working of globe valve – What Is Piping

A globe valve uses a linear motion for moving a disc (closure member) into or out of the seating surface. Globe Valves form a globular-shaped cavity around the port region and hence the name. The disc can be of various shapes and moves perpendicular to the globe valve seat. This disc movement helps in the valve opening and closure. Globe valves in general are used for pipes of size 8 inches or less. In this article, we will explore the globe valve types, parts, working principle, function, symbol, application, advantages, and disadvantages. The opening of the valve varies in direct proportion to the travel of the disc. Such a proportional relationship is suitable for various duties including flow regulation. In addition, depending on the design of the seat and disc, the seating load of globe valves can be positively controlled by a screwed stem. The sealing capacity of the globe valve is high. They can be used for on-off duty if the ﬂow resistance from the tortuous ﬂow passage of these valves is acceptable. Some globe valves are also specifically designed for low ﬂow resistance for use in on-off duty. Because of the short travel distance of the disc between the open and closed positions, globe valves are ideal if the valve has to be opened and closed frequently. Thus, globe valves can be used for a wide range of duties. Globe Valve Parts A typical globe valve constitutes of the following parts: Valve Body (6): Globular shapeBonnet (7)Disc or Plug (10)Stem (9)YokeGland bushing and flange (3)Seat ring (11)Yoke Sleeve (2)Gland Bolts and Nuts (4)Handwheel (1): Mechanical ActuatorBack SeatPressure Seal GasketPacking Typical Globe Valve Types of Globe Valve To meet the varied range of work duties at the lowest cost, numerous variations of globe valves are designed. The three most common types of globe valve configurations available for industrial use are Tee Pattern (T-Type) Globe valves possess the lowest flow coefficient and a higher pressure drop. In severe throttling services, such globe valves can be used. If the pressure drop is not a concern and throttling is required, the Tee type globe valve will be a good choice. Wye Pattern Globe valves can be cracked open for long periods without severe erosion. They offer very low resistance to flow. For start-up operations, Y-type globe valves are extensively used. Angle Pattern Globe valves turn the fluid flow in a perpendicular direction without the use of an elbow. For pulsating flow services, angle type globe valves are used because of their capability of handling the slugging effect. Globe Valve Patterns Globe Valve Seating Globe Valves are provided with either metal seat or soft seat arrangements. To keep the seating stresses uniform, a number of variations of seating design are made. The following figure shows the most frequently employed seating configurations in globe valves. Common Seating Configurations in Globe Valve Function of Globe Valve The main function of a Globe valve is to start, stop, or regulate the flow. As the globe valve opens the disk moves upwards allowing the fluid to flow proportionally. In an open position, the valve stem rises and popped out at the top. Globe Valve is normally used in smaller sizes; mostly 12 inches or less in sizes. With an increase in the globe valve size, the power required to operate increases. In normal practice, a globe valve works as an on-off valve, but it can be used for throttling purposes as well. Working Principle of Globe Valve Refer to the cross-sectional image of the globe valve shown below. As can be seen from the image, when the handwheel is rotated manually or by an actuator, the disc movement is controlled (lowered or raised). When the globe valve disc seats over the seat ring, the flow is completely stopped. Globe Valve Working But when the disk is lifted the flow begins as the upward movement of the disc creates space for the fluid. Globe Valve Disks The position of the globe valve disk and seat can be described schematically as follows: Globe Valve Disc position The Disc of a globe valve is made in three basic designs; Conventional Disc (or Ball Disc), Composition Disc, and Plug Type Disc. Ball Disk: In the conventional type disc, the seat and disc construction is made of a ball-shaped metal disc. This possesses a short taper and it easily fits against a flat-surfaced seat in the body. For low-pressure services, this type of globe valve is quite popular and economical. Even though the ball disc is capable of throttling flow, but still primarily used to start and stop the flow. Composition Disk: A hard, nonmetallic insert ring is used on the disc of a composition disc design to create a tighter closure. For high temperature and pressure applications, composition disks are used due to their sufficient resilience and erosion resistance. The composition disc globe valve is basically an improved version of the conventional disk type globe valve. Composition discs can be replaced or repaired. Types of Disks of a Globe Valve Plug Disk: The plug disc is the best among all three globe valve disk designs and is suitable for throttling services. A long tapered