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AN OVERVIEW OF API 650 STORAGE TANKS

Updated: Mar 31

INTRODUCTION


The API 650 is the American standard for welded flat-bottomed vertical storage tanks. This standard dictates tank design, manufacture, welding, inspection and installation requirements. The API 650 is widely used for tanks designed to withstand low internal pressures (2.5 PSI), for the storage of typical products such as crude oil, gasoline, chemicals and produced water. Advance tank has extensive experience with API 650 tanks ranging in diameter from 8’- 300’ for refineries, terminals, pipeline facilities.


INSTALLATION PROCEDURES

  • Tank Bunds

  • Foundations/ Ring Beams

  • Welding Procedures

  • Layout of Floor Plate

  • Strake Layout

  • Construction of Roof

  • Installation of Nozzle

  • Surface preparation


TANK BUNDS


A tank bund is an area or secondary containment system around storage tanks or storage drums used to hold flammable or toxic liquids. AS 1940 mandates bundled storage for hydrocarbon storage facilities. These are designed to hold 110% of the volume of the corresponding tank size. They are used as well to contain a spillage should any of the fittings become broken or loose. Although each tank must be independent and fully isolatable from the other tanks.


  • Bund Construction

Earthen bund fully lined with a composite Geo-Fabric, Welded HDPE liner, Geo Fabric Membrane installed on a select fill base. Bund walls with a 45 degree angle of repose covered with a concrete filled revetment mattress. Revetment mattress can either be painted or covered with a layer of concrete.

  • Bund liner is pneumatically leaked tested on the overlapping welded seam.



FOUNDATION/ RING BEAM


A ring beam element, a ring beam and a method of constructing a ring beam from a plurality of ring beam elements is disclosed. The ring beam can support a shell or wall of a tank. The ring beam element comprises a reinforced concrete body, preferably having a metal base plate anchored therein, and a plurality of tensile conduits extending circumferential there through. Ideally, the reinforced concrete body will have interlocking surfaces such that the ring beam elements interlock with one another when circumferential juxtaposed. Tensile members can be placed through the plurality of tensile conduits and tensioned and anchored by anchors to clamp a plurality of juxtaposed ring beam elements together. Ideally, the base plates of a plurality of the ring beam elements can be connected or welded together to form an annular ring of base plates. The ring beam elements can be mounted upon piles to provide level surfaces to the base plates so that the metal base plates can be readily connected together to form an annular ring beam.


WELDING PROCEDURES


API 650 section 9 requires all welding on ground storage tanks to be done in accordance with the manufactures Weld Procedure Specifications (WPS) and the supporting Procedure Qualification Record (PQR). Shield Metal Arc Welding (SMAW) or better known as stick (arc) welding was the leading welding process and still very common today for construction of ground storage tanks. The SMAW process is versatile and very well suited to the environment of field erected work. The radio graphic method is used on the shell butt weld joints, annular plate butt joints, and flush type connections with butt joints. Welding operators are tested and certified in accordance with the tank builders welding procedures. Welds on newly constructed above ground storage tanks are subjected to various types of inspection. Section 8 of API 650 provides criteria for determining the number of shots to be taken for each welding operator based upon the thickness, number of shell courses, number of T joints, and the linear feet of vertical and horizontal welds completed.

Weld procedure defined under ASME IX

  • FCAW/Machine Welded Horizontals

  • Vertical Down MMAW Verticals

  • MMAW butt weld with backing plate for fillet to butt weld transition as the floor approaches the shell.

  • MMAW Iron Powder for fillet Welds on floor, roof, wind girder, compensating plates.


LAYOUT OF FLOOR PLATE


Typically floor plate ranges in thickness from 6-10mm without the addition of corrosion allowance. FT&P generally utilize an offset over lapping plate arrangement. Alternatively, an annular plate arrangements often used which allows for thicker plate at the shell interface and simplifies floor replacement.


STRAKE LAYOUT


A section of a tank made by rolling a stainless steel sheet or length of coil into a cylinder and welding the longitudinal seam joint.During construction each strake is added to the tank by positioning the (relative flexible) rolled plate segments on the top edge of the strake below, while at the same time maintaining a fixed gap between vertical edges of adjacent plates. Once positioned, the vertical welds between plates are executed, and then the circumferential weld to the strake below. As, such the strake can be considered to be placed on the top of each other in a deformed (as dictated by the shape of the top edge of the strake below), but essentially stress- free manner. The structural response of the tank as a whole is also affected by a wind girder, which is moved up the tank in a sequence of temporary positions as the strakes are added, until it is fixed in its final position to the top strake of the tank. In its sequence of temporary locations, the wind girder not only provides stability to the tank shell, but it also used to assist in maintaining the cylindrical shape of the tank.


