Equipment Design

Equipment Design (Week 5)

Dr. H.B. Vuthaluru

Some of the material presented in the lecture slides is

adapted from several textbooks and electronic resources

Types of Equipment

• Types of equipment used in process industries– Proprietary

– Non-proprietary

• Proprietary equipment– pumps, compressors, filters, centrifuges, dryers

– designed and manufactured by specialist firms

• Non-proprietary equipment– Role of chemical engineer

• usually limited to selecting and sizing the equipment

Types of Equipment

• Example– Distillation column

– What’s the role of chemical engineer• Determine the number of plates

• Type and design of plates

• Type and design of plate

• Diameter of the column

• Position of the inlet, outlet and instrument nozzles

• What next??– This information is transmitted in the form of sketches,

specification sheets, to the specialist mechanical design groups or fabricating design team for detailed design

Design Methods

• Shortcut approaches (why??)

– Less computer costs and time

– Ability to provide approximate equipment sizing

– Ability to provide quick costing estimates

– Support more detailed engineering designs

– Adequate for initial specification purposes

Equipment and selection method

• Selection charts

– Coulson & Richardson Volume 6

– Walas, Chemical Process Equipment - Selection & Design

– TEMA for heat exchangers

• Tubular Exchanger Manufacturers Association, Inc. (TEMA) is trade association of leading manufacturers of shell and tube heat exchangers, who have pioneered the research and development of heat exchangers for over sixty years.

– Perry’s Handbook for Chemical Engineers

Selection chart - compressors

Heuristics

• What’s the use of Heuristics??

– Provides good initial start

– Rules are based on general engineering practice

• Widely used references

– Jim Douglas (Conceptual Design …)

– Walas (Chemical Process Equipment)

– Ulrich (Chem Eng Proc Design….)

– McKetta (Encyl. Chem Proc. And Design)

Example Heuristics

• Flash drums: 600s liquid residence

• Steam use: 100-250°C, Dowtherm>250°C

• Compressor efficiencies: 76-78%

• Maximum compression ratio per stage: 4:1

• Motor efficiencies: 85%

• Control valves: 30% of total pressure drop or at least 70kPa

Shortcut design methods

• Heat exchangers

• Tanks

• In-process pressure vessels

• Separation columns

• Pumps

• Compressors

Design of heat exchangers

• Procedure– Obtain key specifications from M&E balances (duty, flows, temperatures)

– Estimate/obtain physical properties (, , k)

– Select heat exchanger (TEMA Standards)

– Obtain overall H/T coefficient (Uest)

– Calculate Tm (log mean driving force)

– Use governing equation to get area, A

– Choose appropriate layout (shell/tube side passes etc)

• Then– Calculate individual coefficients than U

– Compare U and Uest

– Calculate P (shell, tube sides)

Heat exchangers - Governing Equation

Q = UA Tm

Tlm = [(T1-t2)- (T2-t1)]/ln [(T1-t2)- (T2-t1)]

Tlm = Ft Tm

where Ft is the temperature correction factor

– Adjusts Tlm for number of shell and tube passes (flow pattern)

Overall Heat Transfer Coefficients

• Many listings or charts– Walas Table 8.4

– Coulson & Richardson Vol 6

– Perry

• Important to consider fouling factors– Tube side

– Shell side

– Overall

– Listings in • C & R Vol 6

• TEMA Standards

• Ludwig, E.E.

Components of U

Shell and Tube HE’s - TEMA Std

c

Typical S&T ExchangersFixed Tube Sheet

c

Typical S&T ExchangersU-tube Exchanger (U)

Typical S&T ExchangersInternal Floating Head (S)

HE’s - Detailed Engineering Design

• Principal references

– TEMA Standards

– Coulson &Richardson

– Walas

– BS3274

– AS1210

– AS3857

Storage Tanks and Drums

• Principal references

– AS1940-1993 (flammable and combustible liquids)

– AS1692 (tanks for flammable and combustible liquids)

– API620 and 650 (large, welded, low pressure storage tanks)

– BS2654

– Perry, Walas, C&R

• Detailed design– Brownell and Young (1959)

Major Storage Tanks

• Atmospheric storage– Cone roof tank– Floating roof tank

• Low pressure storage (<100 kPa (g))– Horizontal, vertical, spherical– Dished, ellipsoidal, hemispherical ends

• High pressure storage (in-process, LPG etc)– Horizontal, vertical, spherical– Pressure vessel codes apply (AS1210)

• Cryogenic tanks (LOX, LIN, LAR, LNG)– Double wall design

Atmospheric Storage Tanks

High Pressure Storage Tanks

Cryogenic, Low Pressure Refrigerated Tanks

In-process drums

Separation Columns

• Type– Packed (random, structured)

