Protecting Oil Tanks from Fire
The Use of Insulating Material as a Means of Protection—Characteristics Such Insulation Should Have—Must be Permanent in Nature
THE protection of oil tanks from outside exposure is a most important part of fire precaution in oil plants. The following article suggests the advantages of insulating material for the purpose of rendering the tanks fire resistant.
Insulation of oil storage tanks has been discussed at length by numerous authorities from the standpoint of vapor losses and their prevention, but little attention has been paid to the fire preventive and fire resistant quali-
ties of the insulating materials used for this purpose. In tests to determine the actual magnitude of evaporation losses and the saving due to tank insulation, widely varying results have been obtained. Sufficient data has been published, however, to indicate that these losses are almost unbelievably large, but the losses caused by preventable fires amount to even greater sums. It is the purpose of this article to call attention to the role played by fire-resisting insulation in the restraint and prevention of disastrous oil storage fires.
It is not the purpose of this article to delve into the causes of the conflagrations. Many petroleum engineers, after a thorough study of the question, have reached the conclusion that the greatest number of fires have been caused by lightning, or the discharge of accumulated static of the roof to the sides of the tank, thereby igniting the vapors always present above an oil body. This cause has been eliminated by the construction of tanks having water sealed tops, and by thorough grounding of the roof members. The complete loss of the contents of one tank, however, would not be such a disturbing factor if we could be sure that the fire would not spread to adjacent tanks and peril the entire refinery or tank farm.
Very often sumps, drains and open storage pools, are ignited throngh carelessness, and the heat generated sets fire to adjacent tanks, resulting in the subsequent loss of equipment and storage or refining units.
Effect of Heat on Tanks
It is to be remembered that heat affects the strength of steel very materially. The side plates of a loaded storage tank are under a considerable stress and since heat reduces the strength of steel, every effort should be made to prevent the tank shell from becoming hot from a nearby fire and eventually failing and inundating the surroundings with oil. At 600° F. the elastic limit of ordinary tank steel is reduced to one-half of its original strength, and at 900° F. it is only one-third of its original value. Ordinary steel plates become very brittle at a temperature of from 200° to 400° and although they regain some toughness at higher tern peratures, thev again become very brittle at a zone of from 800 to 1000° F.
The conditions which we are endeavoring to portray may be visualized by considering a prostrate gasoline storage tank, a duplicate of which is on fire a few feet away. The heat generated would be imparted in part to the unignited tank and the temperature of the contained gasoline would rise, until at least a part of it. would be vaporized. These vapors in contact with the heated shell would exert a high pressure upon the tank and inasmuch as the elastic limit of the steel has been reduced, the possibilities are that the shell gives way at some point and the gasoline escapes, ignites, and either an explosion takes place, or the entire tank bursts into flame. A welded steel tank provided with a suitable breathing valve, may not explode, but the gasoline would become hot, the vapors from the valve would burn like a torch and the tank would undoubtedly be distorted.
Petroleum products have a considerable volumetric expansion with increase in temperature. When the temperature of crude oil of about 25 Be. (.90 sp. gr.) is raised from 60° F. to 120 F., the volumetric expansion is 2.5%. Kerosene of 45° Be. has an expansion of 3% and gasoline of 60° Be. has an expansion of 4% through the same temperature range. A tightly sealed tank of gasoline, when heated, would therefore be under considerable internal pressure, which increases the danger from explosion.
A 30 foot prostrate gasoline storage tank, 7 foot in diameter, has a capacity of 1850 gallons or 245 cu. ft. at 60° F. If the temperature were raised to 120° F., the volumetric expansion would be equivalent to 9.8 cu. ft. The expansion of the steel shell would compensate, to a certain extent, for this volume increase, but at best the oil in storage would be under a high pressure.
It may be easily seen that where a tank, or refining unit, is completely insulated with a fire-resisting material which is a good thermal insulator, the chances of its ignition because of a nearby fire, are reduced to a minimum. A fire in one of a group of nearby insulated storage units would not have the opportunity of igniting the others.
The effectiveness of insulation for the above purpose is almost entirely dependent upon its resistance to intense heat and its thermal conductivity at high temperatures. The selection of materials for this purpose should be done very carefully inasmuch as many materials have a relatively low conductivity at low temperatures, say up to 250° F., but are practically useless at combustion temperatures. Insulating material should be judged on the following characteristics:
Thermal conductivity: Should not be in excess of 1 B.T.U. per sq. ft. per hr. per inch thickness, per deg. F. difference at a mean temperature of 1600 F.
Fire resistance: Should he free from shrinkage at 1600° F. and have no appreciable loss in weight at that temperature. The melting point should he above 2500° F.
