Tube expansion

Methods and Procedures

By

C.P.Parekh

Tube fitting with Tube sheet

By welding only

By expansion only

By welding and expansion

Introduction

Tube expansion is the art of reducing tube wall by compressing the

OD of the tube against a fix container.

Tube

Tube

shee

t

Tube

Tube

shee

t

Before After

Welded

side

Why Tube Expansion Is Required?

To avoid...

Sludge accumulation

Secondary side IGA/SCC

crevices

Corrosion in tube sheet

Damage to the tube due to

vibration

Tube

Tube

shee

t

Stage of tube expansion

Prior to start of rolling, the tube to tube

sheet welds shall have successfully passed

the helium leak test. ( as per spec.no PB-P-

188, para. 3.2)

Before expansion

Cleanliness of the tube inside surface .

Primary side rolling and checking shall

completed before commencing secondary

side of rolling

Methods of tube expansion

1. Mechanical : A mechanical tube expander consists of an odd number

roller (usually 3 Nos.), a taper mandrel to push the roller

outward, a cage to house the roller and a collar to set the

location of the expansion.A typical Expander is shown in

fig.

2. Hydraulic: This is differs from other known anchoring methods in that

the tube is physically deformed simply by means of pressurized

liquid. The amount of pressurized liquid introduced is a function

of the proportion of elastic and plastic expansion occurring in

the tube and tube sheet.In this a mandrel with two O rings at

appropriate location is inserted into the tube sheet.Hydraulic

pressure applied between two O rings causes the tube in that

region to expand.

Methods of tube expansion

3. Explosive Forming:

This is done by detonating a carefully sized charged inside each tube causing it to deform and make contact

with the tube sheet.

4. Hydro / Mechanical:

This system makes use of mechanical expander but the self feed of mandrel is replaced by positive hydraulic

feed using a separate hydraulic circuit apart from the

electric drive used for rotation of mandrel.

Methods of tube expansion

Requirement for Nuclear Steam

Generator

In order to control the cold work imparted to the tube during

expansion, percentage thinning restricted between 3 and 6 %.

Apart from this , the strength of joint should qualify for a

minimum pull out strength 764 - 1530 kg and the leak tightness

has to be ensured by leak test. Leak rate shall not be more than

1 x 10^-7 cc/sec . A tube expansion cross section shown in

figure.

Expansion process by Pilgrim technique

Expansion is carrying out mainly in two stages process.

1. Closing of gap between hole id and tube OD.

2. Thinning against wall.

As clearance between tube OD and hole dia varies, tube ID

cannot decided until sufficient trials will be taken to determine

the limiting torque value.

When the expansion length for the product is more then the

roller length of the tube expander, more than one stage of

expansion has to performed. That type of expansion is called as

Pilgrim technique of expansion.

Cont....

Pilgrim expansion technique

In this technique

Overlapping of stage is automatically performed by

arrangement available in tube expander.

Special control employed in control unit.

After fixed rolling,

The restraining collar is pushed back to the second

groove position and the control switched to pilgrim

mode

controller gives desired(Can be set by timer switch)

no of anti clockwise rotation to expander.

Clock wise rotation starts for actual expansion

Once the torque limits per setting is reached , the

rotation is reversed again to anti clockwise direction.

This continues till the collar of the expander is

touches the tube sheet and switched off.

Pilgrim expansion technique

Advantage of this technique

Faster then other processes

Ensures better control on cold work imparted to

the tube.

Very smooth transition due to design of profile

of roller end resulting in

lesser stress level

hence better resistant to stress corrosion cracking.

Leakage can be minimized between tube and

tube sheet

% Thinning calculation

Percentage thinning = (Do - Di) - (d - Di w)

2S

Where,

Di w = Average tube inside diameter after rolling

Di = Average tube inside diameter before rolling

Do = Average tube OD before rolling

d = Average hole diameter

S = Do - Di

2

Percentage thinning shall be in the range of

3 - 6 %

Testing and Insp. on expanded joints

Check on job

Check completeness of rolling by ensuring tube ID at rolled /Expanded

portion with respect to unexpanded portion by using bore gauge or air

gauge.

1% of rolled tubes surface shall be examined by boroscope on both rolling

length.

In the reference hole % thinning at both rolled places shall be computed at

the middle of expanded region by using formula.

Removal of lubricants to be checked as peer approved cleaning procedure.

Length of rolling to be checked at random.

Check on RCBs

Visual examination of tube surfaces

Length of rolling on each tube

Pull out test for all the tubes. Pull is between 764 - 1530 kg.

% thinning between 3 - 6 %

Qualifying the Procedure And joint

Prepare a procedure qualification block , which has to

simulate the job condition.

Carry out the expansion as per procedures.

Take adequate trial to get the parameter by which %

thinning, Pull out test, helium leak test,visual check meet

the required values.

If the above values are meeting the requirement the process

is qualified to perform on job.

During production control is exercised in rolling quality

control block (RCBs)

Operator Qualification

The person who are engaged for qualifying machine is

automatically qualified for performing on the job.

For other

10 nos. tubes hole to be rolled and following is the test

required on that.

% thinning of that rolled tube within limit.

Pull out test on all tube

Out of 10 tubes 2 tube ID to be sectioning for scratch

free expansion.

List of non conformance

In case of RCB Tube s fails to meet the acceptance criteria

two successive tubes shall be rolled and shall meet the

acceptance criteria before proceeding on the job

If roller breaks during rolling all the previous holes rolled

using the above tool shall be inspected for any damage.

The holes, where rolling is incomplete or interrupted due

to power failure, shall be identified and rolling shall be

performed after all other holes rolling completed.

On job rolling shall be performed only after RCBs

satisfactory results on above holes.

There are few mores other than this.

Record of expansion

Following record to be kept for expansion of tubes

No. of tube rolled (with location R x C) for each shift,

day, operator,equipment and settings.

Correlation of RCBs with the tube locations, equipment

and operator.

Melt and heat no. of tube and block.

Report of % thinning and surface inspection of 1% tubes

in the tube sheet.

Pull out test and % thinning results of RCBs

Certificate of checking of length of expansion and

termination from secondary side .

Non conformance list

Tube cleaning Procedure

Cleaning operation

Pug all the tubes at 450 mm depth from tube end.

Fill 2 storage tank with DM water

In 1st tank add approved cleaning agent to make 5% w/v concentration

and stir the solution and raise temp up to 50c.

In 2nd tank also raise and maintain the temp. between 50c- 55c..

With help of CF pump put the cleaning agent in circulation and inject the

solution in tube at 8-10 kg/cm with injection lance at a depth of 450mm.

Cleaning operation shall be start from top row and finish at bottom row.

Withdraw injection lance towards tube end.

Repeat this for few times.

Check on tube end with help of filter paper / tissues by rubbing them in

ID of the tube for any oil spots.

Stop pump and allow cleaning agent to drain from tube injector.

Cont...

Tube cleaning Procedure

Flushing operation: Close discharge valve from cleaning tank.

Start the pump and open DM water by transverseing the injection

lance in to the tube and back.

Start from top and bottom.

Random check of pH on the flushing at the tube end with

phenolophthalene indication shall not show any pink color.

Inspection : Check all tube end for free from lubricants.

Remove all plugs and with ball test check for free passage.