20-11-2012, 06:30 PM
Fire protection system in tank farms
Fire protection system.doc (Size: 105 KB / Downloads: 50)
STORAGE TANKS:
Storage tanks are an industrial facility for the storage of oil and or petrochemical products & from which these products are usually transported to end users or further storage facilities. There is tag number provided to each tank in the refinery such as 252,253 etc so that in case of fire, valve corresponding to that tank is opened to fight the fire.
On each tank its capacity, type of material stored, tag no. , etc are boldly mentioned on its shell. Such as Tag no. : 253
Material stored: crude oil
Capacity: 3000 cubic meter
Since most liquid can spill, evaporate, or seep through even the smallest opening, special consideration must be made for their safe and secure handling. This usually involves building a binding, or containment dyke, around the tank, so that any leakage may be safely contained. The capacity of dyke is 110% of the capacity of single tank. This dyke is provided so that in case of failure of tank it prevents oil wastage.
TYPES OF TANKS:
There are two kinds of oil storage tanks, they are floating roof type and fixed roof type tank. Tanks are chosen according to the flash point of that substance.
1) Fixed roof tanks: These types of tanks are meant for liquids with very high flash points (eg. Fuel oil, water, bitumen). Cone roofs, dome roofs and umbrella roofs are usual. These are insulated to prevent clogging of certain materials, wherein heat is provided by steam coils within the tanks. Dome roof tanks are meant for having slightly higher storage pressure than that of atmosphere.
2) Floating roof tanks: Floating roof tanks are broadly divided into external floating roof tanks called (FR tanks) & internal floating roof tanks (IFR).IFR tanks are used for liquids with low flash points ( eg. ATS, MS, gasoline). These tanks are nothing but cone roof tanks with a floating roof inside which travels up and down along with the liquid level. This floating roofs are supported with legs which they rest.FR tanks do not have a fixed roof and has a floating roof only. External floating roof tank is a storage tank commonly used to store large quantities of volatile petroleum products with medium flash point such as crude oil or gasoline (petrol). It comprises an open-topped cylindrical steel shell equipped with a roof that floats on the surface of the stored liquid. The roof rises and falls with the liquid level in the tank. An opposed to a fixed tank there is no vapour space in the floating. In principle, this eliminates breathing losses and greatly reduces the evaporative loss of the stored liquid. There is rim seal system between the tank shell and roof to reduce rim evaporation.
The roof has support legs hanging down into the liquid. At low liquid levels the roof eventually lands and a vapour space forms between the liquid surface and the roof, similar to a fixed roof tank. The support legs are usually retractable to increase the working volume of the tank.
Advantages: It is installed for environmental or economical reasons to limit product loss and reduce the emission of volatile organic compound, an air pollutant.
Disadvantages: Rain water & snow can accumulate on the roof, eventually the roof may sink. Water on the roof is usually drained from a flexible hose that runs from a drain-sump on the roof, through the stored liquid to a drain valve on the shell at the base of the tank. The hose often develops leaks and drains both water and product.
CONSTRUCTION MATERIAL:
While steel remains one of the most popular choices for tanks, GRP and thermoplastic, polyethene & fiberglass tanks are increasing popularity. They offer low build costs & greater chemical resistance. There are various relevant standards,
such as British Standards,4994(1989) which give advice on wall thickness, quality control procedure, testing procedure, fabrication & design criteria of final product.
DESIGN CRITERIA:
The following shall be the basic design criteria for a fire protection system:
i) Facilities should be designed on the basis that city fire water supply is not available close to the installation.
ii) Fire protection facilities shall be designed to fight two major fires simultaneously anywhere in the installation.
Fire water requirements will be decided as per guidelines given in calculations.
iii) All the tank farms and other areas of installation where hydrocarbons are handled shall be fully covered by hydrant System.
iv) Class 'A' petroleum storage in above ground tanks shall have fixed water spray system, whether floating roof or fixed roof.
v) Class 'B' petroleum storage tanks of following dimensions shall be provided with fixed water spray.
- Floating roof tanks of diameter larger than 30 mtrs.
- Fixed roof tanks of diameter larger than 20 mtrs
vi) A. Semi-fixed foam system shall be provided for the following tanks.
Floating roof tanks storing Class 'A' and Class 'B' petroleum products.
- Fixed roof tanks storing Class 'A' and class 'B' petroleum products.
- Fixed roof tanks storing class 'C' petroleum products, of diameter larger than 40 mtrs.
B. Automatic Actuated Foam Flooding Proven System may be provided:
- On existing floating roof tanks where Semi-fixed foam system could not be provided.
- On floating roof tank larger than 60 mtrs diameter (as an alternative to halon/its proven equivalent system)