Storage tanks

Introduction

Steel containers are used for storing liquid or gas in a plant. This blog discusses the most common type of storage vessels, plot plan arrangements, safety considerations, spill containment, pumping, and general piping layout, Tank Settlement for Piping Stress Analysis, Tank Bulging effect or bulging effect of tank shells & stress analysis as per API 650 using CARSAR II.

Tanks

         A storage take is a container, usually for holding liquids, sometimes for compressed gases (gas tank). Since most liquids can spill, evaporate, or seep thorough even the smallest opening, special consideration must make for their safety and secure handling. This usually involves building a boundary, or containment dike, around the tank, so that any leakage may be safely contained.

         Some storage tanks need a floating roof in addition to or in lieu of the fixed roof and structure. This floating roof rises and falls with the liquid level inside the tank, thereby decreasing the vapor space above the liquid level. Floating roofs are considered a safety requirement as well as a pollution prevention measure for many industries including petroleum refining.

Type of Storage Tanks

          There are various storage tanks used for storage of raw material and for storage of final products etc. they are mainly fix roof tanks, floating roof tanks, horizontal bullets and sphere and the details are as follows.

A)  Fix Roof Tank

Fix roof tanks are used for liquids where tendency of evaporation is very less and very high flash point. (E.g. fuel oil, water, bitumen etc.) The tank design code is APL 650 and main part are bottom plate, shell and top roof which is conical in shape and used steel section for supporting roof called rafters. Nozzles data will be as per process requirement. Normally nozzles are inlet for tank feeding, outlet which normally connected to pump, drain and vent which are required at the time of maintenance, manholes for ease of maintenance, nozzles level indicator and some spare nozzles. Cone roofs, dome roof and umbrella roofs are usual. These are insulated to prevent the clogging of certain materials, wherein the heat is provided by steam coils within the tanks. Cone roof or fix roof tank is atmospheric tank used for very law pressure. Whereas Dome roof tanks are meant for tanks having slightly higher storage pressure than that of atmosphere (E.g. slop oil).

B)  Floating Roof Tank

Floating roof tank is used for high volatile fluids where evaporation rate is   very high. Roof will float above liquid like ship float above the water and the surface of the roof is touched above the liquid surface which will minimize vaporization of the fluid. There is no any special code for roof design, but shell design is as per API 650

Floating roof tanks are broadly divided into external floating roof tanks (usually called as floating roof tanks: FR tanks) and internal floating roof types (IFR tanks).

IFR tanks: IFR tanks are used for liquids with law flash-points (E.g. ATF, MS. gasoline, ethanol). 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 roof traps the vapor from low flash-point fuels. Floating roofs are supported with legs on which they rest.

FR tanks: FR tanks do not have a fixed roof (it is open in the top) and has a floating roof only. Medium flash point liquids such as naphtha, kerosene, diesel, crude oil etc. are stored in these tanks.

C)  Dome Roof Tank

Dome roof tank is one type of fix roof tanks instead of conical roof, dome roof (Dish type) roof is used which is stronger that conical roof and hence will hold slightly higher pressure than conical roof tank and used for petroleum industry for storage of very high amount of liquid.

 

D)  Horizontal Tanks /Horizontal Bullet       

Horizontal tank design is as per ASME sec Vlll div.I. horizontal tanks installation is very easy and can be installed above ground or if there is space problem then can install underground also. There type of tanks minimizes evaporation loss. Installation of instruments is very easy. Maintenance can be possible by giving platform and ladder or staircase. Mounting of the tank is on saddle.to take care of thermal expansion one saddle should be fix and another should be sliding. foundation height can be decided as per the operating requirement and piping requirement.

 

E)  Spherical Tanks

As flash-points of fuels go very law the tanks are usually spherical (known as spheres), for storage of LPG, hydrogen, hexane, nitrogen, oxygen etc. Spherical tanks are used for holding very high-pressure liquid or gases. Pressure holding capacity is very high because of equal pressure distribution on surface. construction cost of sphere is very high. Also weld points are more in spherical tank which decreases efficiency of the equipment’s. There are some reasons some countries /companies avoid using spherical tanks and horizontal tanks are preferred instead of spherical tanks.

 

Other classification which can be made for storage tanks are based upon their location in a refinery:

1.   COT- crude oil tankages

2.   PIT-product and intermediate storage tankages

3.   Dispatch- dispatch area tankages

4.   Utilities- tanks made in the power plant area, for storage water etc.

5.   OSBL tanks- the first 3 types come under out side battery limit tankages

6.   ISBL tanks- these are usually mini tanks which are found in the production units of a refinery (as neutralization tanks,) water tanks e

 

Typical Arrangement of Tank Piping.

1.   Determination of piping route

2.   Process piping should not cross any other pipes if possible.

3.   Formation of vent pockets

4.   Piping from and to the tanks should run through the shortest routes.

5.   A minimum distance of 500mm shall be secured between the dike and pipe support.

6.   Tank block valves shall be provided with platforms which can also be used as a walkway

7.   Valves shall be arranged by aligning the flange surfaces

8.   Space between two crossing pipes shall be at least 300 mm.

9.   Valve accessories should be considered in determining spaces between closely arranged valves   

Links For Reference

• Stress Analysis of Column Piping System – 01
• Stress Analysis of Column Piping System – 02
• Skirt Temperature/Expansion Calculation