section 02- heat transfer

8/20/2019 Section 02- heat transfer

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EDS 2004/EXC 2-1

Heat Exchangers - Course Content

Design Requirements

TEMA Nomenclature Selection of TEMA Type Layouts

Tube Pass Geometry

Minimizing Leakage Streams

Pass Partitions

Baffle Types

Baffle Selection Criteria

Flanges

Tube to Tubesheet Joints

Impingement Plates

Vapor Belts

Materials of Construction Protective Coatings

Design Temperature & Pressure

Section 2 - Introduction to Shell & Tube Exchangers

Table of Contents - Section 2 Mechanical Elements


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EDS 2004/EXC 2-2

Shell and Tube Heat Exchangers

How do we turn this -

Into this -

EXC-R00-09

The design process for a heat exchanger starts with the process design. The two

streams that need to be exchanged are determined.

The next biggest step is turning that concept in a physical reality with real

equipment. The physical reality of exchangers provides many of the limitations on

what is a possible/practical process design.


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EDS 2004/EXC 2-3

Data Sheet

Process Conditions

Mechanical Data

The design process for a heat exchanger starts with the process design. The two

streams that need to be exchanged are determined. The data sheet shows the

process conditions.

The mechanical section list the type of exchanger, surface area, metallurgy, tube

length, number of tubes, tube diameter, tube thickness and many other important

items of information.

Every process engineer should have a copy of the exchangers in his unit. The API

Data sheet is the starting point for an exchanger evaluation.


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EDS 2004/EXC 2-4

Shell and Tube Heat Exchangers

The exchange has many parts. It is important to learn the terminology. The AES

type shown is the most common exchanger in the refinery

The next biggest step is turning that concept in a physical reality with real

equipment. The physical reality of exchangers provides many of the limitations on

what is a possible/practical process design.


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EDS 2004/EXC 2-5

Design Requirements

Fluid Streams:

– Flow Rate

– Thermophysical Properties of Fluid

– Temperature

– Pressure

– Pressure Drop Limitations

– Phase (Liquid/Vapor/Mixture)

– Boiling/Condensing

– Fouling Potential

– Corrosion Potential

Geometric Concerns – Space Constraints

– Weight Constraints

– Manufacturing Limits

What are the key parameters that are used to develop a

design for a heat exchanger?

What is required for the physical design of an exchanger.

The two streams must be completely defined.

Any physical restraints on the design must be defined. The physical constraints

may be due to the site, equipment at the site for maintenance activities, physical

constraints due to the ability of the fabricator to build the exchanger, or to physical

limitations on being able to ship the exchanger to site.


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EDS 2004/EXC 2-6

Design Requirements(continued)

Operational Concerns – Spare Capacity

– Equipment Failure

– Control

Overriding Concerns

– Economics

– Economics

– Economics

Other concerns include operational concerns in the event of failure, on-line

maintenance such as cleaning, temperature controls, etc.

However, the overriding concern is normally cost. The design of the exchanger

must be based on achieving a fully functional piece of equipment for the lowest

cost.

Cost can be defined as either initial purchase price or as the total cost of ownership.

Total cost of ownership involves determining the cost of the initial purchase,

expected maintenance costs, etc. till demolition and scrap value.

Which cost is used will vary for different users.


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EDS 2004/EXC 2-7

TEMA - Tubular Exchanger Manufacturers Association

Heat Exchanger Nomenclature - Front Heads

EXC-R00-14

A B

NC

D

TEMA produces a standard for the design of heat exchangers. This is a group of exchanger

suppliers getting together to define terminology and practices that all suppliers can agree to.

One of the most useful things in the TEMA standards is a code to describe the overall configuration

of shell & tube heat exchangers. They can come in a vast variety of designs and using this code, theconfiguration is readily conveyed. The code consist of three letters covering the front head, shell,

and rear head of the exchanger.

A Type - Channel with a Removable Cover (Most Common)

•Easy to open for tubeside access

•Extra tube side joint

B Type - Bonnet or Channel with an Integral Cover

•Must break piping connections to open exchanger

•Single tube side joint

C Type - Channel Integral with the Tubesheet with a Removable Cover

•Channel to tubesheet joint eliminated

•Bundle integral with front head

N Type - Channel Integral with the Tubesheet with a Removable Cover (Fixed Tubesheet)

•Fixed tubesheet with removable cover plate

D Type - Special High Pressure Closure

•Special closures for high pressure applications


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EDS 2004/EXC 2-8

TEMA - Tubular Exchanger Manufacturers Association

Heat Exchanger Nomenclature - Shell Types

EXC-R00-15

E J

F K

G X

H

Shell Types

E Type - One Pass Shell

•Most common configuration without phase change.

