unit: 3.6 – materials of construction

3.6

© The Institute of Brewing & Distilling (Dipl. Distil. 3 Revision Notes Version 1 September 2009)

1

UNIT: 3.6 – Materials of Construction ABSTRACT: In this unit we will discuss materials of construction, with an emphasis on stainless steel. Some basics issues around joining stainless steel by welding is also considered.

Candidates are advised that the IBD Book entitled “Principles of Hygiene in the Beverage Industry” (included in these revision notes) contains more information about Materials of Construction, especially from a hygiene perspective.

3.6


2

INTRODUCTION There are many different factors to consider when choosing a material including strength, wear resistance and resistance to bending and buckling, and particularly in distilling the positive effects on spirit character. However, the overriding factor when choosing a material is generally its resistance to corrosion. As a result, the different types of corrosion that can occur are discussed below. In the distilling industry, until about 40 years ago, the major materials would have been limited to mild steel, copper and wood. In recent years, stainless steel has been introduced for continuous stills, mash tiuns, lauter tuns, washbacks and pipework. Much of the discussion below is related to stainless steel and its properties. 3.6.1 COMMONLY USED MATERIALS A selection of materials routinely encountered in the distilling industry is described briefly below. Before choosing a material it is important to check its corrosion resistance in different environments to which it might be exposed. Materials for distillery use: a) Wood b) Cast Iron c) Lined Mild Steel d) Copper e) Mild Steel (Carbon steels) f) Stainless Steel a) Wood Description Wood, larch and Oregon pine, are still used today for the construction of washbacks, but in the past they would have been used for many other pieces of equipment. For example, malt storage bins, grist cases, malt elevators and ducting were all made of timber. Leakage of malt dust was a problem with a timber construction and has largely been replaced with steels for malt handling equipment. However, wooden examples still exist and are still in use today. In the distilling industry, even the continuous column stills have been made from wood. Oak wood is used extensively for the maturation of spirits, where the interaction of the wood with the spirit over a long period of storage is of prime importance. In the production process the residence time is too short for any interaction with the wood, yet spirit receivers have been, and still are, made of wood, but probably more for aesthetic reasons. Advantages

• readily available

• a good insulator, reducing heat loss from the fermentations

3.6


3

• available in long lengths

• aesthetics i.e. “traditional”

• supports bacterial flora, contributes to spirit character

Disadvantages

• vessels have to be flat bottomed, giving poor drainage.

b) Cast Iron

Description

Cast Iron is a ferrous alloy with about 95% iron and the alloying elements of carbon and silicon. It can be cast into panels for making tanks, such as mash tuns, water tanks, chargers and pot ale tanks, or cast as casings for pumps and a variety of other engineering components. It tends to be brittle, unless processed as malleable cast iron, but it is strong and resistant to rusting. Temperature cycling of cast iron tanks can cause cracking and catastrophic failure of the tank so they need to be regularly inspected – which is difficult if the tank is insulated.

Advantages

• good compressive strength

• some resistance to corrosion, OK for water tanks

Disadvantages

• brittle, not resistant to shock

• less tensile strength than steel

• can crack if subjected to temperature cycling

• not resistant to aggressive chemicals.

Figure 3.6.1 Wooden washbacks, complete with wooden lids and held together with steel hoops.

3.6


4

c) Lined Mild Steel

Description

Colclad - is a mild steel bonded to a thin layer of s/s and was invented by Clydebridge Steelworks in Scotland. Itwas a cheap alternative to pure stainless steel. The mild steel provided the strength and the stainless steel the

corrosion resistance. It was used for washback construction and vessels made of Colclad are still in use today.

• Contamination of the stainless layer with mild steel was possible if not carefully welded.

d) Copper and copper alloys Description Pure copper is much used for distilling equipment, and historically it would have been readily available, for example for cooking utensils, and was easily worked into shape to make the pot stills. The main alloys of copper are:

• Brasses – alloyed with zinc

• Bronzes – alloyed with tin

• Cor-ten - alloyed with mild steel

Advantages of copper

• It is soft and easily worked, malleable when cold.

