Subsection 4.3.1. - Design, Construction and Use of Storage Tanks
The amendment provides a means by which existing tanks that are still functional and structurally sound and have many more years of useful life may continue to be used. Without this amendment, any existing tank that does not meet one of the listed tank standards would have to be replaced by August 21, 2000. This was not the original intent of the provision. The criteria set out in Sentences (5) and (7) are similar to those used in the ULC standards on refurbishing (S601A and S630A) and verifiable through inspection. This permission for existing tanks would not grandfather provisions set out in Subsections other than 4.3.1.
Sentences (6) and (8) would require upgrading or replacing of existing tanks that do not meet one of the listed standards, or have not been designed to good engineering practice or the acceptance criteria in Sentences (5) or (7). Changing the reference in Clause (1)(p) replaces a reference to a standard that no longer exists with the current standard. The inclusion of Sentence (9) means that existing tanks that met the former ORD C142.16 will continue to be acceptable.
This Section applies to storage tanks where capacity exceeds 230 liters used for storing flammable and combustible liquids above and below ground. Due to the wide variety of liquids along with their respective properties being stored, there is no one single standard applicable to the design and construction of these tanks. Most aboveground storage tanks are constructed of steel. However, the use of other non-combustible material may be permitted provided the storage tank is used to store a combustible liquid. As well, if properties of the liquid necessitate the type of material the tanks must be constructed from and the tank is protected against fire exposure by an approved method, other non-combustible materials may be used.
Underground storage tanks may be constructed of combustible materials when built in accordance with CAN4-S615-M "Standard for Reinforced Plastic Underground Tanks for Petroleum Products."
Aboveground storage tanks are either designed as atmospheric or low pressure tanks. The term "atmospheric tank" is defined as a tank operating at pressures from atmospheric up to and including 3.5 kPa. A "weak shell-to-roof" joint design (API Standard 650) can be found in many atmospheric tanks. This design allows for the failure of the weak seam of this roof to shell joint in the event of an over pressure buildup within the tank. Tank failure at the top of a tank will permit vapours to be released while containing the liquid contents. Pressure vacuum valves are provided to accommodate normal pressure fluctuations due to the filling and emptying of the tanks and changes in temperature.
Low pressure tanks are shop fabricated and leak tested prior to being shipped as completely assembled units. These tanks are designed for pressures greater than 3.5 kPa up to 100 kPa (gauge). Generally, normal internal operating pressure is 6.9 kPa while 17.2 kPa results in emergency venting conditions. These restrictions recognize that failure of a horizontal low-pressure tank is invariably accompanied by the release of tank contents.
This Subsection outlines the various standards permitted for the design and construction of both atmospheric and low-pressure storage tanks. An existing atmospheric tank may be approved provided its calculated internal and external corrosion loss expected during the design life of the tank does not exceed that provided for in the original design. Where calculations indicate a higher corrosion rate, additional metal thickness, protective coating or approved lining shall be provided such that the corrosion loss standard for the original design is met. Tanks may be lined or sprayed with combustible or non-combustible material to provide corrosion protection. As the quantity of the lining material used is small, the fact that it is combustible or non-combustible does not matter. However, this lining must not increase the risk of ignition from static electricity and must be compatible with and resistant to degradation from the liquid stored in the tanks.
The air space within the storage tank above the flammable or combustible liquid surface will contain the vapours of the liquid. As the ambient temperature rises, more of the liquid will evaporate. These vapours can then be lost to the environment during venting especially when the tank is being filled. To reduce this evaporation, a floating roof was designed to float on the liquid surface. This design is particularly beneficial for Class I liquids and serves as an excellent environmental protection device. A floating roof tank is defined as one that incorporates either:
This floating structure must be constructed of metal (except for the perimeter sealing material) and must be sufficiently buoyant to prevent it from sinking when half of the pontoons or floats are punctured and flooded. Most fires in this design of tank burn only at the seal area and are usually easily extinguished.
A tank with an internal floating pan or one that uses plastic foam for floatation is considered to be equivalent to a fixed roof tank. This is because metal pan roofs are prone to sinking and because foamed plastic and similar floatation devices will not withstand the conditions imposed by fire. Though not recognized by Part 4, these can serve as good conservation and environmental devices.
Experience has shown that tanks having floating roofs are not likely to be involved in serious fires. This is because there is far less liquid surface exposed to the fire.
Each tank must be clearly labeled as to its contents. Lettering size readable from 4.5 metres away or from outside the tank dike area, whichever is greater and from two diagonally opposite tank sides is required. Labeling will assist the firefighters when they arrive on the scene by allowing them to determine the best means to extinguish the fire and the most appropriate extinguishing agent to use. In situations where tank contents are frequently changed, a label indicating "flammable liquid" may be used subject to the approval of the Chief Fire Official. In plants and refineries where qualified personnel, knowledgeable in the various tank contents are on site 24 hours/day and 7 days/week to assist the fire department, a coded identification may be approved.
