Kuppan T. Heat Exchanger Design Handbook

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984

Chapter

is no contact as given by the formulas. For example, the expression for minimum ligament

efficiency

is given by CODAP [37]:

for tubes welded to the tube sheet

for tubes expanded more than

90%

of the tube-sheet thickness

(4)

where

is the tube outer diameter,

the tube pitch, and

the tube wall thickness.

Ductility of Welded Joint in Feedwater Heaters

For applications such as feedwater heaters, the tube sheets are subjected mainly to bending

stresses due to high feedwater pressure, which may be as high as

5000

psi in modern plant

cycles. The tube-to-tube-sheet joints must remain impervious to water under

this

high pressure

while sustaining the cyclic stresses due to tube-sheet flexure, and temperature variations between

the tube and tube sheet caused by such operating conditions

startup, load shedding, shutdown,

and possible abnormal transient operations [31]. According to Lohmeir et al. [31], due to these

reasons, the flexural and thermal stresses can

sufficiently large, even exceeding the elastic

limit, to propagate leakage paths from weld metal flaws. In order to cope with such high stresses

without suffering damage, the welds must have consistent mechanical properties and high ductil-

ity. The main factors to

controlled

ensure the ductility of the joints are [38]:

Combination of tube and tube-sheet materials

Welding parameters

Heat treatment

The carbon content of the basic material and welding alloy (less significant factor)

The following factors ensure welded joints of even quality and high ductility [38]:

Tubes and tube sheets are always of the same material, with a maximum carbon content

0.20%.

The filler is of

special alloy whose main feature is an extremely low percentage

of carbon.

All joints and the electrode are preheated before welding.

When all welding is completed the entire tube sheet

stress relieved.

Welded Mockups

To determine correct welding parameters, use mockups to scrutinize tube-to-tube-sheet weld-

ments. Welded mockups are sectioned

and

prepared for review of the root area and dimension

checks of the weld. Examinations

welded mockups are discussed in detail by Syal

[

1 11. An

expanded and welded tube-to-tube-sheet mockup is examined by the following methods:

Radiography examination: The weld shall be free from porosity, cracks, or other defects

that would result in leakage in service.

Micro and macro examination: to detect defects, metal flow in the annular grooves (if

grooves are present in the tube sheet), fusion, penetration, etc.

Minimum leak path: For strength-welded joints, the minimum leak path shall be the

tube-

sheet thickness, and for seal-welded joints

0.7

times the tube-sheet thickness.

Hardness examination: tube sheet, tube, weld, and HAZ; normal hardness limits are for

carbon steel

220

HV, low-alloy steel

260

HV, and stainless steels

250

HV or as specified

in the specification

[

111.

Tear test for strength welded joint.

Pulling test: Tubes shall be pulled on a tensile testing machine with a pulling load not less

985

Heat Exchanger Fabrication

than 0.1

3000 (kgf) for expanded tubes without seal welding, where

is the

length of expansion joint (cm) and d tube outer diameter (cm)

[

111.

Pressure test as shown in Fig. 21.

Leak Testing of Welded Tube-to-Tube-Sheet Joints

Leak testing of exchangers will allow one to determine if tubes have been adequately rolled

or welded into the exchanger tube sheets. Such testing can be accomplished by pressurizing

the shell side with air to a pressure of 1.0 to 1.25 ksc (gauge) and “soaping” the face of the

tube sheet and looking for bubbles, or by filling the shell with water, pressurizing the channel

with air, and looking for bubbles through the shell-side nozzle. Various leak testing methods

are discussed in Chapter 14 on quality control, inspection, and NDT.

TEMA Standards on Testing

Tube-to-Tube-Sheet Joints. As per paragraph RGP-RCB-7.6,

tube-to-tube-sheet welds are to

tested using the manufacturer’s standard method. Any spe-

cial testing using halogens or helium will be performed by agreement between manufacturer

and purchaser.

Radiographic Inspection of Tube-to-Tube-Sheet Joint Welds

For very critical applications, the acceptance standards for tube-to-tube-sheet welds like the

seal, spigot, and butdfillet weld are generally based on high-definition panoramic radiography.

The small size of the welds and their shape necessitate a very short source to film distance

[39]. This requires the development

highly specialized equipment such as the microfocus

rod anode x-rays set and miniature low-energy gamma sources a unique film cassette for use

on small diameter welds to implement radiography on these welds [39,40]. Figure 23 illustrates

the principles of the radiographic technique to test tube-to-tube-sheet weld.