• Used extensively for pipes and tubes.

• Very good thermal conductivity.

• Wettable surface beneficial to boiling regime.

• Fairly good corrosion resistance.

• Resistant to caustic and many organic acids and salts

• Positive effect on ester formation and removal of sulphur compounds.

• Positive effect on ethyl carbamate removal.

• Attractive appearance,

Disadvantages of copper

• Strong acids and oxidising acid solutions attack copper

• Costly e) Mild Steel, Carbon and low alloy steels

Description

• Carbon steels are alloys of iron with about 0.05% to 1% carbon

• Low carbon steel is known as mild steel and is a commonly used engineering material

• Low alloy steels have other alloying elements at levels usually < 2% mainly to improve mechanical properties

3.6


5

• Carbon and low alloy steels come in a wide range of strength and hardness by simple variations in carbon content, alloying elements and heat treatment.

• Cor-ten - a copper bearing alloy of mild steel, that has been used in distilling for mash tuns and underbacks

Advantages

• Inexpensive

• Available in wide range of standard forms and sizes

• Easily worked

• Can be welded

• Good tensile strength and ductility

Disadvantages

• Iron and steel are not resistant to corrosion – except in certain specific environments

• Often require protective coating even in relatively non corrosive conditions

f) Stainless Steel - See 3.6.2 below 3.6.2 Stainless Steel

Description

Stainless Steel was invented in Sheffield, England in 1913 by metallurgist Harry Brearly and it formed the basis for Sheffield’s cutlery business Steel is only considered to be stainless if it has a minimum Chromium content of 11 – 12%. The chromium content is usually between 11 – 30%. Passivity is the built – in natural resistance which stainless steels possess to combat corrosion. It is chromium that gives stainless steel this passivity, due to the formation of an extremely thin (3 – 5 x10-7mm), uniform, tenacious and stable chromium rich oxide film on the surface. A minimum of 12% chromium is required to ensure that the film is continuous.

3.6


6

CR rich passive film Cr2O3

Parent metal

> ± 11 % Cr 3 - 5 .10-7 mm Thick

Oxygen or

oxidising environment

If a section of the oxide layer is destroyed and exposes the metal below, a new layer forms as shown below:

The chemical film forms spontaneously in air, but chemical oxidation e.g. use of 15% Nitric acid for a set time and temperature followed by a water wash improves the integrity and results in a higher resistance to the initiation of corrosion.

Keypoint Passivity is defined as a state in which a metal or alloy loses its chemical reactivity and becomes inert. Higher Chromium content, the addition of other elements e.g. Nickel and Molybdenum enhance the formation and properties of the passive film.

There are a few main groups of stainless steels. Depending on what country you are in, the same stainless steel can be given different names. Below are some of the more common names that are used and some of the key properties of the different types of stainless steels. Austenitic stainless steels are the most commonly encountered in the distilling industry and are described in detail below.

3.6


7

BASIC PROPERITIES GROUP COMMON

NAMES BASIC

COMPOSITION ADVANTANGES DISADVANTAGES COMMON USES

AND APPLICATIONS

MARTENSITIC • 410

• 420

• 431

• 12 – 18 % Chromium

• Relatively high Carbon Content 0.2 – 1.2% C

•

• Strength and hardness from high carbon content and hardenable by heat treatment

• Moderate corrosion resistance

• Magnetic

• Very poor weldability because of high carbon content and hardenable nature

• Applications that need strength and hardness e.g. knife blades, surgical instruments, nozzles, shafts, spindles, fasteners

FERRITIC • 430

• 409

• 12 – 18 % Chromium

• Lower carbon Content than Martensitics

• Moderate to good corrosion resistance (resistance increases with increasing Cr content)

• Magnetic

• Good strength, non – hardenable (always used in the annealed condition)

• Moderate to poor weldability – thus, only thin gauge

material ± 3mm is welded

• Builders hardware.

• Domestic: sinks, domestic equipment

• Industrial: chute liners, hopper liners, chain conveyors dust extraction etc.

