ValvesDOE-HDBK-1018/2-93TYPES OF VALVESAll elastomeric materials operate best below 150F. Some will function at higher temperatures.Viton, for example, is noted for its excellent chemical resistance and stability at hightemperatures. However, when fabricated into a diaphragm, Viton is subject to lowered tensilestrength just as any other elastomeric material would be at elevated temperatures. Fabricbonding strength is also lowered at elevated temperatures, and in the case of Viton, temperaturesmay be reached where the bond strength could become critical.Fluid concentrations is also a consideration for diaphragm selection. Many of the diaphragmmaterials exhibit satisfactory corrosion resistance to certain corrodents up to a specificconcentration and/or temperature. The elastomer may also have a maximum temperaturelimitation based on mechanical properties which could be in excess of the allowable operatingtemperature depending upon its corrosion resistance. This should be checked from a corrosiontable.DiaphragmValveStemAssembliesDiaphragm valves have stems that do not rotate. The valves are available with indicating andnonindicating stems. The indicating stem valve is identical to the nonindicating stem valveexcept that a longer stem is provided to extend up through the handwheel. For the nonindicatingstem design, the handwheel rotates a stem bushing that engages the stem threads and moves thestem up and down. As the stem moves, so does the compressor that is pinned to the stem. Thediaphragm, in turn, is secured to the compressor. DiaphragmValveBonnetAssembliesSome diaphragm valves use a quick-opening bonnet and lever operator. This bonnet isinterchangeable with the standard bonnet on conventional weir-type bodies. A 90 turn of thelever moves the diaphragm from full open to full closed. Diaphragm valves may also beequipped with chain wheel operators, extended stems, bevel gear operators, air operators, andhydraulic operators. Many diaphragm valves are used in vacuum service. Standard bonnet construction can beemployed in vacuum service through 4 inches in size. On valves 4 inches and larger, a sealed,evacuated, bonnet should be employed. This is recommended to guard against prematurediaphragm failure.Sealed bonnets are supplied with a seal bushing on the nonindicating types and a seal bushingplus O-ring on the indicating types. Construction of the bonnet assembly of a diaphragm valveis illustrated in Figure 15. This design is recommended for valves that are handling dangerousliquids and gases. In the event of a diaphragm failure, the hazardous materials will not bereleased to the atmosphere. If the materials being handled are extremely hazardous, it isrecommended that a means be provided to permit a safe disposal of the corrodents from thebonnet.Rev. 0ME-04Page 27
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