African Fusion

AFRICAN FUSION

November 2015

SAIW Member profile: Hydra-Arc

Fabrication for hygiene

ndustrial plants subject to health

and environmental regulations in-

clude food, dairy, pharmaceuticals,

cosmetics, andbeverage plants process-

ing drinks such as beer and wine, where

bacteria can be directly associated with

bio-film build-up on product-contact

surfaces. Tomeet the demand for great-

er plant hygiene, stainless steel plants

were built in the mid-1960s, which had

an intended life expectancy of 30 years

and more.

Hygienic processing plants are typi-

cally fabricated from polished 304L or

316L austenitic stainless steel, although

duplex stainless steels have recently

been introduced. In linewith regulations,

plant design philosophy requires: free

product flowwithout stagnation; protec-

tion fromthe external environment; and

cleanable plant interior surfaces.

When fabricating plants, standard

plates and pipes are formed and con-

nected together, often by welding, into

systems. Welding, an enabling tech-

nology, facilitates stress alignment of

components and thus lighter structures.

Whilst traditionally welding has always

focused on structural integrity, hygienic

welding introduces new challenges.

Fabrication for processing

Thesemay provemore demanding than

structural integrity, and are an addition

to other operational requirements in-

cluding environmental pressures.

Welded processing plants

Plants generally comprise a closed

system of tanks, heat exchangers,

mixers, distillation columns and inter-

connecting thin-walled pipes. Two par-

allel systems operate in tandem, one

product-related, and the second to fa-

cilitate cleaning of the internal surfaces

of pipes, vessels, equipment, filters and

fittings without disassembly, known as

clean-in-place (CIP). Up to the 1950s,

closed systems were disassembled and

cleanedmanually but with CIP, cleaning

is faster, less labour-intensive andmore

repeatable, posing less of a chemical

exposure risk to people.

The welded area in a plant is a very

small part of the total area of the instal-

lation. Fabrication and design detail,

however, influence plant performance

and cleaning processes, and can result

in lost productive time.

Realistically, hygienic welded fab-

rication will be both challenging and

costly. Although this puts capital bud-

Plants generally comprise a closed system of tanks, heat exchangers, mixers, distillation columns and

inter-connecting thin-walled pipes. Photo courtesy BOC and A&G Engineering.

In this article, Tony Paterson of the Wits School of Chemical and Metal-

lurgical Engineering describes the special welding requirements for

stainless steel piping in plant applications where bio-film build up on

pipe contact surfaces has to be avoided. Amathematical algorithmwas

developed to assess interconnecting pipes against the criteria of a point-

by-point minimum overlap of 80% around the pipe circumference.

gets under pressure, well-designed

and executed fabrication should ease

operational costs.

Factory and site welding

Whilst themajor components inprocess-

ing plants are factory built, many of the

small-bore pipe inter-connections are

made on site.

Factory conditions are easier to con-

trol as both sides of aweld are generally

accessible. During welding of products

accessible fromboth sides, protectionof

the tool side and the opposite side of the

weld by an inert gas is required to avoid

sensitisation. If carried out properly, the

need for post-weld treatments (grinding,

polishing, pickling and passivation) will

be minimised.

Site welding is more difficult as,

after fabrication, the system is closed

as access compromises sterility of the

equipment. The interiors of pipes are

only accessible to fabricators for clean-

ing, gas purging and NDT inspection

purposes. On-site welding is inherently

more difficult to control, more difficult

to monitor and more difficult to repair.

Inadequate welded joints compro-

mise product quality in an otherwise

hygienically designed plant because

poor joints and/or welds will trap

bio-film, thus encouraging bacterial

growth. Bio-film volume increases with

surface roughness, increased tempera-

tures, lower flow speeds and stagnant

areas.

Bio-film formation on inside contact

surfaces of a plant can also exacerbate

local galvanic activity andmicrobiologi-

cally induced corrosion (MIC). Alongside

bio-film, MIC can significantly impact

capital equipment life.

Welding effects include:

• Welding process-related roughness

in the heat affected zone (HAZ).

• Possible sensitisation leading to

roughness and corrosion attack (low

carbon stainless steels overcome

this).