3.6


8

AUSTENITIC • 304, 304L

• 316, 316L

• 321

• 317

• 18% Chromium

• 8% Nickel (addition of Ni changes crystal structure)

• 2-3% Molybdenum (additional corrosion resistance)

• Low carbon content 0.08% max

• Low carbon or ‘L’ grades max 0.03% C

• Stabilised grades have titanium or Niobium to prevent corrosion in region next to weld (see below)

• Excellent corrosion resistance

• Excellent cleanability and hygiene factor (NB for the brewing industry)

• Easily fabricated and formed with ease

• Excellent weldability

• Hardened by cold work, not by heat treatment

• Non – magnetic

• Ability to handle extremely low (cryogenic) and high temperatures (up to

925 °C)

• Maximum temperature of

925°C in oxidising conditions

• Only suitable for low concentrations of reducing acids – reducing acids break down the oxide film and leads to the general corrosion of these steels

• On shielded areas and in crevices, insufficient oxygen is available to maintain the maintain oxide film. This leads to crevice corrosion.

• Halide ions esp. chlorine (Cl-) can break down passive film

• Food/ beverage processing – beer

• Hospital and pharmaceutical equipment

• Low temperature – cryogenic (liquid gases)

• Fabricated tanks, pipework, process vessels, fittings, valves

3.6


9

Because of the limitations of the stainless steels described above, other forms of stainless steels have been developed: Heat resisting stainless steels: developed to handle higher temperatures under oxidising conditions Super ferritic stainless steels: developed to overcome pitting and stress corrosion cracking (see below) problems of conventional Austenitic stainless steels. Austenitic stainless alloys: highly alloyed materials which are extensions of conventional Austenitic stainless steels. They were developed for higher corrosion resistance especially against pitting and stress corrosion cracking (see below). Duplex Stainless steels: Are a mixture of Ferritic and Austenitic resulting in a high resistance to stress corrosion cracking One of the main properties of stainless steel is its corrosion resistance. Materials that arrive on site at a distillery are normally processed in such a way that they have a chemically produced passive film on their surface. However, stainless steels are not indestructibe. It is important that when they are handled on site that the integrity of this passive film is maintained by:

• Avoiding contamination

• Avoiding mechanical damage

• Chemically clean and repassivate any affected areas 3.6.3 CORROSION Corrosion is an important factor to consider in the design of any distillery. The various types of corrosion are discussed below. Although they are discussed with reference to stainless steel, Carbon steels, low alloy steels and other Metals and Alloys also undergo similar forms of corrosive attack. 1) General Corrosion

This corrosion is ‘uniform’ over the entire surface. As far as corrosion goes this is the ‘best’ type of corrosion to have. This is because the rate of corrosion is measurable and predictable which means that it can be allowed for at the design stage.

3.6


10

Atmospheric corrosion is a common form of general corrosion e.g. a mild steel tank situated outside would be subject to general corrosion.

Keypoint: Attack is uniform over the entire surface

2) Galvanic Corrosion

Active Noble Galvanic corrosion occurs when two different metals are in electrical contact and immersed in the same electrolyte. In this case, one of the metals is preferentially corroded while the other is protected from corrosion. The less noble, less passive, more active metal of the two becomes the anode and corrodes at an increased rate e.g. Fe � Fe2+ + 2e-

Using a galvanic series it is possible to determine whether or not corrosion between two metals will occur and how severe it will be:

NOBLE Gold (Cathodic) Graphite

…. 304,316 430 …. Bronze …. Tin Steel ….

Aluminium

(Anodic) Zinc ACTIVE Magnesium

Galvanic Series.

3.6


11

Because stainless steel is high up on the galvanic series, it is not often subjected to increased corrosion as a result of Galvanic corrosion. If a more active material is coupled with stainless steel e.g. a stainless steel pipe with a steel flange, the steel flange will preferentially corrode. An important fact to note with galvanic corrosion is the ratio of the surface areas between the two metals. Electrons flow from the anode to the cathode. Thus, if the anode is small in relation to the cathode a large number of electrons are required from a relatively small area and the anode will corrode more quickly. Ideally, in order to prevent galvanic corrosion a small cathode : large anode is required.