CONSTRUCTION OF ROOF


Fuel Tank & Pipe often build the roof on the ground and lift this on to the completed strakes. A fixed roof tank is a type of storage tank, used to store liquids, consisting of a cone or dome-shaped roof that is permanently affixed to a cylindrical shell. Older tanks, however, are often riveted or bolted, and are not vapor tight. A breather valve (pressure-vacuum valve), commonly installed on many fixed roof tanks, and allows the tank to operate at a slight internal pressure or vacuum. This valve prevents the release of vapors during very small changes in temperature, barometric pressure, or liquid level. Fixed roof tanks without breather valves will generally be freely vented thus the emissions from a fixed roof tank can be non-trivial.


Floating roof tank is widely used to store volatile oil, like gasoline, aviation kerosene, diesel oil, crude oil and other light oil. Floating roof is the core component of floating roof storage tank, which is divided into two types: "external floating roof" and "internal floating roof". Because the flexible floating roof deck can float up and down to close to the liquid level, gas space above the liquid level can be greatly reduced, thereby greatly reducing the evaporation loss of the stored oil. The external floating roof tank has a floating roof deck which is covered on the surface of oil, and floats up and down with the oil level. As there is almost no gas space between floating roof deck and oil level, the evaporation loss of stored oil can be greatly reduced. Internal floating roof tank is a floating roof storage tank which is covered with a fixed roof on the floating roof, which can prevent the tank itself from been suffered from wind and rain, guarantying the quality of stored oil under the floating roof.


INSTALLATION OF NOZZLE


API 653 section 9.8 "Addition or replacement of shell penetration" specifically section 9.8.6 describes a procedure for the installation of a nozzle larger than 2" NPS where the shell plate thickness is greater than 1/2".


The number of tank nozzles is dependent on the operation and fluid handled. The orientation of each nozzle and providing platforms shall be considered together. Normal practice is to collect body (shell) nozzles in one side of the tank and roof tanks in the same direction along with of roof platform. By this design, these nozzles and the valves, connected to each nozzle are easily accessible. Lesser platforms are required in such cases. Hence, the design is economic. However, there are some exceptions like the tank manhole. It is better to locate the manhole in a different direction to other nozzles, the reason being easier entrance and exit. Locate manholes close to dike access way, and far from pipe way, and also accessible from the ground then operators can enter and exists easily.


Sealed and Vented manholes to be provided at strategic points of the tank system when separate major groups of storage tanks, process blocks, and loading facilities are involved. The main process nozzles of a tank are input/output nozzles. The main point, where there is no other process requirement, is the minimization of pipe length (and so pressure drop). Nozzle elevation also affects orientation, by locating elevated input nozzle and output nozzle in the same orientation vapor, which is soluble during the falling of liquid, will make gas traps in piping or cavitations in pumps. For Storage tanks in hydrocarbon services, to permit periodic draw off of water which normally collects in the product, an API low type shell nozzle and a drain valve are normally provided at the bottom of the tank. A manual gauging-sampling well may be used where the stored product, is specified as a static accumulator. Gauge well shall be burr-free. If a manhole or a separate roof connection is used for gauging-sampling purposes it shall be located near roof support. Each tank has a minimum one safety valve and one breathing valves, which are very important for tank safety during its life, installed on top of the roof shall be accessible from a platform. Also valves of an inert gas line which is connected to the tank for pressure balance when considered. When on a tank, a mixer or a tube bundle heater is installed via a nozzle, orientation of it shall provide adequate area for removal and easy movement to the outside of dike. The most important point that shall be considered during the design of nozzle orientation for these systems is the number of nozzles and also this fact that nozzles shall be in the symmetrical arrangement to provide uniform coverage of foams.


SURFACE PREPARATION


In preparing any surface to be coated, all loose paint, dirt, grease, rust, scale, weld slag or any other extraneous material shall be removed and defects repaired, so as to obtain a clean, dry even surface. Sharp or rough welds shall be rounded and contoured. Sharp edges shall be rounded or chamfered. Abrasive shall be kept dry and clean. Compressed air supply quality check shall be done by blotter test at the start and midpoint of each 8 hour shift. Primer or paint shall be mixed in accordance with the paint manufacturer’s datasheet. Mixing of the paint ingredients shall be done in open containers in a ventilated area. All ingredients in any container shall be thoroughly mixed and shall be agitated by mechanically (manual agitation not allowed) frequently during application to keep the solids in suspension.


COMMISSIONING

  • Tank strapping

  • NDT

  • Hydro-testing

  • PV vent

  • Control philosophy

  • Instruments


TANK STRAPPING


Tank Strapping is a method of measuring the deviation of the tank from its cylindrical form. The method employs a laser and provides an accurate circumferential profile of the tank at different heights up the tank. Strapping data is then entered into the tank gauging system providing an accurate means of interpolating the tank volume.


