structurally stable stable
4.5.2.6 EFFECTS OF TYPES OF EFFECTS OF TYPES OF
4.5.2.6 EFFECTS OF TYPES OFEFFECTS OF TYPES OF
SETTLEMENT ON STORAGE SETTLEMENT ON STORAGE TANKS TANKS 4.5.2.6.1 4.5.2.6.1 BackgroundBackground
The excessive settling of a tank can The excessive settling of a tank can cause serious tank
cause serious tank operating problemsoperating problems and lead to tank failure. Therefore, a and lead to tank failure. Therefore, a key
key step in tank design is estimatingstep in tank design is estimating the amount of settlement the
the amount of settlement the tank’stank’s
shell will undergo in its lifetime. The shell will undergo in its lifetime. The desired maximum
desired maximum
Lifetime settlement is usually less Lifetime settlement is usually less thanthan 0.3 m (1 ft.). When
0.3 m (1 ft.). When settlementsettlement exceeds 0.3 m (1 ft.), there may be exceeds 0.3 m (1 ft.), there may be
serious problems
serious problems with the storagewith the storage tanks, shell, annular plate or
tanks, shell, annular plate or bottom.bottom. 4.5.2.6.2
4.5.2.6.2 Types of SettlementTypes of Settlement
The settling of a tank is classified by The settling of a tank is classified by the type of shell settlement
the type of shell settlement and theand the type of bottom settlement.
type of bottom settlement. When aWhen a tank shell settles, the settlement can tank shell settles, the settlement can be classified as
be classified as
Uniform, planar tilt or deviation from Uniform, planar tilt or deviation from planar tilt. When a tank
planar tilt. When a tank bottombottom settles, the settlement can be settles, the settlement can be classified as center-toedge
classified as center-toedge or localor local shell and bottom.
shell and bottom.
The following sections discuss these The following sections discuss these types of settlement: types of settlement: • U • Uniformniform • Planar tilt • Planar tilt
• Deviation from planar tilt • Deviation from planar tilt • Center
• Center-to-edge-to-edge
• Local shell or bottom • Local shell or bottom
4.5.2.6.2.1
4.5.2.6.2.1 UniformUniform
When the tank shell remains level as When the tank shell remains level as the tank settles, uniform
the tank settles, uniform settlementsettlement has occurred. Uniform settlement has occurred. Uniform settlement does not cause
does not cause significant stresses orsignificant stresses or distortions in the tank. This type of distortions in the tank. This type of settlement requires correction only settlement requires correction only when the foundation or
when the foundation or pipingpiping connections develop problems.
connections develop problems. FigureFigure 4-36
4-36illustratesillustrates uniform uniform settlemensettlement.t.
Figure 4-36 Uniform Settlement Figure 4-36 Uniform Settlement
Uniform settlement can cause the Uniform settlement can cause the following:
following:
• Overstres
• Overstressing of the connectingsing of the connecting piping and
piping and associated tank nozzle.associated tank nozzle.
• Blockage of surface water drainage • Blockage of surface water drainage
from the
from the tank pad, which could causetank pad, which could cause corrosion
corrosion of theof the tank shell or bottomtank shell or bottom 4.5.2.6.2.2
4.5.2.6.2.2 Planar TiltPlanar Tilt
When the tank’s shell tilts as the tank When the tank’s shell tilts as the tank
settles and the bottom of
settles and the bottom of the shellthe shell remains in a single plane, planar tilt remains in a single plane, planar tilt settlement has
settlement has occurred. The bottomoccurred. The bottom plane does not distort; it only tilts. plane does not distort; it only tilts. Figure 4-37
Figure 4-37illustrates planar tilt.illustrates planar tilt.
Figure 4-37 Planar Tilt
Figure 4-37 Planar Tilt SettlementSettlement
As the shell tilts, stresses are As the shell tilts, stresses are introduced that change the shape introduced that change the shape ofof the shell. As a result of these stresses, the shell. As a result of these stresses, the top of the tank
the top of the tank becomes elliptical.becomes elliptical. Planar tilt settlement can cause the Planar tilt settlement can cause the following:
following:
• Malfunction of floating roof seals • Malfunction of floating roof seals • Binding of a floating roof
• Binding of a floating roof
• Problems with connecting pipes • Problems with connecting pipes • Problems with surfa
• Problems with surface waterce water drainage from the
drainage from the tank padtank pad
• Buckling in flanges or webs of wind • Buckling in flanges or webs of wind
girders girders 4.5.2.6.2.3
4.5.2.6.2.3 Deviation from Planar TiltDeviation from Planar Tilt
When the shell does not remain in a When the shell does not remain in a plane as it settles,
plane as it settles, deviation fromdeviation from planar tilt or differential settlement planar tilt or differential settlement has occurred.
has occurred. Figure 4-38Figure 4-38 illustratesillustrates deviation from planar tilt.
