Design of piping systems for the food processing industry

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 purchasesale 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

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

Den Rustfri Stålindustris Kompetencecenter

c/o Teknologisk Institut

This guideline is developed with the support of the Danish Ministry of Science,

Technology and Innovation

Published for the Centre by:

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

Contents

1 Domain 5

1.1 Limitations 5

1.2 Definition and use of guidelines 5

2 Choice of steel quality 5

2.1 Recommended material selection for the food industry 6

3 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 Choice of material to be welded 13

7.2 Filler material 13

7.3 Welding from a product hygiene perspective 13

7.4 General welding requirements 14

7.5 Shielding gas 15

8 Welding requirements 18

8.1 Equipment 19

8.2 Welding preparation 19

8.3 Pipe welding 20

8.4 After welding 23

9 Mounting 24

9.1 Drainability 24

9.2 Pipe dimensions 24

9.3 Pipe layout 24

9.4 Seals at pipe joints 25

9.5 Pipe supports 25

9.6 Production stoppage precautions 25

10 Inspection of completed work 26

10.1 Weld inspection for deliveries 26

10.2 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

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.

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

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

AISI 304 min

max 0.08

18.0 20.0

8.0 10.5

18.0 20.0

8.0 10.5

-

- 0.045 0.030

SS 2333 min

max 0.05

17.0 19.0

8.0 11.0

17.0 19.0

8.5 10.5

16.0 18.0

10.0 14.0

2.0 3.0 0.045 0.030

AISI 316 L

EN 1.4404

min

max 0.03

16.0 18.0

10.0 14.0

2.0 3.0 0.045 0.030

max 0.05

16.5 18.5

10.5 14.0

2.0 2.5 0.045 0.030

max 0.05

16.5 18.5

10.5 14.0

2.5 3.0 0.045 0.030

EN 1.4401 min

max 0.07

16.5 18.5

10.5 13.5

2.0 2.5 0.045 0.030

EN 1.4436 min

max 0.07

16.5 18.5

11.0 14.0

2.5 3.0 0.045 0.030

22.0 23.5

4.0 5.5

21.0 23.0

4.5 6.5

2.5 3.5

24.0 26.0

6.0 8.0

3.0 5.0

0.30

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

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

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 filled at all times This means that when a process has ended, a sterile liquid is fed

liquid-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

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

• 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

Figure 4 Examples of correctly wrapped fittings

Figure 5 Examples of correctly stored fittings

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

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 designed 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

well-The normal roughness of a well-performed weld will be approx 1.6 to 4 µm well-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

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