Design of piping systems for the food processing

Guideline no. 5
Design of piping systems for the food processing
industry – with focus on hygiene
Authors:
Folkmar Andersen, Jens; Alfa Laval Kolding A/S
Boye Busk Jensen, Bo; BioCentrum – DTU
Boye-Møller, Anne R.; Danish Technological Institute
Dahl, Michael; Danish Technological Institute
Jepsen, Elisabeth; APV Nordic A/S
Jensen, Erik-Ole; Arla Foods amba
Nilsson, Bo; Senmatic A/S
Olsen, Bjarne; Tuchenhagen GmbH
Thomsen, Willy; Royal Unibrew A/S
Prepared by the flow components task group under the auspices of the competence
centre of the Danish stainless steel industry.
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Den Rustfri Stålindustris Kompetencecenter
c/o Teknologisk Institut
Holbergsvej 10
DK-6000 Kolding
Tel.: +45 72 20 19 00
Fax: +45 72 20 19 19
info@staalcentrum.dk
www.staalcentrum.dk
This guideline is developed with the support of the Danish Ministry of Science,
Technology and Innovation.
Published for the Centre by:
Holbergsvej 10
DK-6000 Kolding
www.teknologisk.dk
© Danish Technological Institute
ISBN: 87-7756-750-1
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Guideline no. 5
Introduction
This guideline provides general advice on the design of piping systems in the food
industry.
The focus will be on the design and installation of stainless pipes in closed hygienic (CIP
cleanable) systems.
The guideline is divided into chapters that can be read independently of each other. If
read in its entirety, the guideline will contain repetitions, which is a deliberate choice.
The guideline is prepared by the flow component task group under the auspices of the
competence centre of the Danish steel industry.
Guideline no. 1: Cabling and electrical cabinets – with focus on hygiene
Guideline no. 2: Check list for the purchase/sale of production equipment – with focus
on hygiene
Guideline no. 3: Conveyors – with focus on hygiene
Guideline no. 4: Stainless steel in the food industry – an introduction
Guideline no. 5: Design of piping systems for the food processing industry – with focus
on hygiene
Guideline no. 6: Installation of components in closed processing plants for the food
processing industry – with focus on hygiene
Enjoy!
Key words
Piping systems, pipe runs, food industry, steel quality, hygiene design, requirements,
welding, fittings, receiving inspection, mounting, inspection, flow, installation,
construction, check list, plant layout
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Contents
1.
Domain ...................................................................................................................... 5
1.1.
1.2.
2.
Limitations....................................................................................................................... 5
Definition and use of guidelines.................................................................................... 5
Choice of steel quality.............................................................................................. 5
2.1.
3.
Recommended material selection for the food industry............................................. 6
Design of piping systems ........................................................................................ 8
3.1.
Drainability or not? ......................................................................................................... 9
4.
Requirements for pipes and fittings ....................................................................... 9
5.
Receiving inspection.............................................................................................. 10
5.1.
Analyses for the inspection of deliveries ................................................................... 10
6.
Storage .................................................................................................................... 11
7.
Welding of stainless steel pipes and fittings ....................................................... 13
7.1.
7.2.
7.3.
7.4.
7.5.
8.
Choice of material to be welded .................................................................................. 13
Filler material................................................................................................................. 13
Welding from a product hygiene perspective ............................................................ 13
General welding requirements .................................................................................... 14
Shielding gas................................................................................................................. 15
Welding requirements ............................................................................................ 18
8.1.
8.2.
8.3.
8.4.
9.
Equipment ..................................................................................................................... 19
Welding preparation ..................................................................................................... 19
Pipe welding .................................................................................................................. 20
After welding ................................................................................................................. 23
Mounting ................................................................................................................. 24
9.1.
9.2.
9.3.
9.4.
9.5.
9.6.
10.
Drainability .................................................................................................................... 24
Pipe dimensions ........................................................................................................... 24
Pipe layout..................................................................................................................... 24
Seals at pipe joints ....................................................................................................... 25
Pipe supports ................................................................................................................ 25
Production stoppage precautions............................................................................... 25
Inspection of completed work ............................................................................... 26
10.1.
10.2.
Weld inspection for deliveries ..................................................................................... 26
Installation inspection .................................................................................................. 27
11.
Overview of other guidelines about piping, etc. .................................................. 27
12.
Applied methods .................................................................................................... 29
13.
Further information and literature......................................................................... 29
14.
Change protocol ..................................................................................................... 29
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Guideline no. 5
1.
Domain
This guideline provides general advice on the design of piping systems in the food
industry.
Special attention points will be illustrated by drawings/photographs.
1.1. Limitations
The focus will be on the design and installation of stainless pipes in closed hygienic
(CIP/SIP cleanable) systems for the food industry.
1.2. Definition and use of guidelines
The guideline can be used by construction engineers in connection with the design of new
and the renovation of old plants. It can also be used by chief installation engineers as a
check list to prevent unsuitable installation. Furthermore, the guideline contains knowledge
for production supervisors of correct handling and storage of the supplied material.
It can also be used by purchasing officers when deciding on the specification of plant
layout and parts.
Finally, the guideline can be used as a communication tool between purchasing officers
and suppliers when coordinating their expectations for the delivery.
