Classification of Valves
Whether you are searching for "classification of valves" or "valves" the following are some of the commonly used valve classifications:
Classification Based on Mechanical Motion
Based on the mechanical or cyclical motion of the valve closure member, valves are classified as follows:
Linear Motion Valves. The valves in which the closure member, as in gate, globe, diaphragm, pinch, and lift check valves, moves in a straight line to allow, stop, or throttle the flow. Table A10.1 lists the valves based on motion of valve-closure member.
Rotary Motion Valves. When the valve-closure member travels along an angular or circular path, as in butterfly, ball, plug, eccentric and swing check valves, the valves are called rotary motion valves. See Table A10.1.
Quarter Turn Valves. Some rotary motion valves require approximately a quarter turn, 0 through 90°, motion of the stem to go to fully open from a fully closed position or vice versa. Refer to Table A10.1.
Classification Based on Valve Size
Valve Size. Valve size is denoted by the nominal pipe size (NPS), which is equal to the size of valve-connecting ends or the flange-end size. In the metric system, valve size is designated by the nominal diameter (DN) of connecting pipe or the connecting flange ends. When a valve is installed with reducers on each end, the size of the valve will be equal to the size of the reducer-connecting ends attached to the valve. The valve size is not necessarily equal to the inside diameter of the valve.
It is a normal industry practice to categorize valves, based upon size, in two classification: small and large.
Small Valves. NPS 2 (DN 50) and smaller valves are called small valves. At times, NPS 2¹⁄₂ (DN 65) and smaller valves are referred to as small valves. As such, the size classification can vary and, therefore, it should not be considered a uniform industry practice.
Large Valves. NPS 2¹⁄₂ (DN 65) and larger valves are classified as large valves. As indicated earlier, NPS 2¹⁄₂ (DN 65) valves may be designated as small valves, depending on the criteria used in classifying small valves.
Classification Based on Pressure-Temperature Rating
Class Ratings. Pressure-temperature ratings of valves are designated by class numbers. Based on the material(s) of construction, the pressure-temperature ratings for each class are tabulated to provide the maximum allowable working pressures, expressed as gauge pressures, at the temperature shown. The temperature shown for a corresponding pressure rating is the temperature of the pressure-containing shell of the component. Items such as a piping system or a portion thereof, a pump, tank, heat exchanger, pressure vessel, valves, et cetera are considered components. ASME B16.34, Valves—Flanged, Threaded, and Welding End is one of the most widely used valve standards. It defines three types of classes: standard, special, and limited. ASME B16.34 covers Class 150, 300, 400, 600, 900, 1500, 2500, and 4500 valves. It also allows valves to be classified as intermediate whose pressure- temperature ratings may fall within those listed for the standard and special class valves. See Table A10.2 for valve classifications and their limitations covered by ASME B16.34. Refer to Table A1.2 in Chapter A1 for metric equivalent (PN) of valve classifications. Tables A10.3 and A10.4 provide a listing of valve classes covered by various commonly used valve standards published by the Manufacturers Standardization Society (MSS) and the American Petroleum Institute (API). Table A10.5 provides a brief summary of valves covered by AWWA standards.
Cold Working Pressure (CWP) Rating. Valves are also rated by the CWP rating. This rating represents the maximum allowable working pressure at the ambient temperature to which the valve may be subjected in normal service. Sometimes it is referred to as cold rating. The pressure rating of a valve at —20 to 100°F (—29 to 38°C), as listed in ASME B16.34 and other valve standards, is considered cold working pressure (CWP) or cold rating. CWP is also designated as the water-oil-gas (WOG) rating. Valves marked with CWP or WOG rating are primarily intended for applications in which the flow medium is maintained at ambient temperature, such as, but not limited to, water, oil, and gas distribution and transmission systems.
General Service Rating. The general service rating may be considered to be the equivalent of the CWP rating. Some valve manufacturers assign two ratings, fire rating and general service rating, to a valve. A UL-listed and FM-approved valve rated for 175 psig (1210 kPa) may have a general service rating of 400 psig (2760 kPa) at ambient temperature. It means that this valve can be used in a fire protection system having a maximum allowable working pressure of 175 psi (1210 kPa) or less, whereas it can be used in other services termed as general services and be subjected to a maximum allowable working pressure of 400 psig (2760 kPa). One must classify the service in accordance with the code of construction, as required. If the service does not fall within the jurisdiction of a code, then prudent engineering judgment may be followed in selecting the valve.
Steam Working Pressure (SWP) Rating. The SWP rating of a valve is intended to define the maximum working pressure corresponding to the steam temperature. A valve assigned a SWP rating must not be used in steam service at pressures and temperatures exceeding the rating.
Dual or Multiple Ratings. A valve may be assigned one or more ratings by the valve manufacturer. Ratings assigned must be marked on the valve. The marking on the valve must be in accordance with the applicable valve standard or standards. A valve may comply with one or more valve standards. For example, a Class 600, NPS 4 (DN 100), butt-welding end, steel gate valve complying with ASME B16.34 may be marked as Class 800 valve in accordance with API Standard 603, provided the valve complies with the design and construction requirements of both ASME B16.34 and API 603. Some valve manufacturers may have proprietary design valves that are rated for specialty applications. Dual- or multiple-rated valves may be used within the pressure-temperature rating(s) conforming to the valve standard referenced in the code of jurisdiction under which the system is designed and constructed. Dual- or multiple-rated valves have increased utility and broad market. The manufacturer benefits from reduced design and production costs.
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