General Information About Tanks

 

 1) Fixed or Cone Roof tank 2) Floating Roof tank 3) Dome roof.

Selection & size of storage facilities

Based on the specification and characteristics of petroleum products (LPG classification is not under this consideration), petroleum products are classified as:

Petroleum Products – A class:    liquid which have flash point below 23^°C.

Petroleum Products – B class:    liquid which have flash point between 23 to 65 °C.

Petroleum Products – C class:    liquid which have flash point between 65 to 93 °C.

Types of Tanks:

Selection criteria for types of tanks are based on the class of product to be stored. This selection is normally based on the vapour pressure of the petroleum products

Ø  Cone Roof Tank:    Vapour Pressure less than 0.05 kg/cm².

Ø  Floating Roof Tank:  Vapour Pressure between 0.05 to 0.85 kg/cm²

Ø  Dome Roof Tank:   Vapour Pressure between 0.85 to 1.25 kg/cm²

 

1 (a) Fixed Roof Tanks

 Fixed or Cone roof tank used for storing products of low volatility (vapour pressure less than 0.05 kg/cm²)

These tanks are made of vertical cylindrical plates with cone roof fixed over the plates supported by internal truss. Depending upon the service, the fixed roof tanks are provided with some or all of the following accessories:

·         Goose neck type vent/PVRV.

·         Man ways for internal access-on the shell and roof.

·         Dipping datum plate for dip measurement.

·         Dip hatch with reference height mark fitted with cover.

·         Level indicator.

·         Mixer for mixing purpose.

·         Steam heating coil with inlet &outlet nozzle.

·         Product inlet and outlet nozzles for receipt and delivery.

·         Drain nozzle for water draining.

·         Foam connection nozzle for fire fighting.

·         Temperature gauge.

 

 1 (b) Floating Roof Tank

Floating roof tanks are used for storing products having vapour pressure between 0.05 to 0.85 kg/cm². These tanks are used primarily to reduce evaporation losses from the tanks and to reduce the spread of hydrocarbon vapour and harmful gases (H₂S) to the atmosphere.

 

            In refinery, floating roof tanks are used for storage of Crude oil, etc. The roof resting on the liquid contributes to the minimization of vapour space between the liquid and the roof bringing about increased operational safety, and minimum evaporation loss. The floating roof is provided with annular pontoon around the periphery. These pontoons make the roof float by increasing the buoyancy of the roof. In certain tanks, along with pontoons, there are hollow cylindrical structures directly welded to the roof, which are called buoys. The purpose of the buoys is to increase the buoyancy of the roof.

 There are two types of floating roofs in use i.e. single deck roof, having pontoons on the periphery and double deck roof, where the entire roof is like a circular box. The double deck roofs are used for highly volatile products and for very large tank.

 Foam type seal is used to seal off the clearance between rim of the roof and tank shell. The roof is supported, when it is not afloat, by a number of tubular legs. Each leg can be within sleeve attached to the roof. The position of the leg can be fixed at one of two points by securing pins. One fixing points corresponds to the minimum height (1.8m) of the tank roof in the lowest working position known as low leg position, other (2.3m) known as high leg position supports the roof giving sufficient clearance between the tank roof and floor for maintenance work to be carried out. Bleeder vent in the roof allow air to escape when an empty or near empty tank is being filled and before the roof is afloat. These vents also serve to protect the tank against drawing vacuum since they are designed to open automatically just before the roof lands on its supports  For draining rain water from the roof a metallic pipe is located inside the tank . This pipe has got swivel joints and is connected to an outside gate valve near the bottom of the shell. This valve is kept open normally to drain off water accumulated on the roof, particularly during rainy season.

 

 

There is also an emergency drain having water seal. In case rain water doesn't flow freely through the roof drain for any reason it can get into the tank through emergency drain. Subsequently this water can be drained from the tank after settling. The height of the emergency drains nozzles should be 4"-6", so that there is no ingress of water into the tank due to heavy rains.

