank sediments also should be kept separate from the bulk.

Records of the ship’s heating log should be provided to the buyer.

Ship loading samples properly market and sealed should be delivered to the buyer

The three previous cargoes carried in a ship’s tank should be declared to the charterer of the tank. The provision should be part of all shipping contracts.

Explanations and Notes

General

Three types of deterioration can occur in oils and fats during the operations dealt with here.

Oxidation

Oxidation of oils results in chemical changes leading to rancidity. While the oil refinery can put right some of the effects of oxidation, processing of oxidised oils are more expensive and rectification may be incomplete.

Oxidation requires the presence of oxygen which is a universal component of the atmosphere. While it would be possible to protect oils totally from contact with air, it would be uneconomical. Much can be gained by reducing contact and this principle is the basis of several of the recommendations. Oxidation proceeds more rapidly at high temperatures. Thus each operation should be carried out at the lowest practicable temperature. Oxidation is speeded up by contact with copper or copper containing alloys and these must be excluded from the systems. Other metals such as iron also have a catalytic effect though it is less than that of copper. There is, therefore, an advantage of providing storage tanks with an inert coating and ensuring the absence of rust.

Hydrolysis

The breakdown of fats to fatty acids is promoted by the presence of water and higher temperatures. Hydrolysis is also promoted by the growth of microorganisms in the presence of water. Tanks should always be clean and dry before use. Free fatty acids also promote hydrolysis. And therefore crude oils, in particular, will increase in acidity during transport. In the case of fully refined palm oil, it has been found that oils shipped with an initial acidity of 0.05% (as oleic acid) or less change very little, whereas oils shipped with acidity above 0.05% show increases.

Contamination

Contamination may be from residues of a previous material handles in the equipment, from dirt, rain or sea water, or through the accidental addition of a different product. In storage installations and ships, particular difficulty may be experienced ensuring cleanliness of valves and pipelines, particularly where they are common for different tanks. Contamination is avoided by good plant design, adequate cleaning routines and an effective inspection service.

Specific Notes

The economics of bulk transport require that ship’s tanks should be versatile with respect to the cargoes that can be carried. Tank coatings protect a mild steel tank from corrosion and the product carried from contamination. Today most ships tanks used for oils and fats are coated. Damage to the coatings can be caused by abrasion, or by unsuitable cleaning methods and may be followed by local corrosion. Regular inspection and where necessary repair of coatings is important.

A large proportion of shore tanks, railcars and tanker lorries in use today are of mild steel and are acceptable to the trade. Nevertheless, it is observed that fully refined oils show some pickup of iron during transport, and as quality standards become more stringent, the advantages of a truly inert construction material or surface coating will become apparent.

An interesting possibility for tanks in shore installations is to fit a stainless steel lining in a mild steel tank. A thin layer provides the benefits of stainless steel at a lower cost.

Stainless steel grades 317 and 304 are also suitable. However, acid oils obtained from the alkali refining process may contain sufficient residues of mineral acid to be corrosive even to stainless steel. Stainless steel is not recommended is the pH of an aqueous extract of the oil is below 5.5, provided this is due to mineral acid and not to water soluble fatty acids or to citric acid. Citric acid is often added to refined oils to protect them against oxidation.

It may be noted that even a low steam pressure of 1.5 kg cm^-2 gauge gives a high coil surface temperature when viewed from the point of view of chemical reactions of the oil. This assumes particular importance if the oil layer in contact with the coil is static for example, as a result of a substantial amount of crystallised fat covering the coils. The value of ‘hairpin’ or side heating coils is that they quickly provide a pathway for convection currents to promote mixing. The provision of a mechanical agitator near the coil surface is claimed to speed up heating rates to 25ºC/24 hrs or more without overheating.

When designing an agitator or an oil recirculation system, it is necessary to avoid incorporating air during mixing. If there is dirt at the bottom, mixing will blend this into the whole consignment, which may be undesirable.

One design incorporates an elbow joint in the inlet pipeline. The outlet end is then arranged to ‘float’ on the tank contents and dip below the surface. A suggestion received for loading ship’s tanks was to use a line to the bottom for about 3/4^th of the filling operation and then to switch to a higher loading line.

Under cold weather conditions, discharge temperatures may need to be higher than those given, to prevent blocking of pipelines. The provision of heating to pipelines is, however, a preferable solution.

The practice was not considered practical by some correspondents but was always practiced by others.

It was pointed out that doubts about cleanliness should be resolved by an additional cleaning procedure. Nonetheless, one is left with the practical problem that the inside of pipelines often cannot be directly inspected. The most positive method for cleaning is undoubtedly a pigging system. Next best is to clean with hot water or steam as available and to ensure complete drainage.

This practice is already a part of some shipping contracts. It received universal support from correspondents. Considerable damage could be caused to an edible oil cargo by a chemical contaminant.