References pump selection

For quick pump selection, manufacturers often give the most essential performance details for a whole range of pump sizes. Figure 10-40 shows typical performance data for a range of process pumps based on suction and discharge pipes and impeller diameters. The performance data consists of pump flow rate and head. Once a pump meets a required specification, then a more detailed performance data for the particular pump can be easily found based on the curve reference number. Figure 10-41 shows a more detailed pump performance curve that includes, in addition to pump head and flow, the brake horsepower required, NPSH required, number of vanes, and pump efficiency for a range of impeller diameters. 

Abrasion is usually a major consideration in pump selection. Except for the mild chemical-type duties already referred to, slurry pumps are of special construction, essentially to resist abrasion to the pump and its shaft seals but often also to permit fast maintenance and liner or component replacement for very abrasive duties. Generally either replaceable rubber lining or specially hard abrasion-resistant alloys are used shaft seals are of special design, and are usually continually flushed with clean water. Impeller tip velocities are kept within proven limits for the application. Power transmission is often through V-belts rather than direct-drive couplings, to permit exactly the required speed to be obtained (rather than synchronous speeds) and to permit speed optimization in service (varying the impeller diameter is not so convenient). 

The term solid-state laser refers to lasers that use solids as their active medium. However, two kinds of materials are required a host crystal and an impurity dopant. The dopant is selected for its ability to form a population inversion. The Nd YAG laser, for example, uses a small number of neodymium ions as a dopant in the solid YAG (yttrium-aluminum-gar-net) crystal. Solid-state lasers are pumped with an outside source such as a flash lamp, arc lamp, or another laser. This energy is then absorbed by the dopant, raising the atoms to an excited state. Solid-state lasers are sought after because the active medium is relatively easy to handle and store. Also, because the wavelength they produce is within the transmission range of glass, they can be used with fiber optics. 

The rotative speed of a pump is dependent upon the impeller characteristics, type fluid, NPSH available and other factors for its final determination. The most direct method is by reference to manufacturer s performance curves. VVTien a seemingly reasonable selection has been made, the effect of this selected speed on the factors such as NPSH required, suction head or lift, fluid erosion and corrosion, etc., must be evaluated. For many systems these factors are of no concern or consequence. 

For a detailed discussion of the factors governing the selection of the best centrifugal pump for a given duty the reader should refer to the articles by De Santis (1976), Neerkin (1974), Jacobs (1965) or Walas (1983). 

We have seen how to determine the driving force (e.g., pumping requirement) for a given pipe size and specified flow as well as how to determine the proper pipe size for a given driving force (e.g., pump head) and specified flow. However, when we install a pipeline or piping system we are usually free to select both the best pipe and the best pump. The term best in this case refers to that combination of pipe and pump that will minimize the total system cost. 

Most chromatographers limit themselves to binary or tertiary gradient systems however, it should be noted that pumps capable of quaternary gradient are available (see reference 3, Chapter 3) and should be considered in the equipment selection process to allow greater versatility in method development. 

D NOE-pumping experiment. E Reverse NOE pumping experiment (bottom) and reference experiment (top). F e-PHOGSY NOESY sequence. The water-selective 180° pulse sandwiched by the first two gradients has a gaussian shape and a duration of 40-50 ms. The mixing time is approx. 2 s. For further details, see Refs. [29, 30]. 

HPLC methods can usually be transferred without many modifications, since most commercially available HPLC instruments behave similarly. This is certainly true when the columns applied have a similar selectivity. One adaptation, sometimes needed, concerns the gradient profiles, because of different instrumental or pump dead-volumes. However, larger differences exist between CE instruments, e.g., in hydrodynamic injection procedures, in minimum capillary lengths, in capillary distances to the detector, in cooling mechanisms, and in the injected sample volumes. This makes CE method transfers more difficult. Since robustness tests are performed to avoid transfer problems, these tests seem even more important for CE method validation, than for HPLC method validation. However, in the literature, a robustness test only rarely is included in the validation process of a CE method, and usually only linearity, precision, accuracy, specificity, range, and/or limits of detection and quantification are evaluated. Robustness tests are described in references 20 and 59-92. Given the instrumental transfer problems for CE methods, a robustness test guaranteeing to some extent a successful transfer should include besides the instrument on which the method was developed at least one alternative instrument. 

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. 

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