Pumps, materials selection

Now let us look at the systems and the components of the systems. Reports in the field are virtually mute on many small but annoying problems, which almost invariably accompany the operation of seawater RO systems. Here are some which have plagued us, starting with the pumps. I will not identify manufacturers, as I have avoided reference to any in this writeup. It is not my objective to accuse anyone. I just offer these comments in the hope to encourage those who I hope are aware of their product problems in the field and failures to rectify those for the benefit of all. I feel that considerably greater attention must be placed on pumps and pump materials selection for seawater RO use in order to improve reliability, simplify maintenance, and reduce failure frequency in the pump operations, and in turn improve on the RO system water supply capability. 

POURBAIX PRIME Belgium Belgium KEE (sucker rod pumps) Materials selection... 

Pump Materials of Construction In the (memical industry, the selection of pump materials of construction is dictated by considerations of corrosion, erosion, personnel safety, and liquid contamination. The experience of pump manufacturers is often valuable in selecting materi s. See section on materials. 

Pump materials shall he selected to suit the particular service requirements. 

Temperature. The temperature of the liquid being pumped is important because it affects the seal face material selection as well as the wear life of the seal face. 

Pumps and compressors are designed per technical specifications and standards developed over years of operating and maintenance experience. Table lO-lO lists some of these standards for pumps and compressors and for related equipment such as lubrication systems and gearboxes which, if not properly specified, could lead to many operational and maintenance problems with the pumps and compressors. These standards specify design, construction, maintenance, and testing details such as terminology, material selection, shop inspection and tests, drawings, clearances, construction procedures, and so on. 

At this point, experiments must be performed. Experimental results for the erosive wear of the selected candidate ceramic materials in coal slurries are presented in Table 8.4. Notice that the wear rate has a very rough inverse correlation with which is consistent with some of the descriptions of erosive wear from the previous section. Any of these ceramic materials is suitable for the piping and pump components based solely on wear rate, with the lowest wear rate for SiC being the most attractive. Formability and economic criteria can be applied to assist in the final material selection. 

Materials selection for centrifugal pumps is contained in API 3 610, Appendixes E and F. Another source of information is API s Guide for Inspection of Refinery Equipment, Chapter II. In addition, both NACE Group Committee T-8 on Refining Industry Corrosion and the API Corrosion Committee publish minutes of their semiannual meetings. An index of Group Committee T-8 s minutes has been produced so that information on any topic discussed can be located. 

Appendix A contains a materials selection guide for aerated freshwater systems. As indicated in Note 27 of Appendix A, in freshwater systems, admiralty brass should be limited to a maximum pH value of 7.2 from ammonia and copper-nickel alloys and should not be used in waters containing more sulfides than 0.007 mg/L The materials selection guide is also satisfactory for seawater, although pump cases and impellers should be a suitable duplex stainless steel or nickel-aluminum-bronze (properly heat treated). Neoprene-lined water boxes should be considered. For piping, fiber-reinforced plastic (up to 150 psi [1,035 kPa] operating pressure) and neoprene-lined steel should also be considered. Titanium and high-molybdenum SS tubes should be considered where low maintenance is required or the cost can be justified by life expectancy. 

The laboratory scale apparatus (reactors, separators, heat exchanger, line, pump etc.) will be fabricated using the materials selected. 

Materials selected for piping in mild to moderately corrosive services are sometimes less conservative than those for vessels, heat exchangers, tanks, and pumps in the same services. In this case, piping materials may be chosen on the basis of a shorter design life. This is usually justified because piping is easier to inspect, both online and offline. Also, piping is usually easier to replace and does not have the long lead-time problem often associated with fabricated equipment, vessels, etc. 

Various types of pumps are to be used in the plant. Confirm that the correct pump has been provided for the required service conditions. The selection of pump, material of constmction of wetted parts depending on the liquid handled, discharge capacity and head, oil seal, water seal, and gland packing should be checked. Direction of rotation of pump and motor must match, and their alignment with each other must be perfect. 

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