F Type - Two Pass Shell with a Longitudinal Baffle

•Countercurrent flow obtained. Baffle leakage problems.

G Type - Split Flow

•Lower pressure drop.

H Type - Double Split Flow

•Thermosyphon reboilers.

J Type - Divided Flow

•Flow can enter or exit via the single nozzle.

•Older reboiler designs.

K Type - Kettle Type Reboiler

•Phase separation integral to exchanger.

X Type - Cross Flow

•Lowest pressure drop.


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EDS 2004/EXC 2-9

TEMA -Tubular

Exchanger Manufacturers

Association

Heat Exchanger

Nomenclature -

Rear Head Types

EXC-R00-16

L M

NP

S T

U W

Rear Head Types

L Type - Same a “A” Type Front Head

M Type - Same a “B” Type Front Head

N Type - Same a “N” Type Front Head

P Type - Outside Packed Floating Head

•Rarely used in refining

W Type - Externally Sealed Floating Tubesheet

•Rarely used in refining

S Type - Floating Head with Backing Device

•Very common

T Type - Pull Through Floating Head

•Very common

U Type - U-Tube Bundle

•Steam service and high ∆P shell/tube


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EDS 2004/EXC 2-10

Selection of Exchanger Type

1) Feed preheater

Low pressure

Tubeside - Steam

Shellside - Light naphtha

Selection Example #1


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Selection of Exchanger Type(continued)

1) Feed preheaterLow pressure

Tubeside - Steam

Shellside - Light naphtha

BEU

Selection Example #1 Answer


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Tubeside - Coker fractionator bottoms

Shellside - Light coker naphtha



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EDS 2004/EXC 2-13



Tubeside - Coker fractionator bottoms

Shellside - Light coker naphtha

AES or AET



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EDS 2004/EXC 2-14


3) Xylene column reboilerLow pressure

Tubeside - Steam

Shellside - Mixed aromatics



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EDS 2004/EXC 2-15


3) Xylene column reboilerLow pressure

Tubeside - Steam

Shellside - Mixed aromatics

A or B, E or H or J or X, U

A or B, K, T or U probably BHU



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EDS 2004/EXC 2-16

Tube Pass Geometry and Pass Partitions

Separation of fluids in each pass must be maintained since

leakage would short circuit the exchanger.

The term “Pass” used in exchangers is not the same as the

term pass is used in fired heaters. The number of passes inan exchanger is the number of times one fluid passes through

the other fluids compartment.

RibbonInlet

Outlet

In

In

Out

Out

In

In

Out

Out

Front head Rear head

EXC-R00-19

When the tubes are divided into more than one pass, this division can be done in

three main ways.

Ribbon where the tubes are divided horizontally or vertically.

In order to divide the flow, the channel (front) and floating or rear head has to have

flow dividing baffles. These are called Pass Partition Baffles. The baffle patterns

are different in the two heads.


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EDS 2004/EXC 2-17

Quadrant

Inlet

Outlet

In 1

In 5

Out 8

Out 4 In 3

Out 2

Out 6

In 7

In 1

In 5

Out 8

Out 4 In 3

Out 2

Out 6

In 7

Front head Rear head

EXC-R00-20

Tube Pass Geometry and Pass Partitions

Quadrant where the division occurs in both the horizontal or vertical direction.

More readily used with larger number of tube passes.

In quadrant layouts, the nozzles are generally placed off the centerline to avoid the

vertical baffle.

Also, it is possible to use a combination of both ribbon and quadrant pass layouts.

As the ends of the front and rear heads need to have sealing gaskets, it can be seen

that the shape of the gaskets and the difficulty in getting them into position becomes

more difficult.


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EDS 2004/EXC 2-18

Shellside Flow Distribution

Fluid flows through the shell in five paths

– A Stream - Through gap between tube and baffle

– B Stream - Between tubes across the bundle

– C Stream - Through gap between bundle and shell

– E Stream - Through gap between baffle and shell

– F Stream - Through pass partition lane

There are 5 significant flow paths that the shell side fluid can take through the shell.

They are listed with a letter to describe them. Other than that the letter has no real

significance.


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EDS 2004/EXC 2-19

EXC-R00-21

This picture shows the flow paths in a cross-sectional view.

The gap between the bundle and the shell often uses a seal strip. This strips come in

pairs. They force the fluid flowing around the edge of the bundle to flow back

toward the center of the bundle.

Notice in the gap in the tube field caused by the pass partition baffle in the channel.