Figure 4-38 Deviation from Planar
Figure 4-38 Deviation from Planar Tilt SettlementTilt Settlement
Deviation from planar tilt settlement Deviation from planar tilt settlement can cause the following:
can cause the following:
• Malfunction of floating roof seals • Malfunction of floating roof seals • Binding of a floating roof
• Binding of a floating roof • Problems with
• Problems with connecting pipesconnecting pipes • Problems w
• Problems with surface waterith surface water drainage from the
drainage from the tank padtank pad
• Buckling in flanges or webs of • Buckling in flanges or webs of windwind
girders girders
• Shell buckling • Shell buckling
• Overstress of the shell or
• Overstress of the shell or bottombottom
plates plates 4.5.2.6.2.4
4.5.2.6.2.4 Center-to-EdgeCenter-to-Edge
When the support under the bottom When the support under the bottom of the tank settles more
of the tank settles more than thethan the support under the shell of the tank, support under the shell of the tank, center-to-edge
center-to-edge settlement settlement hashas occurred.
occurred. Figure Figure 4-394-39 illustratesillustrates center-to-edge
center-to-edge settlement.settlement.
Figure 4-39 Center-To-Edge
Figure 4-39 Center-To-Edge SettlemenSettlementt
Excessive center-to-edge settlement is Excessive center-to-edge settlement is most likely to cause the
most likely to cause the following:following:
• In tanks under 45 m (150 ft.) i • In tanks under 45 m (150 ft.) inn diameter,
diameter, buckling of the bottom shellbuckling of the bottom shell course
course
• In tanks over 45 m (150 ft.) in • In tanks over 45 m (150 ft.) in
diameter, failure
diameter, failure in the bottom platesin the bottom plates
• Inaccuracies in tank gauging • Inaccuracies in tank gauging
4.5.2.6.2.5
4.5.2.6.2.5 Local Shell or BottomLocal Shell or Bottom
When the shell and bottom do not When the shell and bottom do not settle together or if local
settle together or if local areas of theareas of the bottom settle differently from the rest bottom settle differently from the rest of the bottom,
of the bottom, local shell or bottomlocal shell or bottom settlement has occurred.
settlement has occurred. Figure 4-40Figure 4-40 illustrates local shell or bottom illustrates local shell or bottom settlement.
settlement.
Figure 4-40 Local Shell or
Figure 4-40 Local Shell or Bottom SettlementBottom Settlement
In local shell or bottom settlement, In local shell or bottom settlement, significant stress may
significant stress may develop in thedevelop in the bottom plates, their attachment bottom plates, their attachment welds, the
welds, the bottom-to-shell junctionbottom-to-shell junction weld, or the lower section of the
weld, or the lower section of the shell.shell. This stress can cause the bottom of This stress can cause the bottom of the tank to fail.
the tank to fail.
4.5.2.7
4.5.2.7 FUNCTIONS OFFUNCTIONS OF
APPURTENANCES AND THE APPURTENANCES AND THE
OPERATION OF SAFETY OPERATION OF SAFETY SYSTEMS OF STORAGE SYSTEMS OF STORAGE TANKS TANKS
This section discusses the functions of This section discusses the functions of storage tank
storage tank appurtenances and theappurtenances and the operation of safety
operation of safety systems.systems. 4.5.2.7.1
4.5.2.7.1 Functions ofFunctions of
Appurtenances
Appurtenances
Figures 4-41, 4-42, 4-43
Figures 4-41, 4-42, 4-43illustration ofillustration of the various types of
the various types of storage tanks tostorage tanks to identify
identify appurtenancappurtenances.es. The
The appurtenanceappurtenancess that are shownthat are shown and other
discussed in the
discussed in the paragraphs thatparagraphs that follow. The figures also highlight follow. The figures also highlight
several other tank construction details several other tank construction details in order to aid clarity.
in order to aid clarity.
