storage tanks
TRANSCRIPT
ABOVEGROUND STORAGE TANKS &
SPHERES
AN OVERVIEW
Brief History Of Petroleum Storage Tanks
1859 Wooden tanks were used in field production area.
1900 Oblong / Rectangular tanks about 30 feet long X 8 feetwide X 5 feet height
1861 First round oil tanks , Cap 6 to 12 barrels.
1910 First large Iron tank of cap. 8000 barrels1913 First bolted steel tank – By Gante Tank Co.
1920 to 1930 Welded tanks.
1923 First successful Floating Roof (Pan Type) tank designed& constructed by the Chicago Bridge & Iron Works.
Later on Floating Roof has significant Improvement.
Design and constructional codes
1. API standard 650 : Welded steel tanks for oilstorage
2. API standard 620 : Design and construction oflarge , welded low pressure storage tanks.
3. IS – 803 : Welded steel storage tanks for OilStorage
4. BS – 2564 : Vertical steel welded storage tankswith butt welded shell for petroleum industries.
Storage tanks- generalStorage tanks are used to storefluids such as crude oil,intermediate and refined products,gas, chemicals, waste products,aqueous mixtures and water.Factors such as Volatility of stored productand the desired storage pressure, result intanks being built in various types, sizes andmaterial of construction.
Types of Storage tanks1. Fixed Roof Tanks2. Floating Roof Tanks3. Dome Roof Tanks4. Internal Floating Roof Tanks5. Horton Spheres
Selection of tanksFixed Roof Less Than 0.05 Kg/cm2
Floating Roof. Between 0.05 – 0.85 Kg/cm2
Dome Roof Between 0.85 – 1.06 Kg/cm2
Horton Spheres Above 1.06 Kg/cm2
Selection of tanksAdditional aspects that have an impact on the choice of tanktype:
Building Cost
o Cone roof tanks offer building cost advantagetowards the lower tank diameter range.
o Floating roof tanks cheaper towards the largediameter end of the scale.
o In terms of cost per m3 of gross storage capacity afloating roof tank could costs 34% more for an8,000 m3 tank and 9% more for a 20,000 m3 tank.
Selection of tanksCost for storage capacity
o For a given tank size it is the net working storagecapacity of a tank that determines the profitableuse a refinery makes of a tank.
o Also, the minimum stock level represents anamount of dead stock, which is a cost to therefinery in terms of working capital.
o The cost of dead stock increases with thehydrocarbon value of the product, with the relatedyearly interest charge.
Selection of tanks
True Vapour Pressure (TVP)(at ambient or product
temperature, whichever is highest)12.5 psi (86 kPa) maximum
Initial Boiling Point (IBP) Above ambient or product temperature, whichever is highest
Density > 700 kg/m3 at storage temperature
TVP, IBP or density restrictions for floating roof.
Selection of tanksTVP, IBP or density restrictions for floatingroof.
o The density limitation ensures that the roof will havesufficient buoyancy, including the capability to carry a certainamount of rainwater.
o Limitations ensure that the roof will always rest on the liquid.If vapour collect under the floating roof, it might, tilt and sink.
o Even if roof design copes with the evolving vapours themechanical performance of the tank and seal would becomecritical. Any gap between the seal and tank wall would causehigh emission of volatiles and increase the risk of a rim firedue to lighting or any other source of ignition. Also theevaporation through the gap would be a continuous source ofproduct loss.
Main Parts of Storage TankThe Bottom
o Bottom plates function as a membrane and transfer theliquid pressure directly on to the tank foundation.
o They are made of 10-15 mm thick rectangular steel plates,which are interconnected by overlaps.
o Tank bottoms may have conical shape or inverted coneshape, also known as "Apex down". Bottoms are coneddownwards towards the center with a sump of adequatesize at the center of the tank bottom and a siphon drain.The Shell
o The tank shell is made of a number of courses, normally1.5 to 2.5 m wide. The plate thickness gradually decreaseupwards due to the reduction in horizontal loading of theliquid pressure.