A serious fire hazard results when an aboveground storage tank is overfilled with a flammable liquid (Class I). To prevent this, Part 4 requires continuous supervision of the filling process by a qualified person or by the use of an overfill protection device conforming to ULC/ORD-C58-15, "Overfill Protection Devices for Flammable Liquid Storage Tanks". Examples of devices used to prevent overfilling include automatic sensing devices interconnected with shut-off equipment, automatic overfill shut-off devices of a float valve, vent restriction devices, and audible or visual overfill alarm devices.
Overfilling a tank is most likely to occur when liquids are being transferred from facilities having a large capacity in comparison to the tank capacity. Examples of operations where overfilling is likely is pipeline deliveries and deliveries from large marine vessels. To prevent overfilling during such liquid transfer operation, the tank owner shall comply with one of the following requirements:
The instrumentation referred to in b) and c) must be fail safe and supervised electrically to a constantly attended location or by a method approved by the Chief Fire Official.
Written procedures concerning liquid transfers shall be prepared and implemented by the owner. These procedures should include instructions covering methods to ensure the delivery if to the correct tank and the quantity to be delivered does not exceed the available tank capacity. Personnel should be given suitable training, and be monitored to ensure the procedures are properly followed. Procedures should also specify inspection intervals at least every three months and testing intervals at least annually for the high liquid level instrumentation. Any deficiency or malfunction must receive immediate corrective action.
The requirements of this Subsection are to ensure that tanks are installed in locations where they will not present a hazard to structures on adjacent properties and minimize the damage within the property itself in the event of a fire. Tank spacing is determined based on the maximum tank capacity or size as specified in Table 4.3.2.A1.
The location and spacing requirements will also vary depending on the characteristics of the liquid stored, type of tank, protection provided for the tank and protection provided for fire exposures.
The following liquid characteristics are considered in these requirements:
A stable liquid is defined as one that will not undergo violent decomposition or reaction at or near normal temperatures and pressures and is chemically stable when subjected to shock or impact. For tanks containing stable liquids operating at a pressure of 17 kPa (gauge) or lower, the minimum separation distances provided in Table 4.3.2.A1. may be used provided they are equipped with emergency venting to maintain the pressure at or below 17 kPa (gauge). The use of a weak roof-to-shell seam in atmospheric storage tanks meets this requirement. The separation distances in Table 4.3.2.1A. can be reduced by 50% if the tanks are equipped with fixed fire protection as specified in Article 4.3.2.5. Tanks containing unstable liquids operating at pressures under 17 kPa (gauge), may use the separation distances in Table 4.3.2.1A. multiplied by 3, with a minimum spatial separation of 15 metres. The reason for this increased separation is to compensate for the higher potential for a tank containing an unstable liquid to fail violently. Where such tanks are equipped with fixed fire protection as specified in Article 4.3.2.5., the separation distances provided in Table 4.3.2.A1. may be used with a minimum spatial separation of 7.5 metres.
When storage tanks are not equipped with fixed fire protection and contain a stable liquid operating at a pressure above 17 kPa (gauge), the separation distances given in Table 4.3.2.1.Table 4.3.2.A. are multiplied by 1.5, with a minimum spatial separation of 7.5 metres. For the same type of tank equipped with fixed fire protection, the separation distances given in Table 4.3.2.1.Table 4.3.2.A. are multiplied by 0.75, with a minimum spatial separation of 7.5 metres. Where storage tanks are not equipped with fixed fire protection, and contain an unstable liquid operating at a pressure above 17 kPa (gauge), the separation distances given in Table 4.3.2.1.Table 4.3.2.A. are multiplied by 4.5, with a minimum spatial separation of 15 metres. For the same type of tank equipped with fixed fire protection, the separation distances given in Table 4.3.2.1.Table 4.3.2.A. are multiplied by 1.5, with a minimum spatial separation of 7.5 metres.
Sentence 4.3.2.1.(8) places additional requirements on the location of horizontal pressure tanks (commonly referred to as bullets). This sentence specifies that where end failure of horizontal storage tanks may endanger adjacent property, the tanks shall be placed with the longitudinal axis parallel to such property. When these pressure tanks are exposed to fire they tend to fail at the one end resulting in the tank being rocketed along its axis. This requirement is designed to minimize the potential impact should the tank fail.
Greater spacing requirements are applied to tanks storing liquids having boil-over characteristics. Boil-over occurs with liquids that have a wide range of boiling points with volatile components, liquids that contain a highly viscous residue, and water-in-oil emulsions near the boiling point of water. The distances in Table 4.3.2.1.Table 4.3.2.A. are to be used when the storage tank is not equipped with fixed protection in accordance to Sentence 4.3.2.5.(2). Where such protection is provided, the distance provided in Table 4.3.2.1.Table 4.3.2.A. is multiplied by 0.75.
In open pool burning, the radiant energy from the flames just above the vapours heats the liquid driving off more vapours which burn generating more radiant heat. This continues until the liquid is depleted.