It is not the purpose of the guideline to recommend certain types of solutions or suppliers.
2.
Choice of steel quality
Stainless steel of varying quality is the most used material in the food industry for the
construction of machines and processing equipment. This is due to the ability of steel of
forming a chromium oxide layer on the surface, which will appear smooth and whole, and
have good mechanical properties, without the steel corroding. When the chromium oxide
layer disintegrates, the steel will corrode. Basically, stainless steel is not a precious metal
but a material which is more or less inactive to most environments.
Despite the many good properties of stainless steel, it is a complex material in which
corrosion problems may arise due to wrong use or treatment. This can lead to the
premature replacement of processing equipment or machines.
For an introduction to stainless steel, please refer to Guideline no. 4: Stainless steel in the
food industry – an introduction.
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2.1. Recommended material selection for the food industry
Generally, the quality of steel should be selected according to the environment to which is
will be exposed. A change to a more corrosive environment (e.g. changes in the product or
detergent/cleaning procedure) may lead to serious corrosion attacks on the plant.
In the food industry, stainless steel grades below the following requirements should not be
used (see the reason for this limit below and in section 7.1):
Carbon content (C)
max. 0.05%
Molybdenum content (Mo) min. 2.0%
When using grades like AISI 304 and AISI 316, special attention should be paid to the
carbon content. This can be as high as 0.08%, which is too high if the steel is to be
welded. Table 1 shows the most frequently used steel grades in the food industry,
including the contents of the most important alloy constituents. When choosing stainless
steel for welding operations, a low carbon content is crucial to prevent the formation of
chromium carbide. The greater the material thickness, the longer the workpiece will take to
heat during welding, and the lower the carbon content has to be.
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Table 1. Stainless steels in various standards, grouped according to grade with a 2004 price index.
As prices of stainless steel are very dependent on the alloy constituents, the price index will
fluctuate over time.
Steel grade
C
%
Cr
%
NI
%
Mo
%
P
%
S
%
N
%
Price
index
(relative
scale)
Common stainless steel
(austenitic)
AISI 304
AISI 304 L
EN 1.4306
SS 2333
EN 1.4301
Acid-resistant stainless steel
(austenitic steel containing molybdenum)
AISI 316
AISI 316 L
EN 1.4404
SS 2347
SS 2343
EN 1.4401
EN 1.4436
Duplex steel
(austenitic-ferritic
steel)
AISI 904 L
AV 254 SMO
SAF 2304
SAF 2205
(EN 1.4462)
SAF 2507
min.
max
.
min.
max
.
min.
max
.
min.
max
.
min.
max
.
min.
max
.
min.
max
.
min.
max
.
min.
max
.
min.
max
.
0.08
18.0
20.0
8.0
10.5
-
0.045
0.030
0.03
18.0
20.0
8.0
10.5
-
0.045
0.030
0.05
17.0
19.0
8.0
11.0
-
0.045
0.030
0.07
17.0
19.0
8.5
10.5
-
0.045
0.030
0.08
16.0
18.0
10.0
14.0
2.0
3.0
0.045
0.030
0.03
16.0
18.0
10.0
14.0
2.0
3.0
0.045
0.030
0.05
16.5
18.5
10.5
14.0
2.0
2.5
0.045
0.030
0.05
16.5
18.5
10.5
14.0
2.5
3.0
0.045
0.030
0.07
16.5
18.5
10.5
13.5
2.0
2.5
0.045
0.030
0.07
16.5
18.5
11.0
14.0
2.5
3.0
0.045
0.030
25.0
18.0
4.0
5.5
4.5
6.1
-
-
0.03
20.0
20.0
22.0
23.5
21.0
23.0
4.5
6.5
2.5
3.5
0.14
0.03
24.0
26.0
6.0
8.0
3.0
5.0
0.30
0.03
0.01
0.01
min.
max
.
min.
max
.
min.
max
.
100
130
300
400
0.10
170
190
400
Please note that although AISI 304 and EN 1.4301 are often regarded as identical, there
may be small differences in the carbon content.
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3.
Design of piping systems
The design of piping systems must ensure a good future-enabled pipe layout by using the
fewest possible components, and at the same time ensure optimal functioning of the plant.
There are many possible solutions for a pipe run. Often, far more elbows than necessary
are used because the designer did not give the layout enough thought.
An elbow too much means:
•
•
•
costs for the purchase of an elbow
costs for the installation
increased energy costs for the entire lifetime of the plant due to
pressure loss
It is therefore important to think everything through during the design phase!
Figure 1. Example of more elbows than necessary being used, which leads to increased costs.
During replacement of existing piping systems, the existing piping systems are often not
dismounted while the new system is being built. The existing piping system will often
constitute an obstacle for the new system. Therefore, the existing piping system has to be
taken into consideration, and consequently the new system turns out to be less than
optimal. The focus should be on optimising the new piping installation. It is better to
change the old installation temporarily than to mount new pipes around it, with a poor
outcome.
When planning the pipe layout, it will be useful to use 3D drawings or perspective
drawings to illustrate the pipings. It will be easier for the customer and the supplier to
troubleshoot, avoid misunderstandings and find the optimal pipe layout together, before
the final mounting is initiated. This will facilitate the mounting process considerably.
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3.1. Drainability or not?
