PAP Lecture 1.

EQUIPMENT DESIGN CHEMICAL ENGINEERING LECUTRE

by : Dr.Ir.P. Sumardi , SU

The objectives :

1. The roles of chemical engineers responsibility while designing the processes and equipments

2. The roles of Inherently Safer Design

3. Review on

1. Materials / chemicals storage refer to TBS course

2. Thermodynamics Phase equilibrium refer to chemical engineering thermodynamics 1st course

The objectives (cont)

4. Design Variables

5. Equipment Sizing (short cut methode of equipment sizing)

6. Material Construction Selection

7. Mechanical design of equipments

a. Equipment under internal working pressure load

b. Equipment under external working pressure load

The objectives (cont.)

8. Vessel stability ( tall tower/vessel stability )

9. LOPA ( Layer Of (Equipments) Protection analysis

The chemical engineers scope of work in industrial fields

1. Process engineering field process engineers

2. Operation engineering field operation engineers

3. Utility / Fasility engineering field fasilities engineers

4. Managerial field plant manager

Chemical engineers Responsibilities while designing the process or the equipment

1. Responsible to comply the design intention for example equipment designed should comply the design capacity

2. Responsible to the safety of the equipment while operation / start up and intended shut down

“fail safe operation philoshopy “

3. Responsible to the safety of the “environment”

minimizing/eliminating the consequences of the accident / process failure realization

decreasing the likelihood/the frequency of the accident realization using the accurate/suitable of layer of protection (control systems) for the equipment designed

Responsible to comply the design intention

The steps or squences while designing the equipment / process :

1. Should be familiar / full recognize about what the basic principle / the fundamentals around the equipment

2. Equipment sizing

1. Using the best practices short cut method calculation (using soft wares or empirical equation ertc.)

2. Using the “Chemical Engineering Tools “ to provide the “design equations” selecting the design variable sizing the equipment

3. Applying the engineering sense , comparing the size of the equipment found/calculated that it is pratically possible / useable

The safety of the equipment designed “fail safe operation philoshopy “

Fail ??? fail operation

There will be process parameters deviation increasing / decreasing of flow rate, temperature , pressure or composition

Safe or unsafe????

Safe operation (while being fail) , mean no accident realization , no equipment damage or minor damage under the acceptable consequences / risk associated

The safety of the equipment designed “fail safe operation philoshopy “

What should you need to ensure the fail safe operation philosophy??

Your equipment should be completed with

1. Operator intervention procedure

2. Indicators

3. Automatic controllers

Inherent existing in something (property, potencial hazard , quality etc ...., as a permanent and inspasable element, quality or attribute

Safer safer than others design conducted by by other designers or existing design (maybe)

less risk associated , decreasing in potentialy releasing hazard both passively or actively occuring .

Inherently safer design definition :

to conduct the design of equipment , design of process , design of product , concerning to eliminate or to decrease the hazard realization rather than relying on the hazrd control only.

1. Minimizing or attenuation significantly reduce the quantity of hazardous

material or energy in the system or eliminate the hazard entirely if possible

2. Substitute replace a hazardous material with less hazardous

substance or using the hazardous chemistry with a less hazardous one

3. Moderate reduce the hazard of a process by handling

materials in a less hazardous form, or under less hazardous condition , for examples at lower pressure and temperature

4. Simplify eliminate unnecessary complexity to make plants more “user friendly” and less prone to human error and

incorrect operation

1. In many cases it will not clear which of several potential technologies is really inherently safer , and there may strong disagreement about this

2. Chemical processes and chemical plants have multiple hazards , and the different technologies will have different inherent safety characteristic with respect to each of those multiple hazards

Chemical substitution ??? create the later hazards (maybe)

3. Who is to determine which alternative is inherently safer ???

requiring the implementation of inherent technology ?? who determine that this technology is inherently safer ???

4. Some technology choices which are inherently safer locally , but more globally could increase the hazard

Thinking about the opprtunity of implementation of inherently safer design

1. Engineer should think of this at all times in everything they do it should be way of life for those designing and operating chemical plant , technologies etc.