Welded Tube-to-Tube-Sheet Joint Quality Control by Hydraulic Testing

of Feedwater Heaters

The integrity of tube-to-tube-sheet joints is judged by hydrostatic tests and leak testing tech-

niques. The basic limitations in these tests are that they reveal only those defects that provide,

at the time of testing, a flow path completely through the weld. Experience with carbon steel

tube-sheet welds has shown that the leaks that can occur after service operation give no indica-

tion of leakage during shop floor leak testing or hydrostatic testing [31]. One method that has

been successful in detecting subsurface defects involves cycling the heat exchanger to hydro-

Figure

Welded tube-to-tubesheet pressure test model.

986

ChaDter

Figure

22 Radiographic inspection of tube-to-tubesheet joint welds. (a) Panoramic radiographic tech-

nique,

(b)

position of the radiation source and film relative to the butt fillet weld, (c) microfocus rod

anode x-rays set. (From Ref.

39.)

static test pressure a number of times, and providing a thermal shock to the tube-sheet weld

surface in the shop floor. The high tube-to-tube-sheet joint stresses due to pressure cycling

have been particularly successful in completing the potential leakage paths that require a short

propagation distance.

Brazing

used for tube-to-tube-sheet joints that are free from stress concentration and crevice

corrosion problems. For the fabrication of heat exchangers three methods of brazing are in

common use: torch brazing, furnace brazing, and dip brazing.

ASME

Code Section

VIII,

Div.

prohibits brazed joints for lethal service and for unfired steam boilers.

3.10

Heat Treatment

Heat treatment of fixed-tube-sheet exchangers may be done by either

these methods:

With tubes welded in one tube sheet and left free in the holes of other tube sheet

Both ends

the tubes welded with tube sheets

Salient features of these methods are discussed next.

987

Heat Exchanger Fabrication

With Tubes Welded in One Tube Sheet and Left Free in the Other Tube Sheet

The procedure adopted is as follows:

After welding the tubes on one tube sheet, stress relieve the joints by leaving the tubes

free in the second tube sheet.

Weld the tubes to the second tube sheet and stress relieve the second tube sheet by placing

only part of the exchanger in the furnace: however, this stress relieving may be waived if

proper welding procedure with suitable preheating can control the hardness of the weld

metal and HAZ within acceptable limit, provided that PWHT is not a mandatory require-

ment [ll].

Both Ends of the Tubes Welded with Tube Sheets

During heat treatment, the rate of heating and cooling should not exceed 30°C per hour. The

temperature gradient between the outer skin of the shell and the innermost tube should be

narrowed down to an acceptable limit by soaking the exchanger during the heating stages.

Heat Treatment: General Requirements

Prior to PWHT, the exchanger should be thoroughly examined by visual inspection and NDT

methods to avoid welding repair after completion of heat treatment. Consider the following

points while heat treating heat exchangers:

The complete exchanger should be thoroughly cleaned before charging into the furnace.

Any scale formation on the tube sheet and tubes, when tubes are kept free on one side, is

avoided by the circulation of nitrogen. Alternately, tube ends may be plugged with ceramic

fiber plugs, which will not permit the hot gases from entering the tubes, thereby preventing

oxidation.

Exchangers must be heat treated in a furnace that is heated electrically or by

gas

burners.

The furnace must be of such a design as to prevent direct impingement of flame on the

exchanger. Flame selection, i.e., oxidizing, reducing, slightly oxidizing, etc.,

important

to avoid excessive scaling and oxidation.

Sufficient numbers of thermocouples should be provided on the exchanger to verify uni-

form heating; their locations are also important.

Rate of heating, holding temperature, and rate of cooling are generally governed by mate-

rial used and the construction code. PWHT requirements for welds between dissimilar

metals shall conform to the requirements of the material having the more stringent require-

ments.

Ensure that the tubes are adequately supported to prevent sagging of the tube bundle.

Similarly, during PWHT of the exchanger, it may be necessary to support the shell, if its

supports are widely spaced and sagging is likely.

Heat treatment should always be done before hydrostatic test. If for any reason tube end

welding, however small, is carried out after heat treatment, it is recommended to repeat

the heat treatment when PWHT has been specified as requirement.

After heat treatment, the tube sheet and tubes shall be examined to ensure that its surfaces

are free from scale, oxidation, and decarburization, fine cracks, distortion, dimensional

accuracy, etc.

