4 Materials for pumps

Materials for pumps

Extensive knowledge and considerations are required when choosing materials for pumps, even for basic applications like pumping clean water. This section reviews the very wide range of materials used in pump technology, strength of materials, pressure sealing and creep are dealt with as is corrosion, which is a big problem in this field, because of the substantially greater velocities within pumps compared with pipelines. The form of corrosion due to erosion-corrosion, has therefore been given a relatively large amount of space, with regard both to the mechanism of corrosion itself, with build-up of protective film, and to testing at high liquid velocities. pH value is given good coverage with regard to the effect of corrosion on choice of materials.

The section includes checklists and cost relationships for various combinations of materials used in pumps, and also typical applications. The choice of materials for strong acids, hot liquids and liquids containing halogene, or contaminated liquids, usually require a close study of corrosion tables in special literature intended for that purpose.

Introduction

Material losses from corrosion in our society amounts to a very large sum, the old days layman’s terms used to say that the output from every fifth steelworks emerges in the form of rust. The nature of the problems of materials in the field of pumps is generally quite different from that of simple common rusting, because of the complex effects brought about by the pumped liquids themselves. Since pumps practically always comprise part of the auxiliary equipment  of some process, ongoing process development gives continuous rise to new problems for which solutions based on experience do not yet exist.

Examples of these are:

• In catalytic manufacture of sulphuric acid, production rate has been increased by removing various cooling groups. Through this, the temperature rises by 50°C which increases the material problems of the pump by a factor of three.
• Plating workshops are adding fluoric acid to chromic acid baths in order to improve the shine on chromium surfaces.

Materials and choice of materials for pumps are determined by a number of interactive factors, see figure 4a. The end result is usually an inspired compromise between all of these, whilst at the same time unfortunately, some of the factors may be more or less unknown quantities.

Figure 4a Factors with effect on materials for liquid pumps.

Review of materials for pumps

The vast number of materials used nowadays in pumps can be classified within the following main groups:

• Material with iron as its main component
• Material with copper or aluminium as its main component
• Other metallic materials
• Non-metallic materials

The mechanical properties are determined in the usual way by the tensile strength and elongation. Sometimes there are other physical properties which are of interest such as impact resistance and hardness. The pump pressure rating, PN, is largely determined by the strength of materials. For plastics at room temperatures as for metals at higher temperatures, the resistance to deformation must be considered. Creep resistance describes the effect of time on the tensile strength.

Grey cast iron for pumps

Cast iron is manufactured in various qualities and grades specified in ISO 185 Grey cast irons, the grades referring to anticipated minimum and maximum tensile strength. The tensile strength depends primarily upon the content of carbon and reduces as the carbon content increases. The carbon occurs as graphite flakes enclosed in a steel-like matrix. The flake formation means that the values of impact resistance and elastic limit are very low. Grey cast iron should thus be avoided in those pumps which are subjected to external and/or internal shock, water hammer and so on. This is especially important if the medium pumped is hot water at saturation pressure.

At +120°C for example, 5% of the mass of water in the complete hot water system boils off in steam if the pressure falls to atmospheric pressure because of a breakage. The volume of steam at this saturation temperature is about 600 times greater than the corresponding amount of water, so that personnel could suffer very serious scalding if a crack were to occur in the pump. For higher temperatures above 120°C spheroidal or nodular graphite cast iron or cast steel is normally use for pressure stressed systems.

Grey cast iron is the most commonly used material in normal centrifugal pumps. From the corrosion aspect, it has a very wide field of application. Grey cast iron can be used without any great risk in water, or aqueous solutions, with pH values between 6 and 10. Grey cast iron can be used in more acidic media, but it is dependent to a high degree upon what has caused the lower pH value. Temperature dependence is also more accentuated in this region.  Because if its high carbon content, grey cast iron quickly acquires a protective graphite layer, which means, from the strength point of view, that poorer quality cast iron with higher carbon content has better protection against corrosion. Grey cast iron is also superior to normal steel from the corrosion point of view for the same reason.

Spheroidal or nodular cast iron (S.G. iron)

Spheroidal or nodular cast iron is, by analysis, very like ordinary cast iron with regard to carbon and silicon content. In contrast to ordinary cast iron, however, the graphite is not flakey but occurs in the form of small spheroidal nodules in a mainly pearlitic matrix. This is brought about by adding small quantities of magnesium to the smelt before pouring. Because of the spheroidal form of the graphite, spheroidal or nodular cast iron has a substantially greater strength than ordinary cast iron. Furthermore, the elastic limit is greater and the resistance to impact is better. Further improvement in the elastic limit and impact resistance can be obtained by heat treatment, whereby the basic matrix is changed towards a ferritic structure. Spheroidal or nodular cast iron is very suitable for pressure vessels subjected to internal pressures, where these were formerly always assigned to cast steel. Spheroidal or nodular cast iron also has good welding properties. The upper limits of use are about PN 25 or liquid temperature +300°C. The resistance to corrosion is very similar to that of grey cast iron at low flow velocities. At higher flow velocities this material often has a lower resistance than ordinary cast iron.