 Access to the floating roof is provided through a rolling ladder, one end of which is hinged at the gauger platform at the top of the stairway and the other end is free to move on rollers on a runway fixed to the   roof. The tank tops are re-enforced with stiffening rings called girders to take care of the wind load. 

Depending upon the service, floating roof tanks are provided with some or all the following accessories-

 

·         Man ways for internal access---on the shell& roof.

·         Vents-Rim vent and bleeder vent.

·         Gauge hatch with cover and reference height marks

·         Level indicator.

·         Mixer

·         Emergency roof drain

·         Roof drain nozzle

·         Foam Connection

·         Spiral stairway.

 

1 (c) Dome Roof Tank

Dome roof tanks are used for storage of liquids having vapour pressure between 0.85 to 1.25 kg/cm². These are nitrogen blanketed tanks. Products with high vapour pressure viz. Feed Naphtha, and HCU LT Naphtha are stored in dome roof tanks.

 

 

 

 

 

 

2. DESCRIPTION OF TANK ACCESSORIES

All storage tanks are provided with various fittings for operation of the tanks and ensuring safety operating personnel working on tanks and to the equipments as well. Operating personnel who are involved in tank operations should have some ideas about the structure and utilities of the various fittings. Attempts are made to give a brief description of each of the fitting below.

 

2 (a) GENERAL ACCESSORIES

 

·         Tank Pad :

It is a strongly built concrete structure on which a tank is erected. The bottom plate of the tank rest on this structure called tank pad from which shell plates are fabricated vertically one above the other.

 

·         Steam coil & Insulation :

In the storage tanks where a petroleum liquid is require to be kept at higher than the ambient temperature, heating coils are provided inside the tank and shell of the tank covered with insulated materials. The coil is placed approx. 250 mm above the floor bottom and is provided with inlet and outlet valves, Inlet for introducing steam and outlet for withdrawal of steam condensates. The outlet is fitted with steam traps.

 

The steam condensate from the steam coil if do not come out freely and get held up inside the coil for any reason can cause a loud banging sound which can be heard from a distance. This phenomenon is called "Hammering''. This is very harmful to the coils and coils may get ruptured. The steam coils are not welded to the supports to protect the coils against damage due to hammering and to take care of thermal expansion. If hammering starts in the steam coil it is required to drain out condensate from trap bypass and check trap for any damage so as to establish smooth flow of the condensate from coil.

·         Inlet, outlet & mixer nozzles:

Each tank is provided with inlet/outlet manifold nozzle, mixing and drain nozzles. The inlet/outlet manifold nozzle is for receiving and taking out oil from tanks. The mixing nozzle is used for mixing the contents of the tank for homogenizing the density to uniform level using mixers. Drain nozzle is provided for draining water /sludge from the tank. The drain line is downwards and rest slightly above a drain funnel which is connected to an underground OWS network leading to the oil separator system (at WWTP). It is important to know that while draining water from a tank, oil also gets drained out with water.  However, this oily emulsion goes to WWTP (Waste Water Treatment Plant) from where it can be recovered for reprocessing.

 

·         Staircase:

This is a ladder to enable a person to go on to the roof of a tank .It is in the form of a spiral welded to the shell of the tank .The ladder is provided with a railing. Operating personnel while going up or coming down from tank roof must hold the railing with grip to protect himself against falling  for any reason .At the top of the tank where this stair case ends, is provided with platform for landing.

 

·         Dip hatch:

Near the platform at the tank roof, tank is provided with an opening with a projection and cover thereon. This is called a Dip hatch, which is almost all the time to be kept closed. This hatch is used for taking dip measurement, samples and product temperature. The dip hatch has a long pipe going down to the tank bottom only in case of floating roof tanks. At the top of the pipe, a ring of non –ferrous metal is fixed. This is  to avoid  generation of static electricity which may be produce while dip tape is introduce into the tank  sliding over the  pipe edge.