Details on quality control during PWHT of welded components are discussed in Chapter 13

on material selection and fabrication.

988

Chapter

3.1

Assembly of ChanneldEnd Closures

Where a fully pressed end is required, the end closure is purchased from the trade as a pressing

or can be fabricated from a series of petals on the shop floor. (Forming of various types of

heads discussed in Annexure

2.)

Assemble the channelslend closures with the shell assembly

and measure the flatness. Measure tube-sheet flatness across the diameter of the gasket ring

for conformity with the specifications. Inspect fixed tube sheets extended as flanges for defor-

mation due to weld shrinkage.

Bolt Tightening

After the heat exchanger assembly, tighten the bolts in a controlled sequence crisscross pattern

similar to that shown in Fig. 23a [35] or Fig. 23b, as illustrated in TEMA Figure E-3.25,

except for special high-pressure closures when the instructions of the manufacturer should be

followed. It is important that all the flange bolts carry approximately equal load. For ex-

changers in critical services where it is essential to avoid leakage, use the ultrasonic bolt

tensioning devices to ensure that the bolts are tightened uniformly [35].

3.1

Hydrostatic Testing

The hydrostatic test shall be carried out after PWHT and complete assembly and all NDT

performed, prior to applying painting, insulation, coating, and packing, under the supervision

of a quality control

(QC)

engineer andor third-party inspection, by observing the state and

local pressure vessel code. Care has to be taken for safe supporting and complete venting of

the pressure vessel. Welded joints are to be cleaned prior to testing the exchanger to permit

proper inspection during the test.

The shell side and the tube side are to be tested separately in such a manner that leaks at

the tube-to-tube-sheet joints can be detected from one side. When the tube-side design pressure

is higher than the shell-side pressure, the tube bundle shall be tested outside of the shell only

if specified by the purchaser and the construction permits. In case of stacked assembly these

tests will be done in the stacked condition.

Hydrostatic Testing Procedure

The minimum hydrostatic test pressure at room temperature should be

1.5

times the MAWP,

corrected for temperature, except where other code requirements govern. Air lock plays a very

Figure

Bolt tightening sequence. (a)

Danis

[Ref.

351

recommendation; and

(b)

TEMA

recommen-

dation

[2].

Heat Exchanger Fabrication

989

important role. Make sure that air is removed completely before conducting the hydrostatic

test, and the overflow is witnessed by the inspector before closing the nozzle. The pressure

has to be measured with two calibrated pressure gauges, connected independently of each

other. The indication range must not exceed

150%

of the test pressure. Keep a watch on the

pressure gauge dial to spot drop in pressure that indicate leaks. The test pressure shall be held

for at least

min while looking for water oozing from welds, gaskets, and other potential

sources for leaks. For the pressure test, in addition to inspecting the expansion joint for leaks,

the expansion joints shall be inspected before, during, or after the pressure test for visible

permanent distortion.

Hydrostatic Test Fluid

The hydrostatic pressure test is in general carried out with drinking-quality water free from

any corrosive and suspended substances, especially, chlorides and microorganisms. In the case

of bad water quality, temperatures higher than

30°C

combined with longer holding time may

cause corrosion. If required, suitable corrosion inhibitors may be used. Do not test austenitic

stainless steel exchangers with water that has a chloride ion concentration high enough to cause

stress corrosion cracking. Because most potable water supplies are chlorinated, it is necessary

to check the chloride levels. The chloride level in testing water is limited to

ppm for

austenitic stainless steel types

304, 310,

and

321,

Incoloy

800,

and for aluminum, and

100

ppm for Cr-Ni-Mo austenitic steels types

316

and

17. Alternatively, use conditioned deminer-

alized or distilled water for hydrostatic testing.

Use of Fluorescent

Visible Tracer Dyes in Hydrostatic Test Fluids

Fluorescing dye indicators can be added to the water used in hydrostatic pressure tests for

improving the visibility for locating leaks.

Pneumatic Tests

As per ASME Code paragraph UG-100, vessels that cannot be filled with water or cannot be

readily dried and where traces of water are not tolerated can be tested pneumatically at

1.25

times the MAWP to be stamped on the vessel multiplied by the ratio of the stress values of

the test temperature

the vessel

the stress value

the design temperature, except where

other code requirements govern.

Stamping

The vessel shall be stamped as follows:

Shell side Design Pr side.

Tube side Design Pr.