1. R&D engineer inherently safer process based on those chemistry

2. Design engineer / process engineer inherently safer plant design using the selected technology and process minimize the size of equipements containing the hazardous chemical make the plant design “user friendly”

3. Plant operation engineers and operator develop the the inherently safer operating procedures look for opprtunity for enhancing inherent safeety in existing facilities

4. Operator look for inherently safer ways to do all of the tasks involves in day to day operation

STORAGE VESSEL CONTAINMENTS:

1. LIQUID PHASE

1. NON VOLATILE LIQUID

2. VOLATILE LIQUID

2. GASEOUS STATE PHASE

1. UNDER PRESSURE STORGAE

2. ATMOSPHERIC STORAGE

3. LIQUIFIED GASEOUS

1. CRYOGENIC STORAGE SYSTEM

2. NON CRYOGENIC STORAGE SYSTEM

STORAGE CONDITION 1.

STORAGE VESSEL CONDITIONS:

1. LIQUID PHASE

1. NON VOLATILE LIQUID AT AMBIENT TEMPERATURE AND AMBIENT PRESSURE STORAGE TANK

2. VOLATILE LIQUID AT AMBIENT PRESSURE , BELOW ITS BOILING POINT AT AMBIENT TEMPERATURE , ELEVATED PRESSURE

2. GASEOUS STATE PHASE

1. UNDER PRESSURE STORGAE UNCONDENSED CONDITION PRESSURE VESSEL

2. ATMOSPHERIC STORAGE GAS HOLDER

STORAGE CONDITION 2 (CONT.)

1. LIQUIFIED GASEOUS STATE

1. CRYOGENIC STORAGE SYSTEM AMBIENT PRESSURE AND AT CRYOGENIC

TEMPERATURE AND AT ITS BOILING POINT (SATURATED LIQUID AT SLIGHTLY OBOVE ITS BP) ELEVATED PRESSURE , CRYOGENIC TEMPERATURE AND AT ITS BOILING POINT

2. NON CRYOGENIC STORAGE SYSTEM AT AMBIENT TEMPERATURTE AND HIGH

PRESSURE AND AT ITS BOILING POINT (SATURATED LIQUID)

AT ELEVATED TEMPERATURE AND ELEVATED PRESSURE AS SATURATED LIQUID

STORAGE CONDITION 3 (CONT.)

EMPIRICAL EQUATION USED TO DEFINE THE STORAGE CONDITION FOR SATURATED LIQUID

ESPECIALLY FOR LIQUIFIED GAS STORAGE BOTH AT ELEVATED OR CRYOGENIC TEMPERATURE

Antoinne Equation

T

BAP 0ln

Where :

P0 = vapor pressure , psi or cmHg or other unit

A , B = Antoinne constante , according to the liquified gas stored

T = storage temperature , liquid temperature being stored , K or R

STORAGE SYSTEM / LAY OUT

1. STORING LIQUID UNDER AMBIENT CONDITION

1. OUT DOOR STORAGE

2. USING VERTICAL SILINDRICAL TANK , FLAT BOTTOM AND DOOME TYPE TANK , EQUIPPED WITH P/V VALVE CONNECTING TO FLARING SYSTEM (FOR FLAMMABLE LIQUID) , LI OR LC (FOR CONTINOUS PUMPING SERVICES)

2. STORING LIQUID AT ITS BOILING POINT (AT ELEVATED TEMP. AND PRESSURE)

1. OUT DOOR STORAGE USING SPHERE VESSEL (ESPECIALLY FOR THE MOST VOLATILE LIQUID, LNG OR LPG ETC.) USING VERTICAL SILINDRICAL VESSEL BLANKETTING WITH HEAT INSULATION MATERIAL WITH THE TYPE OF DOUBLE WALL VESSEL OR SINGLE WALL VESSEL NEED INSULATION TOO THICK

1. STORING LIQUID AT ITS BOILING POINT (AT ELEVATED TEMP. AND PRESSURE

1. INDOOR SYSTEM STORAGE USING BURRIED HORIZONTAL VESSEL, EQUIPPED WITH P/V VALVE AND LI OR LC FOR CONTINOUS PUMPING SERVICES

2. STORING GASEOUS STATE

1. GASEOUS STATE COMPRESSIBLE FLUID

2. TO ATTENUATE THE VOLUME OF GAS BEING STORED USUALLY USE STORAGE SYSTEM AT ELEVATED PRESSURE BUT THE GAS NOT YET CONDENSED USED SMALL SIZE VESSEL LESS THICKNESS OF VESSEL SHEEL.