When an open top tank containing a boil-over liquid is involved in a fire, the volatile components of the surface layer is evaporated. This layer becomes hotter and denser and sinks below the surface to be replaced by unburned oil. This cycle continues resulting in a deepening layer of very hot oil (93.30C or more) known as a "heat wave". When this hot layer reaches water or water/oil emulsion at the bottom of the tank, the water is superheated and subsequently flashes into steam and boils almost explosively. This steam rises to the surface forming a froth, over-flowing the tank. This condition results in the expulsion of as much as half of the tank's contents spreading it as a burning mass over a wide area.
Note that a boil-over is an entirely different phenomenon from a slop-over or froth-over. Slop-over occurs when water is sprayed onto the surface of hot burning oil resulting in minor frothing. Froth-over on the other hand does not involve a fire. When water is mixed into a tank of hot viscous oil, the flashing of the water into steam causes a portion of the contents to overflow.
Protection of storage tanks against fires or explosions must conform to Article 4.3.2.5. or good engineering practice. Good engineering practice includes assessing the necessary protection for tanks through the use of guidelines or standards published by National Fire Protection Association, Insurer's Advisory Organization (1989) Inc., Industrial Risks Insurers and Factory Mutual Engineering Corporation.
The minimum spacing requirements between any two adjacent aboveground storage tanks is 25% of the sum of their diameters. This is subject to a minimum of 1 metre separation for stable liquids and 2 metres for unstable liquids. The requirements are to permit sufficient space for maintenance and suppression, to permit an orderly and safe arrangement for pipelines and to prevent the spread of fire from one tank to another. Due to the inherent hazards of liquefied petroleum gas (LPG) and compressed natural gas (CNG) a 6 metre minimum separation is required between cylinders of these gases and flammable and combustible liquid storage tanks. Further, storage of these cylinders within the same containment or dike compound of the storage tanks are prohibited. A minimum separation of 3 metres is required from the center line of the storage tank dike wall to a LPG cylinder and 7 metres to a LPG storage tank.
Access to above ground tanks for fire fighting purposes should take into consideration the tank diameters, the tank layout and access routes for fire department vehicles and hose streams. The routes must be located so that fire department vehicles can approach to within 60 m of any storage tank. Arrangements where there are more than two rows of tanks with large diameters will make exposure protection a challenge for firefighters.
Due to the fire hazards associated with large storage tanks, there is a requirement that tanks exceeding 45 m in diameter be provided with protection against fire or explosion as outlined in Sentence 4.3.2.5.(2).
Article 4.3.2.4. indicates that where fire fighting access is not provided, the option of providing protection is available only for those tanks storing Class I or II liquids. If an owner of tanks that store Class IIIA liquids submits a compliance equivalency under 4.1.1.5. using options available for Class I or Class II liquids, the submission should be acceptable. Because Class IIIA liquid is less hazardous than Class I or Class II liquids, other options may also be acceptable, but these must be evaluated on their own merits.
See commentary for Article 4.2.7.7. for additional references to standards that may be used as good engineering practice.
Foundations used to support aboveground tanks used for the storage of flammable and combustible liquids, should be designed to minimize uneven tank settlement and minimize corrosion on the part of the tank in contact with the foundation. Tank supports or foundations for atmospheric storage tanks must be in conformance with Appendix B of API 650, "Welded Steel Tanks for Oil Storage" and to Appendices C & D of API 620, "Design and Construction of Large, Welded, Low-pressure Storage Tanks."
Generally, horizontal tanks are supported well above their concrete foundation to provide a hydraulic head for the purpose of loading the liquid contents. These cylindrical shape tanks are usually supported by a steel saddle contoured to the shape of the tank wall. The number and spacing of these saddles are subject to the allowable designed stress of the storage tank. The ability of these supports and saddles to withstand exposure to fire is very important. Failure of these saddles may result in the tank falling to the ground causing damage to the piping or the tank itself. As well, spilled contents allow the fire to spread resulting in exposure to other tanks or structures. For these reasons this Subsection requires that such supports and saddles be protected by a 2-hour fire-resistance rating. This can be accomplished by encasing the steel members in a minimum of 11/2 inches of wire mesh reinforced concrete or gunite. Saddles which do not exceed 300 mm in height do not require fireproofing.
Tanks float when the level of their contents become less than the surrounding water level. Storage tanks located in areas subject to flooding must incorporate measures to prevent them from floating. Within diked areas the accumulation of rainwater or melting snow can cause an empty tank to float. Procedures and equipment should be provided to drain accumulated water. Water draining operations should be attended at all times. Upon completion, valves, etc. should be immediately shut. When a tank floats it can break any connecting piping resulting in a spill.