Shell side Design Temp.

Tube side Design Temp.

Shell side Hydrotest Pr.

Tube side Hydro Test Pr.

Serial No. Inspection By.

Hydrotested On

3.13

Preparation of Heat Exchangers for Shipment

During plant manufacture, storage, transport to site, and site erection, special precautions are

to be taken to ensure that all the components remain clean and reasonably protected:

Internal and external surfaces are to be free from loose scale and other foreign matter that

is readily removable.

Oil, water, or other liquids used for cleaning and hydrostatic testing should be drained

before shipment. The shell side may be dried out by vacuum pump and the tube side by

blowing through with hot air.

990

Chapter

All exposed machined contact surfaces shall be coated with a removable rust preventive

and protected by suitable cover against damage. Rust-preventive compounds must be re-

garded as contaminants to be removed before the heat exchanger is put into service [41].

All threaded connections are to be suitably plugged.

The exchanger and any spare parts are to be suitably protected to prevent damage during

shipment.

All

tell-tale holes shall be packed with hard grease. In the event of alloy material, the

grease shall be

low

lead and should not contaminate the material.

General guidelines for preparation of heat exchangers for shipment are discussed

TEMA, paragraph

G-6.

Painting

External surfaces shall be painted as per

drawings/specifications.

Use paint that will withstand

the surface temperature while

operation. Stainless steels do not require painting.

Nitrogen Filling

desired by the purchaser, the heat exchanger internals may be filled with dry nitrogen to

protect the internal parts against corrosion.

3.14

Making

Certificates

The following documents may be enclosed

a folder along with the heat exchanger:

Vessel detail drawing together with a materials list

Material certificates of chemical and physical properties for each vessel item

Welding procedure specification and procedure qualification records

Welder qualification records

Records of welding consumables used and their properties

Certificate of radiography and other nondestructive testing methods

Heat treatment charts

Hydrostatic test report

Job inspection report

10.

Stagewise inspection chart

11.

Final inspection report

12.

Guarantee certificate

ANNEXURE

PLATE BENDING MACHINES, PWHT,

AND MANIPULATIVE EQUIPMENT

Roll

Bending Machine

Mechanical and hydraulic three-roll plate bending machines are appropriate for the hot rolling

of heavy plate. A four-roll bending machine offers production advantages, although

is not

particularly suited to hot work [42].

B Vertical Plate Bending Machine

The vertical plate bending machine can be used for all ASME Code pressure vessel work, and

results are claimed to be as good as or better than those obtained with horizontal plate bendiog

rolls. Some of the advantages claimed for vertical roll bending include these [42]:

(1)

absence

of camber

the rolled shell, (2) higher capacity and hence thicker plates can be rolled.

(3)

Heat Exchanger Fabrication

991

less power consumption,

(4)

the weight of the plate has very little effect on the bending accu-

racy, and

(5)

the mill scale experienced on many plates drops clear during the bending opera-

tion, whereas in horizontal machines it tends to get rolled into the plate surface.

PWHT

of Shells

Use spider bracing for adequate support during the stress relieving operation, and thermocou-

ples are fastened to the vessel internally and externally. Temperatures are closely monitored

and controlled during the period of temperature raising, soaking, and relaxing. Test plates

inserted both inside and outside the vessel during stress relieving are later tested to ensure that

the plate has been returned to its proper ductility and tensile strength

[43].

D Manipulative Equipment

In the fabrication

heat exchangers, rotator sets or turning rolls are utilized to manipulate the

shell for various welding positions. They ensure smoother welds. Circumferential welding is

carried out by providing for the components to be rotated on the roller manipulator on the

support bed. Basically these comprise two types

[44]:

(1)

self-aligning rotators and

(2)

conven-

tional rotators. Self-aligning rotators consist of rubber-tired wheels that automatically align

themselves to the change from one diameter of vessel to another. In the case

conventional

rotators the rolls have to be manually positioned across the frames to suit the diameter

the

vessel.

ANNEXURE

FORMING OF HEADS AND CLOSURES

Forming Methods

For thin plates, the cheapest method is by cold spinning, and hot spinning of greater thickness.

For still greater thickness a forming press is used. Where the dimension of the head are such

that a single forming operation is not possible, several plates are pressed to the required contour

and then welded together to a cap. This is known as the crown-and-segment technique.

