29527771-Intro-to-Pressure-Vessel.pdf

7/27/2019 29527771-Intro-to-Pressure-Vessel.pdf

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INTRODUCTION TO PRESSURE VESSELPREPARED BY: MUHAMMAD RIDHWAN ABDUL RASID, MECHANICAL TRAINEE DATE: FEBRUARY 2010

Stresses

In two dimensions, the state of stress at a point is conveniently illustrated by drawing four

perpendicular lines that we can view as representing four adjacent planes of atoms taken from an

arbitrary position within the material. The planes on this stress square shown in Fig. 1 can be

identified by the orientations of their normals; the upper horizontal plane is a +y plane, since its

normal points in the +y direction. The vertical plane on the right is a +x plane. Similarly, the left

vertical and lower horizontal planes are y and x, respectively.

Figure 1: State of stress in two dimensions; the stress square.

The sign convention in common use regards tensile stresses as positive and compressive stresses as

negative. Besides, the stress square must be in equilibrium; therefore this arrow must be balanced

by another force acting on the x (lefty) face and pointed in the x (lefty) direction. Of course, these

are not two separate stresses, but simply indicate the stress state is one of uniaxial tension. It goes

same with y direction for compression.

Figure 2: The sign convention for normal stresses.

Consider now a simple spherical vessel of radius r and wall thickness b, such as a round balloon. An

internal pressure p induces equal biaxial tangential tensile stresses in the walls, which can be

denoted using spherical r coordinates as and . The internal pressure generates a force

towards the spherical wall, which is balanced by the wall stress.

Figure 3: Wall stresses in a spherical pressure vessel.

At the surfaces of the vessel wall, a radial stress must be present to balance the pressure there.


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Figure 4: Free-body diagram for axial stress in a closed-end vessel.

The stresses in the axial direction of a cylindrical pressure vessel with closed ends are found using

this same approach as seen above and yielding the same thing (equilibrium state& equation).

Figure 5: Hoop stresses in a cylindrical pressure vessel.

Note the hoop stresses are twice the axial stresses. This result different stresses in different

directions occurs more often than not in engineering structures, and shows one of the 3

compelling advantages for engineered materials that can be made stronger in one direction than

another (the property of anisotropy). If a pressure vessel constructed of conventional isotropic

material is made thick enough to keep the hoop stresses below yield, it will be twice as strong as it

needs to be in the axial direction.

What is pressure vessel?

Structures such as tank capable of holding internal pressure have been very important in the historyof science and technology. In order to make a better flow of gas and fluid, an aqueduct or tank must

be constructed so they can run all the way from the reservoir to the destination.

However, temperature& pressure differential is dangerous and many fatal accidents have occurred

in the history of their development and operation. These temperature& pressure difference have

created stresses toward the shell.

A pressure vessel is a closed container designed to hold gases or liquids material under internal or

external pressure.

Pressure vessels are designed to operate safely at a specific pressure

and temperature technically referred to as the "Design Pressure" and

"Design Temperature". Design code to design pressure vessel such as

ASME sec VIII and BS standard.

Pressure vessels may theoretically be almost any shape, but shapes made of sections of spheres,

cylinders, and cones are usually employed. A common design is a cylinder with hemispherical end

caps called heads.


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Type of pressure vessel

1. Horizontal vessel on saddle support.a. Shell is primary component that contains pressure. Curved

shape.

b. Vessel always closed by head.c.

Components typically welded together.

d. Saddle supports used for horizontal vessel.- Spreads load over shell.- One support fixed, other slides.

2. Vertical vessel on leg support.a. Most head are curved shape for strength, thinness and

economy.

b. Semi-elliptical shape is most common head shape.c. Small vertical vessel typically supported by legs

- Typically maximum 2:1 ratio of leg length to diameter.- Number, size and attachment details depend on loads.

3. Tall vertical tower.

a. Nozzles used for:- Piping systems- Instrument connections- Manways- Attaching other equipment

b. End typically flanged, maybe welded.c. Sometimes extend into vessel.

4. Vertical reactor.a. Skirt support typically used for tall vertical vessels:

- Cylindrical shell.- Typically supported from gradeb. General support design

- Design for weight, wind, and earthquake.- Pressure not a factor.- Temperature also a consideration for material

selection and thermal expansion.

