Storage Tanks [PDF]

Zitiervorschau

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