Non-alloyed steel castings

Non-alloyed steel castings have the same corresponding strength as commercial steel.  increases in hardness. Corrosion resistance to highly (Micentrated sulphuric acid and solutions of
strength as commercial steel. By analysis, steel castings only differ because of certain additives intended to improve castability. Low alloy steel castings have small additions of nickel and chromium in order to improve their strength at elevated temperatures. Steel castings are used mainly for high pressure and hot liquid pumps but are being replaced more and more by spheroidal or nodular cast iron and alloyed qualities of steel. From the corrosion aspect, steel castings are not as good as grey cast iron and spheroidal or nodular cast iron, because, by preference, the carbon content should not exceed 1.5%, which is insufficient to provide a protective graphite layer.

Alloyed grades of flaked graphite and spheroidal graphite

The properties of flaked and spheroidal cast iron can be altered to some extent by the addition of small quantities of alloying substances affecting strength, work-ability and resistance to corrosion and abration. The following materials can be used:

Copper (for flaked graphite cast iron only)

Copper additives give the basic matrix of cast iron a more uniform pearlitic structure with less ferrite and better form and distribution of the graphite. The tensile strength increases by 10-20 percent with additions of 1-2 percent, with corresponding increases in hardness. Corrosion resistance to highly concentrated sulfuric acid and solutions of sulfuric acid is markedly improved.

Nickel and chromium, Ni and Cr

Small additions of nickel and/or chromium cause the structure to be more even and finely distributed giving advantages similar to those for copper alloys. Furthermore, a more pressure-tight casting is obtained in this way. The additions of 20% Ni (Ni-Resist) causes the matrix to become austenitic, non-magnetic. This quality has shown itself to be very useful for sea-water and other chloride containing liquids, even at high temperatures. The strength is similar to the corresponding grades of grey cast iron and spheroidal or nodular cast iron.

Ni-Resist

With alloying proportions of 6% Ni and 9% Cr (Ni-Hard) a martensitic matrix with chrome carbides is obtained, resulting in hardness figures between 500 and 600 Brinell. The hardness can be further increased by heat treatment or by increasing the chrome content up to 30%. Thus a material can be produced with very good resistance to abrasion and erosion. Because of the considerable hardness, the material can only be worked by means of grinding so that special designs are required for pumps.

Silicon, Si

Silicon alloyed cast iron, silicon iron, is highly resistive to acids, if the silicon content is greater than 13%. This material is resistant to sulphuric acid at all concentrations and also to many other non-organic and organic acids, though  not to hydrofluoric acid, hot concentrated hydrochloric acid, sulphurous acids, sulphites and hot alkalis. An additional dose of 3% molybdenum makes the material resistive also to hydrochloric acid and solutions containing chlorine.

Silicon iron is very difficult to cast and displays poor quality leak-tightness under pressure. It should never, for that reason, be used in pumps with an internal positive pressure greater than 0.4 MPa. As the material is hard and very  brittle, special design principles are normally used for pumps, e.g. enclosure of the silicon iron in a steel cladding so as to take up the pressure stresses.

Stainless steel and acid resistant steel for pumps

It is a general rule for steels that the content of alloying substances shall not exceed 50% of the mixture. A characteristic of all stainless steels is that they contain a minimum of 12% chromium. In the presence of oxygen this forms a thin invisible film of chromic oxide which is chemically resistant to a high degree and which inhibits any direct contact between the surrounding medium and the steel. It is, however, necessary that oxygen is present in order to maintain the film. The steel is said to be in a passive state. In a reducing environment, on the other hand, there can be no build-up of the chromic oxide layer and the steel is then said to be in an active state.

The designation stainless steel is not altogether correct, as these steels can easily be subject to corrosive attack depending on external circumstances relative to alloying content, degree of heat treatment, welding and so on. The designation rust-delaying has been proposed but has not yet been adopted.

The strength of resistance in stainless steels is dependent upon the rate at which the passive oxide layer is dissolved into solution. This rate is slowed down mainly by alloying with metals which themselves have high resistance to corrosive media. Such alloying elements are, in the main, nickel, molybdenum and copper. The effects of chromium are then less important from the point of view of chemical resistance.