Filling and emptying a storage tank creates positive and negative pressures inside the tank. To fill a tank, air and vapours within the tank must be pushed out developing an internal tank pressure slightly above atmospheric pressure. For this reason, tanks are designed to withstand an internal pressure of 2.3 kPa. To empty a tank, the reverse occurs requiring air to enter the tank, developing an internal tank pressure slightly below atmospheric pressure. For this reason, atmospheric tanks are designed to withstand a vacuum of 0.689 kPa. Ambient pressure and temperature changes may also produce a slight pressure or vacuum within the tank. To accommodate this air movement, vents are provided to prevent the tank from exploding or imploding. Normal vents are sized in accordance to Article 4.3.4.1. or API Standard 2000, "Venting Atmospheric and Low Pressure Storage Tanks," Table 1, but in no case should the vent size be less than 30 mm nominal inside diameter.
containing flammable liquids are equipped with venting devices that are normally kept closed for conservation and environmental purposes unless the tank is venting. For pressures up to 6.89 kPa, PV (pressure/vacuum) valves are used to prevent over-pressure and under-pressure conditions. Since the vapour space in a flammable liquid storage tank can be in the explosive range, a flame arrester is used in conjunction with the PV valve to prevent a flame from entering this space. Hard topped floating roof tanks may have open vents for venting the space between the floating roof and fixed roof.
The discharge from vents must be arranged such that it prevents flame impingement on the tank should the discharge vapours be ignited. This is often accomplished using outlet pipes which discharge away from the tank. To prevent accumulation of water in the pipes from rain or condensation which can hinder the operation of the vents, open weep holes are provided. These weep holes must also be located such that the vapours are discharged away from the tank.
Flames contacting aboveground tanks quickly increase the rate of vapour production which in turn increases the internal tank pressure. This pressure must be relieved before the design tank pressure is exceeded. This can be accomplished by either:
Venting requirements for unstable liquids must take into account two-phase (i.e. liquid/gas or foam) flow through the vent. Therefore, the vent size needs to be sufficiently large to accommodate the increased resistance to flow.
may also generate heat and vapour through chemical reaction. "Runaway" chemical reactions may generate considerable heat and vapour. Therefore, the sizing of the emergency vents should also take these characteristics into account.
Flames and radiant heat contacting the outer surface of the tank will result in heating of the contents and generation of large quantities of vapour when the contents boil. Table 3 in API Standard 2000 expresses a venting rate for this vapour in cubic feet of free air per hour as opposed to the size of the tank opening. This is because all venting devices have a specific discharge coefficient. The wetted surface area of the tank is used in this table is calculated as follows:
These capacities are based on the assumption that the liquid has the characteristics of hexane.
Where the storage tanks are provided with protection from fire exposure by insulation that is noncombustible, resists dislodgment by fire fighting activities and does not decompose at temperatures up to 540 oC, the normal and emergency venting capacity can be reduced by a factor as outlined in Table 4 of API Standard 2000.
Although Article 4.3.5.1. references the entire Section 4.4, the intent is that the owner only needs to comply with Subsections 4.4.2., 4.4.3., 4.4.5., 4.4.7. and 4.4.8.
The location of vent pipe outlets from tanks is important because the vapour discharge may be in the explosive range. Should the discharge be exposed to a fire, it can act like a blow torch and therefore must be directed away from all tanks, buildings and structures.
For tanks storing flammable liquids located adjacent to buildings or public ways, the vapour from outlets should be released at a safe point outside the building at a minimum of 3.5 metres from the ground. For tanks storing combustible liquids, the minimum distance is 1.52.0 metres. The vapours should be discharged upward or horizontally away from closely adjacent walls and at least 1.5 metres from any building opening. These requirements will assist in the dissipation of vapours that are almost always heavier than air.
The manifolding of vent pipes for tanks storing flammable liquids with vent pipes for tanks storing combustible liquids should be avoided unless means are provided to prevent flammable liquid vapours from entering combustible liquid tanks. When this mixing occurs, the combustible liquid storage tank should be reclassified to a flammable liquid storage tank.
API Standards 620 and 650 require that all piping connections used for tank filling, emptying or blending operations, and located on the tank shell below its maximum liquid level, must be provided with a shutoff valve. The valve must be located adjacent to the tank shell, constructed of steel and clearly indicate whether the valve is open or closed. If piping from a tank should fail, staff or emergency response personnel should be able to quickly shut the valve to prevent draining of the tank contents. It is recommended that all tank valves be kept in the fully closed position at all times except when filling, emptying or a blending operation is taking place.
Opening of hatches for the purpose of measuring tank contents creates a risk of introducing air and forming an explosive mixture in and near the tank opening. Where hatches are difficult for staff to open and close, there may be temptation to leave the hatch open. Use of hatch counter-weights and strict operating procedures should minimize this risk.
Connections used for the filling and emptying of a tank must be located outdoors. The reason for this is because these connections are frequently broken thus releasing both vapour and liquid. It is preferred that this release occur outside of a building. An exception to this is permitted if a process located indoors is directly associated with a tank.
The requirements of this Subsection are designed to ensure that in a situation where a flammable or combustible liquid from a tank is released, the spill will be contained such that it will not endanger other adjacent areas.