– Trays (sieve, valve)

• Specifications– HK, LK in top and bottom products

– Column pressure (determined by condensers)

• Sizing– Minimum stages via Fenske; Min RR via Underwood

– Performance via ASPEN PLUS

– Actual column based on an efficiency (literature/correlation)

– Diameter based on percent flooding

– Pressure drop based on vapor-liquid correlations

– Shell and head thicknesses via AS1210• Tall columns increase due to bending moment

Pumps

• Selection (Coulson and Richarson, Vol 6)

Type Capacity (m3/h) Head (m)Cenrifugal 0.25-1000 1-50 (300 multistage)

Reciprocating 0.5-500 50-200Diaphragm 0.05-50 5 to 60Rotary gear 0.05-500 60-200Rotary vane 0.25-500 7 to 70

Pumps - shortcut sizing

• Power

ingreciprocat

lcentrifugafor

efficiencypump

smrateflowQ

mNdroppressureP

PQP

P

P

P

P

9.0

7.0

)/(

)/(3

2

Gas moving equipment

• Classification– Fans P<15 kPa

– Blowers 3kPa< P<5 bar

– Compressors P<5 bar

• Types of compressors– Rotodynamic

• Centrifugal

• Axial

– Positive displacement• Reciprocating piston

• Rotary (screw, lobes, blades)

Rotarydynamic Compressors

Compressor sizing

• Polytropic process:

• Work:

• where W = work (W)

Z = compressibility factor

T1 = inlet temperature (K)

M = Mol wt.

P1, P2 = inlet and outlet pressures

n = polytropic coefficient (dependent on design/operation)

f = gas flowrate

R = 8.314 J/mol.K

11

1

1

211n

n

P

P

n

n

M

RTfZW

Compressibility Factor, Z

Compressor sizing

• Temperature out:

• Work: Polytropic exponent

where Ep = polytropic efficiency (0.65-0.80)

m = polytropic temperature exponent

= (-1)Ep/

n = [Y-m(1+X)]

`

X, Y = compressibility functions

XEC

ZRm

P

PTT

pP

m

1

1

212

Reactors

• Heart of a chemical process

– process where raw materials are converted into products

– reactor design is a vital step in the plant design

• Where do you find information on reactor design?

– Rase (1977), (1990) covers practical aspects of reactor and includes case studies of industrial reactors

– Pickett (1979), Rousar et al. (1985) and Scott (1991) covers the design of electrochemical reactors

Reactors

• Design of a any reactor must satisfy the following requirements:

– Chemical factors: Sufficient residence time for the desired reaction to attain required degree of conversion

– Mass transfer factors: Reaction rates are governed by the rates of diffusion of the reacting species with heterogeneous reactions rather than the chemical kinetics

– Heat transfer factors: The removal or addition of the heat of reaction

– Safety factors (important): confinement of hazardous reactants and products, and the control of the reaction and the process conditions

Types of Reactors

• Characteristics used to classify reactor designs include:– Mode of operation

• batch or continuous

– Phase present• homogeneous (G or L) or heterogeneous (L/L, L/S, L/S/G, G/S, G/L)

– Reactor geometry• flow pattern and manner of phase contact

• examples include– stirred tank reactor

– tubular reactor

– packed bed, fixed and moving

– fluidised

Design Procedure

• Gather all the kinetic and thermodynamic data on the desired and side reactions

• Collect the physical property data– either from the literature, estimation or experiments

• Identify the predominant rate-controlling mechanism, kinetic, mass or heat transfer

• Choose a suitable reactor type and materials

• Make an initial selection of the reactor conditions– to arrive at the desired conversion and yield

• Size the reactor and estimate its performance

• Make a preliminary mechanical design– vessel design, heat transfer surfaces, internals

• Cost the prosed design (capital and operating) and repeat to optimise the design

Design of Pressure Vessels

• Mechanical design of pressure vessels

– Chemical Engineer will not usually be called on to undertake this task

– Who does it then??

• this is a specialised subject

• usually carried out by mechanical engineers who are conversant with the current design codes and practices and methods of stress analysis

– What’s the role of chemical engineer?

• Responsible for developing and specifying the basic design information for a particular vessel

• Responsible for developing and specifying the needs to have a general appreciation of PV design to work effectively with specialist designer

Design of Pressure Vessels

• What’s the basic data needed by the specialist designer??