FIXED ROOF TANKSo Vertical cylindrical vessels with conical top.o Used for storing non-volatile materials like
Heavy oils, gas oils, furnace oils and non volatilechemicals.
o Tanks are protected by vacuum vent thatmaintains the pressure difference between tankvapor pressure and outside atmosphere.
o The roof plates are 5 mm thick and areinterconnected by lap-weld on the top side only.
o Above the shell the plates are welded to the topcurb angle of the tank with a continuous sealweld, with the intention that the roof willrupture at the shell roof connection in case of aninternal tank explosion. No damage to the shellbottom connection will then occur with the riskof loss of product.
o Mountings required for access, gauging,dipping and vents are placed on top of the roof.
FIXED ROOF TANKS
FIXED ROOF TANKS
Low – Pressurefixed roof tanks
o Design internalpressure 200 mm ofwater
o Design vacuum of 60mm of water column.
Non- Pressure Fixed roof tanks
o Work at atmosphericpressure.
o Design internalpressure 75 mm ofwater column.
o Design vacuum 25mm of water column.
FIXED ROOF TANKIn Non pressure fixed roof tanks, normalventing takes the form of free- flowatmospheric vent which allow unimpededflow of vapors out of, and of air into the tankand, at the same time, prevent rain andairborne dust from getting into the tank.
In Pressure fixed roof tanks, the contents arestored under nitrogen blanketing to preventingress and to reduce vapor formation.
CONE ROOF TANK APPERTENANCES
VentsOpen Vents
The tank breathes - in air when the tank pressureis lower than the atmospheric pressure andbreathes - out when tank pressure is greater thanthe set pressure.
Open vents are of goose neck type, covered witha 4 to 8 mesh screen. Normal venting takes theform of free- flow atmospheric vent which allowunimpeded flow of vapors out of, and of air intothe tank and, at the same time, prevent rain andairborne dust from getting into the tank.
RAIL ROAD TANK FARM
CONE ROOF TANK APPERTENANCES
VentsPressure Vacuum Relief ValveFunctions:o Minimizing vapor losses that would occur by free
venting.o Protection of the tank from excessive pressure or
over vacuum.
The vents are fitted with flame arrestors to preventpropagation of flame into the tank.
Where tanks are blanketed, breathing-in will befrom the blanketing gas system. Necessary controlvalve are provided for supply of blanketing gas atconstant pressure.
CONE ROOF TANK APPERTENANCESVents
Emergency Venting
When a tank is exposed to fire, vent capacitiesbased on normal breathing may not be adequate tocope with the increased rate of vaporization.
Roof is only lightly attached to shell so that underexcessive internal pressure, the roof is torn awayfrom the shell, leaving the shell and its contentintact.
CONE ROOF TANK APPERTENANCESDip Hatch
Dip hatch or gauge hatch is used for gauging theheight of the liquid in a tank as well as to take outsamples for testing. Gauge hatch are non-sparking(or lined with non-sparking material) and selfclosing type.
Gauge well pipe (with slots) is provided for alltypes of tanks. This should have continuouscontact by means of strips with bottom plate ofthe tank. Continuous contact makes the tank saferwith respect to static charge accumulation and actsas a support for the gauge well pipe.
CONE ROOF TANK APPERTENANCESDatum Plate
o Datum plate a circular/ rectangular steel plateprovided on the floor of the tank just below theDip hatch pipe.
o Datum plate corresponds to the floor of the tank,where the bob of of the dip tape rests.
o The height measured form the tapered notch inthe gauge pipe to the datum plate is termed as“Reference Height”.
FIXED ROOF TANK APPERTENANCESManholes
Number of manholes depend on diameter of thetank (Refer API 650 for details). Minimum of oneflush type clean out manhole is provided for tanksfor access into the tank for inspection andcleaning.
Bottom Drains
Drains are provided in all tanks for draining waterand also for emptying out the tank for cleaning.Apex down tank bottom have one drain connectionlocated at the lowest point near the center of thetank in addition to normal circumferential drains.
