Types of storage tanksTanks are classified based on their roof design. Normally, atmospheric tanks are of fixed roof or cone roof or floating roof or fixed cum floating roof (with or without Nitrogen blanketing) type tanks and low pressure Nitrogen blanketed tanks.

Floating Roof Floating roof may be single deck pontoon roof, double deck or pan roof. Pan roof shall not be used as these are unsafe. For designing these tanks, API 650 guidelines may be followed.

Floating roof tankThe floating-roof tank is the most widely used storage method to safely control product evaporative emissions from volatile petroleum products. Floating-roof tanks remain the safest, most cost effective method of storing petroleum and many petrochemical products.

Fixed RoofFixed roof may be of cone type or dome shaped. The tank may be pressurised (to a few inches of water) type with breather valves. Alternatively, tanks may be provided with fuel gas or inert gas blanketing to prevent oxygen/moisture ingress. Fixed roof tanks for light products (e.g. Motor Spirit) breathing into a neoprene balloon is not acceptable. For designing atmospheric/low pressure tanks, API 650 or API 620 may be followed based on the type of the tank.

Fixed cum Floating RoofThese tanks have a fixed roof over a floating roof. They are used for products having very stringent water content specifications like Aviation Turbine Fuel and products sensitive to oxygen like light intermediate feed tanks. Where oxygen ingress is to be avoided, it is preferable to provide Nitrogen blanketing.

Selection of RoofSelection of type of roof generally depends on ambient conditions and the product handled.

Product HandledFollowing guidelines should be used for specific cases:(a) Tanks used to store finished Aviation Gasoline/ Turbine Fuel shall be floating cum fixed roof to avoid entry of water into product.(b) Where product degradation due to air/moisture ingress is a problem and fixed roof tanks are used, such tanks should be provided with inert gas blanketing.(c) Nitrogen blanketing for internal floating roof tanks/fixed roof tanks should be considered for storing hazardous petroleum products like benzene etc.

LOCATIONTanks may be above ground, on elevated ground or underground. In case of underground storage tanks, protection to the external surfaces of fixed tanks/pipes

shall be provided by a glass or synthetic fibre reinforced hot applied bitumen and by surrounding the heated tank with a backfill of selected sand. This is preferred over solid skin of fine concrete through contraction or subsidence. Cathodic protection should be provided where very high standard of protection is required due to soil condition/geographic location.

CORROSION ALLOWANCECorrosion allowance should be taken depending on the nature of petroleum products to be stored, it's impurities level, atmospheric conditions etc. TANK APPURTENANCES

1. Ladders and HandrailsIndividual tank are provided with access to the roof. A platform with railing provided from the top of the stairway to gaugewell and roof ladder. On floating roof tanks, non-sparking self levelling tread type rolling ladder with suitable earthing connection are provided. 2. StairsStairs should be made of grating. All staircases shall have resting/landing platform preferably for every 5m height. 3. ManholesNumber of manholes shall depend on diameter of the tank (Refer API 650 for details). Minimum of one flush type clean out manhole should be provided for tanks under dirty services. 4. DRAINS(i) Bottom DrainsDrains should be provided in all tanks for draining water and also for emptying out the tank for cleaning. Besides, these are also useful for draining water after a hydrotest or initial flushing during a start up operation. Refer API 650 for number and details of such drains. Apex down tank bottom shall have one drain connection located at the lowest point near the centre of the tank in addition to normal circumferential drains.(ii) Floating Roof Drains Maximum rainfall rate on hourly basis for the past 15 years should be considered for designing the number and size of drains for open floating roof tank. Also drains shall pass the design rainfall when roof is resting at the lowest position. Primary roof drain system shall be closed type using pipe and swing joints and shall include a suitable outlet valve. The inlet for these drains shall have a swing type check valve to prevent product from flowing into roof if pipe drain leaks/fails. (iii) Emergency Roof DrainEmergency drain for floating roof tank shall be provided on the roof to take care of drainage problem and drainage of total water in case of plugging of normal roof drain.

5. Dip HatchDip hatch or gauge hatch is used for gauging the height of the liquid in a tank as well as to take out samples for testing. Gauge hatch shall be non-sparking(or lined with non-sparking material) and self closing type. Storage tank having pressure while in normal operation may pose problem in sampling or taking manual dip. For such tanks, it is

suggested to resort to slot dipping device. This accessory permits sampling/dipping in tanks having pressure upto 300 mm WG. For operating pressure beyond this, it may be necessary to provide appropriate instrumentation with redundancy. Gauge well pipe (with slots) should be provided for all types of tanks. This should have continuous contact by means of strips with bottom plate of the tank. Continuous contact makes the tank safer with respect to static charge accumulation and acts as a support for the gauge well pipe. 6. Walkway on the RoofWalkway with handrail on the roof of the tank should be provided to facilitate inspection/checking of vents/ flame arrestor etc. so that movement of personnel on roof is safer.

