tailieunhanh - Mechanical Science HandbooksMechanical Science Handbooks 20000 Part 5

Tham khảo tài liệu 'mechanical science handbooksmechanical science handbooks 20000 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Valves DOE-HDBK-1018 2-93 TYPES OF VALVES Stem seals are accomplished either with packing in a conventional stuffing box or by means of O-ring seals. Some valve manufacturers particularly those specializing in the handling of corrosive materials place a stem seal on the inside of the valve so that no material being handled by the valve can come into contact with the valve stem. If a stuffing box or external O-ring is employed the fluid passing through the valve will come into contact with the valve stem. Needle Valves A needle valve as shown in Figure 20 is used to make relatively fine adjustments in the amount of fluid flow. The distinguishing characteristic of a needle valve is the long tapered needlelike point on the end of the valve stem. This needle acts as a disk. The longer part of the needle is smaller than the orifice in the valve seat and passes through the orifice before the needle seats. This arrangement permits a very gradual increase or decrease in the size of the opening. Needle valves are often used as component parts of other more complicated valves. For example they are used in some types of reducing valves. Needle Valve Applications Most constant pressure pump governors have needle valves to minimize the effects of fluctuations in pump discharge pressure. Needle valves are also used in some components of automatic combustion control systems where very precise flow regulation is necessary. STEM POINT Figure 20 Needle Valve Rev. 0 Page 33 ME-04 TYPES OF VALVES DOE-HDBK-1018 2-93 Valves Needle Valve Body Designs One type of body design for a needle valve is the bar stock body. Bar stock bodies are common and in globe types a ball swiveling in the stem provides the necessary rotation for seating without damage. The bar stock body is illustrated in Figure 21. Figure 21 Bar-Stock Instrument Valve Needle valves are frequently used as metering valves. Metering valves are used for extremely fine flow control. The thin disk or orifice allows for linear .