RAIL ROAD TANK FARM
RAIL ROAD TANK FARM
FLOATING ROOF TANKSo Economical storage of volatile liquids with
higher degree of safety.
o A Steel roof floats on the liquid and moves upand down as oil is pumped into or out of thetank.
o Absence of vapor space above the liquid.
o Immediate dissipation by roof of any staticelectricity on the liquid surface.
o Each pontoon is a watertight compartment sothat even if there is a leak in one of thepontoons, the liquid is confined to theparticular compartment.
o Each pontoon has a hatch on the top withcovers for checking.
Roof TypesSingle Deck
o Single deck roof, havingpontoon at the periphery
o Virtually Unsinkable.o High degree of stability
& reserve buoyancy.o Depth of Pontoons 0.4 m-
0.8 mo Standard Sizes 10 m-60 m.o Plate size 4.75 mm
Double Decko Entire roof is like a
number of pontoons.o Underside is in contact
with the Product.o Top deck provides
insulation over the entirearea.
o Vapor loss is minimized.
Single Deck Roof
Double Deck Roof
RAIL ROAD TANK FARM
SUPPORT LEG
LOWER PLATE OF ROOF
ROOF COMPARTMENT
SUMP DRAIN
6"x4" SWINGJOINT ROOF
DRAIN
ROLLING LADDER(SELF LEVELING TYPE)
EARTHING CABLE
ROOF LADDER TRACK
PONTOONENTRANCE
OUTLET
INLET
MANHOLEDATUM PLATE
BOTTOM OF THE TANKPROTECTED WITH CATHODIC
PROTECTTION SYSTEM
SUMP TO COLLECTSETTLED WATER
ROOF DRAINOUT LET
SUMP DRAINOUT LET
DIP HATCH
EMERGENCYDRAIN
BREATHERVENT
MANENTRANCE
RIM VENT
WEATHERSEAL
TG
LI
LSHH
FLOATING ROOF TANK
Seals for Floating roof tanks
o Clearance between rim of roof and shell to allowfree movement, distortion & eccentricity of theshell.
o Gap to be effectively sealed to prevent loss ofvapor.
o Sealing to permit lateral movement of tank roofwithin reasonable limits
Resilient Foam Filled Sealo Ease of installation and low initial cost.o Flexible, opened- celled polyurethane foam body
enclosed within a nylon scuff band.o Seal compressed against tank shell by the gentle
force of foam body.o Highly resistant to most liquidso Puncture or failure of cover will not cause sudden
failure of the seal.o Seal protected from weather, falling rust etc by
hinged weathersheilds which also provide earthingcontinuity between tank shell and roof
Supports for Floating Roofo Roof supported by number of tubular legs, when not
afloat.o Each leg free to move in a sleeve attached to the roof.o Position of legs fixed at two points by a securing pin.o Position 1 : Minimum height of tank roof in lowest
working position above the tank floor (0.76m).o Position 2 : Minimum clearance between tank roof and
tank floor for maintenance work. (1.83m).o Auto Bleeder vents permit the escape of air when an
empty tank is being filled and before the roof is afloat.o Protect the tank against vacuum, as they open
automatically just before the roof lands on its support
Roof Drainso Rain falling on to a floating roof must be led away
to the outside of the tank.o The water is directed from roof via a metal pipe
with swivel joints to an outside gate valve nearbottom of the shell.
o Flow of product via drain to the roof is prevented byNRV in the roof drain.
Rim VentTo avoid any damage to the seal due to excessvapor pressure rim vent is provided which allowsthe vapor to escape to atmosphere depending uponthe size 1 to 3 rim vents are provided.
Emergency Roof Draino In case the normal roof drain fails or due to heavy
rains, the normal roof drain is not able to cope with theload, water gets drained though Emergency roof drain.
o A short length of about 250 mm dia pipe passesthrough both decks and extends slightly above theroof.
o The other end terminates in a hanging turf filled withwater and acts as a seal.
o The water going through the drain falls into the tankand prevents overloading and sinking of the roof.
