Fixed, Cone Roof Tanks

Fixed (cone, dome or umbrella) roof tanks are the most common and identifiable bulk storage vessels in the oil & gas industry, typically seen with a wrap around staircase. They range in sizes up to 30 meters tall by 100 meters wide and are used to store liquids with very high flash points (e.g. fuel oil, heavy oil, kerosene, diesel oil, water, bitumen, etc.). The addition of a dome roof reduces environmental emissions and provides additional strength to allow slightly higher storage pressures than that of atmosphere. Float and tape tank gauges can be installed ‘at grade’ on the tank-side or on the tank roof. Servo, radar and other gauging technologies are installed on the tank roof. When installed on the tank roof, a gauge is mounted on a flange that is either permanently affixed to the tank roof or integrated into a manhole cover.

Punto de inflamabilidad

El punto de inflamabilidad es el conjunto de condiciones de entorno en que una sustancia combustible inflamable, está en condiciones de iniciar una combustión si se le aplica una fuente de calor a suficiente temperatura, llegando al punto de ignición. La diferencia entre punto de inflamabilidad y punto de ignición, es que en el primero, el combustible está en condiciones de inflamarse, pero le falta el calor de ignición. Una vez retirada la fuente de calor externa pueden ocurrir dos cosas: que se mantenga la combustión iniciada, o que se apague el fuego por si solo.

Si se consideran unas condiciones normales de presión (presión atmosférica normal de 101,3 kPa), esas condiciones se reducen a una temperatura mínima y una proporción determinada de vapor de combustible en el aire ambiente, que puede darse en una pequeña parte del mismo.

Son importantes tanto la temperatura como la mezcla. De hecho la temperatura puede ser relativamente baja, la mayoría de las veces inferior a las normales en el ambiente, pero a esa temperatura los combustibles líquidos empiezan a desprender vapores que, al mezclarse con el oxígeno del aire u otro comburente, pueden dar las condiciones, para que cualquier chispa que alcance la temperatura de ignición necesaria, inicie el fuego. Entre estas condiciones es fundamental la proporción de los gases con el aire y, tanto si la proporción de gases es escasa, como si es excesiva, no se producirá la ignición.

La diferencia con el punto de ignición es que en ese caso se ha producido ya la inflamación, es decir, se ha aplicado el calor de ignición.

Para medir el punto de inflamabilidad se usa el aparato de Pensky-Martens.

Temperatura de inflamabilidad de algunas sustancias:

Combustible temperatura

Alcohol etílico 12 °C / 53.6 °F

Alcohol metílico 11 °C / 51.8 °F

Alcohol butílico 38 °C / 36.4 °F

Gasolina -40 °C / -45.4 °F

Nafta de petróleo -2 °C / 28.4 °F

Queroseno 38 °C a 72 °C / 100.4 °F a 161.6 °F

Gasóleo 52 °C a 96 °C / 125.6 °F a 204.8 °F

Benceno 20 °C / 68.0 °F

Hexano -28 °C / -18.4°F

Tolueno 9 °C / 48.2 °F

Furfural 62 °C / 143.6 °F

The flash point of a chemical is the lowest temperature where enough fluid can evaporate to form a combustible concentration of gas.

The flash point is an indication of how easy a chemical may burn. Materials with higher flash points are less flammable or hazardous than chemicals with lower flash points.

burning flames

Some fuels and their flash points at atmospheric pressure are indicated below:

Fuel Flash Point

(Farenhait)

Acetaldehyde -36

Acetone 0

Benzene 12

Biodiesel 266

Carbon Disulfide -22

Diesel Fuel (1-D) 100

Diesel Fuel (2-D) 126

Diesel Fuel (4-D) 130

Ethyl Alcohol, Ethanol 63

Fuels Oil No.1 100 - 162

Fuels Oil No.2 126 - 204

Fuels Oil No.4 142 - 240

Fuels Oil No.5 Lite 156 - 336

Fuels Oil No.5 Heavy 160 - 250

Fuels Oil No.6 150

Gasoline -45

Gear oil375 - 580

Iso-Butane -117

Iso-Pentane less than -60

Iso-Octane 10

Jet fuel (A/A-1) 100 - 150

Kerosene 100 - 162

Methyl Alcohol 52

Motor oil 420 - 485

n-Butane -76

n-Pentane less than -40

n-Hexane -7

n-Heptane 25

n-Octane 56

Naphthalene 174

NeoHexane -54

Propane -156

Styrene90

Toluene 40

Xylene 63

Floating roof tanks (as compared to fixed roofs) – Pros and Cons

Floating roof tanks are advantageous, compared to fixed roof tanks, as it prevents vapour emissions (that are highly combustible) that help eliminate the chances of fire or an internal tank explosion. They are usually used for stable liquids (with no dynamic loads acting, as discussed later). However, adverse environmental conditions could affect floating roofs as accumulation of snow and rain water could result in roof submersing in the stored liquid. Nonetheless this static load can be incorporated in making assumptions on the response of the roof, which can be further used in the design of the tank with a significant factor of safety. An area of concern, although, is the dynamic loads that act upon the roof due to constant splashing of water or leaks that result in flooding of roof compartments. This could be partly corrected by having sufficient stiffness in the circumferential direction at the roof, but due to the irregular nature of such loads, it may not be possible to accurately predict its magnitude.

