TANKS Purpose;
a) Feedstock
b) Finished products prior to shipping out to customers and Unfinished petroleum components awaiting for further
c) processing & blending
VERTICAL TANKS
- open top,
- floating roof (Internal and External),
- fixed roof (cone and dome)
1) Atm storage/ Open Top
- Apllied at or near atm pressure/Open TOP
- Fluids that will not evaporate and damage the enviromenet or people
- Type of tank that will not vapouize at ampient temp.
- This type of tank has no roof and shall be used for storing city water, fire water and cooling water
One of the common types found in mining areas is the open roof type tank, usually to store ore slurries. These are the easiest storage tanks to build
2) Fixed Roof – Dome and Roof
- store large volumes of liquid with little or no pressure
- In a fixed roof tank a floating cover can be installed to give a further reduction of vapor losses. These tanks are fitted with breather vents either at the top course of the shell plate or on the roof edge.
- Kerosene, Jet
3) Floating Roof – Internal and External
- Floating tanks are designed to work at atmospheric pressure.
- Low to medium flash point fuels stored
- This type of tank roof minimises the vapour space between it and the liquid surface. Since there is no large vapour space for the liquid to evaporate into, vapour losses are also minimised.
- These tanks are nothing but cone roof tanks with a floating roof inside which travels up and down along with the liquid level.
- naphtha, kerosene, diesel, Gasoline, Solvents, crude oil
4) Pressurized storage/ Mounted vessels
- Vessels designed to withstand pressure sufficient to keep liquid storged, from vapouizing
- The very volatility & high vapor pressure product such as
- Thee vessels are Mounded underground to prevent thermal radiation in case of any fire nearby.
- High pressure HCs – pROPANE, BUTANE, iC5, LPG
SPHERICAL (sphere) - Very high pressure liquid (15 psig and more)
- So they are very strong
- The even distribution of stresses on the sphere’s surfaces, both internally and externally, generally means that there are no weak points.
- As flash-points of fuels go very low the tanks are usually spherical
- An advantage of spherical storage vessels is, that they have a smaller surface area per unit volume than any other shape of vessel. This means, that the quantity of heat transferred from warmer surroundings to the liquid in the sphere, will be less than that for cylindrical or rectangular storage vessels.
- LPG, hydrogen, hexane, nitrogen, oxygen etc.
Cylindrical Pressure Vessel - Cylinders are widely used for storage due to their being less expensive to produce than spheres.
- Very high pressure liquid (15 psig and more)
- However, cylinders are not as strong as spheres due to the weak point at each end. This weakness is reduced by hemispherical or rounded ends being fitted. If the whole cylinder is manufactured from thicker material than a comparable spherical vessel of similar capacity, storage pressure can be similar to that of a sphere.
- Pressure Vessel Heads
- Ellipsoidal Head, Hemispherical Head and Torispherical Head are three types of ASME Pressure Vessel Dished Heads.
GAS CYLINDERS
- To store small quantities of gas at very high pressure, we use gas cylinders.
- These vessels however, are very heavy due to the thickness of the metal of construction.
- When a gas cylinder is in use, a special head is connected which incorporates a regulator with a needle valve for fine adjustment and pressure gauges indicating the pressure upstream and downstream of the regulator.
TYPES OF PRESSURES – FOR THINKING
Atmosphere =
Kpaa = kpa +kpag
Tires pressure = 30 psig (440 kpa)+101 = 540 kpaa
Pressure injected into SAGD =10 psia
Atmospheric pressure at sea level = 101 kpa (14.69 psia)
Calgary atm pressure = 93 kpa (since it is at a higher level than sea level)
The diagram shows all the main features that MAY be found on a storage tank. For example, some tanks may be open and will not need a relief valve. Some will be shallow and will not need an access manhole …etc.
a) Roof Access Ladder - As its name implies, is used for safe access to the tank roof.
b) Access Manholes -Usually fitted at ground level or on the roof for access to the inside of the tank for cleaning, maintenance and repair.
c) Water Drain - For use where water separation occurs in the tank and is to be drained off. Also useful during internal cleaning operations.
d) Transfer Pumps - These are used to transfer some or all of the tank contents to a process or to other storage tanks, ships, vehicles … etc. Depending on their piping arrangement, the pumps may be used for re-circulation (mixing) of the tank contents or pumping liquid into the tank from another source.
e) Bund Walls (or Firewalls) - These can be of two types :
A wall surrounding the tank, high enough to contain the entire tank contents in the event of a burst or severe leak. In this way, flammable, toxic or corrosive substances can be contained safely until recovered.
An alternative to the above, is having the tank situated in a cylindrical hole in the ground, again large enough to contain all of the tank contents in the event of burst or leakage.
f) Relief Valves - Installed where a tank contains pressure and set to relieve excess pressure if it rises to the safe operating limit.
g) Inert Gas Blanket - This facility is installed when a tank contains volatile liquid which may produce vapours that become flammable or explosive when mixed with air. The inert gas injection (often Nitrogen), replaces the vapour above the liquid and will not react chemically nor produce an explosive mixture with the tank’s contents.
h) Foam Injection -In emergency, foam can be sprayed into the tank and over the surface of the liquid. Hazards arising from toxic or flammable vapours can be minimised.
i) Vapour Vent - (Often called the ‘Breather Valve’) - This allows the tank to ‘breathe’ when emptying or filling. If a vent is not fitted, the tank could over-pressure when filling and cause a rupture in the tank and, when emptying, a vacuum would be pulled possibly causing the tank to collapse (implode). (The vapour vent can consist of simply an open hatch where the liquid being stored has no hazards).
.j) For hazardous materials, the vapour vent may consist of a two-way safety valve. This will operate at a rising, pre-set pressure to vent excess gas from the tank to atmosphere or flare system to prevent rupture of the tank. At a falling pre-set pressure, the valve will operate to admit air, gas or inert gas (called ‘Blanket Gas’), into the tank to prevent collapse of the tank if the pressure falls to a vacuum. (Gas or inert gas would be used where ingress of air is undesirable).
k) Sampling & Gauging Hatch - As the name implies, this is installed in order to obtain samples of the tank’s contents for analysis and is also used for dropping a ’ dip-tape ’ or ’ dip-stick ’ into the tank to check the liquid level and also check the automatic level measuring instruments.
l) Earth Connections -An ’ Earthing-strip ’ is connected to tanks (and other equipment) in order to carry away and prevent build up of ‘Static Electricity’ which tends to form during filling and emptying operations. If static was allowed to build up where flammable liquids are being stored, then we would have a potential fire or explosion hazard. The storage of large volumes of liquid can lead to problems arising from the daily and seasonal variations in the weather. The most important variable, is that of the ambient temperature.