heat exchanges & tubesheets - design & verification

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A division of Midas Engineering Group

HEAT EXCHANGERS & TUBESHEETS - DESIGN & VERIFICATION

By Catalin Iliescu, Senior Mechanical Design Engineer

> Structural > Mechanical > Design > Verification > Project Management > Structural > Mechanical > Design > Verification > Project Management

> DISCLAIMER With respect to all the information contained herein, neither CDMS Consulting Engineers, nor any

officer, servant, employee, agent or consultant thereof make any representations or give any warranties,

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contents hereof.

> COPYRIGHT © Passing of this document to a third party, duplication or re-use of this document, in whole or part,

electronically or otherwise, is not permitted without the expressed written consent of CDMS Consulting

Engineers.

> ACKNOWLEDGEMENTS This document is a dynamic record of the knowledge and experience of personnel at CDMS Consulting

Engineers. As such it has been built upon over the years and is a collaborative effort by all those

involved. We are thankful for the material supplied by and referenced from various equipment

manufacturers, vendors, industry research and project partners.


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Introduction

Purpose of presentation

> Presenting the process for the design and design verification of heat

exchangers used in the processing industries

Definition

> A heat exchanger is equipment that allows the transfer of heat from

one fluid (liquid or gas) to another fluid. The fluids are normally

pressurised therefore heat exchangers are classified as pressure

vessels and their design and the equipment requires registration with

the relevant Statutory Authority

Types of Heat Exchangers (HE)

> There are many types of HE and the most commonly used in oil and

gas are: – Shell and tube heat exchangers

– Air cooled heat exchangers (box header types)


Regulatory Requirements

> Many statutory authorities in Australia – e.g. Worksafe (Workcover), Dep’t of Mines & Petroleum, Comcare, NOPSEMA & Dangerous Goods Dep’t

> Generally if the facility is a mine site then Dep’t of Mines has jurisdiction. If equipment is on an offshore facility then NOPSEMA is the authority and each offshore facility operates under a safety case presented by the operator and approved by NOPSEMA

> Not all Australian statutory requirements for pressure equipment are uniform. They vary greatly from state to state and even within each state

> It is imperative that pressure equipment procurement departments and their engineers follow the requirements of the relevant regulatory authority and state regulations prior to purchasing new pressure equipment or repairing / modifying existing pressure equipment


Hazard Level

> AS4343 is the standard for determining hazard levels for pressure

equipment

> Statutory regulations require certain types of plant (classified plant) to have the designs registered and individual items of plant to be registered with the relevant authority

> Not all pressure equipment falling within AS1200 is deemed to be classified plant and therefore not all pressure equipment needs to be registered

> The requirements vary across states of Australia


Quality Systems

> Design verifying bodies, manufacturers and inspectors must have certified

quality systems in place to verify designs, manufacture and inspect pressure

equipment of certain hazard levels

> AS3920.1 - Assurance of Product Quality – provides the relationship between

the equipment hazard level and the requirement for a certified quality system

> Certification to ISO 9001 / 9002 alone does not qualify the organisation to

supply pressure equipment or services relating to design verification /

inspection of pressure equipment under AS1200. The quality system must also

includes certification to AS3920.1

> Currently, design verifying bodies must be certified in Australia and must be

acceptable to the regulatory authority


Mechanical Design

> Pressure vessels (AS1210-2010 / ASME VIII Div. 1, 2 & 3 / PD5500)

> Design by code formulae – shells, heads, opening & reinforcement, vacuum,

supports etc.

> Using other empirical methods such as WRC 107, 297 for nozzle loads

> Design using advanced stress analysis e.g. FEA techniques for complex stress

analysis of components

> Proof testing to prove a design – usually a complex one

> There are 8 classes of construction in AS1210-2010

> Design to consider, low temperative service, stress corrosion cracking, wind induced vibration, external loads such as platforms, pipe supports, vessel supports, piping loads onto nozzles, thermal expansion / contraction, buckling, transport / lifting loads, differential pressure, etc.

