Direct valve replacement

Percutaneous Valve Repair and Replacement for Mitral Regurgitation 124 Percutaneous Aortic Valve Replacement 131 Future Directions 138 References 138... 

It often happens tiiat only a part of the heart, such as the aortic valve, fails and needs replacement. Repair could be made by using foreign tissue (for example, a pig heart valve) or a mechanical heart valve implant to replace a diseased one. About 250,000 valve replacement procedures are performed annually worldwide. In the United States about 45% of the procedures involve a mechanical valve. The most widely used valve is shown in Figure 12.19 . It has two semicircular discs tiiat move to allow blood to flow m the desired direction as the heart pumps, then they fall back together to form a seal against backflow. 

Treede, H. et al.. Six-month results of a repositionable and retrievable pericardial valve for transcatheter aortic valve replacement The Direct Flow Medical aortic vahre. / Jhorac Cardiovasc Surg, 2010. 140(4) p. 897-903. 

Other types of pressure-relief valves do not depend upon the back pressure for their performances. However, to ensure that the safety valves work at their maximum capacity, back pressure is limited to 50 percent of the relief valve set pressure. In the balanced bellows type valve, the spring does not act directly on the disk. Instead, it serves on a bellows first, which in turn acts on the disk. In case of the piston type, it works on the same principle as the bellows type, except that the bellows is replaced by a piston (see Figure 17B). The cross-sectional area of both the piston and the bellows is the same as the inlet nozzle of the valve and the effect of the back pressure on the top and the bottom of the disk creates equal balancing forces. That is, P,A is always equal to F, as shown in Figure 17B. 

The leak could have been stopped as soon as it was detected if an emergency isolation valve (Section 7.2.1) had been fitted in the pump suction line. On the rebuilt plant such valves were fitted on the pump suction lines, more combustible gas detectors and more extensive insulation were installed, plastic pump bodies were replaced by metal ones, and spillages were directed to collection pits. The plant was built in 1972, when these features were not common practice many improvements had been made since then, but they did not go far enough. Most of those made after the fire could have been made beforehand. 

Figure 14.4 Schematic diagram of the cliromatographic system used for the analysis of very low concentrations of sulfur compounds in ethene and propene CP, pressure regulator CF, flow regulator SL, sanrple loop R, restriction to replace column 2 VI, injection valve V2, tliree-way valve to direct the effluent of column 1 to either column 2 or the restriction column 1, non-polar- capillary column column 2, tliick-film capillary column SCD, sulfur chemiluminescence detector FID, flanre-ionization detector.

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

Alternatively, a purpose-built dispenser can be used. One counter-top system stores a BiB container of concentrate in a refrigerated compartment above a water chiller. Concentrate is transferred directly to the dispense valve by a peristaltic pump acting on a tube that is replaced with every BiB. This ensures that the only permanent part of the equipment contacting the juice is the valve. 

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