clean_piping_SS

8/7/2019 clean_piping_SS

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The Aim

The ideal process line will be

designed to give smooth flow

of the product. To achieve this,

the inside bore of the weld

should be consistent through

the entire system.

The bore of the system should

also be smooth with no marksor crevices which may cause

potential contamination areas,

know as ‘bug traps’. ‘Bug

Traps’ are small voids in the

material surface where product

can catch and deteriorate over

a period of time.

By designing a system with

maximum welded joints the

amount of designed problemareas is kept to a minimum.

Mechanical joints naturally have

areas which cause potential

a stainless steel process line is

to ensure that the stainless steel

will not degrade during the

installation operation. Probably

the biggest risk is the degrading

of the material during the

welding operation.

Stainless Steel’s contain

chromium and nickel which give

the material the ‘stainless’properties. If the material is not

adequately protected from the

oxygen in the atmosphere

during the welding process the

material will degrade

dramatically.

Another serious risk is the pipe

lines which have had no post

clean and treatment prior to

service. If a line is not cleaned

prior to being put into

production fabrication debris

can be left in the system. These

‘bug traps so should be kept to

a minimum.

What are the risks

Should the line not be installed

to a satisfactory standard it may

well not last as long as you hope

and can cause serious infection

in the product travellingthrough it.

The first and most important

consideration for installation of

When planning an installation of a new stainless

steel process pipe line it is important to understand

the importance of using the best possible materials

and techniques to ensure it is clean when installed.

How To Achieve

A Clean Process

Pipe LineBy Steve Purnell

Figure 1 – A potential ‘Bug Trap’ on a

welded tube



debris will soon cause and

infection of the product flowing

through the system.

How to Achieve The Aim

The main considerations to help

create a clean process pipe lineare:-

Material Selection

Fabrication Techniques

Welding Techniques

Cleaning & Treatment

Each of these areas has its own

important factors and we will

explore each of these in details

below.

Selection of Tube &

Fittings

To achieve a clean process pipe

line it is important to select tube

and fittings from a high quality

source that can provide mill

certification, chemical analysis

and guarantee the quality of the

tube finish. Tubes should besupplied from a controlled

stockholding in protective

packaging to eliminate any

smooth internal surface on the

tube and fittings. This includes

the inside diameter of the tube

and fittings selected and it is

important that the tube supplier

should be able to give a

guarantee of the quality of the

internal surface of the tube,especial in welded tubes where

the internal surface will be

effected in the weld zone.

Manufacture Specification –

There are many specifications

to which tube can be

manufactured but the most

important regulation is ASME-

BPE 1997. It is important that

the tube should bemanufactured with a SPC

(Statistical Process Control)

production line to give

guaranteed quality tube.

Batch Control – All tube and

fittings should be ordered in on

lot to ensure that the tube and

fittings delivered are from the

same batch of material. This is

ensure that all materials

supplied have an equal sulphurcontent, this in turn will

eliminate any problems with

weld quality.

Traceability – All tube and

fittings should be traceable to

original source. This allows any

problem encountered during

fabrication, installation or

service to be traced and the

problem identified and then theresults of the problem rectified.

Tube Fabrication

Techniques

During fabrication it is

important to follow

recommended fabrication

techniques to keep

contamination of the tube bore

and material degradation to a

minimum. Each stage of

chance of tube contamination

before it is installed.

When selecting and procuring

tube and fittings the following

points should be considered:-

Sulphur Control

Surface Finish

SpecificationsBatch Control

Traceability

We will look at each of these

points in turn and investigate the

importance of each area.

Sulphur Control – Stainless

Steels have a trace of sulphur

which is often not considered as

a problem area. But in practice

the sulphur content has a drasticeffect on the weldability of the

material. Only 0.001%

difference in the sulphur content

of the material can cause a vast

difference in weld profile. This

is due to the Maragoni effect,

which changes the surface

tension temperature coefficient.

This means that the heat

movement in material changes

for different sulphur contentmaterials.

Surface Finish – As mentioned

earlier it is important to have a

Figure 2 – High Quality Fittings. Photo courtesy of Axium Process,Swansea.

www.axiumprocess.com



fabrication of a pipe line has

potential to cause problems in the

final process line. The main

fabrication techniques used arecutting, tacking and welding. We

will look at each stage of the

fabrication process individually.