Blank diameter,

for the pressure vessel heads shown in Fig.

can be determined from

the Welding Engineer Data Sheet Nomograph

[45]

or from the formula given therein:

2dRH

1.33R2

(5)

Spinning

Spinning of ends is applied to a wide range of materials. The forming is done by either hot or

cold spinning. With cold spinning there can be substantial savings in production time, particu-

larly in handling time, over conventional hot spinning or dishing and flanging on two separate

Figure 24

Pressure vessel head formed

using

from blank diameter

(From

Ref.

45.)

992

Chapter

machines [46]. Salient features of spinning of various materials are discussed by Peacock [4]

and include the following:

Mild steel ends up to

in (3.2 mm) thick are cold spun with intermediate anneals, and

ends greater than this thickness are hot spun at 1150°C.

Low-alloy steels are successfully hot spun.

For stainless steels with ends up to

in (15.9 mm) thickness, cold spinning with interme-

diate annealing at 1050°C is preferred. Ends greater than this above thickness are satisfac-

torily hot spun in the temperature range 900-1200°C; due care is to be taken to avoid

carbide precipitation.

Ends can be made from explosion-clad plates by conventional hot or cold forming tech-

niques. For more details on forming of clad plates, refer to the section on cladding in

Chapter 13 on material selection and fabrication.

The spun heads will have a constant thickness in the hoop direction but will vary in the

meridional direction. The thickness will be a maximum at the crown and a minimum in the

knuckle region. The amount of thinning in the knuckle can be as high as

20-30%.

Under

internal pressure, the direct hoop stresses are compressive and hence the heads are susceptible

to buckling [47].

C Pressing

Pressing, either hot or cold, is usually employed for the smaller diameter ends and is not as

flexible in operation as spinning, because slight changes in end dimension require an alteration

to the ring and pressing die. The majority of ends are hot pressed for two main reasons 161:

(1)

The metal to be pressed undergoes less work hardening and is less prone to cracking, and

(2)

the load needed to press the material to the required dimensions is less.

For forming of heads by pressing, two major kinds of equipment are required. They are

(1) the cold forming and dishing press and

(2)

the flanging and knuckling machine [48].

Initially the plate is manipulated into position beneath the ram and progressively rotated and

tilted

successive ram strokes form the required shape. After dishing, the formed head

transferred to the knuckling machine for forming the edge to a more accurate radius.

Crown and Segment (C and

Technique

Large hemispherical heads and torispherical heads are fabricated by the crown-and-segment

(C and

technique. Hemispherical vessel heads are fabricated from a series of hemispherical

orange-peel sections and one dish-shaped section to form a hemisphere, whereas the torisphere

is fabricated by welding a spherical cap to a toroidal portion made of several segments

(or

gores) that have been welded together, as shown in Fig. 25. Two methods are followed

for

making the petals [48]. In one method, the required form is marked around the template

the flat plate; the petal is flame cut and then hot formed. In the more common method, the

plate is rolled to the required petal shape and a template made to the form of the finished petal

is used to mark out the lines to be cut. Then flame cutting

carried out manually and

the

edges are beveled and examined. All the plates are assembled on a supporting structure and

welded. Caps for the hemispheres/torispheres are welded into position using fully automatic

submerged arc welding. At the various welds, there may be local variations in geometry,

although these will be reduced in magnitude by hammering and spinning after the welding

operation. The thickness in the C and

heads will be essentially the same as that of the base

plate material [47].

993

Heat Exchanger Fabrication

CROWN

Figure

Crown

(C)

and segment

(S)

technique

welding pressure vessel head.

(From

Ref.

6.)

Purchased End Closures

Purchased end closures should be accompanied by the following certificates.

Mill certificates of raw material and test coupons.

Process of manufacture.

Raw material plate thickness used for pressing.

Minimum thickness achieved after pressing.

Type of heat treatment carried out.

Stress-relievinghormalizing

chart (time and temperature chart).

As-built dimensions of dished end.

Mechanical test results on test coupons.

NDT

reports.

Important details of a manufactured dished head are shown in Fig.

26.

Calibration

Heads

Heads distorted during the operations after pressing out have to

checked for reduction

enlargement of diameter, ovality, and increase

height

[49].

PWHT

Dished Ends

Dished ends shall be stress relieved if cold formed, and normalized if hot formed.

stress-

relievinghormalizing chart duly certified by

inspection agency shall be furnished. For hot-

formed dished ends, mechanical properties shall be proved on a test coupon after subjecting

Figure

Details

a dished head.