Two different operating situations are predominant in the processing industry:
1. The first is that the plant must be emptied every time production has taken place,
i.e. it is very important that it can be drained of everything before the plant is left.
2. The second, and most widespread in recent years, is that the plant must be liquidfilled at all times. This means that when a process has ended, a sterile liquid is fed
to the plant and left there until the next process is started, when the sterile liquid will
be displaced by the process liquids.
Regardless of the chosen solution, the plant must be designed so that it can be drained.
This is done by constructing the plant with a highest point and falls on both sides in the
process piping system and by incorporating drains to ensure that the plant can be emptied
of air, product and CIP-liquids. It is highly unlike that the plant will never have to be
emptied at some point.
4.
Requirements for pipes and fittings
•
•
•
•
•
•
•
•
The choice of steel quality must match the load and the environment to which it is
exposed. A sensible choice might be EN 1.4401, however with a maximum carbon
content of 0.05% (see Table 1).
Pipes must be round, and fittings must have round branch pipes. There must be no
oval cross sections at the ends of neither pipes nor fittings. Special attention should
be paid to elbows and plungings.
The inside surface roughness is industry dependent. The typical requirement in the
food industry is Ra < 0.8 µm. Please note that according to international norms,
there is a difference between specifying a “max. value” and an ”upper value” (and
similarly for a ”min. value” and a ”lower value”). If e.g. an "upper Ra value” of 0.8 is
specified, it means that 16 per cent of the measurements can be higher than this
value. If, on the contrary, a ”max Ra value” of 0.8 is specified, no measurements
can be higher than this value. Measurements must 1) be based on sufficient
statistical data (a sufficient number of measurements) and 2) be carried out on
uniform flawless surfaces that are inside the basis of the estimate.
The inside surface should be passivated, pickled or electropolished. Special care
should be taken when welding electropolished surfaces as a far better gas
protection is required for welding of surfaces which have been made “shiny” – e.g.
through grinding, electropolishing, etc.
There must be no scratches, holes, porosity or other surface defects on the product
side of the steel.
Pipes must be delivered free of defects and clean on the inside as well as on the
outside. The pipes must be plugged at the ends and wrapped.
Fittings must be delivered flawless and clean on the outside as well as on the
inside, and they must be wrapped, see figure 4.
All pipes and fittings belonging to the same mounting operation should have
identical pipe diameters and material thickness, i.e. be delivered in the same
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standard (DS, DIN, SS and similar). (Later a DIN pipe can, however, be welded
onto a DS pipe. It only requires that the pipe with the smallest diameter be milled to
the same diameter as the other pipe).
5.
Receiving inspection
Receiving inspection should ensure:
•
•
•
•
•
•
•
•
•
•
That the delivered material contains no visible defects and impurities. This may later
on cause problems in the processing equipment where these impurities might
accumulate.
That the material has been delivered in the agreed quality, and that the material
comes with certificates verifying this.
That the supplied pipes are plugged at the ends and dry on the inside.
That the cross section is not oval in the material as this can cause root defects
when welded. Often the case in fittings.
That fittings are delivered in the right degrees/angles that were ordered.
That the inside surface roughness of pipes and fittings complies with the agreed
requirements (often Ra < 0.8 µm).
That pipes as well as fittings are pickled and passivated on the inside. Special
attention should be paid to welds.
That the longitudinal welding in pipes and fittings did not cause discoloration on the
inside, see figure 2.
That fittings and pipes are wrapped.
That surfaces which come into contact with the product are free of scratches, holes,
porosity and other defects that appear as cavities in the surface.
Figure 2. Recently delivered pipes with discoloration in the longitudinal weld. The pipe is a reject. (A mirror is
inserted into the pipe. The picture shows the reflection).
5.1. Analyses for the inspection of deliveries
The following techniques can be used for the inspection of received material and
documentation of surface treatment and finish.
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Guideline no. 5
•
•
•
•
Optical emission spectral analyses (OES analyses) to examine the chemical
composition stated in the accompanying delivery certificates according to EN
10204/3.1B (identical to prEN 10204/3.1a).
Light Optical Microscopy (LOM) to inspect the microstructure.
Scanning Electron Microscopy (SEM) for the inspection and photographic
documentation of the surface finish (topography).
Roughness measurements for the documentation of Ra and Rz values and the
recording of surface profiles, cf. ISO 4288, ISO 4287 and ISO 3274.
6.
Storage
•
•
•
•
•
Once the receiving inspection is completed, the pipe ends must be sealed. This is
to prevent the ingress of impurities and small animals in pipes and fittings.
All materials (pipes, fittings, valves, etc.) must be stored in dry, dust-free conditions
(and not as shown in figure 3).
All materials (pipes, fittings, valves, etc.) must be stored at a temperature
corresponding to that of the mounting site. If this is not possible, the materials must
be brought to the mounting site no later than 24 hours prior to the mounting so that
they may achieve the temperature of the mounting room. This is to prevent
condensation inside the pipes, which may cause welding defects and lead to the
rejection of the welds.
Precautions must be taken to prevent deformation of the stored materials through
collision or insufficient support.
Work in black steel and stainless steel must always be kept separate. This also
applies to storage.
Figure 3. Recently delivered pipes incorrectly stored on site.
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Figure 4. Examples of correctly wrapped fittings.
