Stress Analysis of storage Tank piping

Jeba Anand Nadar

• Introduction• Classification of Tank (fluid, construction type)• Dyke wall• Tank settlement• Bulging of Tank • Tank behavior• Modeling of storage tank in Caesar • API 650 calc data for Analysis• Nozzle check as per API 650• Routing of line connected to tank

Contents

INTRODUCTIONThe storage tanks are vessels that holds liquids, compressed gas, used for the short or long term storage and transportation of different liquid and semi fluid products independently on their form.Based on position & location, they can be vertical and horizontal, aboveground, semi underground and underground, to carry static and dynamic loads, to work under vacuum and over pressure, upon the wind, seismic and temperature influences.The biggest part from this group engineering facility which are widely used in almost all industrial sectors are the Aboveground Steel Vertical Tanks, which are the sheet constructions with cylindrical form.

1. Based on process Fluid Type

Crude oil and its derivatives are potentially hazardous materials. The degree of the hazard is determined essentially by volatility and flash point.The OISD has specified the following classes:

Class A - Liquids which have flash points below 23 ⁰CClass B - Liquids which have flash points 23 ⁰C and above but below 65 CClass C - Liquids which have flash points 65 ⁰C and above but below 93 ⁰C

Excluded petroleum - Liquids which have flash points 93 ⁰C and above

CLASSIFICATION OF TANK

CLASSIFICATION OF TANK

2. Based on Tank Construction

For safe storage of petroleum products, we have to consider the flash point, internal pressure, properties of product, volatility, pour point and others before designing and constructing. Storage tank comes in many shapes and sizes depending on the product to be stored, potential for fire and amount of storage.

Open Top tanks

This type of tanks are used for storing city water, fire water and cooling water.

Open Top Tank

CLASSIFICATION OF TANKS

FIXED ROOF TANKSThese types are divided into two types -Cone roof -Dome roof

This is one of the most common type of storage tanks and are provided with self supporting roof structures either cone or Doom type roofs.

Mainly for storing low pressure fluidsMost common type used for storing countless products, including petroleum, chemicals, petrochemicals, food products and water

CLASSIFICATION OF TANKS

Fixed roof tank with Floating Covers ( Internal Floating roof type)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. Typical feedstock stored are : DPK ( Kerosene, Jet A1)

Floating Roof TankThese tanks are designed to work at atmospheric pressure. Roof of this tank rises and lowers with the stored contents, there by reducing vapor loss and minimizing the fire hazard. Typical Products stored are Crude Oil, gasoline, gasoline components , Solvents.

Horizontal Pressure Tank ( Bullet)

The very volatility and high vapor pressure product such as Iso pentane, LPG & Butane will be stored in a

vessel that will withstand product pressure and prevent any product vaporization. Due to high pressure

the vessels are usually mounted underground to prevent thermal radiation in case of fire nearby.

Horton sphere Pressure Tank

Used to store large quantities of liquids and gases under pressure

CLASSIFICATION OF TANKS

DYKE WALL

Due to risk of failure of storage tanks and primary piping systems, means must be provided to contain the spills. The containment for petroleum storage tanks is in the form of Dyked enclosures.

Dykes may be constructed of earth, steel, concrete or solid masonry. They may be square, rectangular, circular or irregular in shape depending on the natural terrain around them.

Lines penetrating concrete dike walls may be secured against leakage by commonly available seals.

The figure below shows the Seal with a minor line movement

Proper pipe supporting to be provided on either side of the Dyke wall penetration to prevent load on the Dike wall.

DYKE WALL

TANK SETTLEMENT

Settlement in tanks will be in two stages

Settlement during Hydro Test

Long term Settlement

1. Settlement during Hydro Test

This is a one time settlement and usually caused due to the Hydro load during Hydro-testing before

the Pre-commissioning stage.

The Tank is Hydro tested separately before connecting the Lines. Hence this settlement is not

considered in our Analysis

2. Long Term settlement

This settlement which takes place during the entire life cycle of the tank. Most settlement occurs

during hydro testing of the tank. Gradual settlement continue for years since it's a slow process and

it may take more years to reach full settlement. The tank settlement amount depends on soil

characteristics-primarily, consolidation and compressibility.

For piping stress analysis only long term settlement is considered.

TANK SETTLEMENT

Hydro testing of tank Piling provided for foundation Settlement

Differential settlement at the base of a tank Tilted tanks due to uneven settlement

BULGING OF TANK

Tank bulging-radial growth occurs on the shell due to product static head for large diameter tanks. Bulge formations appear because a static head may cause circumferential and longitudinal strains.