3. STORING UNDER AMBIENT PRESSURE GAS HOLDER , NEED LARGE DIAMETER OF GAS HOLDER , USING DOME HEAD/ROOF SMALL SIZE OR USED FLOATING ROOF

STORAGE SYSTEM / LAY OUT

(CONT. 2)

ENSURING THE SAFETY OF THE EQUIPMENT WITH INHERENTLY

LAYER OF PROTECTION

LI LC LLAI

PC

DRAINING SYSTEM

SATURATED LIQUID

HHPAI

LI LC LLAI

DRAINING SYSTEM

SUBCOOLD LIQUID

P/V VALVE

LI LC

LLAI

LI LC

LLAI

SATURATED LIQUID

PC

SUBCOOLD LIQUID

DRAINING SYSTEM

DRAINING SYSTEM

HHPAI

ENSURING THE SAFETY OF THE EQUIPMENT WITH INHERENTLY

LAYER OF PROTECTION

P/V VALVE

Phase equilibrium

liquid

vapor

Equilibrium means :

1. Rate of vapor condenses = rate of liquid boils

2. Equilibrium occurs depend on pressure , temperature and composition

3. The temperature and pressure of each phase are equal Tvapor phase = T liquid phase Pvapor phase = Pliquid phase

4. At spesific composition , equilibrium occurs at soecific P, T , means that the phase composition at equilibrium state depend on T,P

F(Xi, T, P) = 0

Xi = f (T,P)

T = f(Xi, P)

Note :

P = total presure suppress the equilibrium

ROULT & DALTON LAW compliance only for ideal solution

SATURATED

LIQUID , Xi ---n**

3*2

*1

1

*

....... i

i

itotal

pppp

pp

oii

totalii

px

pyp

*

Additional notes;

Yi = mole fraction of component i in the vapor phase

Xi = mole fracti on of component i inthe liquid phase which in equilibrium with its vapor phase

Pio = pure vapor pressure of component i at equilibrium

temperature f(Teq )

you can estimate using Antoinne equation

oii

totalii

px

pyp

*

PHASE EQUILIBRIUM

total

i

i

ii

total

i

i

i

p

p

constmequilibriuphase

x

yK

p

p

x

y

0

0

.

How to estimate Ki, eq ??

1. Estimate pure vapor pressure of i component , using Roult. & Dalton Law

2. EstImate pure vapor pressure , using EOS ( Equation of State for example : PENGROBINSON OR REDLICH AND KWONG

3. YOU CAN ALSO USE EQUILIBRIUM PHASE CHARTS THAT WRITTEN IN ALL TEXTBOOK OF CHEM . ENG. THERMODYNANAMICS

APPLICATION OF PHASE EQUILIBRIUM IN PROCESS DESIGN PHASE

1, ESTIMATION OF DUE POINT TEMPERATURE OF CONDENSABLE GAS MIXTURE THE DEW POINT IS DEFINED BY :

AT SPECIFIED TOTAL PRESSURE OF THE MIXTURE , Ptotal defined

the dew point of this mixture is that the temperature at which the vapor will start to

condensed , so the state of the mixture should be in saturated vapor

At this dew pint , the vapor composition follow this equation.

0,1 iy

2. ESTIMATION OF BUBLE POINT TEMPERATURE OF LIQUID MIXTURE

THE BUBLE POINT IS DEFINED AS FOLLOW : THE TEMPERATURE WHICH THE MIXTURE WILL START TO EVAPORATE

AT THIS CONDITION SHOULD

a. The liquid mixture in saturated liquid state

b. THE SUM OF PARTIAL VAPOR PRESSURE OF EACH COMPONENT (i) BE EQUAL TO TOTAL

PRESSURE OF MIXTURE SYSTEM

IN PRACTICE FOR ESTIMATING THE BOTTOM CONDTION OF THE LIQUID FLOW OUT FROM THE REBOILER

0,1 ix

PROCESS CONDITION DESIGN FOR COLUMN DESTILATION

RULES OF THUMB / SEQUENCES

1. CHOOSE THE VARIABLES DEFINED BY DESIGNER

a. product composition DESIGN intention ( top product or bottom product)

b. Using the mass balance around the column calculate the existing calculated composition

2. CHOOSE THE DESIGN VARIABLE

a. INTERNAL REFLUX RATIO (Lo/D) (1,20 – 1,4) Rmin

b. Number of plate (for existing column) the design work is to optimize the existing column by varying the process condition

3. CHOOSE THE APPROPRIATE AVAILABLE SOURCE OF UTILITIES :

a. STEAM / FUEL AS HEATING MEDIUM

b. COOLING WATER / AMBIENT AIR/ REFRIGERANT AS COOLING MEDIUM

RULES OF THUMB / SEQUENCES (CONT.)

