Storage Tanks

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ABOVEGROUND
STORAGE TANKS &
SPHERES

AN OVERVIEW

Brief History Of Petroleum
Storage Tanks
1859
1900
1861

Wooden tanks were used in field production area.
Oblong / Rectangular tanks about 30 feet long X 8 feet
wide X 5 feet height
First round oil tanks , Cap 6 to 12 barrels.

1910 First large Iron tank of cap. 8000 barrels
1913 First bolted steel tank – By Gante Tank Co.
1920 to Welded tanks.
1930
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 oil
storage

2.

API standard 620 : Design and construction of
large , welded low pressure storage tanks.

3.

IS – 803 : Welded steel storage tanks for Oil
Storage

4.

BS – 2564 : Vertical steel welded storage tanks
with butt welded shell for petroleum industries.

Storage tanks- general

Storage tanks are used to store
fluids
such
as
crude
oil,
intermediate and refined products,
gas, chemicals, waste products,
aqueous mixtures and water.
Factors such as Volatility of stored product
and the desired storage pressure, result in
tanks being built in various types, sizes and
material of construction.

Types of Storage tanks
1. Fixed Roof Tanks
2. Floating Roof Tanks
3. Dome Roof Tanks
4. Internal Floating Roof Tanks
5. Horton Spheres

Selection of tanks
Fixed 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 tanks
Additional aspects that have an impact on the choice of tank
type:

Building Cost

o Cone roof tanks offer building cost advantage
towards the lower tank diameter range.
o Floating roof tanks cheaper towards the large
diameter end of the scale.
o In terms of cost per m3 of gross storage capacity a
floating roof tank could costs 34% more for an
8,000 m3 tank and 9% more for a 20,000 m3 tank.

Selection of tanks
Cost for storage capacity

o For a given tank size it is the net working storage
capacity of a tank that determines the profitable
use a refinery makes of a tank.
o Also, the minimum stock level represents an
amount of dead stock, which is a cost to the
refinery in terms of working capital.
o The cost of dead stock increases with the
hydrocarbon value of the product, with the related
yearly interest charge.

Selection of tanks
TVP, IBP or density restrictions for floating
roof.
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

Selection of tanks

TVP, IBP or density restrictions for floating
roof.
o The density limitation ensures that the roof will have
sufficient buoyancy, including the capability to carry a certain
amount 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 the
mechanical performance of the tank and seal would become
critical. Any gap between the seal and tank wall would cause
high emission of volatiles and increase the risk of a rim fire
due to lighting or any other source of ignition. Also the
evaporation through the gap would be a continuous source of
product loss.

Main Parts of Storage Tank
o
o
o

o

The Bottom
Bottom plates function as a membrane and transfer the
liquid pressure directly on to the tank foundation.
They are made of 10-15 mm thick rectangular steel plates,
which are interconnected by overlaps.
Tank bottoms may have conical shape or inverted cone
shape, also known as "Apex down". Bottoms are coned
downwards towards the center with a sump of adequate
size at the center of the tank bottom and a siphon drain.
The Shell
The tank shell is made of a number of courses, normally
1.5 to 2.5 m wide. The plate thickness gradually decrease
upwards due to the reduction in horizontal loading of the
liquid pressure.

FIXED ROOF TANKS

o Vertical cylindrical vessels with conical top.
o Used for storing non-volatile materials like

Heavy oils, gas oils, furnace oils and non volatile
chemicals.
o Tanks are protected by vacuum vent that
maintains the pressure difference between tank
vapor pressure and outside atmosphere.
o The roof plates are 5 mm thick and are
interconnected by lap-weld on the top side only.

FIXED ROOF TANKS

o Above the shell the plates are welded to the top

curb angle of the tank with a continuous seal
weld, with the intention that the roof will
rupture at the shell roof connection in case of an
internal tank explosion. No damage to the shell
bottom connection will then occur with the risk
of loss of product.

o Mountings

required for access, gauging,
dipping and vents are placed on top of the roof.

FIXED ROOF TANKS
o
o

o

Non- Pressure
Fixed roof tanks
Work at atmospheric
pressure.
Design
internal
pressure 75 mm of
water column.
Design vacuum 25
mm of water column.

o

Low – Pressure
fixed roof tanks
Design
internal
pressure 200 mm of
water

o

Design vacuum of 60
mm of water column.

FIXED ROOF TANK
In Non pressure fixed roof tanks, normal
venting takes the form of free- flow
atmospheric vent which allow unimpeded
flow of vapors out of, and of air into the tank
and, at the same time, prevent rain and
airborne dust from getting into the tank.
In Pressure fixed roof tanks, the contents are
stored under nitrogen blanketing to prevent
ingress and to reduce vapor formation.