5. Spherical pressurized storage vessel.a. Spherical storage vessels typically supported on legs.b. Cross-bracing typically used to absorb wind and

earthquake loads.

6. Vertical vessel on lug support.a. Vessel size limits for lug supports:

- 1-10 ft diameter.- 2:1 to 5:1 height/diameter ratio

b. Vessel located above grade.c. Lugs bolted to horizontal structure.


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Pressure vessel in oil& gas industry

Applications in oil and gas production that use pressure vessel.

1. Pipe receiver and launcher2. Gas lift/ injection launcher and receiver3.

Water injection launcher and receiver4. Production separator

5. Degasser6. Electrostatic coalescer7. Hydrocyclone8. Scrubber9. Glycol contactor10.Flare knock-out drum11.Wet/dry air receiver and launcher12.Deaerator13.Closed/ opened drain vessel

Main components of pressure vessel


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DESIGN

A. Generic Criteria in designing a pressure vessel (PV).The following are the full steps to design a pressure vessel from the beginning, specification towards

the end, shipment and operation.

1. General requirements for PV specificationI. P&ID drawingFrom the drawing, we can determine the type of, function and sizing of the PV which

satisfy the process requirement. Moreover, the datasheet of every PV follows the P&ID.

II. DatasheetAll required information and figures for calculation

III. Scope of SupplyThe supplier shall be responsible for the mechanical design, supply of material,

manufacture, assembly, inspection, testing, painting, protection and delivery of the

pressure vessel.

IV. Regulatory ApprovalsDuring or after the fabrication, the pressure vessel will be inspected and tested for 3rd

party U stamp (for Div 1, allowed to be used for operation) such as ABS and DNV. Theseorganisations have the license to do inspection and award the U stamp after completion.

V. Design LifeDesign life shall be as per client specification.

VI. Location &Environmental ConditionsThe operating environmental conditions shall be specified to determine the protection

of the pressure vessel from the wind (in case there is typhoon) and disasters (earth

quake). The data will be used to design the best support for the vessel.

2. Design basis& manufacturing requirementsAll these are the requirement of design basis in order to fabricate the process PV. They have

to obey the ASME code standard thus it can be approved by the organization for operation.

I. Thickness / pressureBased on datasheet, the supplier will calculate the allowable pressure and required wall

thickness. Therefore, the dimension of the component can be determine and start the

fabrication. The difference in pressure and thickness are based on operation

temperature, stress and young modulus of material itself. The design temperature and

pressure shall be the max value allowed. Below is the formula for required thickness and

allowable pressure:

Cylindrical shell (Longitudinal seam)

Required wall thickness Allowable pressure Sphere or cylindrical shell (circ. Seam)

Required wall thickness Allowable pressure Ellipsoidal head

Required wall thickness Allowable pressure


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Torisperical head

Add data Required wall thickness Allowable pressure

Conical head

Add data = angle

Required wall thickness Allowable pressure Flat ends

Add data d= end type

Required wall thickness Allowable pressure

II. ManwaysVessels that require access or inspection openingsshall be equipped as follows:

a. All vessels less than 450 mm and over 300 mm I.D. shall have at least twohandholes or two plugged, threaded inspection openings of not less than NPS

112 (DN 40).

b. All vessels 450 mm to 900 mm, inclusive, I.D. shall have a manhole or at leasttwo handholes or two plugged, threaded inspection openings of notless than

NPS 2 (DN 50).

c. All vessels over 900 mm I.D. shall have a manhole, except that those whoseshape or use makes one impracticable shall have at least two handholes 100 mm

x 150 mm or two equal openings of equivalent area.

An elliptical or obround manhole shall be not less 300mmx400 mm. A circular manholeshall be not less than 400 mm I.D. A handhole opening shall be not less 50mmx75mm,

but should be as large as is consistent with the size of the vessel and the location of the

opening.

III. Lifting devicesThe size, weight and shape of the PV to be made must consider the available lifting

device (carrier, lorry or crane). If the PV is very big, however there is no lifting device can

lift it, thus the PV can not be transported to the operation site. Some PV are made on

site because of the size are very big.

IV. Attachments/ GaugeSome attachments such as metering gauge (pressure or temperature), piping, and

instruments have to be attached to PV. The designer will determine the location of the

gauge which satisfies with the client.V. Reinforcement (reinforced pad)

When the shell is holed for any attachment (nozzle or piping), there is increased stress

toward the surface nearby the hole. To avoid from the force breaking the shell, the

reinforcement welded to support the surface. Meanwhile the attachment will be welded

on the reinforcement pad.