metal plug is made into a plug disc that seats into a cone generating a wide seating surface. Globe Valve Flow Direction For low-temperature applications, the flow direction in a globe valve is installed so that pressure acts under the disk. This design helps in easy operation, protects the packing, and eliminates a certain amount of erosive action to the seat and disk faces. Whereas, for high-temperature steam service, globe valves are positioned such that the pressure is above the disk. This will avoid the stem contractionupon cooling that may tend to lift the disk off the seat. Globe Valve Symbol The following globe valve symbol is quite popular in piping industry. These are used in P&IDs to differentiate a glove valve from other valve types. Glove Valve Symbol The following image shows the typical globe valve symbol used to denote a by-pass valve in control station. Globe Valve in Control Valve By-pass Globe Valve Diagram The following images shows typical globe valve diagrams. Different globe valve diagrams are used to show different globe valve types. Z type globe valve diagram Y type globe valve diagram Angle type globe valve diagram Advantages of Globe Valve The main advantages of globe valves can be summarized as follows: Good sealing capability.Moderate to good throttling capability.Shorter stroke. Provide a wide range of capabilities as available in tee, Wye, and angle body styles.Machining and resurfacing of the seats are easy.The globe valve can be used as a stop-check valve by modifying the design slightly. Disadvantages of Glove Valve The globe valve disadvantages can be listed as follows: Higher pressure drops due to many directional changes.Greater force or power requirement to seat the valve.Throttling flow under the seat and shutoff flow over the seat. Applications of Globe Valve Globe valve can be used in a wide range of services; both low pressure and high pressure fluid services. The typical applications of globe valves are: Cooling water systems requiring flow regulation.Fuel oil system requiring leak-tightness.Control valve bypass systems.High-point vents and low-point drains.Oil and Gas, Feedwater, chemical feed, Refinery, condenser air extraction, and extraction drain systems.Boiler vents and drains, Steam services, main steam vents and drains, and heater drains.Turbine seals and drains. Globe Valve vs Gate Valve The major differences between globe valve and gate valve is listed below: Globe ValveGate ValveGlobe Valve construction in complex, major internal components are inside the body cavity.Gate Valve construction is simpler, with majority of parts on top of valve body.High Pressure DropLow Pressure Drop.Globe Valve is unidirectional.Gate Valve is a multidirectional valve.Globe Valve is used for Flow Control.Gate Valve is used only for Isolation.The disk of globe valve moves parallel to the flowing medium.Gate valve disk moves perpendicular to the fluid flow.Globe Valve offers more resitance to fluid flow.Gate Valve offers little resistance to fluid flow.Weight of same size and rating Globe Valve is more as compared to gate valve.Gate valve weight is comparatively lesser.Globe valve is Costlier due to complex designComperatively cheaperGlobe valves are typically not used for sizes greater than 12 inch Gate Valves can be used for larger sizes as well.Globe Valve vs Gate Valve Globe Valve vs Ball Valve The following table lists the major differences between a globe valve and a ball valve. Globe ValveBall ValveHigh Pressure DropLow Pressure DropGlobe Valve uses a disk that operates against the flow.Ball valve uses a ball that closes across the flow.Globe Valve is a linear motion valveBall valve is a rotary motion valve.Globe valve provides better control than ball valveThe ball valve provides better shut-off properties than the globe valve.Globe Valve is cheaper than ball valvesBall valve is more complex in design than globe valve and hence costlier.Slow OperationQuick shut off, the possibility of water hammer.Globe Valve vs Ball valve Globe Valve Standards Globe valve manufacturers use the following codes and standards for designing globe valves for industrial applications: API Standards for Globe Valves: API 623 - Steel Globe Valves-Flanged and Butt-welding Ends, Bolted BonnetsAPI 602–Steel Gate, Globe and Check Valves for Sizes DN 100 and Smaller for the Petroleum and Natural Gas IndustriesAPI 6D - Specification for Pipeline valvesAPI 598 - Valve Inspection and TestingAPI 6FA - Fire Test for Valves BS Standards for Globe Valves: BS 1873 - Steel globe valves and stop and check valves, flanged and butt welding endsBS 5152 - Cast Iron globe and globe stop and check valvesBS 5352 - Cast and forged steel gate, globe, check and plug valves, screwed and socket welding ends MSS Standards for Globe valves: MSS SP 80 - Bronze globe valvesMSS SP-85 Cast iron globe and angle valves, flanged and threaded endsMSS SP 61 - Pressure testing of steel valvesMSS SP 25 - Standard marking system for valves, fittings, flanges & unionsMSS SP 45 - Bypass and drain connectionsMSS SP 42 - Class 150 (PN 20) Corrosion Resistant Gate, Globe, Angle, and Check Valves With Flanged and Butt Weld Ends