Secondary containment can be achieved by enclosing a tank or group of tanks within a dike compound or by directing a spill to an impounding basin. This secondary containment must have the capacity to hold the entire volume of the largest tank plus 10% of total volume of all other tanks. The extra 10% allowance is to account for accumulated water or snow that is present or to allow for dike height settlement. Where an impounding basin is used, it must be located such that if the spill ignites, a fire will not damage other tanks, buildings or adjoining property. Grading should be directed away from the tank and away from any adjoining property to assist fire suppression efforts to reduce heat and flame impingement on the tank and other tanks or property. Impounding basins and dike compounds must have water drainage facilities to maintain them free of accumulated water and melting snow as often as necessary. Valves designed to be opened for draining purposes should be closed at all times when not used for this operation.
In situations where it is difficult to provide remote impounding, tank diking may be used to contain liquid spills. A slope of at least 1% directed away from the tank toward the dike base is recommended to keep a minor spill as far away from the tank as possible.
The minimum distance required between a storage tank shell and the interior toe of the dike wall is 1.5 metres.
Access for fire fighting must be provided on a tank farm. Where a diked area extends to the property line, a minimum space of 3 metres must be provided between the outer base of the dike to the property line. This gives firefighters access to provide protection to buildings on adjoining property during a fire in the diked area. Firefighters also need access to valves and tank roofs to control flammable liquid spills.
In situations where the secondary containment wall exceeds 1.8 metres, there is a strong likelihood for hazardous heavier-than-air vapours to accumulate at ground level in the contained area. Access to valves and tank roofs must be provided at a level above the top of the dike through the use of elevated walkways, remote operated valves or other means. Note that testing for vapour accumulation should precede entry into such a contained area. Access roads must comply with Building Code Article 3.2.5.7.
Where drainage facilities are provided to drain water from the diked areas, access must be made available under fire conditions to prevent flammable or combustible liquids from entering any natural water course, public sewer, or drainage system. Combustible materials, compressed gas cylinders, empty or full drums, or barrels cannot be stored in the diked areas as they constitute an additional fire hazard, reduce dike volume and restrict fire department access.
Recently, manufacturers have developed double walled tanks which provide secondary containment in the event of failure of the inner tank. Further, technology now permits the space between the walls to be continuously monitored so that failure of either wall can be detected and prompt action taken.
CAN/ULC-S643, "Shop Fabricated Steel Aboveground Utility Tanks for Flammable and Combustible Liquids", did not have provision for double wall tanks when Part 4 was introduced into the Fire Code in August, 1997, and was not referenced. As provision for double walled tanks was added to the Standard in February, 1998, it is appropriate to reference this standard in Subclause 4.3.7.4.(2)(a)(ii).
The advantages of an underground installation are:
However, the main disadvantage of an underground installation is that tank leakage is not easily detectable, and such leaks can threaten underground water courses or public sewers and adjacent buildings.
Underground tanks should never be built under building walls or foundations because the tanks can be damaged by these stresses. This construction would also make it difficult to replace such tanks. Underground tanks must be installed a minimum of 1 metre from the foundation of any structure so that any settlement of the structure will not damage the tank.
The Chief Fire Official and Chief Building Official must be informed of the location of all underground tanks when a planned expansion is contemplated and a building permit is requested.
Several underground tanks can be buried together side-by-side, as long as there is a minimum separation of 600 mm between the tanks. In this type of installation, backfilling with sand is recommended. A minimum of 600 mm of ground cover is required over underground storage tanks.
A minimum distance of 1.5 metres between underground tanks and a property line is required to minimize the possibility of damage to the tank or to its protective coating and sand envelope by construction activities on the adjacent property.
Underground tanks located in an area subject to vehicular traffic must have a minimum of 1 metre of earth coverage or 150 mm of reinforced or 200 mm of non-reinforced concrete which extends 300 mm beyond the tank perimeter to prevent damage to the tank if a loaded truck is driven over the spot where the tank is buried.
The backfill material of all underground tanks should not contain any rocks, boulders, bricks, blocks, broken concrete or asphalt or other debris as these materials could damage the tank exterior. Only clean sand should be used.
Fiberglass reinforced plastic (FRP) tanks are required to be backfilled with pea gravel as per the tank manufacturer's installation instructions.
Since empty underground storage tanks will float if subjected to a water table or flood, there are requirements to anchor or provide overburden to prevent them from lifting out of the ground. Tank movement can break any connecting pipes or the tank. Any proposed means of anchorage or overburden must be sufficient to resist the buoyant forces on the tanks when they are empty and completely submerged.
Means which have been employed successfully to protect tanks against uplift are:
Anchor or ground straps, where provided, must be installed and tightened in such a manner to not damage the tank or its protective coating.
Underground steel storage tanks are often protected from corrosion by a suitable coating. More recently, tanks have been constructed of fiberglass reinforced plastic (FRP). Tanks are fabricated to Underwriters of Canada (ULC) standards.
