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CONSTRUCTION MATERIALS
91
CONSTRUCTION MATERIALS Normal Construction Materials General service pumps for low and medium pressures (eg 10 bar on single stage split casing pumps and 36 bar on cellular type multistage pumps) have cast iron casings, high tensile steel shafts with bronze rotating and wearing parts. The material specifications are as follows:Cast iron BS1452. High tensile steel shaft BS970 Bronze wearing parts BS1400 LB2, a soft lead bearing bronze. Bronze impellers, diffusers, sleeves, balance discs, etc. BS 1400 LG2 gun metal, or with a greater portion of tin to lead, LG3 or PB3 zincless bronze, according to severity of duty. For higher pressures, the casings are of cast, forged or welded steel and, for the highest heads per stage, stainless steel impellers and diffusers are used. The casing steel forging specification is 14A in general cases, but variations of this occur according to the tensile strength and other properties required. See also BS970 for EN steel. The stainless steel rotating and stationary parts are made to specifications BS 1630, 13chrome steel castings, BS970 for 16-chrome 2 112% nickel forgings and BS 1631 for austenitic stainless steel 18-chrome 8 nickel castings or forgings. High temperature high pressure pumps, (eg boiler feed pumps and hot oil pumps) have forged steel pressure vessels, forged stainless steel shafts with cast stainless steel impellers, diffusers, etc. (See Chapter 33). For temperatures above 120°C, bronze and austenitic stainless steel are unsuitable owing to their low strength and their coefficient of expansion (50% greater than that of mild steel or other stainless steels) which causes difficulty in maintaining correct fits and running clearances at higher temperatures. Bronze is, however, used for the very low stress wearing parts, (eg chamber bushes of multistage pumps).
Corrosion - Erosion Two distinct problems arise in choice of material for pumping duties. The first concerns
92
CENTRIFUGAL PUMPS
the provision of material whose strength is sufficient to withstand any anticipated stress loading without fracture or undue distortion. The second concerns the avoidance of surface pitting due to a combination of chemical attack with the erosion of high speed flow, which may on certain applications contain solid particles. There is reason to suppose that, in general, all normal metals used in engineering construction are attacked by all liquids normally pumped. This attack gives rise to a film of oxide or the salt corresponding to the acid impurities in the liquid pumped, and this film acts in many cases as a protection to the parent metal against further attack under static conditions or low flow velocity conditions, (up to, say, 7 mls). When, however, the flow velocity due to generation of high head per stage increases to the order of 60 mis, the protective film is eroded away, exposing fresh metal to attack. Under these higher velocity conditions consequent upon high head pumping and particularly where corrosive elements are in the liquid and where solid particles may occasionally be handled, it is essential to increase the quality of the material and thereby increase its resistance to attack. It is felt that corrosion and erosion proceed together so that a metal that has high resistance to corrosion is as important as a metal that has a high tensile strength. Local cavitation due to imperfections of surface and of flow can further aggravate these corrosion-erosion problems. The stainless steels possess a combination of high tensile strength and high resistance to corrosion and therefore they are of particular value in dealing with high temperature, high pressure and high head per stage duties. For pump duties, liquid velocities must therefore be considered in any corrosionerosion-cavitation problem.
Special Construction Materials Special duties can be described as those concerned with severe chemical attack, with very high speeds and high stage heads. Severely Corrosive Duties Pumps for handling very corrosive liquids, (eg various acids, etc, used in chemical and process work) are often manufactured in ceramics, plastics or the more noble metals. Such duties normally involve relati vely low heads up to 40 m or so, where the problem is entirely corrosion, since flow velocities are not sufficiently high to cause the combination of erosion and corrosion mentioned above to any marked degree. Some simplicity of pump design is often adopted in order to permit moulding or fabrication in these somewhat special materials. Many pumps are made with metal impellers and casings which are afterwards coated with rubber, plastic or ceramic, but in these applications it is essential that the covering should be entirely free from any flaws or cracks, as otherwise its purpose will be defeated and the liquid will have access to the more vulnerable metal frame. (See Appendix B for materials on chemical duties). Titanium is a most valuable pump material for corrosive duties. At the present time titanium is regarded as a forged or rolled material capable of being welded; small titanium pumps are therefore cut from solid metal and medium sized ones are fabricated. Cast iron
CONSTRUCTION MATERIALS
93
can be poured into a mould lined with titanium to produce a pump which has titanium at all parts in contact with the corrosive liquid and a cast iron outer shell to withstand the forces involved in tightening pump branches on a pipe flanges which are slightly misaligned. A good bond is obtained between the cast iron and the titanium. The combination of high strength, lightness and weldability renders titanium ideal for space rocket pumps. It is clear that the casting of titanium is becoming a commercial proposition which will be of great value to the pump and chemical industries.