– Vessel function

– Process materials and services

– Operating and design temperature and pressure

– Materials of construction

– Vessel dimensions and orientation; Type of vessel heads to be used

– Openings and connections required

– Specification of heating and cooling jackets or coils

– Type of agitator; Specification of internal fittings

Pressure Vessels

• What constitutes a pressure vessel??

– Any closed vessel over 150 mm diameter subject to a pressure difference of more than 1 bar

• Classification of pressure vessels

– Two types• Thin-walled (if the ratio of wall thickness to vessel diameter < (1/10))• Thick-walled (if the ratio of wall thickness to vessel diameter > (1/10))

– Class 1• majority of the vessels used in the chemical and allied industries

– Class 2• for high pressure applications

Blowdown vessel

Prevent vessel reaching the MAWP

API RP 521 recommends reducing the pressure in a vessel to 690 kPag or 50% of the vessel design pressure, whichever is lower, within 15 minutes.

Blowdown or Blowup!

Rapid decrease in vessel pressure causes the inventory to expand and cool

As the inventory cool it absorbs heat from the vessel walls

If the vessel walls cool below their ductile-brittle temperature they will be prone to failure.

Good estimate required for vessel design

Storage tank

Skid mounted carbon steel filter units

Skid mounted carbon steel filter units (side view)

Horizontal pressurised coagulation tank

Clean in space tank Steriliser and Flash Vessel

Steriliser, Flash Vessel and Condenser (different views)

Steriliser, Flash Vessel and Condenser (different views)

Principal Stresses

1 & 2 – Longitudinal and circumferential stresses

3 – Radial stress

• Thin-walled 3 is small and can be ignored 1 & 2 can be taken as constant over the

wall thickness

• Thick-walled 3 is significant 1 will vary across the wall

Pressure Vessel Codes and Standards

• National standards and codes of practice

– covers the design and fabrication of• thin-walled pressure vessels (in majority of the industrialised countries)

– In many countries• codes and standards are legally enforceable

– United Kingdom• British Standard specification for fusion-welded pressure vessels (BS 5500)• or equivalent code “American Society of Mechanical Engineers code (ASME)”

• BS 5500– covers vessels fabricated in carbon and alloy steels and aluminium

• BS 4994– covers the design of vessels in reinforced plastics


• United States of America

– American Society of Mechanical Engineers code (ASME)

– This code is divided into several sections which cover• unfired vessels• boilers• nuclear reactor vessels and• vessels constructed of fibre-glass-reinforced plastics

• What these national codes and standards supply??

– They dictate the minimum requirements and give general guidance • for design and construction

– Any extension beyond the minimum code requirement will be determined by agreement between the manufacturer and customer


• American Society of Mechanical Engineers code (ASME)

– In the textbooks frequent reference is made in the following manner

• ASME Division 1• ASME Division 2

– Division 1 Code will be designated as ASME VIII-1– Division 2 Code will be designated as ASME VIII-2

– Other ASME code sections• Such as Section II Part D will be designated as II-D

– Many design rules in VIII-1 and VIII-2 are identical• These include flange design and external pressure requirements

Codes and Standards

• ASME ‘American Society of Mechanical Engineers’ - From Construction to Post Construction

• NBIC ‘National Board Inspection Code - Repair and Inspection Code’

• API-510 ‘American Petroleum Institute - Pressure Vessels used in Chemical & Petroleum’

• ASNT ‘American Society Nondestructive Testing’ - Nondestructive Examination of Materials

• NEC ‘National Electrical Code [NFPA- National Fire Protection Association] -Electrical Devices

• NFPA ‘National Fire Protection Association’ - Service Conditions & Special Hazards

• API-650 ‘American Petroleum Institute’ - <15-psig to Atmospheric- Aboveground Storage Tanks

• DOT - Department of Transportation - Transportable Pressure Vessels

• UL ‘Underwriters Laboratory’- Aboveground Storage Tanks and Electrical Devices

• ASTM, ANSI,…are material specifications

Codes and Standards

• How these are formulated??

– Are drawn up by committees of engineers in vessel design and manufacturing techniques

– Are a blend of theory, experiment and experience

– Periodically reviewed and revisions will be issued to keep abreast of developments in

• design• stress analysis• fabrication &• testing

– Computer programs are available• to aid in the design of vessels to BS 5500 and ASME code from several

commercial organisations– For eg. Engineering Standards Data Unit in the UK

Shell of revolution

• A shell of revolution is the form swept out by a line or curve rotated about an axis

