Single or double valves at a classification change should be suitable for the more severe line classification on either side of the valve(s) location. At a line classification break, a single block valve may be exposed to both service conditions; therefore, it must be adequate for the more severe of the two services. In the case of a double block valve installation at a line service classification change, it is possible for either valve to be closed against the more severe service condition; therefore, both valves must be designed for it.
Valves around control valves (upstream, downstream, and bypass) should be of the same Class rating as the most severe line classification.
Fugitive emissions usually account for about half of all VOC and toxic air emissions. Valves account for about half of fugitive emissions. Many operating locations require limits on the emissions and also on the allowable leak rate for valves. Purchasing low leak rate valves helps meet the required limits more cost effectively by reducing maintenance (e.g. packing replacement and repeated gland tightening).
Gate and globe valves should have flanged ends, bolted bonnet, and should be outside screw and yoke.