Cutting – Lengths of tube will

need to be cut to length during

every stage of the installation. In

order to maintain the ‘stainless’

properties it is important to control

the heat input during the cutting

process, this would require the use

of an orbital cutting machine (asshown in Figure 3) with a good

condition cutting blade fitted. The

use of other methods or indeed an

orbital cutting machine with a blunt

blade allows heat to build up

during the cutting process and can

degrade the material being cut.

Over Heating during the cutting

process can be identified by the

formation of oxides on the

surface of the tube near to the

cutting area.

Tacking – When tubes are cutto length they will need to be

tacked to fittings or other pipes

to produce the pipe run required.

The atmosphere in the tubes

internal bore should be have an

oxygen content of less than 500

ppm (parts per million) during

tacking as well as welding so that

the material does not degrade.

The tack should also be small,clean and should not fully

penetrate the wall of the material.

Dirty tacks will have a derogatory

effect on the quality of the final

weld. Dirty tacks will be dark

blue or black and in extreme

cases may

have a crusty surface.

Welding – Welding has the most

degrading effect on the propertiesof the material and is one of the

most complex and critical

operations in the fabrication

process. The welding of pipelines

will be discussed in detail below.

Figure 8 - Welded Process

Tube With No Back Purge

Figure 6 - Orbital Welded

Process Tube

Figure 5 - Manual Welded

Process Tube

Figure 4 – An Orbital

Welding System. Photo Courtesy

of Axium Process, Swansea

Welding Process &

Equipment

It is widely accepted that the

most suitable process fromwelding of stainless steel tubes in

TIG (Tungsten Inert Gas)

Welding. This process uses a non-

consumable tungsten electrode

and is shield from the atmosphere

by and inert gas, usually argon.

This process gives a controlled

heat input which can much further

enhanced by the used of a pulsed

arc. To give high quality weldprofiles and to keep material

degradation a consistent weld

speed should be used. The most

effective way to control weld

speed is by the use of and

automatic orbital welding system

Photos courtesy of Delstron System, Burton-on-Trent

Figure 3 - Orbital Cutting

Machine



(As shown in fugure 4). Figures

5 & 6 show the difference

between a weld made by a

manual pipe welder and a weld

made with a orbital welding

system. It can be seen that the

manual welding is significantly

more oxidised and less consistentthan the automatic weld.

It is now considered standard

practice to use orbital welding

equipment with fully enclosed

weld heads for pharmaceutical

process line installation. Fully

enclosed weld heads

encapsulate the entire outside

surface of the weld area. This

area is then filled with a inertgas, usually argon, before the

weld cycle starts. This gives a

guaranteed clean outer surface

and should the weld joint open

slightly during the welding cycle

it is inert gas drawn into the

internal bore and not air. A

enclosed weld head located on

a tube is shown in figure 7.

As well as protecting the

outside diameter of the materialit is also essential that the inside

surface is also protected. If

stainless steel is not protectedFig

from oxygen during the welding

process then the chromium and

carbon in the material will react

with the oxygen to for chromium

carbides, commonly know as

coking (As sown in figure 8).

Once the chromium is burned outof the material in this way it loses

it corrosion resistance. The result

identify the cause if a problem

should occur on a particular weld.

Ideally a data log will be kept for

each weld carried out on the

project. This should identify the

current used, weld travel speed

and date.

Again modern orbital welding

equipment can print or store thisinformation to standard PC cards.

Each log file will include date, time,

weld number, welding current,

travel speed and average arc

voltage. In addition the log files

will detail exact deviation from

the programmed parameters.

The internal bore of the tube

should also be filled with a suitable

inert gas to give a clean internalfinish. It is commonly accepted

that the oxygen level should be

of this will be the break down of

the material in a short period of

time.

