Storage Tank Design Flashcards by Ellie J Bibby

Define ‘Flash Point’

The lowest temperature where vapours form an ignitable mixture given an ignition source.

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Name the 3 main types of storage tanks.

1) Fixed roof
2) Internal floating roof
3) External roof

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When is a fixed roof storage tank suitable? Give 2 examples.

Very high flash points - must not ignite.

eg. water & fuel oil

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When is an internal floating roof storage tank suitable? Give 2 examples.

Low flash points. Has fixed & floating roof suitable for countries with lots of rain.
eg. gasoline, ethanol

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When is an external floating roof storage tank suitable? Give 2 examples of suitable fluids.

Medium flash points. No fixed roof suitable for dry countries.
eg. naptha, kerosene

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What is the purpose of a floating roof?

It sits directly on top of the fluid to prevent vapour escaping.

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When is a spiral staircase required on a storage tank?

If the tank is more than 4m tall.

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What determines whether a roof is self-supporting or supported? What quantities are used for each?

Roof pitch slope must be sufficiently steep to support itself. A 1:5 pitch can be self-supported. A 1:12 pitch uses a roof support

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Name 10 features on a fixed roof storage tank.

1) Roof manhole
2) Shell manhole
3) Dip-tube (sample)
4) Pressure/vacuum vent
5) Staircase
6) Gauge float/liquid level indicator
7) Inlet nozzle
8) Outlet nozzle
9) Railing (if > 4m tall)
10) Fixed roof

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What is the purpose of the pressure/vacuum vent? What vacuum does it operate at?

It prevents build up of vapour & relieves internal pressure build up. It operates at a slight vacuum of 0.19kPa.

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What is the conversion for barrel (bbl) to m3?

1 bbl = 0.1637 m3

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How do you calculate volume in a cylindrical tank?

V = (pi x D^2)/4 x L

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When are short, fat tanks & tall, thin tanks used?

Short, fat = used in windy or seismically active areas & where soil bearing strength is low.
Tall, thin = ground space available small & soil bearing strength higher

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What is the wall & bottom surface area (SA) of a cylinder?

SA wall = pi x D x L

SA bottom = (pi x D^2)/4

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What is the surface area (SA) of a conical roof?

SA = (pi x D)/2 x s

where s = slope.

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Derive the formula for the slope of a conical roof assuming a self supporting roof?

Self-supporting r:h = 1:5.
s^2 = h^2 + r^2
s = sqrt [(13D^2)/50]

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Derive the formula for the slope of a conical roof assuming a supported roof?

Supported r:h = 1:12
s^2 = h^2 + r^2
s = sqrt [(145D^2)/576]

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What L/D ratio does the minimum SA occur for a flat roof?

L/D = 1

What L/D ratio does the minimum SA occur for a self-supporting conical roof?

L/D = 1.01

List 8 features on a external floating roof tank.

1) Rim-seal
2) Access hatch
3) Open top
4) Deck leg (pontoon & centre)
5) Solid guidepole
6) Vacuum breaker
7) Rim vent
8) Overflow drain

What is the purpose of a solid guidepole on an external floating roof tank? How many are usually required?

Allows floating roof to move up & down without spinning, reducing build up of static which could cause an explosion. Only 1 is usually needed.

What is the purpose of the deck legs?

Holds up the floating roof allowing for maintenance so it doesn’t slam down.

What is the purpose of the rim seal? How is it different in an external & internal floating roof?

Stops vapour escaping between joints while allowing roof to move up & down. External roof uses rim-seal around edge of floating roof to prevent rain water entering. Internal roof uses around edge of floating roof, fixed roof & roof columns.

What are the 3 types of floating roof?

1) Single deck
2) Double deck
3) Single deck with central deck buoyancy compartments

List 8 features of an internal floating roof storage tank.

1) Rim-seal 2) Vacuum breaker 3) Deck legs 4) Access hatch 5) Fixed roof support column(s) 6) Fixed roof centre vent 7) Peripheral roof vents 8) Gauge float

How can the fixed & floating roof be accounted for in tank volume?

Effective tank volume should be reduced by adding 0.3m to the L/D ratio to allow space for both roofs when volume is full.

What are the 3 main types of external floating roof seals?

1) Mechanical 2) Liquid filled 3) Vapour/liquid mounted resilient foam-filled seals

How much of a safety margin does the design liquid level provide?

Allow 10% ullage to tank volume. | V (tank) + 10% ullage = V (total)

How much space should be left be in the non-working capacity between the tank bottom & outlet nozzle?

10 inches.

How do you calculate the required height of the bund wall?

V (tank) + 10% ullage = V (total) = V (bund) | V (bund) = h x L^2

What are variable space vapour tanks used for? Give 2 examples & describe how they work.

Used for storing vapours in an expandable vapour reservoir. eg; i) Lifter roof tanks - telescoping roof fits loosely around main tank wall closed by wet seal. ii) Flexible diaphragm tanks - flexible membrane provides expandable volume.

What is the capacity for a horizontal tank?

75,000 L

What is the L/D range for a horizontal tank?

4 < L/D < 6

List 4 types of head that can be used on horizontal tanks. What is the benefit to using these types compared to flat head?

1) Dished 2) Torispherical 3) Ellipsoidal 4) Hemispherical No sharp edges or weak points allows for stronger construction

List 5 features of horizontal tanks.

1) Pressure/vacuum vents 2) Sample/dip tube 3) Manhole 4) Gauge hatch 5) Backfill & Concrete pad (if underground)

What are pressure tanks used for? What counts as high pressure & what shape vessel is most suitable?

Used for storing organic liquids & gases with high vapour pressure. High pressure > 1 bar - spherical tanks are most suitable as there are no sharp edges so no weaknesses.

How is a pressure/vacuum vent set in a pressure tank?

Set to a point to prevent loss from boiling & breathing from daily temperature & pressure changes.

What does the HAZChem label include?

Fire fight code, substance number & telephone number.

What are the 4 key risks that need managing with storage tanks? Give an example for each.

1) Containment - eg. flammable liquids 2) Separation - eg. recommended distance 3) Ventilation - eg. vapours disperse quickly 4) Ignition - eg. sources controlled

What do each of the zones mean when managing storage tank risk?

Zone 0 - explosive gas mixture continuously present. Zone 1 - explosive gas mixture likely present in normal operation. Zone 2 - explosive gas mixture not likely to occur in normal operation.

How big is a manhole?

2ft wide.

What is the surface area (SA) of a domed/hemispherical head?

SA = (pi x D^2)/2

What L/D ratio does the minimum surface area (SA) occur for a domed/hemispherical head?

L/D = 1.5

What L/D ratio does the minimum surface area (SA) occur for a supported (1:12) conical roof?

L/D = 1.002