Figure 5. Examples of correctly stored fittings.
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7.
Welding of stainless steel pipes and fittings
7.1. Choice of material to be welded
Generally, austenitic steels have very high weldability. However, only materials with an
0.05 per cent carbon content or less (dependent on material thickness) should be
selected. This is because chromium carbide may easily form in the grain boundaries. The
higher the C content in the steel, the easier chromium carbide will form. And when
chromium carbide is formed, so are chromium depleted zones. This means that less
chromium will be available for the formation of the passive layer, which may easily result in
intercrystalline corrosion.
7.2. Filler material
When welding stainless steel, never use filler materials with a lower alloy composition than
the base metal. A good rule of thumb is always to upgrade the filler material, e.g. to select
a filler material that is higher in chromium and molybdenum, when welding AISI 316. The
reason is that some of the alloy elements are burnt off in the weld pool. Upgrading the filler
material allows for the burn-off of alloy elements, and at best the welding results in a
surplus of alloy elements which will improve resistance to weld decay.
7.3. Welding from a product hygiene perspective
The design philosophy of equipment is that it should allow the product to flow freely and
unhampered through the piping system. It must be possible to clean the system efficiently.
Therefore, it is crucial that the welding preparations are thorough. Parts that are to be
welded must be thoroughly cleaned and deburred, but appear as sharp-edged (see figure
6, 1+2). Likewise, variations in material thickness, wrong setup, oval pipes or fittings,
wrong design or similar will lead to pockets and zones, where CIP becomes insufficient
(see figure 6, 3).
Incorrect welding may lead to poor hygiene conditions in an otherwise hygienically welldesigned plant. Thus, a residual oxygen content of more than 1 per cent in the backing
gas will lead to irregularities and burrs in the actual welding surface. Similarly, a beginning
corrosion may cause bacterial pockets long before the actual corrosion is detected. Root
defects in the welds must be avoided as they can lead to bacterial pockets.
The normal roughness of a well-performed weld will be approx. 1.6 to 4 µm. The maximum
roughness accepted on the product side is 6 µm. This is accepted as the welded area
constitutes a very small part of the total area of the installation, and at the same time the
surface topography is very “soft” due to the molten pool.
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Figure 6. 1) The pipe is properly deburred and sharp-edged – ready for welding. 2) Shows the opposite of 1).
3) Shows two pipes with identical diameters that are offset to each other. Welding of offset pipes will lead to
shadow zones for the detergent where CIP becomes insufficient. Should it become necessary to weld pipes
with slightly different diameters, the smallest of the pipes must be milled to reduce the diameter difference
between the pipes to < 20% of the material thickness. Likewise, none of the pipes must be oval or out of
flush with each other (eccentric), and the deviation must be < 20% of the material thickness. The distance
between workpieces to be welded must be <0.25 mm). (See also figure 8).
7.4. General welding requirements
All surfaces – inside as well as outside – must be thoroughly cleaned prior to welding, and
all oil and grease-containing material must be removed from the weld zone as it may
otherwise lead to impurities during welding, and consequently reduced corrosion stability.
Only personnel with a valid certificate for welding stainless steel, and the experience that
goes with it, may carry out welding operations on workpieces that are to be used in the
food industry. Please note that there are several norms/standards for the certification of
welders and welding operators, the design and approval of welding procedure, and for
welding requirements (acceptance levels for welding defects). Please refer to the Danish
Standard Association for the currently certified welding norms – see www.ds.dk.
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A well-performed weld will not require any form of grinding but, in so far as possible, it
should be pickled both on the inside and outside of the pipe. This will increase the
corrosion stability. Pickling piping on the inside is difficult, and normally it is not done. This
makes it even more important to be meticulous with the gas protection in the pipe welds
that cannot be pickled.
In general, it should be avoided to mix pipes of different standards (DIN/DS/EN...) as they
may vary in terms of diameter as well as material thickness. Should it become necessary
to weld pipes with slightly different diameters, the smallest of the pipes must be milled to
reduce the diameter difference between the pipes to < 20% of the material thickness.
Likewise, none of the pipes must be oval or out of flush with each other (eccentric), and
the deviation must be < 20% of the material thickness (see figure 6, 3).
All pipes and surfaces that are to be welded must be straight, and to ensure perpendicular
cuts, mechanical cutting devices should always be used for cutting pipes. The distance
between workpieces to be welded must be <0.25 mm).
7.5. Shielding gas
During welding, shielding gas must be used at all times, both on the front and on the
backside of the weld zone. The purpose of the shielding gas is to prevent the access of
oxygen to the front and backside of the weld pool.
Insufficient use of backing gas will lead to oxidation of the heat-affected weld zone. This
will weaken the mechanical as well as the anti-corrosive properties.
The backing gas must be maintained until the surface temperature is below 250°C.
For the gas protection to be full, its residual-oxygen content must be so low that the
discoloration in the weld zone is less or equal to level C in the FORCE Institute report no.
94.30 “REFERENCE ATLAS for validating shield gas quality for welding”. See illustration
from the report in figure 7.