Unless steel tanks are provided with external corrosion protection, they are subject to corrosion and failure. Failure rates vary depending upon several factors such as location of the water table, composition of the backfill and natural soil conditions in the area. For these reasons, the corrosion of steel underground tanks and associated piping has become a major environmental concern throughout North America. CAN/ULC Standard S603.1 requires tanks to incorporate cathodic protection.
Since FRP tanks are resistant to the effects of external corrosion, only the steel components that may be connected to a FRP tank need corrosion protection.
The requirements for vents from underground storage tanks differ slightly from that for aboveground storage tanks. The reason for these differences may be attributed to the following: buried tanks never have a cone roof or a floating roof, they are generally of the horizontal cylindrical type, they cannot be seen when being filled or emptied, and the contents are less affected by weather conditions.
To prevent underground tanks from failing due to pressure changes from filling and emptying operations, a vent is provided to permit the flow of air in and out of the tank. The size of the vent line must be large enough to accommodate the maximum filling or emptying rates without exceeding the allowable stress for the underground tank. It is important that this vent line and its outlet be protected from possible blockage from weather (i.e. snow, rain, ice), dirt, insects and bird nests. Vent piping outlets must be outdoors and located and oriented in a direction that vapour discharge will not accumulate or travel to an unsafe location, enter building openings or be trapped by building eaves and must be at least 1.5 metres from building openings (doors, windows, vents, etc.). Vent outlets for underground storage tanks used to store flammable liquids are required to be at least 3.5 metres above the ground and at least 7.5 metres from any dispenser. Vent outlets for underground storage tanks used to store Class II or IIIA liquids are required to be at least 2.0 metres above the ground. The vent outlet must also be above the top of the fill pipe so that if the tank is over filled, liquid will not be discharged from the vent line. The vent line must be properly supported and protected from mechanical damage such as vehicle impact. Frequently, vent outlets are equipped with return bends, to prevent snow and rain from entering, and with a coarse screen to keep out animals and birds. Screens should be regularly inspected and, if obstructed, should be immediately replaced.
A vent line must be connected to an underground tank at the top of the vapour space. It must not extend down into the tank more than 25 mm, unless equipped with a vent alarm, to prevent the vent line from being sealed off by a high liquid level. Vent lines must be sloped towards the underground tank without any traps to prevent vapour condensate from blocking the vent line. Where necessary, protection should be provided against mechanical damage.
It is not permitted to manifold the vent line of an underground tank containing a flammable liquid with the vent line from a tank containing a combustible liquid. This is because vapours or condensate from the flammable liquid may enter the combustible liquid tank and create explosive conditions and contaminate the product.
To prevent the escape of vapours from an underground storage tank from other than the vent line, all open-ended pipes from such tanks should be capped. Openings for liquid level gauging, the ends of fill and withdrawal lines should all be equipped with a vapour tight cap or cover. This cap or cover should be kept securely in place except when the pipes are in use.
Filling outlets located remote from the tank should be lower in elevation than any of the withdrawal outlets. This allows over-filling to be detected immediately if the withdrawal outlet(s) are not visible from the location of the filling outlet.
During the filling process, the flow of some liquids through a pipe can generate a static charge in the liquid, particularly at high velocities and if the flow involves two or more phases. The manner in which this liquid is added to the tank can cause turbulence and splashing, particularly if the fill pipe terminates well above the liquid surface. The free-falling liquid breaks up into fine droplets that become a charged mist. Where possible, it is considered good practice to extend the fill pipe to within 15 cm of the bottom of the tank to avoid splash filling.
Filling and emptying connections should be located outside a building a minimum of 1.5 metres from any building opening and in a location free of ignition sources. This requirement was created because experience has shown that these connections are a potential source of spills and release of vapours. The only exception for a connection inside a building is for a pipe used to convey a waste liquid to an underground tank. This line must have a liquid trap to prevent tank vapours from escaping into the building.
The term "tanks" used in Part 4 refers only to storage tanks and not to portable tanks or process vessels. If the storage of liquids in outdoor aboveground or underground tanks is not practical due to government regulations, temperature considerations or production requirements, tanks may be permitted indoors when installed in accordance with the provisions of this Subsection. Production considerations that may necessitate indoor storage may include high viscosity, purity, sterility, hygroscopicity, sensitivity to temperature changes and the need to store temporarily pending completion of sample analysis.
The amount of indoor tank storage of flammable and combustible liquids is limited to the amounts shown in Table 4.3.12.A. It should be noted that this Table does not apply to indoor storage tanks used to store ethyl alcohol. Storage of ethyl alcohol is covered in Section 4.9. Tank storage is prohibited below grade such as in basements or cellars where natural ventilation will not be as effective to dispel the accumulation of heavier-than-air vapours. Storage on floors above the first floor is only allowed in buildings where automatic sprinkler protection is installed in conformance with NFPA 13, "Standard for the Installation of Sprinkler Systems". Alternatively, the area may be equipped with an approved equivalent fixed fire extinguishing system. Where there is tank storage of both flammable and combustible liquids within the same room, the total quantity of the two liquids permitted can be calculated using a formula similar to those explained in Subsections 4.2.4. and 4.2.7.