Materials for High Stage Heads (See Ref 27) High heads involve high speeds both of impeller and of liquid flow, so that high metal stresses and severe erosion- corrosion necessarily occur. An attractive material for these duties is martensitic precipitation hardening stainless steel, for example FV520 containing 14% chromium and 5% nickel. This steel has the corrosion resistance and weldability of austenitic stainless steel 304 and the strength and acceptable expansion coefficient of the non- weldable high strength stainless steels 420 and 431. A similar martensitic precipitation hardening stainless steel contains 17% chromium and 4% nickel. Ferralium, 25% chromium is of particular value on offshore duties (see Table II of Appendix B). These steels have assisted the attainment of stage heads up to 2 000 m and show promise of greater heads. For the future, glass and carbon fibres show promise of even greater strength and resistance of erosion.
_ • • •1 s:
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Figure 15.1
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1400
CENTRIFUGAL PUMPS
94
Brittle materials, for example cemented carbides and ceramics, have applications in pump duties since their hardness is valuable for bearings lubricated by abrasive fluids (Fig 15.1, Ref 28). Plastics Plastics offer considerable attractions for chemical services since many of the man-made materials based on synthetic resins are notable for their chemical inertness and thus high resistance to chemical attack. This is particularly true of many of the more recently developed thermoplastics, and thermoset plastics in general. Certain limitations apply to all plastics, notably: (i) Low mechanical strength compared with metals. (ii) Lower moduli, particularly with thermoplastics. (iii) Low creep resistance, particularly with thermoplastics. (iv) Limited top service temperature. Some families of plastics have further limitations which are less apparent at first sight but show up in service such as: lack of dimensional stability, a susceptibility to stress, corrosion cracking in contact with chemically active fluids, and high permeability. To some extent these can be reduced, or even overcome, by modifying the plastic, but the best practical solution is to use a newer plastic which does not exhibit the same limitations. New materials with better properties continue to be evolved, and in recent years there has been a considerable increase in the number of commercial plastics with 'engineering' properties; that is to say, they can be considered as direct alternatives to metals for some applications. The first plastics used on any scale for chemical and process pumps were PVC and other vinyl resin compounds, and polythene, chiefly for linings and coatings. The rigid form of PVC was, and still is to a certain extent, employed for moulded components and casings for small pumps where high strength and rigidity were not required. Neither material fulfilled its expectations and the use of polythene linings, in particular, gave trouble because the material is permeable and allows liquid to penetrate to the underlying metal, causing corrosion and disruption of the bond. Since then the engineering plastics have progressed and the all-plastic chemical pump has become a practical proposition, at least in smaller sizes. The materials favoured now are polycarbonate and epoxy. The only metal component necessary is the shaft, which can be stainless steel or plastic sleeved. Epoxy resins-have better engineering properties since they are thermoset rather than thermoplastic materials. The mechanical properties can easily be raised by filling with, say, glass fibre. Thus, epoxy is coming into greater use for impellers in the larger chemical pumps and even for casings. Owing to the multiplicity of plastic materials with potential applications - some with proven limitations and others still little explored - it is impossible to deal with plastic constructions comprehensively. Table III Appendix B summarizes the chemical resistances of the plastics which bear consideration for chemical duties. This can be read as a general rather than specific guide in most cases since some acids or solvents may attack a material
CONSTRUCTION MATERIALS
95
shown as suitable. Table IV summarizes the leading physical properties of the same materials and shows their current applications. The above description of plastics was taken from an article by R.H. Warring (Ref 29). References 27. MORLEY, J.1. 'An improved Martensitic Stainless Steel', Proc BISRA - 151 Conference, Scarborough, 2-4 June 1964. 28. DUNCAN, P. and MORTIMER, 1. 'Ceramic Turbines - Why Britain is Leading the Race'. The Engineer.26.2.70 and 26.3.70. 29. WARRING, R.H., 'Materials for Chemical and Process Pumps'. Pumps Pompes-Pumpen, Jan 1969. See Appendix B.