Access to the Floating roofo Provided by a rolling ladder, one end is hinged at
the gauger’s platform and the other end is free tomove on rollers on a runway fixed to the roof.
o Rolling ladders not considered essential for lesserdia tanks where occasional descents to the roof isprovided by means of a monkey ladder.
Wind Girdero Open tanks reinforced with stiffening ring called
“Wind Girders” to maintain roundness when tankis subjected to wind loads.
o Can also be used as walkways, in which case thewidth of the girder is more than 0.6m, moreover itshould not be less than 1 m from top of the shellwith handrails on the open side.
o This primary wind girder is located at or near thetop of the tank.
o Secondary wind girder are sometimes required forboth floating and fixed roof tanks, to preventbuckling of the tank shell under wind and/orvacuum condition
o Bund wall surrounds a single tank or a groupof tanks.
o Retains spillages.o While calculating the capacity of a bunded
enclosure, a reasonable assumption is madethat the largest tank will fail but other tankswill be intact.
o Maximum height of Bund wall: 1.8m
Bund capacity equalsTotal capacity of largest tank
-capacity of remaining tanks up to bund height
Bund Wall
o These tanks have a fixed roof over an internalfloating roof.
o The fixed roof is usually a cone.o Internal floating roof constructed of steel
aluminum or plastic.Advantages:
Conservation of vapor. Promote cleanliness of product. Reduce internal corrosion. Permit highly volatile products to be stored at
atmospheric pressure.
Internal Floating Roof Tanks
Following guidelines should be used for specificcases:
o Tanks used to store finished Aviation TurbineFuel shall be floating cum fixed roof to avoid entryof water into product.
o Where product degradation due to air/moistureingress is a problem and fixed roof tanks areused,such tanks should be provided with inert gasblanketing.
o Nitrogen blanketing for internal floating rooftanks/fixed roof tanks should be considered forstoring hazardous petroleum products like benzeneetc.
Internal Floating Roof Tanks
Side Entry MixersIn general mixing facilities in tanks have to meet one ormore of the following duties,
Maintain homogeneityTo maintain homogeneity to ensure uniform specification.This demands complete movement throughout the tankwith top to bottom turnover as well as circulation round thetank.
Batch BlendingTo mix two or more different crudes, feed stocks, blendingcomponents or chemical products to obtain a homogenousblend. This demands complete top to bottom turnover.
Heavy Oil BS&W controlTo control or prevent the accumulation of bottomsediment, high velocity circulation is needed over thebottom of the tank.
Side Entry MixersSide-entry mixing systemsHorizontally mounted side-entry mixers are supplied in twomain types, namely:
Fixed angle mixers, for maintaining homogeneity and forblending;
Swivel angle mixers, for control of bottom sediments instorage tanks.
Side Entry MixersPropeller
The mixing device of a side-entry mixer is apropeller.
An axial flow device and essentially a case-lessscrew pump which develops a high rate ofvolumetric displacement.
The propeller is free to take suction from anydirection. Its discharge behaves like a free jet.The high velocity fluid in this jet entrains thesurrounding liquid, causing the jet to expand.
The fluid velocity also has a tangentialcomponent. The jet gradually breaks into theadjacent stagnant liquid layers until eventuallythe whole tank contents circulate in a rotarypattern.
Side Entry Mixers
Radar Level Gauges
The various software alarms and interlocks are 1 Level alarm high 2 Level alarm low 3 Time to limit alarm 4 Interlock to stop pump on low level 5 Interlock to stop mixer on low level (if
present)
DOME ROOF TANK o Dome roof tanks are provided for storing
hydrocarbons with a relatively higher vaporpressure like Tame Raffinate and Hydro treatedLight naphtha in the Refinery Tank Farm.
o The dome roof tanks are kept under Nitrogenblanketing at a pressure of approximately 0.3kg/cm².
The accessories and instrumentation provided in adome roof tank are the same as a cone roof tank.
Spheres
Product having high pressure such as LPG and propylene are stored in horton sphere .Refinery LPG is one of the most important product which is designed to withstand high pressures.