4. CHOOSE THE PREFERENCE DESIGN PARAMETER TO OBTAIN COLUMN / PROCESS CONDITION

a. OPERATING PRESSURE DEFINED to aim the minimum thickness of shell vessel the operating temp calculated will define the utility need it would be that the utility beyond of the available one

b. Operating temperature both at the top and the bottom according to the maximum achievable for the available utility in the plant top operating temp rely on to the existing / avaliable cooling medium , if cooling water used , so the top condition would be = 35oC + 10oC (as thermal approach) bottom operating temperature rely on to the heating medium available , if steam used , the maximum available steam (from boiler) = 200oC , the maximum temperature achieved by this steam = 200 – 30 (as thermal approach) = 170 oC.

5. CHOOSE THE PREFERENCES OF USING CONDENSOR (TOTAL/ PARTIAL) AND ALSO THE REBOILER TO PROVIDE VAPOR REFLUX TO COLUMN.

Total condensor

Bottom product at its boiling point

destilate product at its boiling point

Partial reboiler

Heating medium

Condensate accumulator

DISTILATION COLUMN USING TOTAL CONDENSOR AND

PARTIAL REBOILER

Conventional distilation column


Partial reboiler

Heating medium

Partial condensor

Vapor product at its dew point


DISTILATION COLUMN USING PARTIAL CONDENSOR AND

PARTIAL REBOILER

Conventional distilation column

Total condensor


destilate product at its boiling point


Vapor reflux

Flue gas

Fuel medium

Total reboiler

(furnace)

DISTILATION COLUMN USING TOTAL CONDENSOR AND

TOTAL REBOILER

Total condensor


Vapor reflux

Flue gas

Fuel medium

Total reboiler

(furnace)

Partial condensor

Vapor product at its dew point


DISTILATION COLUMN USING PARTIAL CONDENSOR AND

TOTAL REBOILER


Partial reboiler

Heat pump assisted DISTILATION COLUMN

with PARTIAL REBOILER

SATURATED VAPOR

SUPERHEATED VAPOR

COMPRESSOR

DESTILATE product at its boiling point

Saturated liquid at high pressure

Throtling valve

Destilate separator

No heating medium need

Bottom product at its boiling point (liquid)

Partial reboiler

Heat pump assisted DISTILATION COLUMN

with PARTIAL REBOILER and vaporas

top productSATURATED VAPOR

SUPERHEATED VAPOR

COMPRESSOR

Saturated liquid at high pressure

Throtling valve

Destilate separator

Vapor product at its dew point at column pressure

HEAT BALANCE

F = D + B

F.Xi,F= D.Xi,D + B.Xi,B

F.HL,F= D.(QCD+HL,D )+ B.(HL,B – QR,B)

MASS BALANCE

QRB = QR/B

REBOILER

B , Xi, B,

HL, B

D , Xi, D, HL, D

Lo , Xi,,O HL,,O

Total condensor

Sat`d vapor

Sat`d liquid

Sat`d vapor

QC, D = QC /D

F , Xi,F,

HL,F

CONVENTIONAL DESTILATION COLUMN

Design equations

TOP SECTION

NTH

plate

N=1

LN , XiN , HL,

N

GN+1 , Yi,N+1 ,

HG,N+1

D , Xi, D,

HL, D

Lo , Xi,,O

HL,,O

Total condensor

RO = LO/D Xi,O = Xi,D HL,O = HD TTOP = TBP,D

Sat`d vapor

Sat`d liquid

H, enthalpi = f (T,P,Xi)

GN+1 = LN + D

GN+1.Yi,N = LN.Xi,N + D.Xi,D

Yi,N = (LN/GN+1).Xi,N + (D/GN+1).Xi,D

(GN+1)/D = LN/D + 1

= RN + 1

(LN/GN+1) = RN/(1+RN)