VI. InsulationThe importance of insulation in PV is to maintain the heat inside the vessel. The heat is

very important for PV boiler application, hence it has a layer of insulator inner shell to


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avoid the heat absorb out to the surrounding. Meanwhile, if the heat is not significant,

the supplier will just build casing as protection in term of economic wise.

VII. Corrosion allowanceFor corrosive operation, the client will set corrosion allowance that supplier should meet

by choosing the suitable material and corrosion protection (chemical inhibitor).

VIII. DeflectionFor horizontal PV, the weight & pressure might cause deflection to the PV itself; hencethe supplier will make sure the calculation of the saddle include the overall weight; PV

and fluid. Meanwhile, the vertical PV might deflect due to wind load, inertia generate by

the movement of the ship (ex FPSO) or earth quake. Therefore the supplier will include

all these external force (wind, G force and seismic load) in the thickness calculation

instead of the inner pressure thus the deflection only in the range below maximum

allowable value.

IX. SupportThis support can counter extreme environmental conditions such as typhoon and earth

quake which create vibration to the PV.

3. Components for fabrication of PVI. Plate

Plate in oil& gas PV commonly made of carbon steel and stainless steel. It will be rolledinto cylindrical shape as PV shell and formed to make the head. The thickness of the

plate is the main point in designing the pressure vessel.

II. PipeThe external force and moments exerted by the attachment such as piping should be

included in the calculation. The joint is another main thing in PV design beside of

dimension and material.

III. ForgingForging is a process which the material (commonly steel) is deformed by giving external

force on it. The forging steel is harder and stronger than the machined cut steel (mild

steel) due to difference in grain structure.

IV. FlangesFlange is an interface between nozzle and pipe. It made of forged steel or mild steel(ASTM A105N). There are 6 types of flange, however only two types commonly use in oil

and gas PV application:

y Slip-on (use in oil& gas)y Welding neck (preferred in oil& gas)y Socket weldingy Threadedy Lap-jointy Blind

V. BoltingBolt and nut joint the flange. The material for bolt is ASTM A193-B7 PTFE Coated; and

the material for nut is ASTM A194-2H PTFE Coated.

VI. GasketsGasket is used between flange to enhance the bolted joint and avoid leakage. There are

two types of gasket material used based on flow pressure:

a. Below 150 pound CNAFb. Below 300 pound SS316, Graphite Filled, SS316 Inner Ring & CS Outer Ring

4. Material selectionThe selection of construction materials for Code pressure vessels has to be made from Code

approved material specifications. It usually specifies the most economical materials of low


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first cost and/ or low future maintenance cost that will be satisfactory under operating

conditions and will meet other requirements. There are many factors must be considered in

selecting the most suitable materials including:

a. Corrosion resistance in the service corrosive environmentb. Strength requirements for design temperature and pressurec. Costd. Ready market availabilitye. Fabricabilityf. Quality of future maintenance.Generally, process equipment is designed for a certain minimum service life under specific

operating conditions. Typical design lives are given below for several types of petrochemical

equipment:

a. 20 years: fractionating tower, reactors, high pressure heat exchanger shells and othermajor equipment which is hard to replace.

b. 10-15 years: carbon steel drums, removable reactor parts and alloy or carbon steeltower internal

c. 5-10 years: carbon-steel piping, heat exchanger tube bundles and various processcolumn internals.

Basically the vessels are specialized to adapt with two different services. Therefore, we can

save money in material selection which uses the material specific to the working operation.

The typical service for pressure vessels are:

I. Noncorrosive serviceIn addition to corrosion resistance, the fundamental material selection criteria are

design temperature and design pressure. So basically, the materials are selected based

on operating temperature. There are 3 level of temperature:

a.

Low temperature low alloy and fine-grain carbon steels which tested for notchtoughness and are found to perform satisfactory.

b. Intermediate temperature low carbon steels are sufficient which they behaveessentially in elastic manner; that is the structure returns to its original dimensions

when applied forces are removed and maximum stress is below than the yield point.

c. Elevated temperature mechanical changes in mechanical properties occur insteels. They begin to exhibit a drop in ultimate and yield strength and cease to be

elastic, becoming partly plastic.