For large-diameter storage tanks( above 36” dia ) with heavy liquid, tank bulging occurs since there is a slight growth of the tank shell in radial direction. When this radial-shell growth occurs at the nozzle location, the nozzle is rotated slightly. Even the smallest nozzle rotation will cause the associated piping to either lift-off from the first support from the tank foundation or excessively compress the pipe at the first support from the tank foundation. This happens even if the vertical displacement of the tank nozzle is in a downward direction. This condition is assumed when the first support is a rigid support from the tank foundation extension.

TANK BEHAVIOUR

TANK SETTLEMENT

THERMAL GROWTH OF TANK

TANK BULGING DUE TO LIQUID HEAD

THERMAL GROWTH OF CONNECTING PIPE

TANK BEHAVIOUR

Bulging of Tank because of pressure head

Tank GA DWG

MODELLING OF STORAGE TANK IN CAESAR

Modeling the storage considering settlement and Bulging effect:-

1. Input the design and operating Temp & Pressure

2. Model the nozzle flange 10-20.

3. Model the projection as pipe element 20-30 with same pipe dia & schedule given in the drawing for the

nozzle up to nozzle pipe and Tank intersection.

4. Define API 650 Nozzle at element 20-30 i.e. Check mark the Nozzle option and select the type as API

650.

5. Fill up the details of the tank in API Nozzle spreadsheet from the Data Sheet provided by the vendor.

6. Fill up the details as nozzle node as 30 (intersection of tank and nozzle) and tank node as 40.

7. Start the next element 40-50 length equal to the radius of the tank with dia & schedule of the tank.

8. Model 40-50 as weightless rigid element with dia & schedule of the tank.

9. Model 50-60 vertically length equal to the elevation of the nozzle up to bottom of the tank as rigid with

no weight and dia and schedule that of the tank.

10. Define the Anchor at 60 with C Node 61.

11. Define the displacement at node 61 DY equal to the Tank settlement in mm and input 0 in all other fields

DX, DZ, RX, RY, RZ.

MODELLING OF STORAGE TANK IN CAESAR

12. For the Tank Bulging effect and radial expansion of tank, define the displacement at node 40 along the

Nozzle axis DZ in mm and rotations RX in degrees and input 0 in all other fields DX, DY, RY, RZ IN

VECTOR 2.

13. Check the Nozzle loads at node 30 against the allowable given by the vendor from the datasheet.

API 650 CALCULATIONS DATA FOR ANALYSIS

Input data Tank Diameter Nozzle Height above bottom Plate Nozzle Outer Diameter Shell Course Thickness Max Fluid Height above bottom plate delta Temperature Modulus of Elasticity Expansion Coefficient Fluid Specific Gravity Reinforcement on Shell or Nozzle

API 650 CALCULATIONS DATA FOR ANALYSIS

Displacement for Settlement of Tank (vector1) Displacement for Bulging & expansion tank (vector 2)

LOAD CASE FOR TANK IN CAESAR

WHERE D1= DISPLACEMENT DUE TO SETTLEMENT D2= DISPLACEMENT DUE TO BULGING EFFECT

NOMENCULATURE OF APPENDIX-P

TANK NOZZLE DISPLACEMENT

NOZZLE CHECK AS PER API 650

For external piping connected to tanks can be used for Tanks size greater than 36m (120 ft) in diameter.

The purchaser can decide for the Tank size 36” and smaller as per API 650 design

The recommendations of this appendix represent accepted practice for the design of shell openings in the lower

half of the bottom shell course that have a minimum elevation from the tank bottom and meet the requirements of

Table 5-6.

SHELL DEFLECTION AND ROTATION 1 Radial Growth of Shell The unrestrained outward radial growth of the shell at the centre of the opening connection resulting from product head and/or thermal expansion shall be determined as follows:

2 Rotation of ShellThe unrestrained rotation of the shell at the centre of the nozzle-shell connection resulting from product head shall be determined as follows:

The allowable Nozzle load can be calculated as per Appendix P- (p.2.7) or the allowable loads can be used which will be furnished by Vendor/ Mechanical Static.

NOZZLE CHECK AS PER API 650

NOZZLE CHECK AS PER API 650

Group of Tanks

ROUTING OF LINES CONNECTED TO TANK

TANK LINE BEHAVIOUR

SUPPORTING OF LINES CONNECTED TO TANK

SUPPORTING OF LINES CONNECTED TO TANK

SUPPORTING OF LINES CONNECTED TO TANK

Note:- 1. In desert conditions Hanger type springs is used to prevent malfunctioning of Spring due to the

accumulation of sand particles

2. for Large diameter tanks, tank bulging plays a important role in nozzle loading and support

selection . Spring support can be advised .

Thank You !!

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