Spheres are selected because of equal distribution of pressure or we can say Maximum volume per minimum area.
Accessories of horton sphere
1. Safety valves 2. Level indicator 3. Level switches and interlocks 4. ROV – ( Remote operated valve ) 5. Pressure gauge 6. Temperature indicator 7. Vapour balancing line 8. Combustible gas detectors. 9. Heat detectors.
Tanks will be in receipt/ settling/ certification/dispatch mode.
o Receipt may be of raw material, intermediate feedstock, blended product or slop oil.
o Transfer/dispatch may be primary feed to processunits, secondary feed to process units, blendcomponents transfer, inter tank transfer or slopprocessing.
o Tank preparation: settling/ water draining/sampling
o Tank certification: Lab will certify the product asper the product specification.
Operational stages
RAIL ROAD TANK FARM
SAFE OPERATION AND MAINTENANCE PRACICE
Tank farm and storage tanks
The capacity, inventory and complexityof Petroleum installations haveincreased manifolds.
As the products handled are highlyflammable and explosive in nature,these installations are extremelyHazardous.
Safe Practices in Operation of Storage Tanks:
Following operational errors may lead toemergencies in a tank farm,
Overfilling Over pressurizing Collapse due to vacuum in the tank Water in hot oil tanks. Sinking of the floating roof Tank fire during mechanical works. Lightning Static Electricity
Principle causes of overfilling
Lack of attention. Errors in level indicators. Wrong setting of valves. Change of service. Gravity filling Absence of philosophy of safe fillings
heights. Failure of manual dip measurement
Causes of over pressurisation The liquid inlet rate is high and the tank vent is not
able to take load of filling rate.
In a blanket tank, it may open up due to pluggingof exit lines of vapour.
The changes in PCV’s setting or malfunctioning orPCV’s in blanketed tank.
Unplugging or de choking of pipelines ifattempted by applying excessive pressure towardstank, can cause bursting of storage tank.
Collapse due to vacuum
The vent is not cleaned.
Plastic bags, rags or bird nests had covered ventand a sudden shower cool will cause tank collapse.
PVRV not properly working
Water in hot oil tanks.
Causes generation of steam due to which oil boilsand rips open the tank roof and overflows .
Accidental introduction of water/condensate fromthe blanketing steam into tank during rains canalso result in such failures.
Failure of steam heating coil in tank can also causeboil over in hot oil tanks
Sinking of the floating roof
Accumulation of rain on the roof creates stresses.This accumulation can occur due to
Plugged roof drains.Plugged roof strainer.Blockage of overflows drains.
The accumulation will tilt tank roof and even sink theroof, exposing petroleum vapour to atmosphere, whichmight find an ignition source outside resulting inflash backfire to the tank.
Tank fire during Hot work
It is very important part of an operation staff toensure that tanks are handed over safely formechanical work. Lot of precaution must beenforced during such work as it is an evident factthat if gas freeing of tank is not done properly. Itmay result into a fire or even an explosion occurreddue to a pocket of gasoline in one of the hollow legsupports of the roof.
Lightening
In case of floating roof tank, small rim fireshave been caused by vapor leaking through theseals. The source of ignition was atmosphereelectricity due to lightning in the rainy season.
Static ElectricityThere are a few fires reported which occurreddue to this cause. The pumping rate was ratherhigh and HC flowing through pump and lineacquired the charge.A spark passed between liquid in the tank andthe roof or wall of the tank, ignited the vapor airmixture.
Inhalation:o Hydrocarbon tanks may have toxic vapour like
H2S. Stand opposite to wind direction whileopening dip hatch.
o Check the gas when going to roof of floating rooftank and use the appropriate PPE’s.
Generalo The dyke wall of the tank farm should be properly
maintained.o The OWS & Storm water channel valves of tank
dyke area should be always kept close.