Yi,N = {RN/(1+RN-1)}Xi,N + {1/(1+RN)} Xi,D

NTH

plate

N=1

LN , XiN , HL,

N

GN+1 , Yi,N+1 ,

HG,N+1

D , Xi, D,

HL, D

Lo , Xi,,O

HL,,O

Total condensor

RO = LO/D Xi,O = Xi,D HL,O = HD TTOP = TBP,D

Sat`d vapor

Sat`d liquidHeat balance :

HG,N+1={RN/(1+RN)}HL,N+{1/(1+RN)}(HD+QCD)

Yi,N = {RN/(1+RN+1)}Xi,N + {1/(1+RN)} Xi,D

Mass balance :

RN = (LN/D) R0 USUALY USED AS DESIGN VARIABLE

N = 0 means represent to total condensor

Sat`d liquid = BOTTOM PRODUCT

GM = LM+1 – B

RM = GM/B

STRIPPING SECTION

GM Yi,M = LM+1 Xi, M+1 - B Xi,B

Yi,M = (LM+1/GM) Xi, M+1 - (B/GM) Xi,B

GM .HG,M = LM+1.HL, M+1 – B(HL,B - QRB)

HG,M = (LM+1/GM).HL, M+1 - (B/GM).(HL,B - QRB)

HG,M = {(RM+1)/RM}.HL,M+1 - (1/RM).(HL,B - QRB)

Yi,M = {(RM+1)/RM} Xi, M+1 - (1/RM) Xi,B

M =2 QRB = QR/B

M =1

Mth PLATE from bottom

LM+1 , XiNM+1,

HL, M+1

GM , Yi,M,

HG, M

B , Xi, B,

HL, B

SATURATED VAPOR

• The mechanical design of a pressure vessel can proceed onlyafter the materials have been specified. The ASME Code does not state what materials must be used in each application

• user to specify the appropriate materials for each application considering various material selection factors in conjunction with ASME Code

Material Selection Factors

The main factors that influence material selection are:

• Strength• Corrosion Resistance• Resistance to Hydrogen Attack• Fracture Toughness• Fabricability

Strength

• Strength is a material's ability to withstand an imposed force or stress.

• Strength is a significant factor in the material selection for a particular application. • Strength determines how thick a component must be to

withstand the imposed loads.• The strength properties depend on the chemical

composition of the material. • Creep resistance (a measure of material strength at elevated

temperature) is increased by the addition of alloying elements such as chromium, molybdenum, and/or nickel to carbon steel. Therefore, alloy materials are often used in elevated temperature applications.

Corrosion Resistance

• Corrosion is the deterioration of metals by chemical action.

• A material's resistance to corrosion is probably the most importantfactor that influences its selection for a specific application.

• The most common method that is used to address corrosion in pressure vessels is to specify a corrosion allowance.

• A corrosion allowance is supplemental metal thickness that is added to the minimum thickness that is required to resist the applied loads.

• This added thickness compensates for thinning (i.e., corrosion) that will take place during service.

Resistance to Hydrogen Attack

• At temperatures from approximately 300°F to 400°F, monatomic hydrogen diffuses into voids that are normally present in steel. • In these voids, the monatomic hydrogen forms molecular

hydrogen, which cannot diffuse out of the steel. • If this hydrogen diffusion continues, pressure can build to high

levels within the steel, and the steel can crack. • At elevated temperatures, over approximately 600°F, monatomic hydrogen not only causes cracks to form but also attacks the

steel.• Hydrogen attack differs from corrosion in that damage occurs throughout the thickness of the component, rather than just at its

surface, and occurs without any metal loss. • In addition, once hydrogen attack has occurred, the metal cannot

be repaired and must be replaced.• Hydrogen attack is a potential design factor at hydrogen partial• pressures above approximately 100 psia.

Fracture Toughness

• Fracture toughness refers to the ability of a material to withstand conditions that could cause a brittle fracture.

• The fracture toughness at a given temperature varies with different steels and with different manufacturing and fabrication processes.

• It is especially important for material selection to eliminate the risk of brittle fracture since a brittle fracture is catastrophic in nature.