– A solid revolution is formed by rotating an area about an axis

• Most process vessels

– are made up from shells of revolution

– they could have different heads

• cylindrical• conical• hemispherical• ellipsoidal• torispherical

Solid revolution is formed by rotating an area about an axis

Fundamental Principles

• Principal Stresses– the maximum values of the normal stresses at the point, which act on

planes on which shear stress is zero

• Theories of failure

• Elastic stability

• Membrane stresses in shells of revolution

• Flat plates (are used as covers for manholes, as blind flanges and for the ends of small diameter and low pressure vessels– Types

• Clamped edges• Simply supported

Fundamental Principles

• Equation for flat plate

where:– t is thickness of the flat plate– C is a constant, which depends on edge support

• C = 0.43 (if the edge is rigid)• C= 0.56 0.43 (if the edge is free to rotate)

– D is the effective plate diameter– f is the maximum allowable stress– P is the pressure load

• Dilation of vessels– under internal pressure a vessel will expand slightly– radial growth can be calculated from the elastic strain in the radial

direction

f

PCDt

Stress Analysis

• In the stress analysis of pressure vessels

– pressure vessel components are classified as primary or secondary

• Primary stresses

– are those that are necessary to satisfy the conditions of static equilibrium

• eg. Membrane stresses induced by the applied pressure and bending stresses due to wind loads

– if they exceed the yield point of the material

• gross distortion and in the extreme situation, failure of the vessel will occur

Stress Analysis• Secondary stresses

– are those that arise from the constraint of adjacent parts of the vessels

– theses are self-limiting

– local yielding or slight distortion will satisfy the conditions causing the stress and failure would not be expected to occur in one application of the loading

• eg. Thermal stress set up by the differential expansion of parts of the vessel, due to

– different temperatures or the use of different materials

– The discontinuity that occurs between the head and the cylindrical section of a vessel is a major source of secondary stress

– Other sources• are the constraints arising at flanges, supports and the change of section due to

reinforcement at a nozzle or opening

General Design Considerations(Pressure Vessels)

• Design pressure and design temperature

• Design stress

• Welded joint efficiency

• Construction categories

• Corrosion allowances

• Design loads

• Minimum practical wall thickness

Design Pressure

• Vessel designed – must be able to withstand the maximum pressure to which it is likely to

be subjected in any given operation

• For vessels under internal pressure– design pressure is normally taken as the pressure at which the relief

device is set• this will normally be 5-10% above the normal working pressure

– when deciding the design pressure• good to add hydrostatic pressure in the base of the column if significant

• For vessels subjected external pressure– should be designed to resist the maximum differential pressure that is

likely to occur in service

Design Temperature

• What effect it has on materials??

– Strength of metals decreases with increasing temperature

– Maximum allowable stress will depend on the material temperature

– What needs to be done from design point of view?

• Design temperature at which the design stress is evaluated

– should be taken as the maximum working temperature of the material

• Do make some allowances for any uncertainty involved in predicting vessel wall temperature

Materials

• Pressure vessels are constructed from– plain carbon steels, low and high alloy steels, other alloys, clad plate

and reinforced plastics

• What factors one must consider while selecting material??– Suitability of the material for fabrication (in particular welding)

– Compatibility of the material with the process environment.

• Refer to the pressure vessel design codes and standards– which includes lists of acceptable materials

• in accordance with the appropriate material standards

– For eg. In UK, carbon and alloy steels for pressure vessels are covered by BS 1501 plates, BS 1502 section etc.

Design Stress

• From design point of view, it is necessary to decide– a value for the maximum allowable stress (nominal design strength) that

can be accepted in the material of construction

• How it is done??– By applying suitable design stress factor (factor of safety) to the

maximum stress that the material could be expected to withstand without failure under standard test conditions

– Design stress factor allows for any uncertainty in the design methods, the loading, the quality of the materials and the workmanship (refer to BS 5500)

• Design stress is based on– yield stress or tensile strength of the material at the design temperature

(for materials not subject to high temperatures)

Welded Joint Categories (ASME VIII-1)

Category A joints consist mainly of longitudinal joints as well as circumferential joints between hemispherical heads and shells.

B joints are the circumferential joints between various components

Attachment of flanges to shells or heads is a Category C joint

The attachment of nozzle necks to heads, shells and transition sections is categorised as a D joint

Welded Joint Categories (ASME VIII-1)

•Four joint categories in VIII-1 do not apply to the following items- jacket closure bars- tube sheet attachments- ring girders (or supports)

•Degree of examination of the welds attaching these components to the shell or head is not covered in VII-1.

•Most designers assign a value an E value of 1.0 when calculating the shell or head thickness at such junctions.