Others safety aspects:
Velocity of the hydrocarbons fluids in the pipelinesdoes not exceed 2 to 3 m/s
Foundation should be firm and properly leveled. All pressure vessels and pressure relief devices
must be tested at specified interval. Prohibition of plastic buckets. Vehicles/ should not allowed without muffler.
Safe Maintenance PracticeFor maintenance and modification in any tankfarms requires system and procedures with a fairdegree of formality. All maintenance jobs in tanksmust be carried out under permit system and allmodifications should require a system whichidentifies, checks and records modifications.
Maintenance Procedures
Isolation of Tanks:
The tank must be isolated from pipelines, power source and other electrical equipment
Emptying liquids
Once the tank has been isolated, the next step is to removeits contents. This is usually achieved by pumping away toanother tank or drain.
Removal of sludge from tank bottom
The tank sludge may be flammable or toxic or pyrophoric.Specific procedures must be developed safely dispose off thetype of sludge encountered in each case.
Preparation for entry and hot work
Ventilation of the tank should be established The atmosphere should be shown to be free of flammable or
toxic gas Repeat gas test shall be done
Gas freeing of Tanks :
Various methods of gas freeing are forced ventilation,water washing and steam cleaning.
Entry Permits
No one may enter or remain in a confined space which hasany time contained fumes liable to cause a person to beovercome unless:
He is wearing a suitable BA set He has been authorized to enter by a responsible person He is wearing a safety harness with life line A stand by person is available outside the confined space and
understand the emergency procedures
Alternatively a person may enter or work in a confinedspace without breathing apparatus provided that :
Effective steps have been taken to avoid ingress ofdangerous fumes
Sludge or other deposits liable to give off dangerous fumes. The confined space has been adequately ventilated and
tested for fumes. There is a supply of air adequate for respiration. The space has been certified for entry by a responsible
person as being safe for entry for a specific period with outBreathing Apparatus.
A stand by person is available outside the confined spaceand understand the emergency procedures
Safety Features and Automation in Tank Farm
o Fire & Gas Systemo ESD (Emergency Shut down System)o TIS (Tank Inventory System)o OMIS (Oil Movement Information System)o TAS (Terminal Automation System)
Fire & Gas System
The F&G system is designated to:
o Provide visual and audible alarms in the respective controlcenter, fire station.
o Initiate preventive/shutdown actions at an early stage inorder to mitigate the consequence of fire or gas release.
o Alert personnel to danger.o Initiate protection systems.
Interfaced with:
o PA system: Toxic/ flammable gas and fire alarmautomatically connected to the PA system.
o Beacons: High intensity visible beacons automaticallyblink if alarms generated
o ESD System: Actuate the ESD as per system logic.o Sprinkler system: Automatic actuation of DV’s as per logic
and can be actuated from control room.
ESD Systemo Is a triple modulator redundant system dedicated
for plant safetyo Communicates with DCS like any other DCS
control processoro Works on simple ladder logic generated based on
cause and effect chartso All cause are connected as inputs to ESDo Out puts of the logic will be desired effect for plant
safety, which are driven to the field equipment.
TIS (Tank Inventory System)
The general functions of TIS are:
o Scanning the field equipment valueso Manage tank statuso Manage on/off grades statuso Manage tank limit data and ‘time to limit’ alarmo Perform inventory calculation and ullage/
pumpable volume.o Interface with OMIS and TAS system
TAS (Terminal Automation System)
The main tasks of the system are:o Accurate and uninterrupted product loading of tank truck
and tank wagons.o Traffic management by controlling access to tank truck
loading area.o Perform plant control functions such as pump control,
measurements, interlocks etc.Interface with:
o Load rack computer for control and data acquisitiono Reliance application server (SAP) for data
upload/downloado ESD system for to handle emergency situationso Weigh bridge computerso OMIS system
OMIS (Oil Movement Information System)OMIS is used to control and monitor the transfer of product to from and within the tank farms.Objectives:
o Manage the product movement line-ups.o Automatically execute all specified types of product
movements.o Identify non-performing field equipmento Product movement controlo Automatically routing of products.o Start- up execution and closing phases with sequenced
operations