Also, while the liquid exits the tank, the floating roof steadily approaches the bottom leaving behind a wet shell (liquid droplets that are left behind as the level drops). This results in the evaporation of liquid droplets to the atmosphere and is termed as the withdrawal loss, a form of emissions similar to hydrocarbon leaving the fixed roof tanks. A flawless rim seal (closure between the roof and the shell) could impede the loss of liquid but most seals have a loss factor associated to them that is calculated based on tank diameter and wind blowing over the tank.

Another alternative to external floating roofs is an internal roof that combines the concept of conical fixed roof tanks that lie on top of pontoons. They too are affected by the withdrawal and storage losses that are mitigated using similar means. Most recently, engineers have been designing floating roof tanks with secondary seals to mitigate such emissions and prevent any seal friction caused by using tighter seals (a simpler solution to prevent any vapours to exist)

Fixed roof tanks storage

During the process of storing crude oil, light hydrocarbons such as natural gas liquids, volatile organic compounds, hazardous air pollutants and some inert gases, vaporize and collect between the liquid level and the fixed roof tanks. As the liquid level in the tank varies, these gases slowly release out to the atmosphere. A solution to prevent this from occurring is by installing vapour recovery units. These units capture the BTU-rich units for sale or use it onsite as fuel.

Another solution could be the use of foam chambers. These are designed to cover flammable hydrocarbon or water miscible liquids with low expansion foam or fire extinguishment or vapour suppression. The foam occupies the vacant space that was initially filled with air, one of the main sources of combustion, to prevent any potential hazards. They have the advantage over ground based monitors of directing all their foam directly onto the flammable liquid surface regardless of weather conditions.

The foam generator made foam by introducing air into a foam solution stream that was delivered to the top pourer system (TPS) in a variety of ways. The inlet of the TPS is fitted with a venturi jet designed to draw air into the stream through a series of holes located around the foam generator. The foam solution is obtained from mobile foam proportioning equipment located far away from the tank and routed back to it through pipelines.

The governing rules for the placement, construction, materials, inspection and fireprotection for diesel fuel and the storage of almost all other similar liquids is the NFPA Standard #30

Capacidad de tanque

There are no standards regarding "spare capacity" they are completely dependent on local needs, commercial strategy, company policy etc., etc.

Some of the things we consider is:

01. Most important, and most frequently overlooked factor for reliable system operation: settling time at least 24 h, preferred 48/72 h - ALWAYS USE FLOATING SUCTION.

02. Volume loss due to water condensation (bottom drains): typically the bottom 30 in (760 mm) of the tank are unusable. May be higher in high humidity locations (above 70%)

03. Is tank heating needed due to ambient conditions? add another 30 in (760 mm) on top of the water condensation allowance

04. Cycle frequency (start/stops) vs. tank refill opportunity: if the cycle is operated once a week and the fuel is brought in by barge/truck once a month: need fuel enough for 4 starts/stops with some additional capacity.

05. Operational experience: how frequent are the trips? how frequent are failed starts? Need to add to the start frequency

06. Light diesel oil might be needed for shutdown as well to avoid having all the fuel passages blocked after cool down. Need to know the normal shutdown sequence as well

07. Refill supply: how often and how secure is it? Commercial decision: how many days of normal operation must be guaranteed without primary fuel?

08. How critical is the service? are the penalties associated with not running or not being able to start higher than the extra fuel storage capacity? What if there is a problem with the tank (e.g. suction is damaged) what are the project requirements for redundancy of critical equipment?

09. Is there a re-circulation line? might need an external cooling system

10. Are there any space (including spill basin volume) constraints?

11. The consideration of the factors above vs. the budget and customer specifications may dictate the need to supply two smaller tanks instead one big one. The added flexibility, availability, reliability and maintainability (FRAM) will exceed by far the situation with just one tank.

Note: settling time is so important that this factor combined with starting frequency alone may dictate the need for redundant tanks.

Fuel

Density - ρ - Specific Volume - v -

(kg/m3) (lb/ft3) (m3/1000 kg) (ft3

Anthracite 720 - 850 45 - 53 1.2 - 1.4

Bituminous coal 690 - 800 43 - 50 1.2 - 1.5

Butane (gas) 2.5

Charcoal, hard wood 149 9.3 6.7

Charcoal, soft wood 216 13.5 4.6

Coke 375 - 500 23.5 - 31 2.0 - 2.7

Diesel 1D 54.6

Diesel 2D 53

Diesel 4D 59.9

Gas oil 835 52 1.2

Gasoline 44.9

Fuel Oil No.1 54.6

Fuel Oil No.2 57.4

Heavy fuel oil 930 58 1.1

Kerosene 790 49.9 1.3

Fuel

Density - ρ - Specific Volume - v -

(kg/m3) (lb/ft3) (m3/1000 kg) (ft3

Natural gas (gas) 0.7 - 0.9

Peat 310 - 400 19.5 - 25 2.5 - 3.2 90 - 115

Propane (gas) 1.7

Wood 360 - 385 22.5 - 24 2.5 - 2.8 90 - 100