If the flow is normal to the gap, there is not loss of efficiency. However, if the flow

is parallel to the gap, there will be a tendency to bypass the tubes in the bundle.

This stream is sometimes blocked with dummy tubes or maybe tie rod.


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EDS 2004/EXC 2-20

EXC-R00-22

This picture shows the flow paths in an elevation view.

Each of these leakage streams needs to be minimized as they result in a loss of heat

transfer efficiency.

The A stream is reduced by having a small gap (1/32" to 1/64") between the tube

OD and the Baffle ID. This clearance cannot be made too tight as it may result in

damage due to tube vibration.

The E stream is reduced by having the bundle close to the shell. This is a loss of

efficiency for floating head type bundles versus fixed tubesheet exchangers. T type

exchangers are worse than S type in the size of this stream. Making this gap too

small can make it difficult to pull and reinsert a tube bundle. Gap varies from 1/8"

to 7/16" as bundle diameter increases.


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EDS 2004/EXC 2-21

Minimizing Leakage Streams

A Stream

– Tight tolerance of tube OD to baffle hole ID

— TEMA RCB 4.2

1/32" (0.8 mm) if baffle spacing < 36" (915 mm)

1/64" (0.4 mm) if baffle spacing > 36" (915 mm)

E Stream

– Tight tolerance of bundle to shell ID

— TEMA RCB 4.3

1/8" (3.2 mm) for shell ID 6-17" (152-432 mm)

3/16" (4.8 mm) for shell ID 18-39" (457-991 mm)

1/4" (6.4 mm) for shell ID 40-54" (1016-1372 mm)

5/16" (7.9 mm) for shell ID 55-69" (1397-1753 mm)

3/8" (9.5 mm) for shell ID 70-84" (1778-2134 mm)

7/16" (11.1 mm) for shell ID 85-100" (2159-2540 mm)

Each of these leakage streams needs to be minimized as they result in a loss of heat

transfer efficiency.

The A stream is reduced by having a small gap between the tube OD and the Baffle

ID. This clearance cannot be made too tight as it may result in damage due to tube

vibration.

The E stream is reduced by having the bundle close to the shell. This is a loss of

efficiency for floating head type bundles versus fixed tubesheet exchangers. T type

exchangers are worse than S type in the size of this stream. Making this gap too

small can make it difficult to pull and reinsert a tube bundle.


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EDS 2004/EXC 2-22

C Stream

Block steam with sealing strip.

only one shown

EXC-Roo-23

The gap between the bundle and the shell often uses a seal strip. This strips come in

pairs. They force the fluid flowing around the edge of the bundle to flow back

toward the center of the bundle.


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EDS 2004/EXC 2-23

F Stream

Normal to flow - No bypassing problem Parallel to flow - Install dummy tubes

– Dummy tubes are tubes on the shell side only.

They physically block the flow but have no

holes in the tubesheet. No tubeside fluid

passes through them.

The F stream is the flow in the gap in the tube field caused by the pass partition

baffle in the channel. If the flow is normal to the gap, there is not loss of efficiency.

However, if the flow is parallel to the gap, there will be a tendency to bypass the

tubes in the bundle. This stream is sometimes blocked with dummy tubes or maybe

tie rod.


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EDS 2004/EXC 2-24

Baffle Types

EXC-Roo-24

The most commonly used shell side baffle is the single segmental baffle. The baffle

consists of alternating flat plates with a portion cut off. The flow is block by the

solid portion of the baffle but allows flow to pass it through the cut out window.

Tubes occupy the entire shell circle. The tubes in the center of the baffle are

supported by each baffle. However, the tubes in the window are supported by every

other baffle.

A variation is the double segmental baffle. In this case the baffle plate is cut so that

the flow is over both sides of some baffles and through a central gap in the other

baffles.

The next variation is the use of single segmental baffles but tubes are eliminated in

the windows. This is a No Tube In Windows design. It is used for exchangers with

vibration problems as each baffle doubles as a tube support.


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EDS 2004/EXC 2-25

Baffle Types(continued)

Triple Segmental Baffles Philips Rod Baffles

Holtec NEST Baffles

EXC-Roo-25

Spiral Baffles

Four other types of baffles are sometimes used.

The first is another variation on the segmental type using a triple segmental baffle.

These have lower pressure drop but are difficult to set up properly due to all the

pieces.

Phillips Petroleum patented a Rod Baffle Design. Here steel rods are used instead

of plates. The rods are placed in an alternating pattern of vertical and horizontal

supports with a circular edge rod to hold the structure together. This is a very good

system for low pressure drop and high viscosity designs. However, anyone that

uses this design must pay a royalty for the privilege.