Keypoint: Galvanic Corrosion occurs when two different metals are in electrical contact and immersed in the same corrosive solution

3) Erosion & Cavitation Corrosion

ErosionCavitation

Impingement Corrosion Abrasive particles in suspension or high flow velocities remove the normally friable products of corrosion from the surface thus exposing fresh metal surface to corrosive attack.

Force of fluid flowing down

moves point of corrosion down

3.6


12

Cavitation Corrosion This is a special form of erosion corrosion and it destroys the efficiency of pump components. It results due to the formation of bubbles in the fluid due to local pressure changes. The steps required are as follows: (a) Bubbles form due to pressure differences (b) Bubbles come into contact with metal surface (c) Bubbles implode against the surface causing shock waves 1.

Bubble

Passive Film

2.

3.

Implosion of bubble and

removal of surface material

Always occurs in same

place ∴eventually looks

like pit

3.6


13

These 3 steps are repeated. The resulting metal surface looks like pits. Presence of turbulence enhances the cavitation process.

Keypoint Mechanical form of corrosion that removes products of corrosion from surface exposing fresh metal below to corrosive attack

4) Intergranular corrosion This type of corrosion is best described in a series of examples: (a) Iron in Aluminium

AluminiumIron

Iron segregates to the grain boundaries and ‘Galvanic’ corrosion occurs. It is not called galvanic corrosion because it is not physically the coupling of two dissimilar metals. Again the situation exists of a small anode and a large cathode which is undesirable. Eventually there is nothing holding the grain of metal in place and the grains drop out. (a) Stainless steels - Sensitisation

Pipe Pipe

Weld Bead

Temperature profile across stainless steel:

3.6


14

1450

850

450

Room

Temp

Weld

Decay

During welding, the temperature of the weld bead reaches a melting point of

approximately 1450°C. The area of concern is a short distance from the weld

called the area of ‘weld decay’. In this temperature range 425 –815°C the stainless steel is subject to sensitisation (intergranular corrosion) a form of corrosion which is detrimental to stainless steel. There is a great affinity for the reaction between chromium and carbon to form Cr23C6 (chromium carbide). This precipitates and forms a ‘blob’ in the steel. The problem occurs in the surrounding areas that are now low in chromium because of the chromium that has been used for the formation of chromium carbide. This leads to increased corrosion in these areas because, as described before, a minimum chromium content is required to form a stable passive layer. The addition of titanium, niobium reduces the effect of sensitisation – it drops the top temperature at which sensitisation occurs such that it only occurs in a much smaller range.

Below 425 °C there is insufficient diffusion to allow the chromium and carbon

to come into contact. Above 815°C Cr23C6 starts to break down. This compound is very stable, essentially inert so there is not much of a galvanic effect.

Keypoint This corrosion is as a result of a change in the localised composition of the stainless steel

5) Pitting corrosion

3.6


15

Pitting can be caused by a variety of circumstances, any situation that causes a localised increase in corrosion rate may result in the formation of a pit. It is not always possible to see the pitting because it is below the surface and it can occur in environments that are not normally particularly detrimental. It is a very aggressive form of corrosion and unfortunately it is very common. It is the chloride ion that is usually the cause of pitting corrosion. Under the right conditions, it has the ability to attack any localised weak points within the passive film. The chromium in the passive layer is dissolved leaving an active site with iron that is very susceptible to corrosion. The negatively charged Cl2- is very mobile and is easily attracted to any pit area formed on the corroding metal surface at the base of the pit. Hydrochloric acid is formed at the base of the pit and, being extremely aggressive, accelerates the corrosion in this area. Like crevice corrosion, it is promoted under stagnant conditions because the halide ions (e.g. chloride ions) have the opportunity to concentrate in the pit. Pitting corrosion is prevalent in passive alloys such as stainless steel

// //

Large Cathode

Small Anode

Large Cathode

Passive Film

Cl2-

HCL

Formed

Pitting can also occur if the composition of the metal is not uniform.