Joint Efficiency Factors

• As per ASME VIII, Div 1 and 2

– All major longitudinal and circumferential butt joints must be examined

• By full radiography with few exceptions

– VIII-1 in particular permits various levels of examination of these joints

– Why these joints are examined?• To detect the internal defects in the weld

– Examination varies• From full radiographic to visual

– Depending on various factors specified in VIII-1 and by the user


• The degree of examination influences

– The required thickness through the use of Joint Efficiency Factors (E)

– Sometimes these are called as Quality Factors or Weld Efficiencies

• Serve as stress multipliers applied to vessel components

– When some of the joints are not fully radiographed

• These multipliers results in an increase in the factor of safety as well as the thickness of these components


• In essence, VIII-1 vessels have variable factors of safety

– Depending on the degree of radiographic tests of main vessel joints

– For eg. Joint Efficiency Factor in a fully radiographed butt-welded joints in cylindrical shells have a E = 1.0

• E= 1 corresponds to a safety factor of 4 in the parent material

– Non-radiographed longitudinal butt-welded joints have an E value of 0.7

• This reduction in E corresponds to factor of safety of 5.71 in the plates

– Highest factor of safety due to a nonradiographed joint results• In a 43% increase in the required thickness over that of a fully

radiographed joint


• Factors used to design a component are

– Dependent on the type of examination performed at the welds of component.

– For eg. The Joint Efficiency Factor in a fully radiographed longitudinal seam of shell course E = 1.0

• Taking the factor E as 1.0 implies that the joint is equally as strong as the virgin plate

• However, this number may have to be reduced, depending on the degree of examination of the circumferential welds at either end of the longitudinal seam.

• Several handbooks show some typical components and their corresponding Joint Efficiency Factors.

Category C weld which identifies the attachment of a flange to a shell, can be either fillet, corner, or butt welded

Joint Efficiency Factors apply only to the butt-welded joint in sketch c

The factors do not apply to sketches a and b.

Category C weld which identifies the attachment of a flange to a shell, can be either fillet, corner, or butt welded

Joint Efficiency Factors apply only to the butt-welded joint in sketch c

The factors do not apply to sketches a and b.

The categories refer to a location within a vessel rather than detail construction.

Eg. Category C weld which identifies the attachment of a flange to a shell, can be either fillet, corner, or butt welded as illustrated on the next slide.

Corrosion Allowance

• This is the additional thickness of metal added to allow for material lost by corrosion and erosion or scaling– Allowance to be used will be based on the agreement between the

customer and manufacturer

• Corrosion is a complex phenomenon– it is not possible to give specific rules for the estimation of the

corrosion allowance required for all situations

• How do deal with this matter?– The allowance should be based on experience with the material of

construction under similar service conditions to those for the proposed design

– For carbon and low-alloy steels 2 mm (where no severe corrosion is not anticipated); 4 mm for severe conditions

• Most design codes and standards specify a minimum allowance of 1 mm

Design Loads

• A structure must be designed – to resist gross plastic deformation and collapse under all the conditions of

loading

• Classification of loads– Major loads– Subsidiary loads

• Major loads– design pressure including any static head of liquid– maximum weight of the vessel and contents under operating conditions– Loads supported by, or reacting on, the vessel– maximum weight of the vessel and contents under hydraulic test

conditions– Earthquake loads and wind loads

Design Loads

• Subsidiary loads– Local stresses caused by supports, internal structures and connecting

pipes

– Shock loads caused by water hammer

– Bending moments caused eccentricity of the centre of working pressure relative to the neutral axis of the vessel

– Stresses due to temperature differences• subsequent effect arising due to the differences in the coefficient of

expansion of materials

– Loads caused by fluctuations in temperature and pressure

• A vessel will not be subject to all these loads at the same time

• Designer must determine – possible combinations of these loads likely to result in worst situation

and design for that loading condition

Minimum Practical Wall Thickness

• Why we need to bother about this??– It is important that there is a minimum wall thickness required

• to ensure that any vessel is sufficiently rigid to withstand its – own weight

– any incidental loads

• General rule– the wall thickness of any vessel should not be less than the values

given below

– these values include a corrosion allowance of 2 mm

Vessel diameter (m) Minimum thickness (mm)1 5

1 to 2 72 to 2.5 9

2.5 to 3.0 103.0 to 3.5 12

Basic Mechanical Details

• vessel openings (nozzles, stub ends)– compensation

• support design (saddles, ladders, walkways)

• flanges and ratings– flat face (FF)

– raised face (RF)

– slip-on/Van Stone

– ring

– spigot/socket

• internals (weirs, supports, plates, distributors etc.)