 

·         Reference point:

A guide made of non - ferrous metal with a sloping notch enough to accommodate the tape of the dip tape is fitted. This guide or a marking on the rim of the ring is said to be the reference point.

 

·         Datum plate:

Just below the dip hatch, a steel plate is provided on the floor of the tank. This is the plate on which the bob of the dip tape rest while dipping. Calibration of the tank is done based on this plate.

 

·         Reference Height :

Reference height is the distance between the datum plate and the making on the rim of the dip hatch. This is for guiding operating personnel to take the correct measurement. One must know the reference height of a tank to have correct dip measurement – especially when the tank has developed huge quantity of solid sludge at the tank bottom.

 

·         Flame arrester:

This is an opening in the center of the roof on which flame arrester is fitted. This is for arresting flame front propagation while allowing the gas vapors (flammable, non flammable) to flow without significant pressure drop as well as any flash back of spark of flame. There are two types of flame arresters namely the in-line arrester and the end of-line arrester .The arrester element, which is sandwiched between two flanges (mostly wire mesh type) can be of different designs/ metallurgy depending on requirements .In storage tank normally end of-line arresters are installed. In line arrester are required for vapor control system e.g., at the upstream of any fire source like incinerator, flare etc. Apart from fire protection, the flame arrester also helps in reducing the volatile organic chemicals (VOC s) or hazardous air pollutants (HAP s).

 

·         Pressure vacuum relief valve(PVRV):

This is very important equipment fitted to the roof of a tank. The valve is called pressure cum Vacuum relief valve. It has got dual function to perform.

When liquid is received into the tank, the air & gases inside the tank are expelled, thereby allowing no pressure to build up inside.

Similarly when liquid is taken out from the tank, air from atmosphere allowed to enter through this valve into the tank thereby allowing no vacuum to be created inside.

If for any reason a vacuum is created inside the tank, shell of the tank will buckle. Again during receipt of liquid into the tank, if the PVRV doesn't function, the pressure inside the tank doesn't get regulated and increases beyond atmospheric pressure and the tank may blow off. In such cases, in order to minimise the damage to shell, joints where roof is welded to shell are kept comparatively weak in order that only the roof and not the shell will get blown up .It is very important that the PVRVs are checked regularly and periodically serviced to ensure smooth functioning all the times.

 

·         Tank Blanketing

"Tank Blanketing" or "Padding" allows the use of a low-pressure blanket of gas, such as nitrogen, to maintain a protective gaseous environment above liquid stored in a tank. The low-pressure gas blanket fills the void vapour space above the liquid stored in the tank. A gas blanketing system reduces the high-pressure source of gas to a lower pressure forming a blanket over the liquid. Low-pressure blanketing systems commonly protect tanks containing volatile organic liquids. The positive pressure gas blanket helps prevent outside air, moisture, and other contaminants from entering the storage tank. In addition, the positive pressure of the system provides a head pressure above the liquid to reduce vapour loss. This helps protect the tank from corrosion.

 

·         Fischer Assembly

Blanketing in tanks is provided with the help of Fischer assembly. A Fischer assembly consists of pressure sensing tapping provided on tank top for sensing the pressure inside the tank. This sensing is given to PCV fixed in blanketing gas (N₂).  In case of lower pressure in the tank than PCV setting, PCV will open to allow the blanketing gas to enter inside the tank for maintaining the pressure.  In case of high pressure inside the tank, PVRV will release the extra vapour from the tank. When the tank suddenly cools, the vapours inside condense causing the tank pressure to decrease. This causes PCV to open which allows the blanketing gas into the tank. Blanketing gas PCV also maintains a constant tank pressure while removing liquid from the tank. The positive pressure prevents the tank from collapsing.