In everyday speech, distinction is drawn between stainless steel and acid resistant steel, the difference being based on the presence of molybdenum in acid-resistant steel. The structure of the iron matrix is changed by the various alloying additives. The type of structure is used to divide stainless steel and acid resistant steels into the following groups (see also figure 4b Modified Schaeffler diagram for determining structure of a stainless material):

• ferrites
• martensites, which are hardened or can be hardened
• austenites, which are totally non-magnetic
• ferrite-austenites, which are para-magnetic

The austenitic stainless steels have lower tensile strength and greater toughness than the other groups. This makes them easier to manufacture into the various supplied forms, such as sheet metal and piping. Because pumps
are largely produced from castings, the analysis can be chosen with greater freedom.

Using the same main analysis there are today a number of standardized variants of materials for pumps. The main reason for this is the granular erosion which can occur around a weld. Granular erosion is eliminated in the various standard qualities in the following ways:

• by low or extra-low carbon content
• by the addition of titanium or niobium in proportions of 5 to 10 times the carbon content

Copper alloys

Copper alloy castings are used largely for salt water and weak cold chloride solutions (for properties of sea-water and choice of materials for use with sea-water see Section for Fluids). The alloys have varying casting characteristics associated with solidifying intervals, which in turn are determined by the alloying elements. For tin-bronze with about 10% tin, the interval is 200-250°C.

The prime result of this is that a certain amount of microscopic porosity cannot be avoided in the solidifying alloy. This porosity need not mean that the casting is not leak-tight under pressure. The wide solidifying interval makes the alloy relatively easy to cast.

Brass with 30% to 35% zinc and aluminium-bronzes with about 10% aluminium have solidifying intervals between 2 and 10°C. The structure is fairly non-porous although large cavities due to coring may occur which can be eliminated by careful metal feed. These alloys with narrow solidifying intervals are more dependent upon the design shape of the casting and involve higher costs.

Red metal made up of about 5% each of tin, lead and zinc, is of an ordinary and easily cast quality. The pressure-tightness quality increases with lead content up to about 7%.

From the point of view of corrosion, copper and its alloys give widely varying results. Here, as with other materials, it is a solid oxide film which provides the protection against further corrosion. The rate of build-up of this protective film varies and is variously affected by flow velocities. In figure 4.1c there are some commonly used alloys.

Aluminium alloys

Aluminium and magnesium alloys, figure 4c are not used to any great extent as materials for pumps, because of their low resistance to corrosion. The exceptions are those areas where low weight is a prime factor, for example in building site ground drainage, and also where manufacturing costs are important as in domestic pumps. The range of application can be increased by inhibiting electrolytic corrosion or actually making use of it, for example by:

• insulating from more noble materials such as steel
• connecting to less noble materials, so-called sacrificial anodes of zinc or magnesium

Figure 4c Some pump materials with aluminium or copper as the main component. At elevated temperatures the design stress and pressure rating must reduced accordingly (see relevant standard).

Nickel alloys

Nickel base alloys, as also pure nickel itself, come as a natural supplement at a point where stainless steels no longer hold up from the corrosion point of view. They are not easy to cast as a rule, and require high annealing temperatures, about 1200°C, in order to maintain the correct structure and grain size which is necessary for good corrosion resistance. Nickel alloys are dominated by a group usually called by the trade name of Hastelloy. Similar alloys are manufactured under different trade names. The commonest nickel alloys (see also figure 4d) are as follows:

Hastelloy B has high resistance in strong hydrochloric and sulphuric acids. Unfortunately, the alloy is sensitive to high flow velocities•and is rarely used in centrifugal pumps.

Hastelloy C is the most common of the nickel alloys because of its resistance in both oxidising and reducing atmospheres. It is used mainly for chloride solutions, liquids containing free active chlorine, bleaching fluids for example, of the type hypochlorite and chlorine dioxide solutions, where the chlorine should not exceed 15 gram /liter at room temperature. It also has a good resistance to acids at temperatures below +70°C.

Hastelloy D possesses a good resistance to erosion, due to its great hardness, approximately 400 Brinell. It has its best application in sulphuric acid, against which it has high  resistance at all concentrations and high temperatures, and within certain limits for boiling acids also. Maximum corrosion resistance is achieved only after about six weeks in operation, which is the time it takes for a protective sulphate film to build up on the surface.

Monel is more resistant than nickel under reducive conditions and more resistant than copper under oxidising conditions. In summary, it can be said that Monel is more resistant to corrosion than are its main components.