CONSTRUCTION MATERIALS Normal Construction Materials General service pumps for low and medium pressures (eg 10 bar on single stage split casing pumps and 36 bar on cellular type multistage pumps) have cast iron casings, high tensile steel shafts with bronze rotating and wearing parts. The material specifications are as follows:Cast iron BS1452. High tensile steel shaft BS970 Bronze wearing parts BS1400 LB2, a soft lead bearing bronze. Bronze impellers, diffusers, sleeves, balance discs, etc. BS 1400 LG2 gun metal, or with a greater portion of tin to lead, LG3 or PB3 zincless bronze, according to severity of duty. For higher pressures, the casings are of cast, forged or welded steel and, for the highest heads per stage, stainless steel impellers and diffusers are used. The casing steel forging specification is 14A in general cases, but variations of this occur according to the tensile strength and other properties required. See also BS970 for EN steel. The stainless steel rotating and stationary parts are made to specifications BS 1630, 13chrome steel castings, BS970 for 16-chrome 2 112% nickel forgings and BS 1631 for austenitic stainless steel 18-chrome 8 nickel castings or forgings. High temperature high pressure pumps, (eg boiler feed pumps and hot oil pumps) have forged steel pressure vessels, forged stainless steel shafts with cast stainless steel impellers, diffusers, etc. (See Chapter 33). For temperatures above 120°C, bronze and austenitic stainless steel are unsuitable owing to their low strength and their coefficient of expansion (50% greater than that of mild steel or other stainless steels) which causes difficulty in maintaining correct fits and running clearances at higher temperatures. Bronze is, however, used for the very low stress wearing parts, (eg chamber bushes of multistage pumps).
Corrosion - Erosion Two distinct problems arise in choice of material for pumping duties. The first concerns
92
CENTRIFUGAL PUMPS
the provision of material whose strength is sufficient to withstand any anticipated stress loading without fracture or undue distortion. The second concerns the avoidance of surface pitting due to a combination of chemical attack with the erosion of high speed flow, which may on certain applications contain solid particles. There is reason to suppose that, in general, all normal metals used in engineering construction are attacked by all liquids normally pumped. This attack gives rise to a film of oxide or the salt corresponding to the acid impurities in the liquid pumped, and this film acts in many cases as a protection to the parent metal against further attack under static conditions or low flow velocity conditions, (up to, say, 7 mls). When, however, the flow velocity due to generation of high head per stage increases to the order of 60 mis, the protective film is eroded away, exposing fresh metal to attack. Under these higher velocity conditions consequent upon high head pumping and particularly where corrosive elements are in the liquid and where solid particles may occasionally be handled, it is essential to increase the quality of the material and thereby increase its resistance to attack. It is felt that corrosion and erosion proceed together so that a metal that has high resistance to corrosion is as important as a metal that has a high tensile strength. Local cavitation due to imperfections of surface and of flow can further aggravate these corrosion-erosion problems. The stainless steels possess a combination of high tensile strength and high resistance to corrosion and therefore they are of particular value in dealing with high temperature, high pressure and high head per stage duties. For pump duties, liquid velocities must therefore be considered in any corrosionerosion-cavitation problem.
Special Construction Materials Special duties can be described as those concerned with severe chemical attack, with very high speeds and high stage heads. Severely Corrosive Duties Pumps for handling very corrosive liquids, (eg various acids, etc, used in chemical and process work) are often manufactured in ceramics, plastics or the more noble metals. Such duties normally involve relati vely low heads up to 40 m or so, where the problem is entirely corrosion, since flow velocities are not sufficiently high to cause the combination of erosion and corrosion mentioned above to any marked degree. Some simplicity of pump design is often adopted in order to permit moulding or fabrication in these somewhat special materials. Many pumps are made with metal impellers and casings which are afterwards coated with rubber, plastic or ceramic, but in these applications it is essential that the covering should be entirely free from any flaws or cracks, as otherwise its purpose will be defeated and the liquid will have access to the more vulnerable metal frame. (See Appendix B for materials on chemical duties). Titanium is a most valuable pump material for corrosive duties. At the present time titanium is regarded as a forged or rolled material capable of being welded; small titanium pumps are therefore cut from solid metal and medium sized ones are fabricated. Cast iron
CONSTRUCTION MATERIALS
93
can be poured into a mould lined with titanium to produce a pump which has titanium at all parts in contact with the corrosive liquid and a cast iron outer shell to withstand the forces involved in tightening pump branches on a pipe flanges which are slightly misaligned. A good bond is obtained between the cast iron and the titanium. The combination of high strength, lightness and weldability renders titanium ideal for space rocket pumps. It is clear that the casting of titanium is becoming a commercial proposition which will be of great value to the pump and chemical industries.