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[Billedtekster: Level A (oxygen conc. 21 ppm) Level C (oxygen conc. 100 ppm Level E (oxygen
conc. 1,000 ppm). From REFERENCE ATLAS for validating shield gas quality for welding]
Figure 7. Excerpts from the FORCE Institute report no. 94.30 “REFERENCE ATLAS for validating shield gas
quality for welding” showing the discoloration in the weld zone at level A, C and E respectively (from left to
right).
In pickled dairy pipes, this corresponds to:
When using argon gas:
When using formier gas:
max. oxygen concentration 30 ppm
max. oxygen concentration 100 ppm
Note: Please note that shining surfaces are much more prone to discoloration than pickled
surfaces and thus require a significantly better gas protection (lower oxygen concentration
in the shielding gas).
Note: Complete gas protection is required for tacking as well as welding!
The most frequently used shielding gasses are argon and formier gas. In recent years,
Noxal has also gained a footing among “skilful” welders. Often, argon is used in the
welding gun and formier gas as gas protection inside pipes. Table 2 shows the properties
and possible applications for the three gas types (argon, formier and Noxal).
Generally, it is recommended to use argon shielding gas in the welding gun and formier
shielding gas as backing gas when welding pipes.
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Table 2. Overview of properties and applications for various backing gasses.
Shielding gas
Properties
Application
Argon
Argon is a colourless and odourless
The use of argon is due to its inert
noble gas which forms 0.93 vol. % or
nature.
atmospheric air. Argon is
Shielding gas for MIG welding of
incombustible and nontoxic.
aluminium and copper, and for TIG
Argon is heavier than atmospheric air.
welding.
Flushing gas for cleaning metal smelting
installations.
Shielding gas for heat treatment of
metals.
Shielding gas in the electronics industry.
Formier
Formier gas is a reducing gas mixture
Gas protection with a reducing effect to
consisting of nitrogen and a small
shield workpieces from atmospheric air
amount of hydrogen.
during welding, brazing, brazing and
Formier gas (90% N2/10% H2) is
tempering.
Formier gas is recommended as root
nontoxic and combustible in certain
shielding (backing gas) in connection with
mixing ratios with air.
Formier gas is lighter than atmospheric welding of stainless steel, including
duplex steel.
air.
Noxal is a gas mixture of argon and
Noxal 2 and 4: Shielding gasses for TIG,
Noxal
hydrogen. The mixtures are
MIG and plasma welding of stainless
colourless, odourless and nontoxic.
steel.
The hydrogen content gives reducing
Noxal 6: Plasma cutting.
properties to the mixtures.
As only mixtures with less than 5%
hydrogen are incombustible when
mixed with air, Noxal 4 and 6 are
categorised as flammable and
explosive.
Noxal 2 = 97% Ar and 3% H2
Noxal 4 = 93% Ar and 7% H2
Noxal 6 = 80% Ar and 20% H2
Noxal is heavier than atmospheric air.
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8.
Welding requirements
Longer operating hours and the use of more aggressive detergents and disinfectants are
contributing factors towards increasing the strain on the installations. This increases the
requirements for material selection and plant design. In this respect, the quality of the
welding is crucial.
In connection with new plants or alterations of existing plant, the client (the food
manufacturer) selects the steel grade based on the anticipated environmental strains the
steel will be exposed to. Once the steel grade is selected, the requirement will be that the
welding of the plant does not reduce the quality of the installation.
With respect to corrosion, the welds must in principle withstand the same environmental
strain as the base material throughout the entire lifetime of the installation. This means that
if there is corrosion over time in a weld, but not in the base material, it will be deemed as
failure on part of the supplier of the plant. Therefore, the supplier must also provide a
warranty for weld stability. Such warranty should not be less than 5 years. Should there
be corrosion in the welds (the weld zone), but not in the pipes, the supplier of the plant,
who undertook the welding, will be liable in damages. In the event of corrosion of both
welds and pipes, the company/the customer is liable. (The weld zone is the area within a
30 mm distance to both sides of the actual weld, as shown in figure 8).
Svejsezone = 2xZ,
Z ~ 30 mm
a < 0,15 * t
b < 0,15 * t
t, godstykkelse
a og b målene skal overholdes såvel
udvendigt som indvendigt i rørene
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[Billedtekster: Weld zone. A and b measurements must be observed on the inside as well as the outside of
the pipes. Material thickness. (Husk at ændre komma i decimal tallet 0,15 til punktum (altså 0.15).]
Figure 8. Pipe ends must be cleaned of dirt and grease in a minimum distance of 30 mm from the weld. The
concavity/convexity of the weld must be < 0.15 x material thickness.
With respect to hygiene, the welds on the product side must be as smooth as possible
without burrs, crevices and the like. A well-performed inside weld should have a roughness
of the seam of Ra < 3 µm, and the absolute highest acceptable roughness is Ra < 6 µm.
The following sections contain guidelines for the process steps. Preparation, execution
and inspection of stainless steel welds.
8.1. Equipment
The following equipment is expected to be required:
•
•
•
•
•
TIG welding unit with pulse box.
Oxygen meter to check the residual oxygen content in backing gas.
Flow meter to control the gas supply.
Shielding gas (argon or formier gas). (It is recommended to use argon on the
outside and formier gas on the inside of the pipes.)
Pipe cutter.
8.2. Welding preparation
For piping installations, observe the following guidelines:
•
•
•
•
•
Pipe ends must be cleaned of both dirt and grease in a minimum distance of 30 mm
from the weld.