Keypoint

3.6


16

Very localised form of corrosion which results in small holes or perforations

6) Crevice (shielded) corrosion

A crevice is just a gap between two pieces of material. The smaller the gap, the more of a crevice there is and the increased chance of crevice corrosion. Corrosion is often increased in the small sheltered volume created by contact with another material e.g. under washers or bolts heads, in the threads of bolts or pipes fittings, under sediments or settled solids. When there are crevices present, solutions, particularly those containing chloride ions can cause crevice corrosion. The ions enter the crevice and build up in concentration. In addition, the lack of free access to oxygen prevents the formation of the passive layer.

Keypoint Attack of the metal surface where it is shielded

7) Microbiologically induced corrosion

It is similar to both crevice and pitting corrosion. Many aqueous solutions, especially in untreated dam or river water contain a large amount of bacteria. If there is an area on the metal surface that attracts and anchors aerobic bacteria, a slime build-up will occur. This can become thick enough to lower the oxygen level under the slime. A situation similar to a crevice results allowing anaerobic bacteria to grow. They produce aggressive metabolic byproducts that attack the passive film on stainless steel. Because of the lack of oxygen, the passive layer cannot reform. One of the most common is sulphur fixing bacteria which produces sulphuric acid which is very corrosive. Areas of concern in the distillery are heat exchangers and effluent lines especially in welds or near drain lines.

Keypoint Corrosion resulting from the microbiological action of bacteria.

3.6


17

8) Stress corrosion cracking (SCC)

Corrosive

Environment

StressTemp

In order to have stress corrosion cracking all three of the above factors need to be present.

• Stress Tensile stress is a measure of the basic strength of a material. For stress corrosion cracking to occur the strength can be applied or residual e.g. seam welding of a stainless steel pipe if you cut along the weld the pipe would open up due to residual stresses. However, SCC will not occur under conditions of compressive stress.

• Temperature Temperature is an important factor. The higher the temperature, the

higher the risk. SCC does not often occur below 60°C. Temperature fluctuations are also detrimental as this leads to additional stresses because of different thermal expansions within the steel. Like all reactions, increasing the temperature also increases the rate of corrosion.

• Corrosive environment SCC also need a corrosive medium particularly the halide ions of which chloride is the most common. Other mediums such as caustic soda and hydrogen sulphide can cause SCC under the right conditions.

SCC can be made to occur with the different types of stainless steels described above. However, in practice it occurs predominantly in Austenitic stainless steels – the group most commonly used in the industry. The mode of attack is as follows:

• Pitting corrosion begins

• Once a pit is formed, the tensile stress is concentrated at the base of the pit. The finer the pit, the higher the stress concentration factor

• This leads to mechanical rupture of the metal at the base of the pit, exposing unpassivated metal.

• This leads to accelerated corrosion at the base of the pit and the process repeats itself.

3.6


18

The initiation of SCC can take some time but once initiated crack propagation can occur quite quickly.

Keypoint The fracture of a metal with a static tensile stress in the presence of specific environmental conditions.

3.6


19

The figure below shows severe stress corrosion cracking in a hot water pipe, where the elements of temperature, tensile stress (NB near a weld) and chloride ions in the water were all present.

Pipe sample, with an outside diameter of 105 mm and a 2 mm wall thickness. Water leaked through this side of the pipe as a result of the development of branched stress corrosion cracks (SCC) (red arrows) at the discoloured areas. Only a few of the SCC cracks penetrated through the 2 mm wall to cause the leak. Most of the cracks developed close to the outside surface of the pipe.

Circumferential weld

3.6


20

3.6.4. INSULATION Insulation is an example where poor heat transfer is a virtue. The vast majority of insulation systems rely on the poor thermal conductivity of air. However to prevent convection currents causing a pathway for the heat, it is necessary to immobilise the air by way of a fibrous or porous structure. One example of a fibrous material used to provide insulation is the natural hair or fur of animals. However wrapping fox fur around steam pipes is both expensive and ecologically unsound! Widespread use is made of porous materials in the form of organic foams One exception to the use of air to achieve insulation is the use of vacuum insulation for low temperature duties such as cryogenic gas storage tanks and, in some cases, liquid gas transfer lines. Insulation Selection Insulation materials are selected on the following basis:

• Suitable for the temperatures and other conditions set by the application.