CONE ROOF TANK APPERTENANCES
Vents

Open Vents
The tank breathes - in air when the tank pressure
is lower than the atmospheric pressure and
breathes - out when tank pressure is greater than
the set pressure.
Open vents are of goose neck type, covered with
a 4 to 8 mesh screen. Normal venting takes the
form of free- flow atmospheric vent which allow
unimpeded flow of vapors out of, and of air into
the tank and, at the same time, prevent rain and
airborne dust from getting into the tank.

CONE ROOF TANK APPERTENANCES
Vents

Pressure Vacuum Relief Valve
Functions:
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 prevent
propagation of flame into the tank.
Where tanks are blanketed, breathing-in will be
from the blanketing gas system. Necessary control
valve are provided for supply of blanketing gas at
constant pressure.RAIL ROAD TANK FARM

CONE ROOF TANK APPERTENANCES
Vents
Emergency Venting
When a tank is exposed to fire, vent capacities
based on normal breathing may not be adequate to
cope with the increased rate of vaporization.
Roof is only lightly attached to shell so that under
excessive internal pressure, the roof is torn away
from the shell, leaving the shell and its content
intact.

CONE ROOF TANK APPERTENANCES
Dip Hatch
Dip 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 are non-sparking
(or lined with non-sparking material) and self
closing type.
Gauge well pipe (with slots) is 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.

CONE ROOF TANK APPERTENANCES
Datum Plate
o Datum plate a circular/ rectangular steel plate
provided on the floor of the tank just below the
Dip 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 in
the gauge pipe to the datum plate is termed as
“Reference Height”.

FIXED ROOF TANK APPERTENANCES
Manholes
Number of manholes depend on diameter of the
tank (Refer API 650 for details). Minimum of one
flush type clean out manhole is provided for tanks
for access into the tank for inspection and
cleaning.

Bottom Drains
Drains are provided in all tanks for draining water
and also for emptying out the tank for cleaning.
Apex down tank bottom have one drain connection
located at the lowest point near the center of the
tank in addition to normal circumferential drains.

RAIL ROAD TANK FARM

RAIL ROAD TANK FARM

FLOATING ROOF TANKS

o Economical storage of volatile liquids with
higher degree of safety.

o A Steel roof floats on the liquid and moves up
and down as oil is pumped into or out of the
tank.

o Absence of vapor space above the liquid.
o Immediate dissipation by roof of any static
electricity on the liquid surface.

o Each pontoon is a watertight compartment so

that even if there is a leak in one of the
pontoons, the liquid is confined to the
particular compartment.

o Each pontoon has a hatch on the top with
covers for checking.

Roof Types
Single Deck

o
o
o
o
o
o

Single deck roof, having
pontoon at the periphery
Virtually Unsinkable.
High degree of stability
& reserve buoyancy.
Depth of Pontoons 0.4 m0.8 m
Standard Sizes 10 m-60 m.
Plate size 4.75 mm

Double Deck
o
o
o

o

Entire roof is like a
number of pontoons.
Underside is in contact
with the Product.
Top
deck
provides
insulation over the entire
area.
Vapor loss is minimized.

Single Deck Roof

Double Deck Roof

FLOATING ROOF TANK
DIP HATCH

WEATHER
SEAL

ROLLING LADDER
(SELF LEVELING TYPE)
EARTHING CABLE

RIM
VENT

ROOF LADDER TRACK

MAN
ENTRANCE

PONTOON
ENTRANCE

LSHH

BREATHER
VENT
EMERGENCY
DRAIN

SUPPORT LEG
ROOF COMPARTMENT
LOWER PLATE OF ROOF

SUMP DRAIN

6"x4" SWING
JOINT ROOF
DRAIN

LI

TG

OUTLET
BOTTOM OF THE TANK
PROTECTED WITH CATHODIC
PROTECTTION SYSTEM

INLET

SUMP TO COLLECT
SETTLED WATER

DATUM PLATE
MANHOLE

RAIL ROAD TANK FARM

ROOF DRAIN
OUT LET
SUMP DRAIN
OUT LET

Seals for Floating roof tanks
o

Clearance between rim of roof and shell to allow
free movement, distortion & eccentricity of the
shell.

o

Gap to be effectively sealed to prevent loss of
vapor.

o

Sealing to permit lateral movement of tank roof
within reasonable limits

Resilient Foam Filled Seal
o
o
o
o
o
o

Ease of installation and low initial cost.
Flexible, opened- celled polyurethane foam body
enclosed within a nylon scuff band.
Seal compressed against tank shell by the gentle
force of foam body.
Highly resistant to most liquids
Puncture or failure of cover will not cause sudden
failure of the seal.
Seal protected from weather, falling rust etc by
hinged weathersheilds which also provide earthing
continuity between tank shell and roof

