VALVE SEAT

The principal uses of PCTFE plastics remain in the areas of aeronautical and space, electrical/electronics, cryogenic, chemical, and medical instmmentation industries. AppHcations include chemically resistant electrical insulation and components cryogenic seals, gaskets, valve seats (56,57) and liners instmment parts for medical and chemical equipment (58), and medical packaging fiber optic appHcations (see Fiber optics) seals for the petrochemical /oil industry and electrodes, sample containers, and column packing in analytical chemistry and equipment (59). 

W. Broadway, "Development of Cryogenic Butterfly Valve Seat," paper presented atMJOE 71 st Mnnual Meeting Cyogenic Equipment Session, Miami, Nov. 12-16,1978. 

Hexachloroethane, like carbon tetrachloride and 1,1,1-trichloroethane, can be used to formulate extreme pressure lubricants (149,150). For example, lubricating oils containing 0.02—3.0 wt % (as halogen) of hexachloroethane reduce the abrasion of exhaust valve seats in internal combustion engines (151)... 

FIG. 20-54 Details of valve seat of the Gaulin liigh-pressure homogenizer, type CD. 

Figure 8.4 Corroded aluminum-alloy casting valve seat. The seat was in contact with water having a pH between 9 and 10 for 3 months. Red marks were drawn on the surfaces to assist metallographic preparation.

Figure 11.6 Split valve body showing metal loss at the valve seat.

Highly localized metal loss at the valve seat is apparent in Fig. 11.6. Figure 11.29 shows the same component close up. Wasted surfaces have a bright, metallic luster free of corrosion products or deposits. Metal loss along the edge of the throttling nut is also apparent (Fig. 11.30). 

Figure 11.29 Erosion-corrosion of a valve seat (bright metal in center of photograph).

An oversized relief valve may also chatter since the valve may quickly relieve enough contained fluid to allow the vessel pressure to momentarily fall back to below set pressure only to rapidly increase again. Rapid cycling reduces capacity and is destructive to the valve seat in addition to subjecting all the moving parts in the valve to excessive wear. E.xcessive back pressure can also cause rapid cycling as discussed above. 

G-VG G-VG Molded by all processes, fluidized bed coatings High-temperature valve seats, chemical resistant pipe, coated vessels, insulation... 

A typical pilot-operated PR valve is illustrated in Figure 5. Under normal operating conditions, the vessel pressure acts on the main valve seat at the bottom of the free-floating differential area piston, and by means of the pilot supply line is also applied to the top of the piston and under the pilot valve disc. Since the top area of the piston is larger than the nozzle area at the lower end of the piston, there is a large net load holding the piston down on the nozzle. Under static... 

Valve. seat Fails to block flow when closed. Possible leakage of SIS fluid into system and reduced operability. Need leakage taps on system to determine if there is leakage between series valves. Sense temperature of pipe. Inspection. 

Long-life special alloy valves have chromium-plated stems, hardened shrink-fit valve seats, and replaceable guides. 

Clearance is the volume remaining in a cylinder end when the piston is at the end of its stroke. This is die sum of the volume between the head of the cylinder and the piston, and the volume under tlie valve seats. The total clearance is expressed in percent of the total piston displacement, normally between 4 and 30%. 

Ventil-kiiken, m. valve plug. -SI, n. valve oil. -schlauch, m. valve tubing, -sitz, m. valve seat. -Stahl, m. valve steel, -stopfen, n. stopper with a valve, valve stopper, -zelle,/. (Elec.) valve cell. 

Popping Pressure the pressure at which the internal pressure in a vessel rises to a value that causes the inlet valve seat to begin to open and to continue in the opening direction to begin to relieve the internal overpressure greater than the set pressure of the device. For compressible fluid service. 

Closing Pressure the pressure established as decreasing inlet pressures when the disk of the valve seats and there is no further tendency to open or close. 

This is the total volume remaining in the cylinder at the end of the piston stroke. This consists of the volume between the end of the piston and the cylinder head, in the valve ports and the volume in the suction valve guards and the discharge valve seats. See Figures 12-12,12-17A, and 12-17B. 

The main function of the relief valve is to act as a back up to a pressure governor to prevent the pre-set downstream pressure being exceeded. In particular, it will relieve the small amount of creep that will occur by slippage past the valve seating in a governor. 

Cleanliness in hydraulic systems has received considerable attention recently. Some hydraulic systems, such as aerospace hydraulic systems, are extremely sensitive to contamination. Fluid cleanliness is of primary importance because contaminants can cause component malfunction, prevent proper valve seating, cause wear in components, and may increase the response time of servo valves. Fluid contaminants are discussed later in this chapter. 

The valve consists primarily of a movable poppet, which closes against the valve seat. In the closed position, fluid pressure on the inlet side tends to hold the valve tightly closed. A small amount of movement from a force applied to the top of the poppet stem opens the poppet and allows fluid to flow through the valve. 

Impingement attack Copper may occasionally suffer this form of attack in systems where the speed of water flow is unusually high and the water is one that does not form a protective scale, e.g. a soft water containing appreciable quantities of free carbon dioxide . Ball valve seatings may also suffer an erosive type of attack. The corrosion of ball valves, including the effect of chlorination of the water, has been studied by several workers... 

Brass water fittings give no trouble except that dezincification may occur in acid waters or waters of high chloride content, especially when hot. This dezincification has three effects. Firstly, the replacement of brass by porous copper may extend right through the wall of the fitting and permit water to seep through. Secondly, the zinc which is dissolved out of the brass may form very voluminous hard corrosion products and eventually block the waterway —this is often the case in hot soft waters. Thirdly, and often the most important, the mechanical properties of the brass may deteriorate. For instance, a dezincified screwed union will break off when an attempt is made to unscrew it and a dezincified tap or ball-valve seat is readily eroded by the water. 

Service stop valves on small compressors may also carry a connection for a pressure cut-out or gauge, or for the temporary fitting of guages or charging lines when servicing. The valve backseats to close off this port while gauges are being fitted. Valve seats are commonly of soft metal or of a resistant plastic such as PTFE. 

An alternative way to use flash steam is to provide several, small self-contained FSHR systems, keeping the system pipework to a minimum and lagging the pipes to avoid unnecessary heat losses. Flash steam forms at the point where the pressure drops, which is at the BD valve or the valve seat of the steam trap. From this point the flash steam and condensate travel together until the flash vessel is reached. The vessel then acts as a steam-water separator. 

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