This reaction can be avoided by

filling the internal bore of the tube

with an inert gas, this method is

called back purging and a range

of special systems are available

to insert into the tube to localisethe area at which the gas is applied

and to avoid filling large lengths

of tube. These systems not only

save on the amount of gas used

but also the amount of time taken

to get the oxygen level of the back

purge to a suitably low level. (As

shown in Figure 9)

Application & Controlof Welding

All welding on the systems should

be carried out to suitable

standards and each weld should

be tested and certified to that

level. If orbital welding equipment

is used the equipment and

operators will be to be certified.

An approved weld procedure willbe submitted with the certification

and this weld procedure should

be adhered to at all times.

Modern orbital welding

equipment has the facility to store

the weld procedure and limit the

amount the operator can change

to parameters to the level

indicated on the standard.

Test Coupons should besubmitted and approved prior to

the welder commencing work on

the installation. Further test

coupons should be submitted to

ensure that quality is being

maintained over the duration of

the contract.

All welds carried out on the

systems should be uniquely

identifiable on the system drawing

with a log of when welds were

carried out and by who. This helps Figure 7 – Fully Enclosed Weld

Head Located on a Process Tube.

Figure 9 – Principle of a Internal Purge

System. Courtesy of Huntingdon Fusion

Techniques.



below 500 ppm before welding

commences.

This in another feature of modern

orbital welding equipment, which

can be connect to a oxygen

analyser to prevent the operating

starting the weld cycle before the

oxygen level is acceptably low.Post Cleaning &

Treatment of Pipe Line

As a result of the above

fabrication techniques debris

will remain in the system and

these will need to be flushed

out. In addition all weld areas

should be passivated as the

metal crystalline configurationat the surface changes during

cooling after welding resulting

in depletion of the chromium

oxide layer. The recommended

procedure for post installation

treatment is detailed below

The system is flushed with

water to remove debris, swarf

etc. A Detergent is then

circulated to remove oil, greaseand organic muck. The system

is flushed with water again to

remove all traces of detergent.

Valve bodies, diaphragms and

other areas of potential debris

accumulation are swabbed and is

then inspection under UV and

white light to demonstrate that

organics have been removed.

Nitric Acid is then circulated to

dissolve iron oxide and other

contaminants adhering to the

steel. This is particularly

important at weld areas where

the metal profile is relatively

rough compared with polished

internals and where there is a

possibility of iron inclusions.

Stainless steel is naturally

resistant to nitric acid and is not

attacked by this chemical. Nitricacid is an oxyacid which has the

property to ‘oxidise’ the

chromium layer at the metal

surface to resistant chromium

oxide.

Even in the case of electro

polished so called ‘bio tubes’

with the use of closed cup

automatic welding it is essential

that the system is passivatedafter erection to enrich the

chromium oxide layer at the

metal surface of the weld.

Where nitric acid is unavailable

or is to be avoided for safety or

environmental reasons other

chemicals can be used involving

mixtures of citric acids and

organic chelants.

The systems is finally flushedwith de-mineralised water to

remove all traces of nitric acid.

On completion a certificate is

issued for validation purposes

and ferroxyl tests, if required,

are carried out at weld areas.

The ferroxyl test is an extremely

sensitive test which will detect

very low levels of iron giving

an intense blue colour in the

presence of iron.

Conclusion

So in conclusion it can be seen

that a whole range of factors must

be considered in order to achieve

a clean process line on completion

of the project.

It is important to use high quality

materials and equipment to give

a process line that will last for a

long period of time. This selection

of high quality goods should be

maintained throughout the project

as one lapse in quality can lead to

the whole line being contaminated.

To allow the contractor to give aguarantee of the quality of the

system at the end of the project,

full documentation will be

required. This will outline

drawings, material quality

certificates, weld data logs and all

relevant specification

conformance documentation.

This all indicates that to achieve a

process line that will give good

service and longevity of life allfactors must be considered at

initial design stage.

Orbimatic GmbH, PO Box

416, Peterborough, PE7 3XE.

Tel 01733 244063, Fax 01733

244463. Email -

[email protected].

Delstron Systems Limited,

Paget Street, Burton-on-Trent,DE14 3TQ. Tel 01283 565120,

Fax 01283 541859. Email

[email protected]

Axium Process Limited,

Hendy Industrial Estate,

Hendy, Swansea, SA4 0XP.

Tel 01792 883882, Fax 01792

886049. Email

[email protected]

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