HEAT_EX..pdf

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4.2 SHELL- AND- TUB E EXCH ANGE RS: CONS TRUC TIO N 4.2.6 Mechanical Design and Fabrication 4.2.6-l 1

circumferential welding in the convolution itself. Thick-

ness is usually in the range 4 to 13 mm, with a convolu-

tion depth of 100 to 150 mm, but because of its stiff-

ness the movement of one convolution is limited to

about 2 to 5 mm. Larger movements can be accommo-

dated by adding more convolutions, but the cost willthen be higher than the thin-wall type. Its chief advan-

tage is that the rugged construction allows the complete

exchanger to be handled easily without the need for

bellows protection or restraining bars.

The thin-wall bellows, shown in Fig. 25, has convo-

lutions in stainless steel or Incoloy, formed cold, either

by rolling or hydraulic forming. Convolution depth is

about 24 to 75 mm, with a single-ply thickness of 0.5

to 2 mm, although multi-ply construction is used for

higher pressures. This design of bellows offers greater

scope than the thick-wall type with regard to movement

and pressure. It has the disadvantage that it requirescareful handling at all times, and it is usual to protec-

tive liners both inside and outside. It is also essential to

provide external restraining bars to keep the complete

unit rigid during handling. Once installed, the restraining

bars must be released to permit axial thermal movement.

The hot-formed medium-wall bellows offers a com-

promise between the thick and thin wall types, being

more flexible than the thick wall, yet offering a more

rugged construction than the thin wall. Material is

usually a chrome-molybdenum alloy or stainless steel,

2.0 to 4.5 mm thick, with a convolution height of 50

to 63 mm.

H. Hydrostatic testing

If tube end welding is involved, the welds are given a

low-pressure air or halogen test on the shell side at about

0.07 before the fmal hydrostatic tests. It is

Nuts r eleased

after i nstall ati on

I scr ews

easier to make sound tube end weld repairs if the ends

have not been in contact with water. If a multipass tech-nique has been used for the tube end welding, then each

pass must be given a low-pressure test before proceeding

to the next.

Upon completion the exchanger must be hydro-statically tested with water to check the soundness of all

welded seams, tube end, and joints. The shell

side and tube side of the exchanger must be tested inde-

pendently to their code test pressures, unless

the exchanger has been designed for a differential pres-

sure, in which case both sides must be pressurized to-

gether, taking care that the differential test pressure is

not exceeded.

The fabricator prefers to test the tube ends with

pressure on the outside of the tubes, as this allows every

tube end to be checked for tightness at the outer

sheet face and leakers can be positively identified. Thisis acceptable if the shell-side test pressure is equal to or

greater than the tube-side test pressure. When the re-

verse is the case, the customer may insist that the tube

ends be tested with pressure on the tube side, which

means that removable bundles must be tested outside

the shell. In this event leakage is detected as drips from

the bottom of the bundle at the inner tubesheet face,

but positive identification of the actual leakers is more

difficult, particularly with the tube configurations com-

monly used. The fabricator endeavors to minimize un-

necessary rerolling of tube ends, but some may be

inevitable.

Split straight tubes and outer U tubes are removedand replaced if practicable, but otherwise they are

plugged. Inner U tubes must be plugged. Explosive

plugging is available for high-integrity services.

Testing of tubesheet exchangers is straight-

forward in that there are only two tests to carry out.

The shell side is pressurized first, with the heads or head

covers removed, to check the shell welding, tube ends,

- External cover ( free at

Restrai ni ng bolts

Restrai ni ng f l ange

I nternal sl eeve Stub ends ( i n same metal

as shell barrel)

Figure 25 Thin-wall bellows.

1983 Hemisphere Publishing Corporation

.

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4.2.6-12 4.2 SHELL-AND-TUBE EXCHANGERS: CONSTRUCTION 4.2.6 Mechanical Design and Fabrication

f l ange

at rear head

Observe l eaksFloati ng tubesheet

Figure 26 Test flange and gland for floating-head exchanger.

and for split tubes. Unless the exchanger has been de-

signed for a differential pressure, the shell side is

drained, the heads or head covers fitted, and the tube

side pressurized to check the head welding and all

head joints. Although this again tests the tube

ends, leaks can be revealed only as drips from bottom

nozzles.

In order to test and T-type floating-head bundles

inside the shell, with the test pressure outside the tubes,

a special test flange and gland is required at the floating

head as shown in Fig. 26. This allows tube ends to be

inspected at the outer face of the floating tubesheet.The construction of and P-type floating-head

la: in gaskets fitted

To check tube expansions and split

Test when test tube-side test pressure

Test 2: in tubes-Service gaskets fitted

To check weldingTo check iointTo checkTo check channelTo check channel/cover

changers is such that they have built-in test flange and

gland facilities. It should be noted that to test T-type

floating-head bundles inside the shell, using the test

flange and gland, the bolt holes in the floating tubesheet

must be plugged temporarily. If the T type has no rear

head flanges, the test flange and gland cannot be used,

neither can it be used for T-type kettles. Sometimes a

separate test shell is made to suit these cases.

When testing any removable bundle inside the shell,

with the test pressure outside the tubes, a joint must be

made between the shell and stationary tubesheet to con-

tain the shell-side pressure. There must also be access tothe outer face of the stationary tubesheet to inspect the

lb: Pressure in tubes

To check tube expansions and split tubes

Test when test pressure shell- ride test

3: Pressure in shell--Service fitted

check shell end shell cover weldingTo check shell/shell cover jointTo check plate

Figure 27 Hydrostatic testing of split-backing-ring floating-head exchanger.



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4.2 SHELL-AND-TUBE EXCHANGERS: CONSTRUCTION 4.2.6 Mechanical Design and Fabrication 4.2.6-13

tube ends. Bobbin-type heads satisfy these requirements,

but bonnet-type heads do not and they must be omittedfor the test. Unless the tubesheet is flanged, it is neces-

sary to replace bonnet-type heads by a special test ring

or backing flange on the tube side of the stationarysheet to hold the joint.

For testing the bundle outside the shell, with the

pressure inside the tubes, a joint must be made between

the stationary head and stationary tubesheet to contain

the tube-side pressure. Unless the tubesheet is flanged,

or integral with the head, a similar test ring will be re-

quired on the shell side of the stationary tubesheet to

hold the joint.

The test procedure for an S-type floating-head ex-

changer is shown in Fig. 27 from which the special testring will be seen. A similar testing procedure is used for

other removable bundle exchangers. If any

joints have to be broken during testing, test gaskets are

used instead of the service gaskets, these being stored

until the service joints are finally made.



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