Pipes must be shortened in the pipe cutter so as to give the pipes a perpendicular
straight cut. Manual shortening with a hacksaw is only acceptable if use of a pipe
cutter is not possible, and it has been previously agreed with the customer.
The shortened pipe ends must be free of burrs.
Pipe ends that are welded must have identical inside and outside diameters. In
case of any difference, the smallest pipe must be milled to the same inside
diameter.
When using fit-up clamps to secure pipe ends, the contact face must be stainless.
The directions in figure 8 must be observed.
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8.3. Pipe welding
•
•
•
•
•
•
•
•
•
Welding of product pipes must only be carried out by certified welders.
During welding, gas protection must be used both inside and outside the pipe. On
the inside of the pipe, the gas protection must be full1 (see section 7.5)
Tacking must always be carried out under full gas protection.
When the weld has been completed, full gas protection must be maintained until the
temperature of the weld is below 250°C.
In so far as possible, all welding operations must be carried out in a vice bench.
When using a filler material, make sure its alloy composition is not lower than that of
the base metal.
The weld must be performed such as to be free of pore defects, root defects,
penetration defects or the like.
To the extent possible, the welder himself must check all welds for correct backing
gas and correct weld root (see figures 9-13).
When welding materials with a thickness of 3 to 40 mm, DS/EN 288, no. 1 to 8 must
be observed.
In pickled dairy pipes, this will typically correspond to:
When using argon gas: max. oxygen concentration 30 ppm
When using formier: max. oxygen concentration 100 ppm
(Please note that shining surfaces are much more prone to discoloration than pickled
surfaces and thus require a significantly lower oxygen concentration in the backing gas).
If sufficient backing gas has been used in accordance with the directions above, there will
be no significant discoloration. Therefore, any discolorations that are level C or higher in
“REFERENCE ATLAS for validating shield gas quality for welding” will not be accepted
(see figure 7).
Figures 9-13 show examples of good and poor gas protection with discoloration and weld
root.
1
For the gas protection to be full, its residual-oxygen content must be so low that the discoloration
in the weld zone is less or equal to level C in the FORCE Institute report no. 94.30 “REFERENCE
ATLAS for validating shield gas quality for welding”.
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Figure 9. Tacking without backing gas The blue colour and the formation coke/cinder in the tack root is clear
to see.
Figure 10. After all-welding the tack point with the formation of coke in the weld root is clear. (A mirror is
inserted into the pipe. The picture shows the reflection).
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Figure 11. Good welds with gas protection: No discoloration and a nice uniform weld root.
Figure 12. Poor weld with insufficient gas protection (blue colour) and with a very poor root.
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Figure 13. Poor weld with an unacceptable gas protection (notice the dark colour), but with a nice uniform
root.
8.4. After welding
•
•
•
•
•
After welding, the welder must perform a visual check of the welds on the inside as
well as on the outside of the pipe.
Internal inspection of piping systems is performed by endoscopy.
The inspection of the welds must show no signs of oxidation and weld splatter
around the concavity/convexity of the weld as shown in figure 12.
The weld is pickled on the outside with pickling paste. When possible, it is also
pickled on the inside.
Any subsequent internal inspection of piping systems is performed by endoscopy.
The supplier of the plant must prepare a welding specification describing how the
requirements for the welding procedure are met and how compliance with the procedures
is checked regularly during the entire construction phase.
The client determines the procedure for control and inspection of the finished plant/part of
plant. If the delivered plant ”fails” at the inspection, i.e. does not meet the specified quality
requirements, the inspection is intensified beyond what was agreed. All defects found must
be repaired.
The client pays the costs of the established procedure for control and inspection, provided
no defects are found. In the event of defects, the supplier of the plant must normally pay
for repairs of defects found and bear the costs of the preliminary as well as of the
intensified inspection.
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See enclosed example of weld inspection of deliveries in chapter 10.
9.
Mounting
9.1. Drainability
•
•
Pipes must be mounted so as to be fully drainable (minimum incline of 3°).
If the location prevents drainability, drain valves must be built in at all low points of
the installation.
9.2. Pipe dimensions
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•
•
•
•
•
To avoid undrainable pockets, an eccentric cone must be used to change pipe
dimensions in horizontal pipings. For vertical pipes a concentric cone is used.
There must be no change of pipe dimension without the use of a cone.
Long piping runs must allow pipe expansion in connection with temperature
changes, without this causing unnecessary tension in the pipes. This is ensured by
building in a lyre or an expansion joint. Stainless steel expands by more than 1 mm
per meter per 100°C heat increase.
If lyres are built in, these must be large enough to allow for the expansion and
contraction without deformation to the pipe.
If expansion joints are built in, they must be designed to allow cleaning and
disinfection.
The pipes must be dimensioned so that the product is not damaged during the flow.
This means that the pipe layout must be such as to minimise any pressure loss, i.e.
contain as few elbows etc. as possible (resistance also entails unnecessary waste
of energy).
9.3. Pipe layout
•
•
•
•
With regard to pumps, it must be ensured that there is a straight piece of pipe
immediately before the pump with a diameter of five times that of the pipe. This is to
ensure that the liquid flow is sufficiently developed before it hits the impeller as it
would otherwise cause vibrations in the pump.