There are two main types of valves used in positive pressure tank blanketing systems: direct-operated and pilot-operated. Direct-operated valves for blanketing sense the tank vapour pressure, and this pressure registers directly on the valve's diaphragm. When the tank's vapour pressure decreases below the system's set point, the diaphragm moves the valve disk away from its seat, allowing gas to flow into the tank.

The position of the disk relative to the seat regulates the amount of flow. As vapour pressure in the tank increases, the disk moves closer to the seat and shuts off the flow completely when the pressure rises above the set point. Direct-operated systems respond quickly to changes in tank pressure.

 

·         Foam Chamber

This is a firefighting equipment fitted on the tanks. These are fitted on the opening in the tank shell near the roof and connected to a pipe which runs along the height of the shell leading itself to a location away from the tank .In some cases, these pipes come right up to the nearest road .In new tanks, these pipes coming from the same tank are hooked up in a single manifold situated on the road side.  End of the individual pipe or manifold is fitted with 2.5" dia quick fit male coupling.

 The foam chamber is fitted with a diaphragm inside it. It is a coated thick paper which can get ruptured under pressure. It prevents gas from inside the tanks to escape. Periodic checks should be made to ensure that the diaphragm is intact. If it is damaged, the gases from inside will escape and besides causing a fire hazard, it will add to loss of petroleum   product .In case of fire, the foam tender connected to the couplings of the foam lines and foam is pumped into the tank. A foam blanket is formed over the surface of the liquid thereby cutting off the oxygen and the fire gets extinguished.

 

·         Water Draw:

 For draining water from tanks, water drains are provided. The arrangement of water draw is different for different tank. Following types of water draw arrangements are normally used.

a) In one type of tank, the tank floor is of concave or saucer type in which there is depression ending in to a pit in to the centre. Because of the slop towards the centre water gets collected in the pit. Water drain line, which enters the tank shells through a gland right up to the pit. Once the drain valve outside the tank is opened, the pressure of the water column collected in the tank pit, forces the water through the drain pipe.

b) In this design, the bottom of the tank is convex, i. e like inverted saucer. In this type water gets collected towards the periphery. Hence the water draw pipe in such tanks is 45°angle with the tank floor. In some tanks, with convex floor a hole of 80mm or 100mm diameter is made in the tank floor near the shell and a pipe welded to it from the bottom of the tank to draw out water.

 

·         Earthing:

All tanks are grounded through earthing strips welded to the shell of the tank. This is to eliminate static electricity charges generated during the movement of petroleum products in the tank at the very high velocities. The static charges produced depend upon the velocity of the moving hydrocarbon and its purity. Kerosene and ATF containing water moving at considerable high velocity will produce much more electric charges than that produced by dry kerosene or ATF. Water draw connection with the inlet line of a tank is very much helpful in receiving a petroleum product in a tank through it. When an empty tank is required to be commissioned the risk of generation of static electricity is very high and this may cause explosion. Since the nozzle of the inlet line is at a height, any liquid pumped into an empty tank will have a free fall on the bottom of dry plate thereby generating tremendous amount of static electricity charges. This risk can be eliminated, if the liquid is received into the tank from the lowest point at a slow rate, till the inlet nozzle gets submerged into the liquid.

 

·         Bund wall:

Bunds made of earth are situated at calculated distance on all sides of tank. The height and area of the bunds are such that in the event of collapse of a tank, the contents of the tank should be accommodated within the bund area.

 

·         Tank mixers:

Mixers are provided in the tank storing heavy oil products like Crude, IFO, and RFO etc. Whenever a product is to be prepared by blending, i.e. by mixing two or more components that may vary in densities, the components are to be mixed up thoroughly to make single homogeneous mass of uniform density. For this purpose, tanks are provided with mixing arrangements.