Supports for Floating Roof
o
o
o
o
o
o
o

Roof supported by number of tubular legs, when not
afloat.
Each leg free to move in a sleeve attached to the roof.
Position of legs fixed at two points by a securing pin.
Position 1 : Minimum height of tank roof in lowest
working position above the tank floor (0.76m).
Position 2 : Minimum clearance between tank roof and
tank floor for maintenance work. (1.83m).
Auto Bleeder vents permit the escape of air when an
empty tank is being filled and before the roof is afloat.
Protect the tank against vacuum, as they open
automatically just before the roof lands on its support

Roof Drains
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 near
bottom of the shell.
o Flow of product via drain to the roof is prevented by
NRV in the roof drain.
o

Rim Vent

To avoid any damage to the seal due to excess
vapor pressure rim vent is provided which allows
the vapor to escape to atmosphere depending upon
the size 1 to 3 rim vents are provided.

Emergency Roof Drain
In case the normal roof drain fails or due to heavy
rains, the normal roof drain is not able to cope with the
load, water gets drained though Emergency roof drain.
o A short length of about 250 mm dia pipe passes
through both decks and extends slightly above the
roof.
o The other end terminates in a hanging turf filled with
water and acts as a seal.
o The water going through the drain falls into the tank
and prevents overloading and sinking of the roof.
o

Access to the Floating roof
Provided by a rolling ladder, one end is hinged at
the gauger’s platform and the other end is free to
move on rollers on a runway fixed to the roof.
o Rolling ladders not considered essential for lesser
dia tanks where occasional descents to the roof is
provided by means of a monkey ladder.
o

Wind Girder
Open tanks reinforced with stiffening ring called
“Wind Girders” to maintain roundness when tank
is subjected to wind loads.
o Can also be used as walkways, in which case the
width of the girder is more than 0.6m, moreover it
should not be less than 1 m from top of the shell
with handrails on the open side.
o This primary wind girder is located at or near the
top of the tank.
o Secondary wind girder are sometimes required for
both floating and fixed roof tanks, to prevent
buckling of the tank shell under wind and/or
vacuum condition
o

Bund Wall

Bund wall surrounds a single tank or a group
of tanks.
o Retains spillages.
o While calculating the capacity of a bunded
enclosure, a reasonable assumption is made
that the largest tank will fail but other tanks
will be intact.
o Maximum height of Bund wall: 1.8m
o

Bund capacity equals

Total capacity of largest tank
capacity of remaining tanks up to bund height

Internal Floating Roof Tanks

These tanks have a fixed roof over an internal
floating roof.
o The fixed roof is usually a cone.
o Internal floating roof constructed of steel
aluminum or plastic.
o

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 specific
cases:
o

Tanks used to store finished Aviation Turbine
Fuel shall be floating cum fixed roof to avoid entry
of water into product.

o

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.

o

Nitrogen blanketing for internal floating roof
tanks/fixed roof tanks should be considered for
storing hazardous petroleum products like benzene
etc.

Side Entry Mixers

In general mixing facilities in tanks have to meet one or
more of the following duties,
Maintain homogeneity
To maintain homogeneity to ensure uniform specification.
This demands complete movement throughout the tank
with top to bottom turnover as well as circulation round the
tank.
Batch Blending
To mix two or more different crudes, feed stocks, blending
components or chemical products to obtain a homogenous
blend. This demands complete top to bottom turnover.
Heavy Oil BS&W control
To control or prevent the accumulation of bottom
sediment, high velocity circulation is needed over the
bottom of the tank.

Side Entry Mixers
Side-entry mixing systems
Horizontally mounted side-entry mixers are supplied in two
main types, namely:
 Fixed angle mixers, for maintaining homogeneity and for
blending;
 Swivel angle mixers, for control of bottom sediments in
storage tanks.

Side Entry Mixers

Propeller

 The mixing device of a side-entry mixer is a
propeller.
 An axial flow device and essentially a case-less
screw pump which develops a high rate of
volumetric displacement.
 The propeller is free to take suction from any
direction. Its discharge behaves like a free jet.
The high velocity fluid in this jet entrains the
surrounding liquid, causing the jet to expand.
 The fluid velocity also has a tangential
component. The jet gradually breaks into the
adjacent stagnant liquid layers until eventually
the whole tank contents circulate in a rotary
pattern.

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 vapor
pressure like Tame Raffinate and Hydro treated
Light naphtha in the Refinery Tank Farm.
o The dome roof tanks are kept under Nitrogen
blanketing at a pressure of approximately 0.3
kg/cm².

The accessories and instrumentation provided in a
dome 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.

