ASME B16.34-2009 pdf download.Valves Flanged, Threaded, and Welding End .
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2.1.2 Standard Class Valves. Valves conforming to the requirements of this Standard, except for those meeting the additional requirements of section 8 for Special Class valves or of Mandatory Appendix V for Limited Class valves, shall be designated Standard Class valves. Ratings shall not exceed the values that are listed in Table 2-1.1 through Table 2-3.19 with an identifying label “A- Standard Class.” 2.1.3 Special Class Valves. Threaded- or welding-end valves that conform to all the requirements of para. 2.1.2, and in addition have successfully passed the examinations required by section 8, may be designated Special Class valves. Pressure- -temperature ratings shall not exceed the values that are listed in Table 2-1.1 through Table 2- 3.19 with an identifying label “B – Special Class.” Special Class ratings shall not be used for flanged-end valves. 2.1.4 Limited Class Valves. Welding- or threaded-end valves NPS 2V/ and smaller that conform to the require- ments of Mandatory Appendix V may be designated Limited Class valves. Pressure-temperature ratings shall not exceed the values calculated in accordance with Mandatory Appendix V. Limited Class ratings shall not be used for flanged-end valves. 2.1.5 Intermediate Rated Valves. A Standard Class or Special Class welding- or threaded-end valve or a Standard Class 400 flanged-end valve may be assigned an inter- mediate pressure-temperature rating or Class in accor- dance with para. 6.1.4, provided all other applicable requirements of this Standard are met.
2.3.3 Fluid Thermal Expansion. Under certain condi- tions, some double-seated valve designs are capable of sealing simultaneously against pressure differential from the center cavity to the adjacent pipe in both direc- tions. A circumstance in which the center cavity is filled or partially filled with liquid and subjected to an increase in temperature can result in an excessive buildup of pressure in the center cavity that may lead to pressure boundary failure. An example is a piping system in which liquid from the condensing, cleaning, or testing fluids accumulates in the center cavity of a closed valve. Such accumulation may result from leakage past the upstream seat of the valve. If, during subsequent startup, the valve is not relieved of the liquid by partial opening of the valve or by some other method, the retained liquid may be heated during warm-up of the system. Where such a condition is possible, it is the responsibility of the user to provide, or require to be provided, means in design, installation, or operation procedure to assure that the pressure in the valve will not exceed that allowed by this Standard for the attained temperature.