NON DESTRUCTIVE ENGINEERING


Vacuum box test

Vacuum testing shall conduct as follow;

Vacuum testing is conveniently performed by means of a metal testing box, 6 inches wide x 30 inches long, with a glass window in the top. The open bottom is sealed against the tank surface by a sponge-rubber gasket. Suitable connections, valves, and gauges should be provided. Approximately 30 inches of the seam under test is brushed with a soap solution or linseed oil. The vacuum box is placed over the coated section of the seam, and a vacuum is applied to the box. Bubbles or foam produced by air sucked through the welded seam will indicate the presence of porosity in the seam. A vacuum can be drawn on the box by any convenient method, such as connection to a gasoline or diesel-motor intake manifold or to an air ejector or special vacuum pump. Vacuum box tested using a test pressure at least 3 psi gauge or as specified on company specification.


Radiography test Radio-graphic examination method shall be in accordance with the ASME section V, Article 2. Radio-graphic inspection is required for shell butt weld, annular-plate butt welds, and flush-type connection with butt weld. Inspection by radio-graphic method is not required for roof-plate or bottom-plat welds, for welds joining roof plates to top angle, the top angle to the shell plates, shell plates to bottom plates. Number and location of radiographic shall minimum as specified on API standard 650, Section 6. The radiographers shall be certified by the manufacturer meeting the requirement as outlined by ASNT Recommended Practice SNT-TC-1A and its supplement. The penetrometer image as result of radiography shall clearly enough for visual examination on a radiograph viewer. The acceptance criteria of radiographs result shall be judged as specified on ASME Section VIII Div. 1, par.UW-51 (b).

Other tests includes: Magnetic Particle Inspection (100% of joints), Dye Penetrant (100% of Joints), and Air Testing of compensating plates.


HYDRO TESTING

Hydro test depends on the availability of clean water. For diesel tanks hydro-testing subjects the tank to a structural load higher than the service load due to difference in SG of water and Diesel. Hydro Testing allows testing of tank settlement Often the previous methods are used in lieu of Hydro-testing (Vacuum Box, MPI, Dye Penetrant on every joint). On a 1.75Ml tank, the cost of clean water at a remote mine-site was $0.40 per litre. Pragmatism prevailed, and Hydro testing excluded. FT&P tender exclusions include provision of clean water and have experienced considerable algae growth on tank internal surfaces following hydro testing and resulting in significant additional costs arising out the cleaning the tank.



PV VENTS/ SAFETY VALVES


A vacuum condition can lead to catastrophic failure of a tank. It can be mitigated by the PV Valve. A plastic bag secured over a tank vent during routine maintenance led to tank failure in a UK refinery during a crash cooling event (rain storm). Pressure is maintained at 0 kPa gauge and vacuum as 1.3 kPa gauge. Vent sizing in accordance with AS1940 and sizing of vents will be undertaken by manufacturer.


CONTROL PHILOSOPHY

Checks

  • Tank Selection control, duty.

  • Tank Selection control, filling.

  • High Level Alarm

  • HH Alarm

  • Double redundancy of above.

  • Low Level Arm


INSTRUMENTS


Level meter

A level meter is a device continuously measures the volume of liquid, powder, or granulated materials. (This is also referred to continuous level meter, liquid meter, or powder meter.) Although a level switch can detect the presence of material at a certain point, a level meter outputs a reading of the total liquid volume in a tank, usually from 0 to 100%. So, a level meter can monitor and control the volume in greater detail than a level switch. The level meter usually outputs its reading as a small current of 4 to 20mA.


Level Switches

A level switch is a sensor that detects the presence of liquids, powder, or granulated materials at a specific location. For example, when a tank is supplied with liquid, it overflows. In order to prevent overflows, a level switch is installed near the top of the tank. When the liquid touches the level switch, it outputs a signal to keep the liquid from overflowing the tank. (To detect that a tank is filled with material is referred to full detection, or upper limit detection.) On the other hand, when the liquid level drops as the liquid is discharged, a level switch is used to prevent completely draining the tank. This is referred to as an empty detection or lower limit detection.



Universal Engineering Services has more than a decades experience in the pressure vessel design for clients in UAE, Oman, Qatar, Kuwait, Bahrain, Saudi Arabia, and many more countries all over the world. UES work to many ASME standards to design and validate pressure vessels, boiler, fittings and piping systems. We are also experts in Stress analysis of piping, structural design, ASME Joint review, Design of Storage tanks, code calculations, FEA/FEM, and spotless service on design management.



**The content of this article is taken from web open source. The blogs are intended only to give technical knowledge to young engineers. Any engineering calculators, technical equations, and write-ups are only for reference and educational purposes.

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