Operational stages
o
o

o
o

Tanks will be in receipt/ settling/ certification/
dispatch mode.
Receipt may be of raw material, intermediate feed
stock, blended product or slop oil.
Transfer/dispatch may be primary feed to process
units, secondary feed to process units, blend
components transfer, inter tank transfer or slop
processing.
Tank preparation: settling/ water draining/
sampling
Tank certification: Lab will certify the product as
per the product specification.

RAIL ROAD TANK FARM

SAFE OPERATION AND
MAINTENANCE PRACICE

Tank farm and storage tanks

The capacity, inventory and complexity
of
Petroleum
installations
have
increased manifolds.
As the products handled are highly
flammable and explosive in nature,
these
installations
are
extremely
Hazardous.

Safe Practices in Operation of
Storage Tanks:
Following operational errors may lead to
emergencies 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 plugging
of exit lines of vapour.
 The changes in PCV’s setting or malfunctioning or
PCV’s in blanketed tank.
 Unplugging or de choking of pipelines if
attempted by applying excessive pressure towards
tank, can cause bursting of storage tank.

Collapse due to vacuum
 The vent is not cleaned.
 Plastic bags, rags or bird nests had covered vent
and 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 boils
and rips open the tank roof and overflows .
 Accidental introduction of water/condensate from
the blanketing steam into tank during rains can
also result in such failures.
 Failure of steam heating coil in tank can also cause
boil 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 the
roof, exposing petroleum vapour to atmosphere, which
might find an ignition source outside resulting in
flash backfire to the tank.

Tank fire during Hot work
It is very important part of an operation staff to
ensure that tanks are handed over safely for
mechanical work. Lot of precaution must be
enforced during such work as it is an evident fact
that if gas freeing of tank is not done properly. It
may result into a fire or even an explosion occurred
due to a pocket of gasoline in one of the hollow leg
supports of the roof.

Lightening
In case of floating roof tank, small rim fires
have been caused by vapor leaking through the
seals. The source of ignition was atmosphere
electricity due to lightning in the rainy season.

Static Electricity

There are a few fires reported which occurred
due to this cause. The pumping rate was rather
high and HC flowing through pump and line
acquired the charge.
A spark passed between liquid in the tank and
the roof or wall of the tank, ignited the vapor air
mixture.

Inhalation:

o Hydrocarbon tanks may have toxic vapour like
H2S. Stand opposite to wind direction while
opening dip hatch.
o Check the gas when going to roof of floating roof
tank and use the appropriate PPE’s.

General

o 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 pipelines





does 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 Practice

For maintenance and modification in any tank
farms requires system and procedures with a fair
degree of formality. All maintenance jobs in tanks
must be carried out under permit system and all
modifications should require a system which
identifies, 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 remove
its contents. This is usually achieved by pumping away to
another 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 the
type 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 has
any time contained fumes liable to cause a person to be
overcome 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 confined
space without breathing apparatus provided that :
 Effective steps have been taken to avoid ingress of
dangerous 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 out
Breathing Apparatus.
 A stand by person is available outside the confined space
and understand the emergency procedures

Safety Features and Automation in
Tank Farm
o
o
o
o
o

Fire & Gas System
ESD (Emergency Shut down System)
TIS (Tank Inventory System)
OMIS (Oil Movement Information System)
TAS (Terminal Automation System)

Fire & Gas System
The F&G system is designated to:
o Provide visual and audible alarms in the respective control
center, fire station.
o Initiate preventive/shutdown actions at an early stage in
order 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 alarm
automatically connected to the PA system.
o Beacons: High intensity visible beacons automatically
blink 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.

o
o
o
o
o

ESD System
Is a triple modulator redundant system dedicated
for plant safety
Communicates with DCS like any other DCS
control processor
Works on simple ladder logic generated based on
cause and effect charts
All cause are connected as inputs to ESD
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:

Scanning the field equipment values
Manage tank status
Manage on/off grades status
Manage tank limit data and ‘time to limit’ alarm
Perform inventory calculation and ullage/
pumpable volume.
o Interface with OMIS and TAS system

o
o
o
o
o

TAS (Terminal Automation System)
o
o
o
o
o
o
o
o

The main tasks of the system are:
Accurate and uninterrupted product loading of tank truck
and tank wagons.
Traffic management by controlling access to tank truck
loading area.
Perform plant control functions such as pump control,
measurements, interlocks etc.

Interface with:

Load rack computer for control and data acquisition
Reliance
application
server
(SAP)
for
upload/download
ESD system for to handle emergency situations
Weigh bridge computers
OMIS system

data

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 equipment
o Product movement control
o Automatically routing of products.
o Start- up execution and closing phases with sequenced
operations

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