There are two different type of mixer encountered in the operations of tanks. This are-

a) Side entry mixer

b) Jet Mixer

 

a) Side Entry Mixer:

Side entry mixers are used for mixing the tank contents to achieve homogeneity in densities of different layers. The equipment consists of a long shaft at the end of which three curved blades are fitted while the other end is connected to a motor outside the tank shell. The motor drives the fan like structure inside the tank. With its rotation in correct direction it throws the heavy liquids towards the top thereby setting up a circular motion in the vertical plane thus mixing the contents without allowing the same to settle down at the tank bottom. With the side entry mixer provided in tank containing crude oil, it has been possible to minimize accumulation of sludge and sediment at the tank bottom thereby reducing the frequencies of tank cleaning. In our refinery side entry propeller mixers are used for following tanks viz.  Three mixers in each crude storage tanks, one mixer in each CDU/VDU IFO and Refinery IFO storage tank.

 

b) Jet Mixer:

This mixer is operated by drawing a heavier liquid from a tank through tank outlet with a pump of adequate capacity and discharge pressure, and discharging the same through a comparatively smaller diameter line which enters the tank through separate nozzle. About a meter from the shell inside the tank, the line turns up about 45°C tapering into a narrow open mouth. The nozzle is pointing towards the centre of the tank roof. Thus the heavier portion drawn from bottom is thrown with force towards the top of the liquid column. The heavier portion thus mixes with the tank contents while going up .In this way a circulation is set up in motion resulting in thorough mixing. Time for making a given mass of liquid homogenous depends upon the density and quantity. Normally for a full tank of 5000 m³ capacity, it takes about eight hours. Jet mixers should not be operated if the liquid level is low in the tank otherwise there will not be sufficient bulk of oil to offer adequate resistance to the jet and the jet of oil may go through the layer of oil and hit the roof causing damage.

 

 

2. (b) Floating Roof Tank Accessories

 

·         Water Draw:

The tank floor of floating roof tank is of convex shape. Water draws are either of siphon type or gravity type.

 

·         Foam chamber:

Foam chambers are provided as part of the extinction measure. Here the foam from deflectors fall on the roof seal and spreads along the periphery. Foam dams are provide on the roof to contain the foam in the zone of roof seal.

·         Tank cooling arrangements:

Towards the top of the shell a circular ring of pipe to which spray nozzles are fitted is provided. This is again connected with pipe from fire water system for supply of water to the spray nozzles. The valves for the water pipes are located outside the tank bund area. Such cooling arrangement is popularly known as water sprinkler.

 

·         Wind girder:

A stiffening ring, called wind girder is welded at the top of the shell from outside to protect the circular shape of the upper part of the shell which can get disturbed due to wind velocity.

 

·         Roof drain:

The pan of the floating roof tank slopes towards centre where a pit is provided. The pit is covered with a perforated loose cover, which acts as a strainer. The pit opens through a check valve into the assembly of steel pipes. This assembly folds when the roof comes down and unfolds when the roof goes up. The rain water accumulating on the roof comes out through this pipe from an outlet provided at the tank shell at bottom with a valve .In the event of any damage sustained by this pipe assembly, oil get into the pipe and will come out along with rain water through the outlet. There should be regular checking of the drain water for presence of oil. In case of any oil detected in the roof drain, it is to be inferred that the swivel joint in the drain line is leaking and required to be attended.

 

·         Emergency Roof drain:

It consists of a short length of about 50mm diameter pipe which passes through the pan. The height of this pipe above the pan is less than the height of pontoon boxes. The other end of the pipe terminates in a hanging tray, which is always kept, filled with water to form a water seal If this seal is broken the oil from inside will come on the roof when it is moving. The portion of the pipe on the roof is provided with a cover fitted with metallic net which act as strainer. If for any reason the roof drain fails or fails to cope up with heavy rain water accumulation, water starts flowing through the emergency water drain, thereby saving the roof from sinking due to heavy weight of accumulated rain water. To prevent ingress of rain water into the tank through emergency drain, the height of the emergency drain maintained normally 4"-6". The height is calculated based on the diameter of tank.