• materials of construction/gasketing/finishing

Entries and Nozzles

Inlet Standard nozzleSS nozzle & welded neck joint(Std ANSI 150)

Inside Pressurized Shell and Pipe

Codes: VIII div1 A2000, UG-32, UCS-79(d)

Inside Pressurized Heads

Codes: VIII div1 1998 A2000 section UG-32, App. 1-4, UCS-79(d),

UG-32 (f) and UG-37 1(a)

Fittings

Lifting Lugs Stiffening Rings

Vessel supports

Saddle supports Welded legs

LEG SUPPORT DESIGN

Vertical vessel mounted on beams

B16.5 Flanges

Codes: ASME VIII div1 2001, UW-15(c) -- ASME B16.5 1996

EQUIPMENT SCHEDULE:

EQUIPMENTNUMBER

SPECIFICATIONNUMBER

PLANNUMBER

DESCRIPTION LOCATION PURPOSE ORDER NO SUPPLIER

T-xxx ES-xxx PFD-xxx Pressurized NG storage Bulk storage facility TBA TBA

EQUIPMENT SCHEDULEDEPARTMENT OF CHEMICAL ENGINEERING

Project: Space Launch SystemsCLIENT: XXXXXX Specification No: ES-1001

Initiator: Checked: Approved: Date: 16/09/04 Ver. A Page1Of 1

Requirements for Sizing and Specification

• Tanks and General Pressure Vessels

– Type (Cone roof, floating, cylindrical PV) Type (Cone roof, floating, cylindrical PV)

– Capacity (m Capacity (m3)

– Length, height, diameter (m) [aspect ratio]

– Operating/Design Pressure, temperature

– Orientation (vertical, horizontal, spherical)

– Nozzles Nozzles – size (NB), type, rating and location • Inlets, outlets, drains Inlets, outlets, drains

• Instruments ( Instruments (LGs LGs, P, L, T), sampling, PVRV

• Foam entry points (storage tanks)

– Supports (Saddle, legs, plinths, pads)

– Materials selection Materials selection

Requirements for Sizing and Specification

• Heat exchangers– Type (S&T Type (S&T- TEMA Std, plate, spiral, compact)

– Duty (kW) & Heat transfer area (m2)

– Length, height, diameter (m)

– S&T Orientation (vertical, horizontal)

– Shell side & Tube side operating /design conditions

– Nozzles Nozzles – size (NB), type, rating and location • Inlets, outlets (process flows)

• Steam / Cooling water

• Instruments (LGs, P, L, T)

– Supports (Saddle, legs, plinths) Supports (Saddle, legs, plinths)

– Materials selection Materials selection

Sample Data Sheet (Appendix H)

Case Study

Heat Exchanger Details

Tube pitch options

Triangular: higher heat transfer, higher pressure dropSquare: lower HT, lower pressure drop, easier cleaning

Shortcut design procedure - HE’s

Look up for U and tube counts from Tables

Mechanical Design of Process Systems

Major references:

• McKetta, J., “Encyclopedia of Chemical Processing and Design”

• Ulrich, G., “A Guide to Chemical Engineering Process Design and Economics”

• Escoe, A.K., “Mechanical Design of Process Systems” Vols 1 and 2

• Backhurst, J. and J. Harker, “Process Plant Design”

• Coulson, J., Richardson, J. and R. Sinnott, “Chemical Engineering Vol 6 Design”

• Chemical Engineering, (bimonthly magazine from McGraw Hill) TN1.M43

• Australian Standards & British Standards

Utilities & Design Considerations

• These are the items required in any process plants– also called process consumables– can be divided into several of the following categories

• Water• Steam• Electric Power• Refrigeration• Compressed air• Inert Gas• Miscellaneous

• Water– What’s the use of this utility in a Chemical Process

Plant??• Used in chemical reactions and in washing, extracting,

dissolving and similar processing operation, for drinking, sanitary and general clean-up, washing etc.

Utilities

• Types of water

– Fresh water, treated or untreated well or city water– in some cases distilled water or deionized water may be required

• This utility could be used for extracting or adding heat to the system

• Steam

– Some unit operations (such as removal of HC’s from oil mixtures or steam reforming etc.) require large amounts of steam

– Usually supplied by an in-house gas fired boiler– Radiation and line losses must be taken into account

while design estimates are made

Utilities

• Electric Power– Several process equipment's and motors require electric

power– can be accessed from the local power generating company– Additional power can also be generated within the plant

• either gas or fuel based• to reduce the load or dependence on the main supply

(partially)• use for temporary power failures

• Refrigeration– Used mainly to supply cooling water– Examples

• for single-stage ammonia compression which requires cooling water at 85°F

• Steam jet refrigeration used to obtain cooling water at 50°F

(steam jet refrigeration is especially advantageous with cheap low-pressure exhaust steam and cooling water available)

Utilities• Compressed air

– used for pneumatic transport, pumping air into the system (power utility) etc.