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II. Corrosive servicesFor high corrosive operation, commonly we use stainless steel for petro-chemical plant.

Since the price of stainless steel is very high, which triple of the carbon steel, there are

three method of attaching the alloy to the carbon steel plate:

a. Integrally applied cladding integrally clad or roll clad is fabricated in a steel mill byhot rolling of assemblies of carbon or low alloy steel plates (backing) and corrosion

resistant sheet (linear) which have been welded at the edges.

b. Strip or sheet lining (explosion cladding)The stainless steel which can not use conventional welding such as titanium steel are

attached to the plate using explosion method.

c. Weld overlay cladding.The stainless steel which can be weld using conventional welding is weld on the plate

line by line until the surfaces are totally covered up.

III. Bolting materialsa. Bolt for pressure connections Bolting materials for pressure connections must

conform to the specifications listed in the Code. Bolts are designed not only for

strength but also for tightness at the joints. In order to prevent leakage of a bolted


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joint, the total force exerted by the bolts must exceed the sum of the force due to the

operating fluid pressure and the force necessary to keep the joint tight.

b. Bolt for structural connections bolting for structural non pressure parts is designedfor strength only, since the tightness of the joint is not important.

5. FabricationI. ShellThe shell commonly made of carbon steel and stainless steel. The steel plate rolling

process will make it become cylindrical shaped. Because there is limitation in forming

and rolling machine width, thus the design should be based on the available width,

meaning the PV shall be divided into several parts.

II. HeadsThere are several types of head:

a. Semi ellipsoidalb. Torisphericalc. Hemisphericald. Flate. Conical

The head is formed into shapes by two types of forming:a. Hot formed heads are formed at temperature suitable to the material. This

method basically to reduce residual stresses, to temper or soften the material

itself. Typically carbon steel heads are hot formed at 899oC and stainless steels

are hot formed at 1038oC.

b. Cold formed heat treatment is usually required after cold forming to removebrittleness including stress relief.

III. NozzleThe nozzle existence will affect PV load calculation. The loads are strictly forces and

moments imposed on the nozzle by the attached piping. Before applying these load

numbers, check the piping layout connected to the nozzle to make assumption about

the loading combination. Then, refer to supplier/fabricator range of dimension which

suit with load.


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Type ofgrooS

e weld

ii. Fillet WeldA fillet weld is a weld with an approximately triangular crosssection,joining

two surfaces at right angles.

iii. PlugweldA plug is a circular weld madeeither by arc or gas welding through one

member of a lap or teejoint.

welding


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Some form of welded joint in combination with different weld types

c. Welding process.i. Shielded Metal Arc Welding (SMAW)

This form of welding is widely used. The heat for welding is produced by the

resistance of the arc air gap to the flow of electric current. Also called stick

electrode welding, SMAW is almost always done manually. As the electrode

heats, the core wire which conducts the current to the arc melts and provides

filler metal for the welded joint. The coating of the electrode breaks down to

form a gaseous shield for the arc and weld puddle as well as small amount of

slag, which protects the weld as it cools. Shielding is very important for the

quality of weld, since it prevents the loss of alloying elements during the transfer

of molten metal through the arc.

ii. Submerged Arc Welding (SAW)This process, almost always fully automatic, is used in the fabrication of main

vessel seams. It gives excellent welds at low cost. However, it can be used for

horizontal positions only. A continuous consumable wire coil is used as

electrode. Weld puddle and arc are protected by liquid slag, formed form

granular mineral flux deposited ahead of the arc.

iii. Gas Metal Arc Welding (GMAW)A consumable continuous wire is used as an electrode which melts and supplies

the filler metal for the welded joint. A protective shield of inert gases (helium,

argon, CO2, or a mixture of gases) is used. The process produces excellent welds

at less cost than the GTAW process (see below) with higher weld deposition

rate.

iv. Gas Tungsten Arc Welding (GTAW)This process is used when the highest-quality welding with difficult to weld

metals is required. An arc is formed by a nonconsumable tungsten electrode,

which carries the electric current; the filler metal, if required, is added


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separately form a rod or a continuous wire. Inert gas flows around the arc and

the weld puddle to protect the hot metal.

v. Gas WeldingHeat of fusion is generated by burning a flame of gas with oxygen. Different

gases are used, as described below:

a. The oxyhydrogen process (OHW) uses hydrogen for combustion.b. The oxyacetylene process (OAW) uses acetylene gas:

- Flame cutting- Flame machining

vi. Resistance WeldingThe heat of fusion is generated by the resistance at the interface to the flow of

electric current. No shielding is required. Pressure must be applied for good

metal joining. Resistance spot welding (RSW) or resistance seam welding (RSEW)

are used to fix corrosion-resistant linings to the wall of a vessel shell.

