Special Application Valves

Joint design and selection generally involves compromising between the ideal and practical. A number of manufacturers produce patented or proprietary joints that embody many ideal characteristics. Some are excellent products and are well suited to special applications. Valves and fittings are often available with proprietary joints that have gained wide acceptance however, consideration should be given to the possible impact on product delivery time and cost. 

Surface tension also affects re-entrainment. The lower the surface tension of the liquid the easier it is for gas velocity to break away small droplets from the collecting surfaces. Generally, the valves of surface tension dot not change enough in oilfield operations to materially affect separation efficiencies. In special applications it may be necessary to consider the surface tension of the liquid to be separated. 

Gain Scheduling Adaptive Control is a special application of this procedure. For example we may have a control valve whose characteristic (input signal/valve stem position relationship) is non-linear. In this case, the valve stem position would be measured in order to obtain the gain of the valve (the appropriate relationship must be known) and the valve gain is used then to adjust the gain of the controller. If the auxiliary variable relationships are more complicated then it may be necessary to employ a Programmed Adaptive Control procedure. 

In summary, the applications engineer should first review the potential methods of eliminating cavitation. These would include adjustment of process conditions, revision of valve type, or change of installation layout. If none of these techniques can guarantee the complete elimination of cavitat-ing conditions, chokes or special anticavitation valves should be installed that can last for some reasonable period, even if some cavitation is occurring. 

The bubble-cap tray was the workhorse of distillation before the 1960s. It was superseded by the sieve and valve trays. Presently, bubble-cap trays are specified only for special applications, while sieve and valve trays are the moat popular types. 

Since certain aerosols can be sterile and non-preserved, microbiological risks may have to be checked at the dispensing orifice, including the possibility of grow back . Special double valve systems and other alternatives are being continuously evaluated for such applications. Innovation related to aerosol adaptors for nasal, ear, eye, etc. applications are typical examples of evolving technology. 

These requirements are met by diffusion pumps or even better by cryopumps and by turbo molecular pumps which are also usually applied in practice. In order to maintain the gas discharge, a gas inlet, for instance in the form of a needle valve, has to be provided. The process, itself runs on a flow-through principle. Noble gases, usually argon, are used as the working gas. However, for special applications, almost any other gases and gas mixtures can be used. 

Inherently, all injection valves have one thing in common, that is, they have moving parts that might eventually become worn. Yet, the recent generation of FIA valves, made of a suitable combination of polymers, have proven very reliable in continuous use—in fact, van der Linden [1059] has reported that rotary valves subjected to exhaustive tests even after 50,000 switches still functioned properly, and no wear or tear was noticeable provided that the sample and carrier stream were virtually free from particles. Even better performance is feasible with valves if powered by stepper motors, which results in superior reproducibility of injection and excellent duration. Nevertheless, it would, particularly for special applications, be beneficial to be able to meter the sample volume and subsequently transfer it reproducibly into the carrier stream by means requiring no mechanically moving parts. 

Welding-end Valves are used for special applications where leakage is prohibited or, as one refiner puts it, if the valve is not replaced more often than every three to five years. Considerable savings in valve cost can be had considering weld-end valves replacing flanged or screwed-end types. 

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