– usually supplied from an air compressor– Air compressor

• A machine which usually sucks in air and compresses same so that it ultimately occupies a smaller volume.

• The resulting air will not only occupy a smaller volume, it will have a higher pressure and a higher temperature.

• Inert Gas– N2, CO2, Ar and He are examples of inert gases

– used for start-up and purging operations

• Miscellaneous– This includes gaskets, lubricants, paint, test chemicals etc.

Cooling Water Systems

• Three systems normally used are:– Once through– Open evaporative recirculating– Closed non-evaporative recirculating

• Once through systems– Cooling water passes through the heat exchanger once.

– Once through systems can be used when plenty of cheap cool water is available and adequate facilities for disposal of warm water exist.

Advantages:

– No cooling tower system; No water treating

Disadvantages:

– Corrosion; Fouling, Waste of water and Thermal pollution of river


• Open evaporative recirculating systems– Cooling water evaporate about 1% water

• Water is reused after make up.

Advantages:

• Less water required

• Enhanced corrosion control feasible

Disadvantages:

• Higher capital cost than once through

• Large cooling towers may be unacceptable

• System purge may pose environmental problems


• Closed nonevaporative recirculating systems– Cooling water is cooled in a secondary

(air) heat exchanger.

– No evaporate

– No makeup.

Advantages:

– Water remains clear

– Cooling water temperature above 100oC is possible

Disadvantages: – High capital cost

– Limited by air temperature

– Open evaporative systems are usually used


• Evaporation in the cooling tower causes

– a build up of suspended/dissolved solids which can inhibit heat transfer by building up on heat exchanger surfaces - usually mould steel.

• Two problems in cooling water system

– Fouling

• silting/sedimentation (particles in source water, e.g. sand)

– scaling (precipitation of salts)

– biological growth (heat, oxygen, phosphates promote biological growth)

– corrosion


• Cooling water treatment is required – to overcome fouling and corrosion problems.

– purpose of water treatment is to control fouling and corrosion.

• Environmental considerations may restrict – the disposal & choice of treatment chemicals,

• e.g. chromate treatments are widely applied in view of their corrosion protection. However, the discharge of chromate treated water is viewed with increasing concern.

• Inlet water quality must be first known:– e.g. pH, total dissolved solids, suspended solids, Ca++,

SO4--,

Scale formation

• Precipitation of the least soluble salts may occur

– for eaxmple e.g. CaCO3, CaSO3.

– Ca++ + 2(HCO3)-- CaCO3 + H2O + CO2

• High concentration of Ca++ and SO4-- may result in

– calcium sulphate scale (CaSO4).

• What are the effects of scale formation?– Scale affects heat transfer efficiency

• With stainless steel, scaling may promote stress corrosion cracking

– Pumping cost will increase

– Energy requirements will be higher

Scale prevention

• Scale prevention – Higher system purge

• to reduce CF – at the expense of higher water/chemical costs.

– Soften makeup water: • using external ion exchangers.

– Acid treatment to reduce [CO3--]:

• with water of medium to high CaCO3, i.e. > 800 mg/l, reducing the alkalinity to 20 - 40 mg/l will reduce CO3

-- below the scaling level. H2SO4 or HCl are normally used.

– Scale inhibitors: • modify crystal scale growth

– inorganic: polyphosphates

– organic: phosphorous compounds

Steam

• Steam is used as a medium for transferring and transporting energy.

– Heating by steam condensation (heat exchanger) – Mechanical work done by steam expansion (through turbine) – Energy stored by latent heat and pressure

• In plant environment, typically a steam system includes:

– Central steam boiler

– Steam main circuit around plant

– Heat exchangers for process heating

– Condensate return line to boiler

Steam

• Issues for steam distribution – Distribution pressure – Pipe expansion – Heat loss – Condensate/air removal

Distribution pressure

• High pressure– Advantages: smaller mains; low installation cost; less insulation

required

– Disadvantages:

• high pressure heat exchanger equipment or local pressure reduction valves required

• difficult to recover low grade heat (low temperature) as regenerated steam

Issues of Steam Distribution

Pipe expansion

– Difference in pipe dimension

• when in use and when not in use

– Expansion allowance required

– Expansion fittings:

– full loop; horse shoe; sliding joint

Heat loss prevention

– Steam is hotter than surroundings, therefore heat loss is inevitable.

– Lagging is used to prevent heat loss

– Typically lagged pipe heat loss 5-10% of that from bare pipe


Condensate/air removal (steam traps)

– Condensate collects at low points in pipe system.