The maximum static head imposed on a storage tank located inside a building is limited to 70 kPa (gauge) when the tank is filled to its maximum capacity. This will prohibit very high tanks or long vent lines. The total pressure on the bottom of a tank must include the static head that would be created if the tank were overfilled, thus, filling the vent line with liquid.
Normal and emergency venting of indoor tanks must meet the requirements of Subsections 4.3.4. and 4.3.5. or good engineering practice. However, emergency venting through the use of weak roof-to-shell seams is not permitted inside a building. All venting from indoor tanks must terminate outside the building at least 1.5 metres from any building opening and be sized to prevent the tank from exceeding its design stress limits.
Article 4.3.12.2. is intended to apply to tanks having a capacity less than 230 L, including integral tanks, used for storing Class I liquids as fuel supplies for stationary engines. Tanks greater than 230 L and located inside a building should conform to Subsection 4.3.12. Class II fuels used for emergency generators are regulated under the Energy Act and are therefore exempted from Part 4. Class II fuels used in other stationary engines are not regulated under the Energy Act and are therefore regulated by Part 4.
The intent of Clause 4.3.12.7.(1)(a) is to ensure that the tank room has provisions to contain a spill equal to at least 100% of the volume of the largest tank.
For the design of normal and emergency venting of indoor storage tanks, Sentence 4.3.12.8.(1) refers to Subsection 4.3.4., which in turn refers to API 2000, "Venting Atmospheric and Low-Pressure Storage Tanks". API 2000, however, is intended for outdoor tanks rather than indoor tanks. The venting rate reduction factors for water spray on the tank surface, or drainage rates for spilled liquids, should not be used to calculate the emergency venting rate of a storage tank inside a building. The effects of water spray cooling, and room drainage on the calculated emergency venting rate must be worked out according to good engineering practice. Increased emergency venting capacity may be required because the heat transfer characteristics to a tank located inside a building are potentially greater than outside.
For the same reasons described in Subsection 4.3.11., it is considered excellent practice to extend the fill pipe to within 15 cm of the bottom of the tank to avoid free falling liquid and splash filling. It is a requirement of this Subsection that all indoor tanks, piping, pumps and discharge equipment be bonded and grounded to prevent a static spark discharge.
Where an indoor storage tank is suspended, rather than supported on a foundation, the supports shall be designed and installed in conformance with good engineering practice. Good engineering practice should meet the intent of Subsection 4.3.3. as far as possible. Such factors as the provision of adequate fire resistance for supports, the need to prevent over-stressing the tank shell or its supports, and resistance to earthquake forces in areas subject to such forces, should be taken into consideration.
The intent of Clause 4.3.13.1.(1)(b) is to ensure that the tank room has provisions to contain a spill equal to at least 100% of the volume of the largest tank.
Article 4.3.13.3. now sets the minimum standard to which explosion venting must be provided when required by the Fire Code.
inside buildings are only permitted in areas at or above grade level that have adequate drainage and are separated from other parts of the building by a fire-resistance rating of at least two hours. The intent here is to separate the tank storage area from the process area so that neither presents a fire exposure to the other and to prevent a spill flowing from the storage area into the process area. This separation must be constructed of liquid tight walls, floors, curbs, and sills. Ramps shall be provided at floor level at least 100 mm high and the room size must be capable of containing 100% of the volume of the largest tank in the room. This approach is analogous to the diked area around an outdoor tank. Another approach for liquid spills is to provide a drainage system sized to handle the largest probable spill and directed to a safe location, such as a catch tank or basin. Regardless of the approach used, the fire safety and environmental impact of an indoor spill should receive greater design consideration than for an outdoor spill.
Rooms used to house tanks of flammable and combustible liquids require ventilation meeting the requirements of Subsection 4.1.7.
Minimum clearances of at least 550 mm must be provided between all indoor tanks and the walls of the room to allow access for fire fighting and maintenance. A posting must be provided in the area outside of the room, indicating the capacity of each tank and whether the liquid being stored is flammable or combustible. This will assist firefighters and to warn personnel of the dangers of the area. The building's fire safety plan provided to the Chief Fire Official must include a list of all such indoor tanks, their capacity and content.
If the dispensing of Class IA or IB liquids is carried out in this storage tank room, then explosion venting in conformance with Subsection 4.2.9. is required due to the potential for an explosive vapour/air mixture to be present during the operation. This requirement does not apply to dispensing ethyl alcohol.
Standpipe and hose systems are required throughout the floor area where storage tanks of flammable and combustible liquids are located. These small diameter hose stations are not intended for fighting a flammable or combustible liquid fire. Rather, they are intended to be used for prompt suppression of a small fire in ordinary combustibles. Flammable or combustible liquid fires should be fought using fog nozzles or foam rather than solid water streams, as water streams may spread the liquid and worsen the situation.