Figure 4-41 Appurtenances on Cone Roof Tank Figure 4-41 Appurtenances on Cone Roof Tank
Figure 4-42 Appurtenances on Floating Roof Tank Figure 4-42 Appurtenances on Floating Roof Tank
Figure 4-43 Appurtenances, Low-Temperature Tank Figure 4-43 Appurtenances, Low-Temperature Tank
Appurtenances are auxiliary Appurtenances are auxiliary equipment items that are added to equipment items that are added to storage tanks to perform specific storage tanks to perform specific functions. The types and
functions. The types and number ofnumber of appurtenances that are installed on a appurtenances that are installed on a specific tank
specific tank depend on the storagedepend on the storage tank type, size, and service. The tank type, size, and service. The paragraphs that follow highlight the paragraphs that follow highlight the function of the following function of the following appurtenances: appurtenances: • • VentsVents • • DrainsDrains • • SealsSeals • •HeatersHeaters • • MixersMixers •
•Access HatchesAccess Hatches
•
• Platforms, Ladders, and StairwaysPlatforms, Ladders, and Stairways
•
•Gauging devicesGauging devices
Later modules discuss some of these Later modules discuss some of these appurtenances in more
appurtenances in more detail.detail. 4.5.2.7.1.1
4.5.2.7.1.1 VentsVents
Vent connections on fixed roof Vent connections on fixed roof atmospheric storage tanks prevent atmospheric storage tanks prevent both an excessive positive pressure both an excessive positive pressure
and a vacuum condition within the and a vacuum condition within the tank when it is filled and emptied or tank when it is filled and emptied or during fire emergency conditions. during fire emergency conditions. Safety valves on pressurized storage Safety valves on pressurized storage
tanks ensure that the tank’s internal tanks ensure that the tank’s internal
pressure does not exceed its design pressure does not exceed its design capabilities.
capabilities.
Rim vents on floating roofs relieve Rim vents on floating roofs relieve excessive vapor pressure that can excessive vapor pressure that can build up under the floating roof seal. build up under the floating roof seal. Automatic bleeder vents on floating Automatic bleeder vents on floating roofs open automatically when the roofs open automatically when the roof approaches its lowest level of roof approaches its lowest level of operation and close automatically operation and close automatically when the roof rises above this level. when the roof rises above this level. These bleeder vents allow ventilation These bleeder vents allow ventilation of the vapor space that is created of the vapor space that is created between the liquid level and the roof between the liquid level and the roof when the roof is at its lowest position when the roof is at its lowest position and rests on its
Figure 4-44 Rim vents Figure 4-44 Rim vents
The design data for the venting design The design data for the venting design is as follow:
is as follow:
Design Filling Rate,Design Filling Rate,
Design Emptying Rate,Design Emptying Rate,
The vacuum venting (In-Breathing) The vacuum venting (In-Breathing)
The The venting venting capacity capacity forfor
maximum liquid emptying will maximum liquid emptying will be 15.86
be 15.86
/h of free air for/h of free air for each 15.9each 15.9
h of maximumh of maximum empty rate.empty rate.
Flow rate of free air for liquidFlow rate of free air for liquid
movement, movement,
==
/ 15.9 *15.86/ 15.9 *15.86The pressure venting
The pressure venting (out-Breathing)(out-Breathing)
The The venting venting capacity capacity forfor
maximum liquid filling will be maximum liquid filling will be 17
17
/h of free air for each/h of free air for each 15.915.9
h of maximum Fillingh of maximum Filling rate.rate.
Flow rate of free air for liquidFlow rate of free air for liquid
movement, movement,
==
/ 15.9 *17/ 15.9 *17 Cross sectional area of ventCross sectional area of vent
Q=Q= maximum flow ratemaximum flow rate
P = Pressure differentP = Pressure different
= Specific weight of air= Specific weight of air
= Cross sectional area of= Cross sectional area ofvent vent
k= Discharge Coefficient, 0.62k= Discharge Coefficient, 0.62
for circular for circular 4.5.2.7.1.2
4.5.2.7.1.2 DrainsDrains
Drains in the roof of external floating Drains in the roof of external floating roof tanks remove accumulated roof tanks remove accumulated rainfall and thus prevent accumulation rainfall and thus prevent accumulation of excessive rainwater which could of excessive rainwater which could overload a floating roof and damage it overload a floating roof and damage it or cause it to sink.
or cause it to sink. Figure 4-45Figure 4-45 illustrates a typical floating roof drain illustrates a typical floating roof drain system. A sump is located in the system. A sump is located in the center of the roof.
center of the roof.
The sump has a pipe Connection and The sump has a pipe Connection and check valve installed in it. Either check valve installed in it. Either articulated pipe with swivel joints (as articulated pipe with swivel joints (as shown in
shown in Figure 4-45Figure 4-45) or flexible hose) or flexible hose connects the pipe to an outlet nozzle connects the pipe to an outlet nozzle that is located in the bottom shell that is located in the bottom shell course. When the tank is filled and course. When the tank is filled and emptied, the Flexibility of the pipe or emptied, the Flexibility of the pipe or hose permits the needed free vertical hose permits the needed free vertical roof movement.
roof movement.