It must be ensured that changes in pipe direction do not block future expansions of
the pipe bridge and the like, which would otherwise lead to poor layout of the
expansion and the subsequent risk of cavitation.
The installation must not contain any dead pockets/blind ends as these would be
difficult to clean. In branches where valves are used for the opening/closing of each
branch, a valve in the actual branch can be used instead (three-way valves, but not
ball valves), to direct the liquid to one branch or the other without causing dead
pockets.
Optimal pipe layout means that the plant is designed to treat the product as
carefully as possible with the easiest possible cleaning and the lowest possible
energy consumption.
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•
The risk of ingress of air must be minimised, as it would mix with the product and
possibly create air pockets in the piping system.
9.4. Seals at pipe joints
• No metal-to-metal towards the product stream.
• Metal-to-metal stops on the backside of gaskets to avoid overcompression.
• No sharp edges where the gasket may expand. This could cause the gasket to be
cut off, e.g. by thermal expansion of the gasket.
9.5. Pipe supports
•
•
•
•
•
•
Pipe supports must be designed to prevent impurities from settling and subsequent
falling into the product, i.e. there must be no horizontal surfaces on the construction.
Supports should have a 45° incline.
Pipe supports on which a profile has been plunged to make the support stronger,
may cause pockets where impurities and liquid may settle. This may lead to
bacterial growth and corrosion. Pipe supports with smooth flanges are
recommended.
Pipe supports must be mounted in a distance that prevents cavitation in the pipes
and at irregular distances so as not to increase the oscillation frequency of the
pipes. The distance between pipe supports is selected as needed, typically 1.5-2 m.
In vertical pipe layouts it must be ensured that pipes are suspended in pipe hangers
that fix the pipes and prevent them from sinking and creating a backward slope in
the horizontal section of the pipe, so that it is no longer drainable.
Stainless steel expands by more than 1 mm per meter per 100°C heat increase. As
pipe expansions are thermal, it must be ensured that the pipes can move during the
expansion, as the pipe supports would otherwise break at the fixing point. Pipe
supports must fully encompass the pipe or be cups.
If pipe cups are used, the pipes should be fixed at regular distances to ensure that
they stay in the cup if an accident in the process creates a water-hammer that
causes the pipe to lift.
9.6. Production stoppage precautions
Pipe systems should not be emptied prior to CIP cleaning, but the product should be
displaced by water. In connection with short production stoppages (a few days), the pipes
should be liquid-filled (with water or similar). The reason is that it may be difficult to drain
piping systems of air. When a piping system is emptied prior to CIP, there is a risk that air
is left in the pipes, and air pockets during CIP means that some zones are not cleaned
sufficiently.
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10. Inspection of completed work
10.1.
Weld inspection for deliveries
Much annoyance can be avoided and money saved if the chief installation engineer
checks the daily work on the installation. The example below is a suggestion for a weld
inspection.
Example for weld inspection for deliveries
A. Specified requirements for welds on piping installations in stainless steel must be observed. Special
B.
C.
D.
E.
F.
G.
H.
attention is drawn to the requirement that only certified welders carry out welding of stainless
installations. Welders who carry out welding operations during installation at a site must take a welding
test which is approved by engineer NN or his assistant, before the actual work can commence. The test
must be a welding of 2” pipes at a 45° incline.
A minimum of 5 per cent of all pipe welds must be endoscopied. Endoscopied welds must subsequently
be individually identifiable just as the endoscopy must be videoed, and copies of the video sent to
engineer NN or his assistant. The selected welds that are endoscopied must be representative of the
welders involved. Engineer NN or his assistant must have the right to select up to 40 per cent of welds
for endoscopy.
If defects are detected, an extended endoscopy inspection is made on 10 of the welder’s latest welds. If
further defects are found, the welder in question must have his certificate renewed. Before the welder
can perform welding operations on installations again, he must perform three supervised, error-free
welds on setups selected by engineer NN or his assistant.
If defects are detected during B, further endoscopy inspection must be carried out on 5 per cent of the
other welds that were not referred to under C.
Endoscopy must be carried out regularly – e.g. weekly – and video copy hereof must be handed to
engineer NN or his assistant, as the inspections are performed.
The customer reserves the right to carry out his own endoscopy to an extent chosen by him, and
documented defects must be subject to the procedures in B, C and D.
The supplier inspects and approves the endoscopied welds mentioned in B, C and D. Engineer NN or
his assistant has a two-week right of protest from the receipt of the video. In matters of dispute, an
impartial party will be chosen as arbiter. The losing party shall bear the costs for said impartial party.
The supplier shall bear the costs for any of the above endoscopies, videos etc. performed by the
supplier. The customer shall bear the costs for any of the above endoscopies, videos etc. performed by
the customer as control inspections.
Requirements for weld quality and inspection methods should be established during the offer phase.
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10.2.
Installation inspection
It is inspected whether the layout of the mounted pipe runs is sensible and according to
the agreement. Among other things, it is inspected that there are no more elbows than
necessary, as they result in pressure loss, i.e. energy loss, throughout the service life of
the plant!
It is inspected whether the selected pipe dimensions, valves and other equipment are
sensible and mounting requirements and guidelines have been observed.