> Different materials and / or thicknesses may mean different classes of construction are required


Mechanical Design

> Support design needs special consideration especially when high

external loads such as piping loads are carried through the vessel

supports

> Proprietary software for pressure vessel design or component design

often has errors or can be too simplistic to use in some cases and

users must have a good understanding of code formulae to be able to

make a sound engineering judgement on whether results are valid

> Advanced analysis (FEA) is becoming more common. This tool must

be used by experienced designers who are well conversant with stress

extrapolation and classification against code limits. Boundary

conditions are often modelled incorrectly


Design Verification

> The design verifier is required to keep all designs submitted for verification as

strictly confidential and must only disclose the details to the statutory authority,

certifying body or a person authorised by the owner of the design

> The design verifier is not permitted to have any involvement in the design other

than for verification purposes

> The design verifier must have the necessary qualifications and experience (per

AS3920.1 CL 4.5 (b) and the design verifying body must have the necessary

certified quality systems

> In some instances (for equipment with hazard levels C & D), the design and

design verification can be carried out by the same organisation as long as it has

the certified quality systems to ISO 9001 / AS3920.1 and the design verifier had

no involvement with the design


Design Registration

> In WA where pressure equipment is imported and requires design registration,

the importer takes on the role of the designer and must sign design registration

application forms taking responsibility for the design

> The design verifier must also sign a design verifiers statement and/or supply a

certificate with the design registration application form

> For design registrations under the Department of Mines WA, only AS1210

designs are acceptable. Be aware that with most suppliers of imported

equipment, they may not be able to fully comply with the requirements

necessary to satisfy AS1210 and applicable codes referenced therein. Some

other states have similar requirements

> In some circumstances, applications for exemption to meeting the statutory

requirements e.g. design code, may be granted by the regulatory authority


Mechanical Design of Air Cooled Heat Exchangers

Sketch of air cooled heat exchanger in two typical configurations


Air Cooled Heat Exchangers (ACHE)

> Consist of 2 box headers connected by tubes.

> Can be single or multi-pass

> They are normally designed by ASME VIII Div 1 Appendix 13

> Formulas from this appendix calculate stresses at the junctions and

middle of the plates. These stresses are amplified considering the weld

and/or ligament efficiencies

> This approach is often conservative since the tube holes in the tube

plate may be placed at lower moment locations

> A more accurate procedure is to model a strip of the section and

calculate the moments at the hole and junction locations as for a frame

with uniform distributed load. This can be done using a structural

software or FEA with plate elements


Possible Problems for the Design of ACHE

> Inlet /outlet nozzles may be special shape (swaged) and requires

calculation using FEA. Due to high deformation during forming may

require heat treatment (normalising for C/S and solution annealing for

austenitic S/S)

> Nozzle loads are normally defined at the flange face. This requires that

the flange to be checked for internal pressure + nozzle loads, as well

as the top plate, nozzles and tubes

> Tubes have different mean metal temperatures and there is differential

thermal expansion that must be considered


Example of ACHE Design

Besides internal

pressure, the

tube rows

operate at

different

temperatures

and the nozzles

have imposed

piping loads


Example of ACHE Design (cont)

> Deformed shape of the tube bundle due to differential thermal

expansion.



> Due to the complex shape of the inlet nozzle and external piping loads,

the design was checked using FEA



Results


Shell and Tube Heat Exchangers

Main components of a shell and tube heat exchanger


Shell and Tube Heat Exchangers (cont.)

> Various configurations of HE

> By tubesheet arrangement they can be: – With two fixed t/s or

– Stationary ts+u-type ts

– Stationary ts + floating head

EX. 1: Straight Tube, Fixed Tubesheet, Type BEM, AEM, NEN, etc.

This TEMA category, especially the NEN, is the lowest cost

TEMA design per square foot of heat transfer surface

Advantages

> Less costly than removable bundle designs

> Provides maximum amount of surface for a given shell

> and tube diameter

> Provides for single and multiple tube passes to assure

> proper velocity

> May be interchangeable with other manufacturers of the

> same TEMA type

Limitations

> Shell side can be cleaned only by chemical methods

> No provision to allow for differential thermal expansion

> May require an expansion joint

Applications

> Oil coolers, liquid to liquid, vapour condensers, reboilers, gas

coolers

> Generally, more viscous and warmer fluids flow through the shell

> Corrosive or high fouling fluids should flow inside the tubes



EX. 2: Removable Bundle, U-Tube, Type BEU, AEU, etc.