Holtec similarly developed a Nest type baffle. Similar in concept to the Rod Baffle

design, except for the use of strips of metal rather than rods, this design allows theuse of triangularly pitched tubes.

The spiral baffle is made of four quadrant pieces of flat plates that direct the shell

side flow in a more uniform spiral flow pattern. This provides better flow

properties and uses the tube surface more efficiently in some situations.


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EDS 2004/EXC 2-26

Flow Patterns

empty space empty space

empty space empty space

FULL

TUBEFIELD

FULL

TUBEFIELD

PARTIALTUBEFIELD

FLOW PATTERNS

max unsupported

tube span

max unsupported

tube span

max unsupported

tube span

max unsupported

tube span

max unsupported

tube span

max unsupported

tube span

FULL

TUBEFIELD

FULL

TUBEFIELD

PARTIALTUBEFIELD

EXC-Roo-26

The side view of the flow patterns for single and double segmental baffles as well as

NTIW designs.

The double segmental pattern allows for a more axial flow with fewer sharp turns

resulting in a lower pressure drop.

The No Tube In Windows design is also shown to be a more truly cross flow pattern

with space in the window for the fluid to remix.


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EDS 2004/EXC 2-27

Baffle Selection Criteria

Single Segmental Baffles

– Standard Arrangement

– Inexpensive

– Used if neither ∆P nor tube vibration is a problem

Double Segmental

– Used if ∆P must be limited and tube vibration is not aproblem

NTIW

– Used if ∆P must be limited and tube vibration is aproblem

Triple Segmental

– Provides lower ∆P than Single or Double Segmental – Has a large unsupported tube span

Summary of reasons to use the various baffle types.


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EDS 2004/EXC 2-28

Baffle Selection Criteria(continued)

Phillips Rod Baffles

– Provides very low ∆P

– More expensive than plate baffles

– Only used on square pitch

– Good for high µ liquids on shellside

– Provides good tube support

Continued summary of reasons for using the various baffle types.


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EDS 2004/EXC 2-29


Holtec NEST Baffles

– Very low ∆P


– Any pitch down to TEMA minimum

– Good for revamps (low ∆P)


– Good tube support



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Spiral Baffles – Provides lower ∆P


– Improves heat transfer


– Reduces shellside dead spaces



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EDS 2004/EXC 2-31

Flanges

Flanges are bolted joints to allow inspection and

maintenance

Significant potential for leakage to surroundings

Gaskets used to minimize leak potential

Flanges are more important and difficult to design in exchangers than in vessels or

piping due to the thermal variations that can occur in the flange.

Further flanges can be as large as the diameter of the exchanger.

Three types of flanges are described.


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EDS 2004/EXC 2-32

Flange Facing Types

Nubbin

1/64” 1/64”

1/64”

EXC-R00-28

The portion of the flange that the gasket comes in contact with is called the facing.

The facing area is usually machined with extra care to get a good surface for the

gasket to seal against.

The surface may be extra smooth or have a specified amount of roughness.

Sometimes a bump is left on the surface. This is called a nubbin. The nubbin

pushes against the gasket concentrating the force applied to the gasket.

The joint in the bottom right is a ring type joint. These were popular years ago as

giving a tight seal however, they are difficult to install and remove since the flange

has to be spread more than a standard type joint. UOP does not normally use ring

type joints.


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EDS 2004/EXC 2-33

Tube to Tubesheet Designs

Most difficult mechanical

design component

Large potential for

internal leakage

– 3 main types

— Expanded

— Seal Welded

— Strength Welded

EXC-R00-29

The weakest part of the heat exchanger design is typically the tube to tubesheet

joint. This is the place where most leaks occur inside an exchanger. This is due to

the high stresses present in a typical joint and typical fabrication requirements.

There are 3 types of joints.

Expanded - Tubes are placed inside the hole of the tubesheet and expanded

(increase in diameter) till a tight bond occurs. This is done typically by hydraulic

rollers pushing against the tube ID. Hydraulic balloons have been used as well as

explosives to accomplish the same goal.

Seal Weld - After the tubes are expanded, a small weld is made on the portion of the

tube projecting beyond the tubesheet. This provides a supposed second line of

defense against leakage. However, the heat of the welding can loose the expansion.

Rerolling the tube after welding can crack the weld. This often creates amaintenance nightmare in that all tubes may have to be worked on for a small

number of leaks. UOP does not recommend this practice.

Strength Weld - Here the weld joint is substantial enough to contain the pressure

between the two sides. The tubes are usually expanded enough to minimize the

crevice between the OD of the tube and the ID of the tubesheet hole.