• It occurs without warning the first time the necessary combination of critical size defect, low enough temperature, and high enough stress occurs.

Fabricability

• Pressure vessels commonly use welded construction.

• Therefore, the materials used must be weldable so that individual components can be assembled into the completed vessel.

B. Maximum Allowable Stress

• Maximum allowable stress is the maximum stress that may be safely applied to a pressure vessel component

• The design of a pressure vessel must ensure that these internal stresses never exceed the strength of the vessel components.

• Pressure vessel components are designed such that the component stresses that are caused by the loads are limited to maximum allowable values that will ensure safe operation.

Note that the allowable stresses at temperatures between-20°F and 650°F are the same as the allowable stress at 650°F for each material presented

Carbon Steel Plates and Sheets

Mechanical design

• The mechanical design of a pressure vessel begins with specification of the design pressure and design temperature.

• Pressure imposes loads that must be withstood by the individual vessel components.

• Temperature affects material strength and, thus, its allowable stress, regardless of the design pressure.

All pressure vessels must be designed for the most severe conditions of coincident pressure and temperature that are expected during normal service.

Normal service includes conditions that are associated with:

· Startup.· Normal operation.· Deviations from normal operation that can be anticipated

(e.g., catalyst regeneration or process upsets).· Shutdown.

Pressure

Operating Pressure

The operating pressure must be set based on the maximum internal or external pressure that the pressure vessel may encounter.

• Ambient temperature effects.• Normal operational variations.• Pressure variations due to changes in the vapor

pressure of the contained fluid.• Pump or compressor shut-off pressure.• Static head due to the liquid level in the vessel.• System pressure drop.• Normal pre-startup activities or other operating

conditions that may occur (e.g., vacuum), that should be considered

in the design.

Design Pressure

• The specified design pressure is based on the maximum operating pressure at the top of the vessel, plus the margin that

the process design engineer determines is suitable for the particular application.

• A suitable margin must also be provided between the maximum operating pressure and the safety relief valve set pressure.

• This margin is necessary to prevent frequent and unnecessary opening of the safety relief valve that may occur during normal variations in operating pressure.

• The safety relief valve set pressure is normally set equal to the design pressure.

• Pressure vessels, especially tall towers, may have liquid in them during normal operation.

• The maximum height of this liquid normally does not reach the top of the vessel.

• The liquid level that is required for design is specified by the process design engineer.

Temperature

Operating Temperature

• The operating temperature must be set based on the maximum and minimum metal temperatures that the pressure vessel may encounter.

• The operation and vertical length of tall towers, and the presence of liquid in the bottom section, sometime result in large temperature reductions between the bottom and top of the vessel.

Design Temperature

The design temperature of a pressure vessel is the maximum fluid temperature that occurs under normal operating conditions, plus an allowance for variations that occur during operation.

Mechanical Design for Internal Pressure

1. in a cylindrical shell under internal pressure this is an idealized stat

2. the ASME Code Formulas have been modified to account for nonideal behavior.

Summary of ASME Code Equations

Typical types of closure heads.

Elliptical Heads

• The 2:1 semi-elliptical head is the most commonly used head type high pressure vessel used

• Half of its minor axis (i.e., the inside depth of the head minus the length of the straight flange section) equals one-fourth of the inside diameter of the head.

• The thickness of this type of head is normally equal to the thickness of the cylinder to which it is attached.

Torispherical Heads

• A torispherical (or flanged and dished) head is typically somewhat flatter than an elliptical head and can be the same thickness as an elliptical head for identical design conditions and diameter.

• The minimum permitted knuckle radius of a torispherical head is 6% of the maximum inside crown radius.

• The maximum inside crown radius equals the outside diameter of the head.

Hemispherical Heads

•In carbon steel construction, hemispherical heads are generally not as economical as elliptical or torispherical heads because of higher fabrication cost.

• The required thickness of a hemispherical head is normally one-half the thickness of an elliptical or torispherical head for the same design conditions, material, and diameter. • Hemispherical heads are an economical option to consider when expensive alloy material is used.