 

·         Roof legs:

When the roof does not remain afloat on the liquid inside the tank, it rests on legs called roof legs .A roof leg consists of a movable pipe with two different positions for adjusting the height. The leg pipe passes through the roof and a sleeve on the roof. Once the roof floats, the legs  are pulled up on the roof and fixed in lower position making length of the leg equal to the 1/3^rd of the total length of the leg, thereby minimizing the height of the roof to enable the liquids inside to be brought down to level as low as possible. Again the length can be adjusted towards higher side so that the roof can be made to stand on the legs at height sufficient enough to enable a man to work under it comfortably during cleaning or any other maintenance job.

  The zone where liquids start touching the roof and till it is completely floating is called critical or inaccurate zone. Any dip measurement taken when the roof is in this zone will give misleading reading. Once the floating roof is put into operation it should not be landed unless and until the tank is required to be taken out of service for cleaning or repair. Every time a floating roof is landed or lifted there is a risk of damage to the roof. Hence, it is strictly forbidden to land floating roof during day to day operation

 

·         Rolling Ladder:

 Most of the floating roof tanks are provided with rolling ladders which move from the shell towards the centre as the roof comes up. The rolling ladder and the gauging column act as an anti rotation device for the floating roof.

 

·         Foam Dam Plate:

This is a vertical plate 2' in height, welded on the periphery of the floating roof, with minute sweep holes at the contact with the roof. This plate holds the foam between the roof and the shell when it is poured in case of fire and the holes allow the water to go out.

An electrical wire and the shell connect the floating roof and the shell again properly grounded.

 

·         Rim seal

The Rim Seal system of a floating roof tank consists of two seals- primary seal and secondary seal.          

 

Primary seal:

Floating Roof is provided with an annular pontoon around the periphery, the pontoon encircling the single centre deck. Primary Seal, generally of foam type, is used to seal off the clearance between the rim of the roof and the tank shell, so as to prevent loss of product by evaporation. It is of polyurethane foam material, pre-formed to the shape of either a `D', Hexagon or Square, and held in position with hangers. The cover is of neoprene for normal hydrocarbon service, while for high aromatic products like Benzene, it is of Viton rubber. The primary seal arrangement is protected by weathering shields made of galvanized MS sheets, bolted to the rim angle at the top of the pontoons. The gap between the pontoon periphery and the tank shell is normally 200 mm. The foam width is 300 mm, but is compressed to 200 mm.

It consists of Core Foam and Foam Casing.

Core Foam:

Core Foam is a prime quality polyurethane foam and of octagon shape. Foam is light weight having appropriate elasticity, little permanent compressive strain and shall have excellent resistance to oil and chemicals.

Rim space of 150 mm and 200 mm is provided for tanks up to 30 m dia. and above 30 m dia. respectively. Foam is octagonal shape and of size 254 mm and 305 mm (across flat ends) for rim space 150 mm & 200 mm respectively.

Foam Casing:

The casing material is oil and water resistant. Material of foam casing is Nylon fabric equally coated on both sides with nitrile butadiene rubber (NBR)

 

Secondary Seal:

Secondary Seal shall have synthetic rubber wiper seal, continuous vapor barrier membrane, galvanized compression plate of 16G thickness, SS electric shunt and all necessary hardware for installing on rim of external floating roof. Vapor barrier membrane has polyurethane coating on both sides of nylon fabric. Minimum thickness of vapor barrier is 0.4 mm.

 

·         Rim Vent:

 This is a vent provided for releasing excess gas pressure trapped between the roof seal and the shell of the tank.

 

·         Gauging Column:

Runs from the rim of the shell to about 1.5' distance from the tank bottom. This is used for gauging and sampling of the product. The gauging column must have holes or perforation at regular intervals to have a free flow of the liquid inside the gauging column.

 

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