• Environmentally acceptable.

• No detrimental effect on the equipment.

• Ease of application.

• Acceptable cost.

• Have adequate mechanical strength.

• Have an acceptable fire performance. With regard to temperature of application, the types of insulation systems necessary for process operation above and below ambient temperature are different, as illustrated in Figure 3.6.4.1. showing the insulation system best suited to each situation..

Figure 3.6.2 Types of Insulation System for operating above and below Ambient Temperature.

(a) Above Ambient Temperature (b) Below Ambient Temperature

3.6


21

Air is a very poor conductor of heat and good thermal insulation can be achieved by materials which trap very small pockets of air within their structure. These pockets of air are so small that the convection currents are totally suppressed and heat can only be transferred by conduction. The thermal insulating properties of such materials are destroyed if they become waterlogged, since water is relatively a much better conductor than air. For process plant operating above ambient temperature, the insulation system used should have an open pore structure which will allow any water vapour in the insulation to permeate out. Fibrous insulations, such as glass wool, are admirable materials for such a situation, as they simultaneously satisfy the dual requirements of excellent thermal insulation and low resistance to water vapour permeation. For process plant operating below ambient temperature, the material chosen must combine the properties of excellent thermal insulation with a high resistance to water permeation, as in this case, the humidity in the surrounding atmosphere will tend to permeate through the insulation and condense on the cold surface of the vessel or pipe. An insulation with a closed pore structure, such as foamed plastic, is a suitable material for this situation and foamed plastics have now virtually replaced cork. As an additional safeguard to prevent water permeation into the insulation, the outer surface of it is covered in a material which has a very high resistance to water permeation. This is known as a vapour barrier.

Suitability of Materials (a) Temperature Particularly in the case of organic foams, the deciding factor is their temperature suitability. Polystyrene foam will melt at temperatures in excess of 80°C and polyphenolic foams will degrade to a powder at temperatures over 120°C. In low temperature duties, there must be consideration to the vapour permeability of the material. With a permeable material, water vapour will migrate through the insulation to the equipment surface where condensation will occur. Under mild conditions, the insulation will become saturated and loose some of its insulation properties. An extension to saturation is the nuisance created by water dripping from the insulation onto floors etc below the pipes or equipment. In refrigerated systems, the condensed water would freeze on the equipment surface. The increased volume of the ice would then damage the insulation and well as reduce the insulation effect. To overcome this problem, foam insulations with a closed cell structure have a reduced permeability to water vapour over fibrous materials. However all insulation systems on cold duties should include a vapour barrier, either applied to the insulation as a coating, or included in the insulation during manufacture as a plastic or foil layer.

3.6


22

Figure 3.6.3 shows a prefabricated foil coated mineral fibre insulation applied to a cold water pipe. Note the self-sealing tape used to ensure a continuous vapour seal along the length of the section. Figure 3.6.3 Foil backed mineral fibre insulation

(Photograph by courtesy of ‘Rockwool’) (b) Mechanical Strength Mechanical strength is another factor to be considered. The contact between the pipe or equipment and the support needs to be insulated. Conventional foam insulation would be of insufficient strength for this purpose. However, higher density foam inserts with a higher compressive strength are available. Figure 3.6.4. shows an insert in position before completion of the main insulation. Figure 3.6.4. Pipe support inserts

(photograph by courtesy of Kingspan)

3.6


23

(c ) Environmental Health Risks It is well known that asbestos fibres represent a hazard to health, which clearly excludes asbestos from use. Many older distilleries will have had asbestos in place on steam systems. The current policy is that where this insulation is in good condition, with adequate external covering for example, plaster (Keenes cement), then the risks are minimal and there is no need for replacement. However if and when maintenance is required then removal and disposal must be undertaken only by a licensed contractor to the approved methods. Older polyurethane foam insulation presents a potential toxic hazard in the event of fire when carbon monoxide and hydrogen cyanide may be released.