– If not removed, pipe network will eventually be liquid filled.

– "Water hammer":

• Fast moving gas meets slow moving slug of liquid resulting in rapid vibration of pipe work.

– Condensate accumulation controlled by deliberate slops in pipe work with intermittent drain points.

– Drain points are known as "Steam Traps".


Types of steam traps– Thermostatic steam traps

– Based on temperature difference between steam and condensate.

• Liquid expansion steam trap

– Bimetallic steam trap

– Balanced pressure steam trap

– Mechanical float steam trap

• Based on density difference between steam and condensate.

Air removal

• When the steam system is shut down– the pipe network is usually air filled

– Air can be purged using thermostatic steam traps

– because the temperature of air is lower than that of steam.

Boiler Feed Water Treatment

• Loss of water from steam system due to:– Steam leaks

– Loss of condensate

– Blowdown

• Therefore, makeup of fresh water to the boiler is required.– Fresh water contains impurities:

• Suspended solids

• organic matter

– dissolved salts

• These will lead to fouling and corrosion (as with cooling system). – Here the major concern is for boiler which is operated

at high temperature and high pressure.

Nitrogen

• Use of nitrogen:

– Blanketing flammable or O2 sensitive mixture

– Purging flammable volatile vapour from empty vessels

– Transferring flammable or O2 sensitive liquids between vessels

– Process gas stream diluent

• N2 sources:

– Delivered from N2 manufacturer

– On site generation

Nitrogen

• Delivered N2

– Advantages:

• high purity supply possible

• flexibility in volume supplied

• little or no capital investment in N2 producing equipment

• standard delivery pressure up to 10 atmosphere

• liquid N2 supply as coolant

• Deliver options depends on volume required – Gas cylinders (up to 150 kg/day) or Dewar flasks (liquid

N2, 18 ton/day)

Nitrogen

• On-site N2 generation

– Cryogenic purification

– Pressure swing adsorption

– Membrane separation

• Selecting N2 supply

– Decision depends on

• Flow rate required

• Purification of N2 required

• Pattern of demand

• Cost of electrical power available on-site

• Temperature (coolant) and pressure of supply required

• Liquid or gas N2 required

Compressed Air

• Use of compressed air

– Pneumatic instruments/controllers (2.5bar)

– Pneumatic driven equipment, e.g. pumps, drills, etc. (6bar)

– Pneumatic conveying (granulated solids)

– Aeration of fermentation (O2 supply)

– Drying solids

– Air stripping (organic removal from H2O)

• For these purposes the air used must be – Free of particulate (dust, fume, rust)

– Free of condensibles (moisture, hydrocarbons)

– free of other contaminants (SO2, H2S, etc.)

Compressed Air

• What happens if you have particulate material??

– Particles in the air are carried into compressor

– trapped in lubricating oil

• can be very damaging to the bearings

• conveyed around air distribution network – contamination of product

• damage pneumatic instruments/control valves

• abrasion damage to pneumatically driven mechanical equipment

Compressed Air

• Filters used on air intake– Oil wetted filters: labyrinth filter, oil bath filter

• particles forced to impinge on surface wetted with oil

• particles trapped in the oil film

• oil droplets may be entrained and carried through to compressor– therefore use same oil as compressor lubricant

• regular cleaning and maintenance required

– Fabric filters

• "deep pleated" woven and nonwoven fabrics (gives large filter area)

• supported by wire mesh

• used to oil free air supplies

• cleaned by back flushing and/or solvent rinse

– Paper filters are similar to fabric filters

• mechanically less strong

• needs to damp pulsations that occur with reciprocating compressors

Compressed Air• Condensibles

– water and lubricant oil present as vapour

– initial compression reduces air capacity for H2O at ambient temperature, therefore condensation occurs

– subsequent fluctuation in air temperature (e.g. indoor to outdoor lines) during distribution can cause condense of H2O

– water precipitate problems

• freezing in lines/equipment (damage)

• corrosion of lines and equipment • oil/water emulsions clogging of equipment • water hammer in pipe work vibration damage

– Removal of water

• Air dryers, eg. packed column of desiccant (e.g. silica gel)

• regenerated by hot stream of air in reverse flow - Temperature Swing Adsorption

Compressed Air

• Air filters/water separators

– inlet air encouraged to follow rotating motion

– particles/droplets impinge on wall and settle at base of chamber

– splash guard prevents re-entrainment

– exit air forced through a filter element to remove fine particles

• Water traps

– As with steam distribution, liquid must be released periodically.

– Mechanical float traps used

Documents

Equipment Design