Other pump designs

In a special type of short-stroke piston pump the piston does not convey the mobile phase directly. The piston moves within an oil-filled channel, and oil and eluent are separated from each other by an elastic steel membrane. By this design the piston is not in contact with the mobile phase which may be aggressive on the other hand, the eluent cannot be contaminated by abrasives from the piston seal. 

For flow rates in the microliter per minute range, special two-piston pumps are available. With such instruments it is even possible to run rather precise high-pressure gradients if the total flow is 50 pi rnin or higher. 

Pneumatic amplifier pumps are used for column packing and for applications where a pressure of over 500 bar is needed. With this design, a relatively low gas pressure activates the larger cross-sectional area of a piston which at its other end is in contact with the eluent over a much smaller area. The force is the same on both sides of the piston but the pressure is higher on the smaller area, according to the ratio of the two areas. The pressure amplification can be as high as 70-fold this means that a gas pressure of 10 bar can generate an eluent pressure of 700 bar. 

For preparative separations there are pumps on the market which convey up to SOOmlmin even at a pressure of 300bar. 

For very low solvent flows a pump type can be used which does not convey the liquid by rapid strokes but which acts like an oversized syringe. A slow-moving piston pushes the eluent directly from its reservoir. This flow is 

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Specialty Pumps. There are a multitude of other pump designs in the family of kinetic pumps. Many of these designs have been around for years others continue to emerge. Some of these special-purpose pumps are described herein. 

Although time-pressure, auger, and piston systems are the most commonly used methods of needle dispensing, some other pump designs exist. For the purposes of fuel cell stack assembly, however, the considerations listed above apply to most technologies. 

Pumps are designed to give trouble-free operation for a long period of time. The ANSI B73.1M pumps are designed for a bearing life of no less than two years (29), and API 610 pumps for a minimum of five years (30). However, in real appHcations, a typical mean time between faUures (MTBF) is often found to be significantly less, and sometimes it is as short as a few weeks. Whereas in some instaUations the seals last from three to four years, in others these are replaced monthly. The reason for such wide variations in pump component life is often not poor pump design but equipment misappHcation. 

As we ve di,seussed, system design is responsible for much of cavitation. Yet, the maintenanee mechanic is responsible for stopping and preventing eavitation. And certainly, it s the maintenance mechanic who has to deal with the results of cavitation, the constant changing of bearings, meehanical seals, damaged impellers, wear rings and other pump parts. 

However, all pump discharges are subject to friction and other losses, and it is therefore necessary to refer to various pump curves and design data in order to establish the correct pump design and to provide a precise match in terms of pump capacity, FW temperature, head demands, and mode of pump operation. 

When liquids are being pumped, it is important to keep the pressure in the suction line above the vapor pressure of the fluid. The available head measured at the pump suction is called the net positive suction head available (NPSHA). At sea level, pumping 15°C (60°F) water with the impeller about 1 m below the surface, the NPSHA is about 9.1 m (30 ft). It increases with barometric pressure or with static head, and decreases as vapor pressure, friction, or entrance losses rise. Available NPSHA is the characteristic of the process and represents the difference between the existing absolute suction head and the vapor pressure at the process temperature. The required net positive suction head required (NPSHR), on the other hand, is a function of the pump design (Figure 2.121). It represents the minimum margin between suction head and vapor pressure at a particular capacity that is required for pump operation. Cavitation can occur at suction pres-... 

Rotary piston pump studies made by Sadler12 demonstrated that the above attributes of piston pump design helped them outperform vane design pumps both in speed and vacuum achieved (Sadler compared pumps of comparable size). On the other hand, piston pumps had greater vibration. 

To change a diffusion pump fluid requires the complete removal and cleanup of all previous fluid before adding a new, different oil. Before adding a new oil, be sure the type of oil (or mercury) that you wish to use will work in the pump that you have. Some oils require specific tolerances or pump designs for optimum performance. One example (in the extreme) is the use of mercury in an oil pump or vise versa. Mercury may work in some types of pumps, but the performance will not be very good. Oil, on the other hand, will not work in any mercury diffusion pump design. 

Indoor residential sampling can be restricted because of available space or by homeowner objections. Equipment noise can also be an issue, depending on the size of the space being monitored, the acoustics of the area and the presence of occupants. Noise from sampling equipment used in residences, schools, offices and other relatively noise-free areas should be limited to 35 dB (1 sones) (ASTM International, 2003e). Many battery-operated portable pumps designed... 

As liquids are essentially incompressible, less energy is stored in a compressed liquid than a gas however, it is often worth considering power recovery from high-pressure liquid streams (>15 bar), as the equipment required is relatively simple and inexpensive. Centrifugal pumps are used as expanders and are often coupled directly to other pumps. The design, operation, and cost of energy recovery from high-pressure liquid streams is discussed by Jenett (1968), Chada (1984), and Buse (1985). 

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