Materials for High Stage Heads (See Ref 27) High heads involve high speeds both of impeller and of liquid flow, so that high metal stresses and severe erosion- corrosion necessarily occur. An attractive material for these duties is martensitic precipitation hardening stainless steel, for example FV520 containing 14% chromium and 5% nickel. This steel has the corrosion resistance and weldability of austenitic stainless steel 304 and the strength and acceptable expansion coefficient of the non- weldable high strength stainless steels 420 and 431. A similar martensitic precipitation hardening stainless steel contains 17% chromium and 4% nickel. Ferralium, 25% chromium is of particular value on offshore duties (see Table II of Appendix B). These steels have assisted the attainment of stage heads up to 2 000 m and show promise of greater heads. For the future, glass and carbon fibres show promise of even greater strength and resistance of erosion.
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0 0
Silicon carbide
30
45
20
30
N
C
:.:::. ~
c
B
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CJ
+oJ
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a.
I
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0
15
+oJ
en en
~
en 0 900
1000
1100
1200
Temperature (OC)
Figure 15.1
1300
1400
CENTRIFUGAL PUMPS
94
Brittle materials, for example cemented carbides and ceramics, have applications in pump duties since their hardness is valuable for bearings lubricated by abrasive fluids (Fig 15.1, Ref 28). Plastics Plastics offer considerable attractions for chemical services since many of the man-made materials based on synthetic resins are notable for their chemical inertness and thus high resistance to chemical attack. This is particularly true of many of the more recently developed thermoplastics, and thermoset plastics in general. Certain limitations apply to all plastics, notably: (i) Low mechanical strength compared with metals. (ii) Lower moduli, particularly with thermoplastics. (iii) Low creep resistance, particularly with thermoplastics. (iv) Limited top service temperature. Some families of plastics have further limitations which are less apparent at first sight but show up in service such as: lack of dimensional stability, a susceptibility to stress, corrosion cracking in contact with chemically active fluids, and high permeability. To some extent these can be reduced, or even overcome, by modifying the plastic, but the best practical solution is to use a newer plastic which does not exhibit the same limitations. New materials with better properties continue to be evolved, and in recent years there has been a considerable increase in the number of commercial plastics with 'engineering' properties; that is to say, they can be considered as direct alternatives to metals for some applications. The first plastics used on any scale for chemical and process pumps were PVC and other vinyl resin compounds, and polythene, chiefly for linings and coatings. The rigid form of PVC was, and still is to a certain extent, employed for moulded components and casings for small pumps where high strength and rigidity were not required. Neither material fulfilled its expectations and the use of polythene linings, in particular, gave trouble because the material is permeable and allows liquid to penetrate to the underlying metal, causing corrosion and disruption of the bond. Since then the engineering plastics have progressed and the all-plastic chemical pump has become a practical proposition, at least in smaller sizes. The materials favoured now are polycarbonate and epoxy. The only metal component necessary is the shaft, which can be stainless steel or plastic sleeved. Epoxy resins-have better engineering properties since they are thermoset rather than thermoplastic materials. The mechanical properties can easily be raised by filling with, say, glass fibre. Thus, epoxy is coming into greater use for impellers in the larger chemical pumps and even for casings. Owing to the multiplicity of plastic materials with potential applications - some with proven limitations and others still little explored - it is impossible to deal with plastic constructions comprehensively. Table III Appendix B summarizes the chemical resistances of the plastics which bear consideration for chemical duties. This can be read as a general rather than specific guide in most cases since some acids or solvents may attack a material
CONSTRUCTION MATERIALS
95
shown as suitable. Table IV summarizes the leading physical properties of the same materials and shows their current applications. The above description of plastics was taken from an article by R.H. Warring (Ref 29). References 27. MORLEY, J.1. 'An improved Martensitic Stainless Steel', Proc BISRA - 151 Conference, Scarborough, 2-4 June 1964. 28. DUNCAN, P. and MORTIMER, 1. 'Ceramic Turbines - Why Britain is Leading the Race'. The Engineer.26.2.70 and 26.3.70. 29. WARRING, R.H., 'Materials for Chemical and Process Pumps'. Pumps Pompes-Pumpen, Jan 1969. See Appendix B.