Valve stem

C 0.15 C treatment easily machinable freezers valve stems, plugs, and gates... 

Various accessories can be suppHed along with the control valves for special situations. Positioners ensure that the valve stem is accurately positioned following small or slowly changing control signals or where unbalanced valve forces exist. Boosters, which are actually pneumatic amplifiers, can increase the speed of response or provide adequate force in high pressure appHcations. Limit switches are sometimes included to provide remote verification that the valve stem has actually moved to a particular position. 

Enichem and DuPont ate two suppHers of acryHc elastomers. It is estimated there ate - 9000 t/yr of these elastomers used in the United States. The principal uses of acryHc elastomers ate in automotive appHcations requiring both oil and heat resistance. These include transmission, valve stem, crankshaft, pinion, and odpan gaskets and seals. In addition hose, tubing, toUs, and belts ate made from acryHc polymers. 

Parts made from fluoroelastomers ate used ia appHcations that justify their high cost, usually where the maintenance and replacement costs are high enough to offset the initial cost of the part. These include automotive appHcations such as valve stem seals, fuel injector components, radiator, crankcase and transmission seals, and carburetor needle tips. Numerous seals and gaskets in the marine, oilfield, and chemical processing industries employ fluoroelastomers. In addition, many hoses in the automotive and chemical industry are made entirely of fluoroelastomer compounds or have a veneer of the fluoroelastomer as a barrier exposed to the harsh environment. Seals and gaskets in military appHcations and the binder for flares and missile appHcations ate made with fluoroelastomers. 

Uses. Uses for copper—siUcon alloys are siUcon bronze, UNS C 87200, as bearings, pumps, valve parts, marine fittings, and corrosion-resistant castings siUcon brass UNS C 87400 as bearings, gears, impellers, valve stems, and clamps and siUcon brass UNS C 87500 as small propellers, valve stems, gears, and bearings. 

Valve Positioners The valve positioner, when combined with an appropriate actuator, forms a complete closed-loop valve-position control system. This system makes the valve stem conform to the input signal coming from the process controller in spite of force loads that the actuator may encounter while moving the control valve. Usually, the valve positioner is contained in its own enclosure and is mounted on the control valve. 

While most turbines have a 10-year-availability record in the range of 95 to 99 percent, troubles may develop in any number of places. The most common are vibration, cycling governor, sticking valve stems, leaky packing, temperature bow, erosion of blading, loss of power, and bearing problems. 

Sticking of valve stems is common if solids are present in the steam. The steam mast be without sohds. (Note comments later under loss of power.) It is important that units operating on a steady load for long periods be checked for sticking stems at regular intervals. The records show that in several cases deposits have caused the stem of both the governor valve and the trip valve to stick when there was a loss of load. The effect of the loss of load was destructive overspeed. 

Valve stem packings on valves in the suction piping. 

Tighten all pump packing rings and all valve stem packings on suction piping. 

At high pressure experiments the reactor should be installed in a pressure cell. All check valves before it, and the filter with the flow controller after it, can be kept in the vented operating room. As a minimum, the bypass valve and the flow controller must be accessible to the operator. This can be done by extended valve stems that reach through the protecting wall. Both the operating room and the pressure cell should be well ventilated and equipped by CO alarm instruments. 

Valve plug may have broken off from the valve stem. 

To minimize the effects of deposit buildup on valve stems of trip or trip and throttle valves, a pull-to-close design is preferred. While somewhat more expensive, the added feature of hydraulic-assisted closure is desirable, particularly for the larger sizes. 

Amongst the many other applications for acetal resins should be mentioned links in conveyor belts, moulded sprockets and chains, blower wheels, cams, fan blades, check valves, pump impellers, carburettor bodies, blow-moulded aerosol containers and plumbing components such as valve stems and shower heads. 

The physieal realization of a system to eontrol room temperature is shown in Figure 1.6. Flere the output signal from a temperature sensing deviee sueh as a thermoeouple or a resistanee thermometer is eompared with the desired temperature. Any differenee or error eauses the eontroller to send a eontrol signal to the gas solenoid valve whieh produees a linear movement of the valve stem, thus adjusting the flow of gas to the burner of the gas fire. The desired temperature is usually obtained from manual adjustment of a potentiometer. 

Fuel and Low Cutout - The RBV should be instrumented so that when the valve stem travel reaches 50% of the closed position, it will ... 

Stange, /. stick rod, bar pole (of sulfur) roll (of gold) ingot (of a valve) stem. 

Major leaks at pump seals, valve stem packing, flange gaskets, etc. 

Figure 10-13 illustrates the flow versus valve stem travel characteristic for various typical valve trim functions (Fisher Controls, 1987). The quick opening characteristic provides the maximum change in flow rate at low opening or stem travel, with a fairly linear relationship. As the valve approaches the wide open position, the change in flow with travel approaches zero. This is best suited for on-off control but is... 

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