Design for Internal Pressure

A. What are the minimum required thicknesses for the two cylindrical sections?

DESIGN INFORMATIONDesign Pressure = 250 psigDesign Temperature = 700° FShell and Head Material is SA-515

Gr. 60Corrosion Allowance = 0.125"Both Heads are SeamlessShell and Cone Welds are DoubleWelded and will be SpotRadiographedThe Vessel is in All Vapor ServiceCylinder Dimensions Shown areInside Diameters

Solution

1. The required wall thickness for internal pressure of a cylindrical shell is calculated using the following equation

2. Since the welds are spot radiographed, E = 0.85 (from Figure 4.5)

3. S = 14,400 psi for SA-515/Gr. 60 at 700°F (from Figure 3.2)

P is given as 250 psig.

For the 6 ft. - 0 in. shell, calculate r (including corrosion allowance)

r = 0.5D + CA = 0.5 x 72 + 0.125 = 36.125 in.

t = 0.872 in. required including corrosion allowance

For the 4 ft. - 0 in. shell, calculate r (including corrosion allowance)

r = 0.5 x 48 + 0.125 = 24.125 in.

t = 0.624 in. required (including corrosion allowance)

B. For the same vessel, what are the minimum required thicknesses for the top and bottom heads?

Solution

1. Since both heads are seamless, E = 1.0.

2. Top Head - Hemispherical head (Equation from Figure 4.6)

r = 24 + 0.125 = 24.125 in.


3. Bottom Head - 2:1 Semi-Elliptical Head

D = 72 + 2 x 0.125 = 72.25 in.


Design for External Pressure and Compressive Stresses

• Pressure vessels are subject to compressive forces such as those caused by dead weight, wind, earthquake, and internal vacuum.

• The failure type of this vessel is due to elastic instability, which makes shell weaker in compression than in tension (due to under internal working pressure)

• In failure by elastic instability, the vessel is said to collapse or buckle.

• These basic principles also apply to other forms of shells as well as to heads and to compressive loads other than external pressure.

Overview

• The critical pressure that causes buckling is not a simple function of the stress that is produced in the shell.

• An allowable stress is not used to design pressure vessels that are subject to elastic instability.

• Instead, the design is based on the prevention of elastic collapse under the applied external pressure

• This applied external pressure is normally 15 psig for full vacuum conditions.

• The maximum allowable external pressure can be increased by welding circumferential stiffening rings (i.e., stiffeners) around the vessel shell

Shells

• The allowable external pressure of a cylindrical shell is a function of material, design temperature, outside diameter, corroded thickness, and unstiffened length.

Heads• The allowable external pressure of a head is a function of

material, design temperature, outside radius, head depth, and corroded thickness.

• The head thickness is increased as required to achieve the required external pressure.

DESIGN INFORMATIONDesign Pressure = Full VacuumDesign Temperature = 500° FShell and Head Material isSA-285 Gr. B, Yield Stress = 27 ksiCorrosion Allowance = 0.0625"Cylinder Dimension Shown is Inside Diameter

External Pressure Calculation

The vendor has proposed that the wall thickness of this tower be 7/16 in., and no stiffener rings have been specified. Is the 7/16 in. thickness acceptable for external pressure?

Solution

1. First, calculate the unstiffened design length, L, and the outside diameter, Do, of the cylindrical shell, both in inches.

L = Tangent Length + 2 x 1/3 (Head Depth)

The tangent length = 150 ft.

Since the heads are semi-elliptical, the depth of each head is equal to ¼ the inside diameter of the shell.

Head Depth = 48 /4 = 12 in.

L = 150 x 12 + 2/3 x 12 = 1,808 in.

Do = 48 + 2 ´ 7/16 = 48.875 in.

Next, determine the ratios L/Do and Do/t.

Accounting for the corrosion allowance,t = 7/16 – 1/16 = 6/16 = 0.375 in.

Do/t = 48.875 / 0.375 = 130

L/Do = 1808 / 48.875 = 37

2. Determine the value of A using Figure 4.12 and the calculated Do/t and L/Do.

Note: If L/Do > 50, use L/Do = 50.

For L/Do < 0.05, use L/Do = 0.05.

Factor A

Calculate maximum allowable external pressure for the value of t, psi.

Where

E = Young's modulus of elasticity at design temperature for thematerial, psi. Do not confuse this parameter with the weld jointefficiency, E, that is used elsewhere.

Figure CS-1

E = 27 x 106 psi from Figure CS-1 (Figure 4.13) at T = 500°F

PAP Lecture 1.

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