Bonding and grounding is required for the indoor tanks, associated piping and dispensing equipment. This is to prevent the accumulation of static electricity which could become a source of ignition.
A shutoff valve must be provided immediately adjacent to the tank shell for any piping connection to a storage tank inside a building. The valve should clearly indicate whether it is in the open or closed position. Since most leaks occur in the tank's piping systems, these valves minimize the chance of an uncontrollable spills. All tank valves should be kept in the fully closed position at all times except during filling, emptying or blending operations.
Opening of hatches for the purpose of measuring liquid levels or sampling contents creates a risk of introducing air and forming an explosive mixture in and near the tank hatch. Where hatches are difficult for staff to open and close, there may be temptation to leave the hatch open. Use of hatch counter-weights and strict operating procedures should minimize this risk. Alternate methods of measuring liquid levels should be investigated for tanks located within buildings.
Filling an indoor tank with a flammable liquid must be done in such a manner that the liquid does not free fall inside the tank. For the same reasons described in Subsection 4.3.11., it is considered good practice to extend the fill pipe to within 15 cm of the bottom of the tank or be designed and installed in a way to minimize the generation of static electricity.
Tanks storing flammable and combustible liquids inside buildings shall be equipped with a device or other means shall be provided to prevent overflow of tank contents in the building as per Subsection 4.3.1. Suitable devices include limit switches, float valves, a preset meter on the fill line, a valve actuated by the weight of the tank contents, a low head pump incapable of producing an overflow or a liquid tight overflow pipe at least one pipe diameter larger than the fill pipe, discharging by gravity back to the outside source of the liquid or to an approved location of adequate capacity.
Before being placed in service or whenever a leak is suspected, all tanks must be tested in accordance with the applicable provisions of the code under which they were built. Refer to the next Subsection 4.3.16. concerning leakage detection. A label is affixed to the tank shell indicating that the tank was made and tested to an applicable code. Where such a label is not provided, that tank must be tested before going into service, in accordance with good engineering principles. Where it is suspected that an existing aboveground storage tank is leaking at the bottom, and the bottom cannot be visually inspected, the tank shall be taken out of service and non-volumetric testing conducted. Such testing may include acoustical, tracer and external product detection methods. The location of leaks in the bottom of a tank can also be determined by using a vacuum box and soap suds. It is suggested that all testing be conducted by individuals or companies experienced in these test procedures.
Aboveground tanks may be hydrostatically tested by filling the tank with water. Pneumatic leakage tests are not performed on such tanks due to the risks inherent in a tank rupture involving compressed gases. However, empty underground storage tanks including their fill and vent lines, may be pneumatically tested at a pressure of not less than 35 kPa (gauge) and not more than 70 kPa (gauge). The source of pressure is then removed and the tank must maintain pressure for two hours. If the pressure drops within the two hour period, and the pressure drop cannot be reconciled, the storage tank has a leak. Test pressures must be measured by an instrument calibrated in 1 kPa increments.
Any storage tank found to leak shall be replaced and re-tested in the case of an underground tank and repaired or replaced and re-tested for any aboveground storage tank before being put back into service.
The testing of underground storage tanks cannot be carried out until the fill and vent lines have been installed and all tank openings sealed. Testing must include the hydrostatic head pressure that would be developed if the tank were accidentally overfilled. However, under no circumstances should the pressure exceed the design pressure of the tank.
Other piping associated with storage tanks must be pressure tested in accordance with Subsection 4.4.6.
Where an underground storage tank is tested using a test liquid, the tank shall be considered to be leaking when, after compensating for any volume differences due to temperature variations and shell distortion, the test indicates a loss of liquid.
This Code requires measurement to be taken of the liquid level in all storage tanks at least once per week to detect leakage. Because Clause 4.1.1.2.(2)(d) exempts locations to which the Gasoline Handling Act and the Energy Act apply, this requirement does not apply to fuel dispensing stations regulated under the Gasoline Handling Act.. For underground tanks, a measurement is also required to be taken of the water at the bottom of the tank. A comparison of these liquid levels with meter readings of liquid flow shall be performed. Any discrepancies between the measured tank liquid levels and meter flow readings that cannot be reconciled is a clear indication that the tank has a leak. When there is significant liquid movement in the tank on a daily basis, the general practice is to take level readings on a daily basis. A record for each storage tank showing the liquid level measurements must be retained for examination by the Chief Fire Official.
Where underground tanks are used, there is an additional requirement to measure the water level in the bottom of the tank. If the tank develops a leak, the hydrostatic pressure of the outside groundwater could force water to enter the tank. If the water level in the tank bottom exceeds 50 mm, then immediate action must be taken.
This Subsection requires written records of these liquid levels to be maintained for a period of at least two years.
Any tank found to be leaking requires immediate remedial action and reporting to the Chief Fire Official within 24 hours.
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