Re-precipitation of dissolved copper cannot occur, since nickel, copper and monel lie very close to each other in the electrolytic series (galvanic series). Monel however, is somewhat difficult to cast and often results in porous castings.It is therefore used mainly for impellers and shafts. Monel has very good resistance to salt water especially at higher flow velocities. In stationary sea-water, ocean organisms may collect on the metal, causing local concentrations of oxygen which lead to local attacks.

Figure 4d Some materials for pumps with nickel as the main component.

Other metallic materials

A variety of metallic materials are available for special purpose use. Amongst these are:

Lead and lead alloys for pumps

Because of its poor mechanical properties, pure lead is not used as a material for pumps. A 10% to 25% addition of antimony (Sb) improves its strength characteristics considerably. The alloy is called hard lead. Pump casings manufactured in this material need to be enclosed in a load-bearing steel casing for higher pressure applications.

Lead has a very good resistance to acids, with the one exception of nitric acid. In sulphuric acid, lead sulphate is built up on the surface of the lead which is difficult to dissolve and acts as a protective film. Lead can be used for sulphuric acid at a concentration of 30% and temperature +100°C, but temperatures have to be reduced at higher concentrations. In hydrochloric acid a film of lead chloride builds up, which protects the underlying metal to some extent. If the acid is in motion however, the surface layer wears away easily, so that lead pumps are not recommended for hydrochloric acid.

Titanium for pumps

Titanium has low density, 4500 kg/m3, and very good tensile strength, (650 N/mm2). Titanium is relatively difficult to work, however, It is cast centrifugally in vacuum chambers in the presence of protective gas with mould and graining material of graphite.

The metal titanium is being used more and more in the chemical industry mostly because if its good corrosion resistance to solutions containing free active chlorine. Corosion is completely excluded in such solutions as chlorine dioxide in water, hypochlorites, chlorates and metal chlorides.

Tantalum

Such exotic metals as tantalum, hafnium, niobium and germanium have lately come more into their own for modern industrial products, where very high corrosion resistance is required from the particular aspect of reliability. The metal tantalum has a corrosion resistance very nearly equal to that of glass. It’s resistive to boiling sulphuric and hydrochloric acids. The density of tantalum 16600 kg/m3.

The ability to cast and weld tantalum is extremely limited, so that parts with complex shapes do not lend themselves to economic production. Tantalum is generally used to provide an internal pump lining.

Plastics

Plastic materials can be divided into two main groups, which in certain ways have differing properties but which, above all, are manufactured differently. Hard plastics with vinylester, polyester, phenol or epoxy as the base, mixed with a hardener. Fillers and reinforces are, however, nearly always included in order to increase hardness and strength, the materials used depending upon the desired characteristics. Fillers and reinforces can be quartz, soap-stone, fiberglass, carbon fiber and so on. If the liquid-contacting surface is not covered with pure resin, then the filler medium must also be resistive to the pumped liquid.

Hard plastics can be used up to about 150°C with a somewhat smaller range of liquids than the thermoplastics. Sometimes, cavitation and air in liquids can give rise to serious damage, so that the greatest care must be taken with regard to the pump intake conditions.

Thermoplastics such as polyvinyl chloride, polypropylene, polyethylene and PTFE are used as blow-moulded parts and also in sheet or tubular form for coating. Liquid temperatures above 80°C (200°C in the case of PTFE) can  seldom be used because of the temperature at which plastics soften.

Figure 4e gives a rough review of plastic resistance to various liquids. The figures in the table may be changed by various additives. The upper regions of use assume that swelling, creep, cracking and so on can be tolerated to a certain extent.

The specific properties for plastic materials must be taken into account in plastics for pumps as follows:

• Pressure class must always be balanced against resistance to creep in the plastic, which means in practice an additional safety factor of 3 to 10 compared with metallic materials.
• Thermal expansion, especially in the axial direction of the pump, has to be taken into account when routing the pipeline. Similarly, the pump must be relieved of pipeline forces to the fullest possible extent.
• Temporary overheating, resulting, say, from a packing box being too tight or when driving against a closed valve, must be avoided. Alternative solutions there would be mechanical seals and by-pass flow regulation.

Other non-metallic pump materials

There are many other materials available for special purposes. Rubber in various grades of vulcanization, because of its resistance to chemicals and soft rubber because of its resistance to abrasion. For the use of rubber for shaft seal components, see section for seals.

Carbon and graphite , because of their resistance to chemicals and because of their low of friction, for shaft seal components. Ceramics and glass, because of their resistance to chemicals. Glass is better then ceramic for mechanical stress and for sudden changes of temperature.