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EDS 2004/EXC 2-34

Impingement Plates

Used to avoid damage to tubes from incoming fluids on shell side

TEMA Guidelines

– Based on ρV2 on inlet nozzle, provide impingement protection

when -

— For non-corrosive, non abrasive, single phase fluids if ρV2 >

1500 lb/ft-sec2

— For all other liquids (including liquids at its boiling point) if

ρV2 > 500 lb/ft-sec2

— For all gases and vapors (including saturated vapors)

— For all liquid and vapor mixtures

EXC-R00-30

Impingement baffles are used to deflect a high energy stream from damaging the

tubes upon entry to the shell side of an exchanger.

TEMA rules are listed here.


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Vapor Distribution Belts

For distribution of large vapor flows

Slotted holes in shell plate to provide even lower

velocity flow of vapor into the bundle

Reduces chance for vibration, erosion or tube damage

Changes flow and heat transfer characteristics in first

baffle space

EXC-R00-31

An alternate approach for shell side entry with large vapor flows is to use a

distribution belt. This allows for entry areas into the bundle that are bigger than a

single nozzle can provide.


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EDS 2004/EXC 2-36

Materials of Construction

Materials must suit temperature and fluids

Example of selection criteria

High H2 partial pressure

– Killed carbon steel

High temperature, high H2 partial pressure

– 410 or Cr-Mo

Sea-water cooling

– Admiralty brass or 70-30 Cu-Ni

Wet H2S service

– Killed carbon steel (Special heat treatment)

Hot HF service

– Monel

Hot acids

– Alloy 20, 904L, 316 SS, Titanium

Typical materials of construction for various service applications.

Exchangers use a wide variety of materials and the designer needs to be aware of

the ramifications of their decision on material selection both on fabrication and

maintenance costs.


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Protective Coatings

Lower alloy requirements can be used if protected from process

with a “coating”. Coatings can take various forms. – Cladding: Cladding is welding a high alloy material on a

lower alloy material. The low alloy provides the mechanical

strength while the cladding provides corrosion protection.

– Refractory: Refractory is a high temperature insulating

material. By attaching it to a low alloy material, the

temperature the material is exposed to is reduced allowing

the use of a lower alloy.

– Paints: Besides external painting for environmental

protection, paint and similar coatings are used occasionally.

Cooling water is a common use of this process.

Cost reduction techniques such as cladding, refractory linings, and paints can reduce

the amount of alloy being used in the exchanger.


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Design Temperature and Pressure

Dictated by worst case conditions for hot and cold use.

Worst case conditions of both temperature and pressure. This

may not be the highest temperature but a combination of hightemperature and higher pressure.

Minimum design metal temperature (MDMT) is also required.

High pressure at low temperatures may result in failure of

brittle materials. Lowest ambient or auto-refrigeration

temperatures normally used.

Typical Safety Margins:

Temperature

– 50°F (28°C) margin on maximum operating temperature.

Pressure

– 25 psi (170 kPa) or 10% margin on maximum operating

pressure (whichever is greater).

The design temperature and pressure reflect the mechanical design conditions used

to set the thickness of the heat exchanger components.

The temperatures reflect both high and low temperature conditions that the

equipment must deal with. Exchangers have failed in cold climates due to the

brittleness of the steel at low temperatures was not considered.


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Protection of Low Pressure Side

Tube Rupture causes high pressure fluid to

flow into low pressure side

Hot high pressure fluid mixing with cold low

pressure fluid may result in vaporization

API 521 provides guidance

10/13 rule based on ASME BPVC

Alternative is relief device on low pressure side

Consider surrounding equipment & piping

The low pressure side of heat exchangers has to be protected from being

overpressured and failing due to a failure of the tubes or leakage at the tubesheet.

The engineering design criteria for evaluating this case is found in API 521 on

pressure protections systems. This situation is typically addressed by adding a relief

device on the low pressure side or by adjusting the design pressure of the low

pressure side.

Based on the ASME Boiler and Pressure Vessel Code, the hydrostatic test pressure

is 130% of the design pressure. Therefore, by setting the design pressure of the low

pressure side to 100/130 of the high pressure side, the low pressure side is

hydrostatically tested at the design pressure of the high pressure side. A leak will

therefore not introduce a pressure greater than the low pressure side cannot

withstand.

A tube failure will create a dynamic situation as the fluid pressurizes the low

pressure side, including vaporizing an all liquid stream. This pressure surge must be dealt with not only in the exchanger but in upstream and downstream equipment

and piping. Failures are known to have occurred outside of the exchanger.

section 02- heat transfer

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