If a plant inspection shows inside discolorations from welding, and there is no root defect
in the welds, the discolorations can be removed by pickling. If pickling of the inside of
pipes is initiated, a pickling procedure must be implemented!
The pickling procedure must ensure:
•
•
•
•
that rubber gaskets, shaft seals for pumps and similar materials are not damaged
that pickling does not represent any kind of danger or hazard to persons and the
environment
that no equipment is damaged
that essential equipment such as condensers and the like is avoided.
The pickling procedure must be approved by the customer and his safety representative
before it can be implemented!
11. Overview of other guidelines about piping, etc.
EHEDG Doc. 2, 2004: A method for assessing the in-place cleanability of food processing
equipment
The test is used to find areas of processing equipment with poor hygienic design where
dirt and microbes can be protected from cleaning. The method is based on a comparison
between the cleaning effect in a piece of test equipment and a straight pipe piece.
The document contains a detailed description of how the test is performed.
The test can be bought at EHEDG: http://www.ehedg.org.
EHEDG Doc. 2, 2004: A method for assessing the in-place cleanability of food processing
equipment
The test is used to find areas of processing equipment with poor hygienic design where
dirt and microbes can be protected from cleaning. The method is based on a comparison
between the cleaning effect in a piece of test equipment and a straight pipe piece.
The document contains a detailed description of how the test is performed.
The test can be bought at EHEDG: http://www.ehedg.org.
The guideline provides basic design principles that should be observed to obtain as
satisfactory hygienic design. The document contains general guidelines for hygienic
design, requirements for materials, function as regards cleaning, decontamination,
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avoidance of ingress of microorganisms, compatibility with other requirements (e.g.
process requirements) and assessment of the hygienic design of equipment.
The guideline can be bought at EHEDG: http://www.ehedg.org.
EHEDG Doc. 10, 1992: Hygienic design of closed equipment for the processing of liquid
food
The guideline deals with specific examples of good and poor design of different
components and piping systems in closed processing equipment. Some these are:
gaskets and couplings, shaft insertions and dead pockets.
The guideline can be bought at EHEDG: http://www.ehedg.org.
EHEDG Doc. 16, 1997: Hygienic pipe couplings
The guidelines describes the design of gaskets and seals that ensure hygienic pipe
couplings. The gaskets must be cleanable, disinfectable, impermeable to microorganisms
and durable. It includes critical design parameters for gaskets made from elastomers and
accurate descriptions of gasket behaviour during heat influence.
The guideline can be bought at EHEDG: http://www.ehedg.org.
EHEDG Doc. 25, 2002: Design of Mechanical Seals for hygienic and aseptic applications
The guideline deals with mechanical seals used in the food production. It provides general
criteria for hygienic design and basic requirements for sealing materials. The guideline
compares different seals in relation to cleaning, microbial impermeability, disinfectability
and pasteurizability. The guideline contains several illustrations.
The guideline can be bought at EHEDG: http://www.ehedg.org.
Champden & Chorleywood Technical Manual no. 17 (ISBN nr. 0905942078) 2. edition,
1997: Hygienic Design of Liquid Handling Equipment for the Food Industry
The guideline is quite comprehensive and contains hygienic design of equipment for the
production of liquid products such as milk and other dairy products. It collects advice from
the EU directive 89/392/EEC and the Machine Directive, and guidelines from EHEDG
publications on equipment for liquids. It touches on subjects such as material selection,
including stainless steel, construction, instrumentation of equipment, agitator, piping
systems, including taps and pumps, and CIP systems.
The guideline can be bought at http://www.campden.co.uk/.
3-A Accepted Practices for Permanently Installed Product and Solution Pipelines and
Cleaning Systems Used in Milk and Milk Product Processing Plants, Number 605-04, 1994
The guideline deals with installation and cleaning of fixed pipe lines used in processing
equipment for the production of milk and other dairy products. The guideline defines
hygienic design, construction, material, manufacture and installation criteria for the fixed
pipe lines for cleaning liquids, and the central CIP unit. The guideline does not apply to
cleaning systems at the milk producer or piping systems containing dry matter.
http://www.techstreet.com/3Agate.html.
ASME BPE-2002 Bioprocessing Equipment
This standard deals with the requirements of the bioprocessing industry, covering directly
or indirectly the subjects of materials, design, fabrications, pressure systems (vessels and
piping), examinations, inspections, testing, and certifications. Items or requirements that
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are not specifically addressed in this standard cannot be considered prohibited.
Engineering judgements must be consistent with the fundamental principles of this
standard. Such judgements shall not be used to override mandatory regulations or specific
prohibitions of this standard. http:///www.techstreet.com/.
12. Applied methods
Group members’ experience and knowledge was collected and structured from 2004 to
2005. Group meetings and visits to companies were held at the authors’ companies.
13. Further information and literature
Furthermore, the www.staalcentrum.dk knowledge portal contains a wide range of
relevant links to authorities and organisations, etc. The portal also offers a clear picture of
the guidelines, standards, legislation etc. available for specific fields/types of equipment
and locations. It is easy to search the material and read a short description of the actual
contents. From the relevant links it is possible to order material from the source.
FORCE Institute report no. 94.30 REFERENCE ATLAS for validating shield gas quality for
welding.
14. Change protocol
This is the first edition. Future changes will be listed here.
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