The check valve is oriented to permit The check valve is oriented to permit rainwater to flow off the roof but to rainwater to flow off the roof but to
prevent any stored liquid from getting prevent any stored liquid from getting on the roof. Stored liquid could get on on the roof. Stored liquid could get on the roof if the pipe or hose failed. The the roof if the pipe or hose failed. The block valve at the shell outlet nozzle is block valve at the shell outlet nozzle is opened as needed to drain any opened as needed to drain any accumulated rainwater.
accumulated rainwater.
Figure 4-45 Typical Floating Roof Drain System Figure 4-45 Typical Floating Roof Drain System
Atmospheric storage tanks are Atmospheric storage tanks are frequently equipped with an internal frequently equipped with an internal water draw off sump and drain
water draw off sump and drain FigureFigure 4-46
4-46. Water can enter a tank from. Water can enter a tank from atmospheric breathing, with the atmospheric breathing, with the stored hydrocarbon as either free or stored hydrocarbon as either free or dissolved water. Water can also enter dissolved water. Water can also enter a tank when water that has a tank when water that has accumulated on top of an external accumulated on top of an external floating roof leaks into the tank. The floating roof leaks into the tank. The sump and drain are used to remove sump and drain are used to remove the water that collects at the bottom the water that collects at the bottom of the tank under the hydrocarbon of the tank under the hydrocarbon that is being stored. A water draw off that is being stored. A water draw off valve located at the tank shell is valve located at the tank shell is opened periodically to drain opened periodically to drain accumulated water.
accumulated water. Figure Figure 4-464-46 illustrates the water draw off sump illustrates the water draw off sump and valve.
and valve.
If water that has entered the tank is If water that has entered the tank is not periodically removed, it not periodically removed, it accumulates until it is drawn out with accumulates until it is drawn out with the hydrocarbon. This accumulated the hydrocarbon. This accumulated water can result in a product being out water can result in a product being out of specification, or it can disrupt the of specification, or it can disrupt the
operation of downstream process operation of downstream process units. In addition, the presence of units. In addition, the presence of water in the bottom of tank increases water in the bottom of tank increases bottom corrosion, promotes biological bottom corrosion, promotes biological growth, and can significantly increase growth, and can significantly increase the number of sulfate-reducing the number of sulfate-reducing bacteria, which can cause severe tank bacteria, which can cause severe tank bottom pitting.
bottom pitting.
Figure 4-46 Internal Water Draw off System Figure 4-46 Internal Water Draw off System
4.5.2.7.1.2.1
4.5.2.7.1.2.1 Drain Pipe DesignDrain Pipe Design
The
The Rain Rain Fall, Fall, RH RH (m/hr) (m/hr) (given)(given)
The Drainage (Q) = RH x deck area The Drainage (Q) = RH x deck area (m³/hr) (m³/hr) A A = = Q Q / / V V , , V V ==
∑∑
ff = Flow Coefficient,= Flow Coefficient,
=pipe diameter=pipe diameter
= Pipe Length,= Pipe Length,
=elevation=elevation - Select the D- Select the Drain Pipe (Ex. 4” Schedulerain Pipe (Ex. 4” Schedule
80) 80)
4.5.2.7.1.3
4.5.2.7.1.3 SealsSeals
Seals are located between the roof Seals are located between the roof outside diameter and the tank inside outside diameter and the tank inside diameter on external and internal diameter on external and internal floating roof tanks. These seals close floating roof tanks. These seals close the gap between the roof and shell. the gap between the roof and shell. Closing the gap in this manner Closing the gap in this manner minimizes hydrocarbon emissions and minimizes hydrocarbon emissions and permits free vertical motion of the permits free vertical motion of the roof as the stored liquid
roof as the stored liquid level changes.level changes.
Seals typically have either a metal Seals typically have either a metal plate or a flexible membrane between plate or a flexible membrane between
the roof pontoon and the tank shell. the roof pontoon and the tank shell. When a metal-plate-type of seal is When a metal-plate-type of seal is used, a counterweight system is used used, a counterweight system is used to press the metal plate against the to press the metal plate against the tank wall.
tank wall. Figure 4-47Figure 4-47 illustrates thisillustrates this type of seal.
type of seal.
Figure 4-47Metal-Plate-Type Floating Roof Seal Figure 4-47Metal-Plate-Type Floating Roof Seal
Other seals consist of tubes that are Other seals consist of tubes that are filled with solid foam, liquid, or air. filled with solid foam, liquid, or air. Figure 4-48
Figure 4-48 illustrates a typical seal forillustrates a typical seal for an internal floating roof.
an internal floating roof.
Figure 4-48 Typical Seal for Internal Floating Roof Figure 4-48 Typical Seal for Internal Floating Roof
4.5.2.7.1.4
4.5.2.7.1.4 HeatersHeaters
Heating coils or heaters are installed in Heating coils or heaters are installed in tanks that store products that have a