(d) No Detrimental Effect on the Equipment All insulation materials contain some chloride ions. There is a risk that these chloride ions could migrate to stainless steel surfaces and cause chloride corrosion. The older fire inhibited polyurethane foams contained a chloro-organic fire retardant, which made this risk particularly great, especially if the insulation became saturated at any time. To overcome this problem, there should be a ‘chloride barrier’ either as a protective paint, applied to the equipment, or a chloride barrier incorporated into the insulation. The risk is reduced further, by having an effective vapour barrier.

(e) Ease of Application With installation costs being of major significance, insulation materials are available manufactured in a form to ease installation. Figure 3.6.4.5 shows mineral fibre insulation pre-cut to allow fitting around a pipe bend. The sections are then secured by wire mesh.

3.6


24

Figure 3.6.4.5 Method of fitting insultation to pipe bends

Figure 3.6.5 shows the method of fixing a flat sheet of insulation to a large diameter pipe or vessel. Note that the insulation panel is cut with mitred cuts on one side to facilitate fixing to the curved surface. Figure 3.6.6. shows a diagram of the fitting of a mitred cut insulation panel to a large diameter pipe.

3.6


25

3.6


26

Figure 3.6.5. Fixing insulation to vessels or large pipes

Figure 3.6.6. Diagram of the fitting of the insulation to a large diameter pipe

3.6


27

Insulation Systems Insulation is rarely fitted in isolation. There are often several components to the system: each performing a separate function. A typical insulation would comprise, starting from the equipment surface:

1) Chloride barrier as a paint or foil 2) Second insulation layer (if required) with joints staggered in relation to

first layer. 3) Vapour barrier as either an applied compound or an impervious

membrane. 4) Outer layer as protection against damage from the weather, animals

(including birds) and personnel. This outer layer is often chosen for ‘appearance’, especially if the equipment is visible to the public. Polished aluminium or stainless steel is often used although plastic and plastic coated steel are used in less observable areas.

Figure 3.6.7 shows a typical plant room with insulated pipes and valves clad in polished aluminium.

Figure 3.6.7 A typical view of insulated and clad pipework

Table 3.6.1 summarises the insulation types likely to be encountered with their applications.

3.6


28

Table 3.6.1. Summary of insulation types and applications Polyurethane/

polyisocyanurate

Polyphenolic Mineral / glass

Foam glass Calcium silicate

Physical form

Foam Foam Fibre Foam Fibre

Temp range -180°C to 140°C

-180°C to 120°C

Up to 700°C Up to 480°C Up to 1050°C

Thermal conductivity

0.021 0.018 0.032 0.039 0.07

Structure Closed cell Closed cell Open fibre Closed cell Compact fibre

Product form

Prefabricated sections or foam in situ.

Prefabricated sections

Prefabricated sections with resin binder



Applications Process, refrigeration and utility use up to 120°C

Process, refrigeration and utility use up to 120°C

Process and utility use at elevated temperatures, e.g. steam and hot water.

Used where high mechanical strength is required.

High temperature applications such as boilers and flue systems.

Note: 1) In some cases, the temperature limitation is set by any coating, for example the vapour seal material. Vacuum Insulation For extremely low temperatures, such as cryogenic gas storage tanks and pipe-work, vacuum jacketing is often used. Here, the cost of the insulation is balanced by savings in product loss due to heat in-leak and vaporisation. Vacuum insulation relies on the principle that the thermal conductivity is reduced by separating the molecules to the extent that molecule to molecule conduction is almost zero. Figure 3.6.8 shows a sectioned diagram of a vacuum insulated storage tank. Figure 3.6.9 shows a sectioned vacuum insulated pipe showing the inner pipe for the cryogenic liquid and fitted with bellows to absorb any differential expansion. Typically, the pressure in the jacket is reduced to the order of 10 Pa. (0.1 mm of mercury).

3.6


29

Figure 3.6.8 Diagram of a sectioned cryogenic storage tank

Figure 3.6.9 Diagram of a sectioned vacuum insulated pipe

End of Unit 3.6

unit: 3.6 – materials of construction

Documents