7. INSTRUMENTATION (i) Level Tanks should be provided with level instruments along with high/low level alarms with independent primary sensing device are recommended. (ii) TemperatureWhen rundown temperatures are likely to be higher than 100 degree C, a remote temperature indicator with alarm should be provided in addition to local indicators.

TANK PROTECTION1. EarthingEvery storage tank, including its roof and all metal connections, should be electrically continuous and be effectively earthed. In case of floating roof tanks, stainless steel shunts may be provided across the peripheral seals to ensure earthing of floating roof. Alternatively, the pontoon, ladder and shell of the floating roof tank shall be continuously bonded (electrically continuous) with copper cable and the shell shall be independently earthed. Refer OISD-RP-110 on Recommended Practices on Static Electricity.

DESIGN CONSIDERATION FOR TANK FARMS/MANIFOLDS1. GENERALThe dyke wall may be of earth, masonary or stone.The purpose of a tank dyke is to contain the petroleum product, in the event of the tank rupture. For details of tank farms, refer OISD-STD- 118 on "Layout for Oil and Gas Installations". 2. Tank Farm DrainsTank farm drainage / spillages / rain water shall be routed either to oily water sewerage or storm water channel. Provision should exist for diversion valves located outside the dyke. In case of clear rain water, the same shall be diverted to open channel. Should a tank rupture, the contents shall remain within the bundwall and gradually be diverted to oily water sewer. In case of high wax content product or high pour crude, the tank oil drains could be separated and pumped to crude/slop tanks. Depending on capacity, a group of tanks can be considered. The separator shall have steam heating arrangement and auto start/stop for pump can be provided for. In this regard refer OISD Standard 109 on "Blow down & Sewer System".

3. Fire ProtectionThe details of the fire protection are covered under OISD Standard - 116 on "Fire Protection Facilities for Petroleum Refineries & Oil/Gas Processing Plants" and OISD Standard - 117 on "Fire Protection Facilities for Petroleum Depots and Terminals". Where large tank farms are involved (especially in refineries/crude terminals or marketing installations in thickly populated areas) hydrocarbon detectors may be located in selected tank farms with remote alarms in control stations.

4. MANIFOLDSFor safety considerations, it is desirable to keep the number of inlet/outlet connections to the tank shell to minimum. This reduces the number of flanges/valves close to the tank. In case of more number of lines, it is desirable to take a single header and form as manifold away from the tank.

Tank manifolds shall be located outside the dyke area. The floor underneath should be paved, have curb walls and connected to drainage system. Crude and other tanks, where water contamination can lead to unit upsets, additional suction at two elevations may be considered so that top outlet can be lined up initially. Alternatively, floating suction shall be installed. After tank settlement, a depression is normally formed on tank pad along the circumference. The same should be effectively made up with proper slope to avoid rain water accumulation and subsequent corrosion. Where large settlement is anticipated, it is desirable to use flexible joints/spring supports for piping to nozzles.

TANK HEATERS/MIXERSTank heating can be accomplished either by steam heating or electric tracing or hot oil circulation.

1. Steam Heating2. Electric Heating3. MIXERS

Heating flues using fired burners are not recommended as these are not safe.

TANK OPERATIONEntry on floating roof is permitted only if all the following conditions are fulfilled:

1. The roof is at least half way to the top.2. Gas test shows no presence of H2S and gas concentration is below 10% of the

lower explosive limit.3. One man is standby at the top of platform with a cannister mask / breathing

apparatus readily available.4. A life line with safety belt is used for the man going on the roof. The other end of

the line held by the standby at the top platform.5. The tank is not under receipt or delivery.6. No gauging or sampling of tanks should be undertaken during thunder or hail

storms.7. Flow velocity at tank inlet should not exceed 1 m/s until the inlet is completely

submerged.8. Conductive footwear e.g. leather soles or electrically conducting rubber soles,

should be worn while gauging, sampling or taking temperatures. Nylon rope shall not be used for lowering sample bottles in the tank.

9. One of the most common sources of leaks and spills is mobile storage tanks, such as diesel fuel tanks used for construction machinery. It is desirable to dig a small pit or construct temporary dyke around the tank.

10. In case of large tank farms effective communication is essential. Telephone with loud hooters may be provided on roadside at various locations.

LOADING/UNLOADING FACILITIES

(i) Pumps shall be located in an exclusive paved area with drainage facilities. To avoid wide variation in pressure, leading to a 'kick' or 'hammering' in header and hoses, it is necessary to choose pumps with flat characteristic curves.

(ii) It is desirable to have separate pumps for truck loading and not combined with wagon loading as the latter are normally of much higher capacity.

(iii) Receiving lines as well as discharge lines shall be provided with thermal safety relief valves to relieve pressure due to ambient temperature rise. Whenever isolation valves are used to isolate TSV, isolation valve with lock open provision should be considered.

(iv) Safety relief valves may vent into a tank or may be piped to a collector drum having level indicator/alarm or to OWS located in safe area. When connected to tank, it should be provided with isolation valve on either side and break flange/union on tank side.