V. Heat TreatmentHeat treatment intended to be done on individual parts of a vessel after they have beenshaped for assembly. Post hotwork/ coldwork heat treatment (heating about 2/3 of

material melting temperature) followed by natural cooling to remove the residual stress.

VI. Clad ConstructionIntegrally clad materials are used for fabricating equipment of reactive metals

economically, robustly and durably. Bonding these materials onto lower cost, stronger

backers extends the economical service of these materials into higher pressures and

temperatures and more challenging service environments than are feasible with solid

alloy construction.

VII. Tolerances.The entire components have their percentage of permissible dimensional difference

which provided in the code.

6. Inspection, examination and testing.The inspection must be run during the fabrication. This because to ensure the vessel is in the

best quality and satisfy with the required operating requirement.

Inspection and Test Plan (ITP)It is basically the test planning before the fabrication of the pressure vessel. Refer Table

1 for an example of ITP

Material Identification and Certificationi. Material

Make sure the materials that supplied by the suppliers are as stated in the

datasheet.

ii. DimensionalThe inspectors have to ensure that the dimension of all fabricated plate, shell,

nozzle and head are same as stated in the datasheet.

iii. SizingThis matter also same with above. Running of size is the biggest problem during

fabrication. It will lead to other problems. I.e. leakage and deform.


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Inspection and Examinationi. NDT

- Visual&optiTaltesting

Must be performed in suitableconditions, however, finecrac U s may remain

invisible.- Magneti V parti V le examination (MT)

It is the best method for the detection of finesurfacecrac W s. This method is the

most sensitive and it reveals manycrac W s that are not detected by other methods

with a lower defect sensitivity. AC yoke WMFT is preferred over DC or prod

methods. DC methods are not assensitive and prod methods may leave arcstrikes

that, if not ground out, can serve ascrack initiators.

- Ultrasoni X examination (UT)With straight and angle-beam probes is the main method for the detection of

subsurface and deeper surface defects ofsufficient size. UT shear waves,

longitudinal waves, and crack-tip diffraction ultrasonic inspections are used. UT

may also be used for theexamination of a vessel under pressure without

discharging of the liquid gas from thevessel.

- Radiographi Y testing (RT)- Itis used incases ofvolume defects revealed by ultrasonic

as an additional method for thecharacterization the

defects. Incases of weld repair, it is always recommended

to use RT.

- Dye-penetrant examination (PT)It is used only occasionally, is lesssensitive thanMT, it is

however, easier to perform. In case, somecomplicated

areas thatMT, UT and RT are not accessible, the PT is used.

ii. Hydrostatic testDuring a hydrostatic test, a pressurevessel is placed inside a closed system, usually

a testjacket filled with water, and a specified internal water pressure is applied to

thecontainer inside thisclosed system. The applied internal pressurecauses an

expansion of thecontainer being tested, and the total and permanent expansion

that thecontainer undergoes is measured. Thisvolumetricexpansion

measurement, in conjunction with an internal andexternal visual inspection of the

container, are used to determine if a pressurevessel issafe for continued use, or

hassuffered from a degradation in itsstructural integrity and must becondemned.

7. MarkingThe manufacturer s markings on a pressurevessel include information regarding the date of

manufacture, the manufacturers name or registered mark, theserial number of the unit and

thespecification or exemption to which thecontainer complies. For pressurevessels that

have been re-qualified one or more times, additional markings indicate the date(s) of any

previous hydrostatic retests and identification markings of the retest facility.

RTsetup


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8. Preservation and protection for shipment.This preservation and protection basically to protect the pressure vessel from accidental

deformation, leakage and rust initiator during transportation to the site. The vessel might be

very big and heavy thus very dangerous to residential if something happen. The action below

taken to overcome this matter:

y Rust Prevention.y Protection and safety during delivery.y Transport Saddles.


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Table 1 (from inspection, examination and testing)


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29527771-Intro-to-Pressure-Vessel.pdf

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