> Especially suitable for severe performance requirements with

maximum thermal expansion capability. Each tube can expand

and contract independently

> Suitable for larger thermal shock applications

Advantages

> U-tube design allows for differential thermal expansion between

the shell and the tube bundle as well as for individual tubes

> Shell circuit can be inspected and steam or mechanically

cleaned

> Less costly than floating head or packed floating head designs

> Provides multi-pass tube circuit arrangement

> Capable of withstanding thermal shock applications

> Bundle can be removed from one end for cleaning or

replacement

Limitations

> Because of u-bend, tubes can be cleaned only by chemical

means

> Because of U-tube nesting, individual tubes are difficult to

replace

> No single tube pass or true counter current flow is possible

> Tube wall thickness at the U-bend is thinner than at straight

portion of tubes

> Draining of tube circuit is difficult when mounted with the vertical

position with the head side up

Applications

> Oil, chemical and water heating applications

> Excellent in steam to liquid applications

Source: Tubular Exchange Manufacturers Association



> Double tubesheet HE. Used when mixing of fluids from both sides cannot be tolerated (e.g. water and

TiCl4)

> Because tubesheets operate at different temperatures their radial thermal expansion is different

> This forces the part of the tubes outside the shell to bend significantly

> Based on the value of differential thermal expansion the gap of the double tubesheet must be sized, to

avoid failure of the tube/tubesheet junction



Design data to be provided to the Mechanical Engineer

> Thermal design details in the form of TEMA or an equivalent

specification sheet.

> TEMA class, type of TEMA shell, channels/heads (if required to

comply with TEMA)

> Shellside and tubeside passes

> Number, type, size, and layout of tubes

> Diameter and length of shell, channel/head, and its configuration

> Design temperatures and pressures

> External pressure if the equipment is under external pressure or is

under internal vacuum

> Worst-case coincident conditions of temperature and pressure



> Nozzle, wind, and seismic loads, impact loads

> Superimposed loads due to insulation, piping, stacked units, etc.

> Corrosion properties of the fluids, the environment in which the unit will

be installed and the expected service life. This will help to specify

corrosion allowances or better material selection to reduce the material

loss due to corrosion

> Materials of construction except tube material, which is arrived at

during the thermal design stage

> Fouling characteristics of the streams to be handled by the exchanger.

This will determine if closures are required for frequent cleaning of

internal parts of the exchanger. Many fixed tubesheet heat

exchangers, if not specified otherwise, may be of welded head and

shell construction

> Special restrictions imposed by the purchaser on available space,

piping layout, location of supports, type of material, servicing

conditions, etc.



> Construction code and standard to be followed

> Installation - vertical or horizontal

> Installation and operation considerations like start-up, transients,

shutdown and upset conditions that decide tubesheet thickness

> Handling of lethal or toxic fluids, which demand more stringent welding

and NDT requirements



Considerations for the design of shell and tube HE > Partition gasket must be considered in the body flange calculation

> If there are nozzle loads on channel nozzles their resultant at the body flange location must

be considered for flange design

> For a flange pair with different pressures on shell and tube sides the flanges must be

designed for the highest bolt load

> For HE with two fixed tubesheets the differential thermal expansion must be considered for

tubesheet, shell and tube design. Depending on the conditions the shell may be required to

be fitted with an expansion joint which also requires special design.

> For double tubesheets operating at different temperatures, the differential thermal

expansion for periphery tube rows must be considered to allow for extra flexibility of the

tubes between the adjacent tubesheets

> Various combinations of tube side and shell side pressure and temperatures must be

considered for the strength design in corroded and uncorroded conditions, to decide the

worst case operating scenario

> Design based only on differential pressure between tubeside and shellside is normally not

allowed unless the same stream of fluid passes through both sides unblocked by valves or

other equipment



Screenshot with software

input for design

combination cases



> Example of page from a HE datasheet showing the design conditions

and materials



> Example of mean metal temperatures for tubes and shell for HE with

fixed tube sheets (datasheet extract):



> Example of HE sketch with dimensions and orientation (datasheet

extract)



> Example of tube layout pattern (datasheet extract)


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