Electrical conductivity: Should be a non-conductor, thereby minimizing danger from fire as a result of accidental contact with live wires.
Salvage value: It should he possible to remove or dismantle the insulation and use it over again on other equipment when necessary.
Permanency: The insulation should last as long as the tank itself, and protect the tank, lowering the maintenance costs, such as yearly painting, scaling, etc.
Application: The insulation should be easily applied without interfering with the use of the tank or its contents. The costs of application should be low and are usually compensated within a short period of time by reduction of evaporation losses alone.
Method of application: The insulation should not come in contact with the contents of the tanks and have no effect on the rapid charging and discharging of the tank.
Thermal conductivity is the characteristic which determines the amount of heat which would flow through a specific thickness of the material. The reciprocal of thermal conductivity is the thermal resistivity of the material, or the resistance which heat must overcome. Fire-brick, for example, have a thermal conductivity from 8 to 10 times as great as that of the most effective heat insulators, therefore, the temperature back of 2 inches of insulation would be lower than that back of even 13 inches of firebrick. The thermal conductivity of most materials increases rapidly with increase in temperature, and the insulating materials chosen for work in which the temperature may be expected to go to high extremes, should be tested at the expected temperatures and not judged by their comparative insulating values at low temperatures.
The insulating material should be inorganic and not suffer deterioration at high temperatures due to the charring or carbonization of ingredients upon exposure to high heat. Hair, for example, is an effective insulator at low temperatures but one would hardly call it an insulator at temperatures above 500° F. The effectiveness of materials at high temperatures can be observed through their loss in weight upon exposure to these heats, a loss in weight usually indicating a loss of some constituent which is responsible for the efficiency of the material at lower temperatures.
The most effective material should be permanent, and not change in character with age. This insures low maintenance costs and. very often makes it possible to remove insulation from a unit being withdrawn from service so that it may be used again on a new unit or otherwise.
A material coming up to these strict specifications is sold under the trade name Sil-O-Cel. It is furnished in the shape of brick 9″ x 2 1/2 x 4 1/2″ and 9″ x 4 1/2″ x 1 1/4″ in size, plastic insulating cements and blocks measuring 9″ x 24″ or 36″ x 1 1/2″, 2″ or 3″ in thickness. This material is a silicious mineral with a melting point of about 2900° F., and has no shrinkage at temperatures of 1600° F. The thermal conductivity is well below 1 B. T. U. per sq. ft. per hour even at 1600° F.
Methods of Tank Insulation
The methods of construction in applying this insulating and fire resisting material to prostrate and circular tanks, are described as follows: Usually about 2″ of insulation is sufficient to insure fire protection and at the same time reduce the evaporation losses to a minimum. For this purpose split the insulating brick referred to, 9″ x 4 1/2″ x 1 1/4“, are applied direedy against the steel shell, covered with chicken wire mesh, then covered with about 3/4“ of plastic insulating cement and then waterproofed with a bituminous material or with canvas which may then be painted with aluminum or other suitable paint. A fire resisting construction of this character will withstand a severe exterior fire very successfully and will only require a new covering of canvas to place it in its original condition.
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Protecting Oil Tanks From Fire
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Small tanks may be covered with brick insulation or with blocks. Fig. 1 gives the construction recommended for a prostrate storage tank. The insulating brick are placed directly against the steel shell with the aid of a sticking cement and wired in place with No. 11 galvanized iron wire. One or two inch galvanized mesh is then tightly wrapped around the tank and about 1/2″ of plastic insulating cement trowelled on and into the mesh to a smooth finish. This in turn may be covered with a hard finish cement or with eight ounce canvas which is treated with a preservative before being applied.
Large vertical tanks, such as shown in Fig. 3, should have their exterior surface divided into sections by means of vertical angle irons spot-welded to the shell at approximately 15 foot intervals. The insulating brick are then placed against the shell and held there by means of No. 11 gauge galvanized iron wires anchored to adjacent angle irons. One or two inch galvanized iron wire mesh is then stretched between the angle irons or completely around the tank and attached to the angles, and covered with to 1 inch of plastic insulating cement, trowelled to a smooth finish. This may then be covered with canvas, and painted.
Inasmuch as there is a certain amount of weaving or movement to the steel shell of large tanks, it is well to divide the surface into sections by means of both vertical and horizontal angles, as shown in Fig. 4. These may be heavily coated with a bituminous material which would effect a joint and at the same time allow for the expansion. Another advantage of this sectional application of insulation is that the insulation may be removed from one section very easily in case the shell leaks at that point, the leak repaired, and the insulation replaced without disturbing other sections of the insulating covering.