Basic knowledge of globe valve
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Globe valves are now used everywhere and have been a fixture in the field of fluid control for 200 years. The main purpose of the globe valve is to regulate the flow of fluid, however, in some applications, the globe valve design can also handle the complete cut-off of the fluid. The valves that commonly dot the exterior of houses and commercial buildings are a good example of the use of shut-off valves for opening/closing and regulating.
The lifeblood of the Industrial Revolution was steam and water, but these potentially powerful fluids needed to be controlled. The first valve required to do this job efficiently is the stop valve. The design was so effective and popular that the first patents granted to most of the major traditional valve manufacturers (Crane, Powell, Lunkenheimer, Chapman, and Jenkins) were for the globe valve design.
Although the globe valve can be used as a cut-off or isolation valve, it is designed to be partially opened to regulate flow when regulated, while the gate valve is designed to be used in a fully open or fully closed position. When considering the shut-off valve of an on-off valve isolating an operating valve, attention should be paid to the design choice, as it is difficult to maintain a tight seal under the strong thrust action on the disc. When the fluid flows down from the top, the force of the fluid assists in achieving a positive seal and makes it easier to achieve a seal.
The regulating function of the globe valve makes it ideal for control valve applications, where the positioners and actuators attached to the globe valve cover and stem make very precise regulation possible. In these applications, they are referred to as the "ultimate control element" and provide superb service in a variety of fluid control applications.
Indirect flow path
The Globe (English for globe valve) is named for its original circular design, which still hides its unique and somewhat zigzag flow channels. Unlike a fully open gate or ball valve, a fully open globe valve still creates a lot of friction or resistance to fluid flow, and its upper and lower channels are zigzag. This angled flow creates fluid friction, which slows down the flow through the valve.
The flow through the valve is measured by its flow coefficient and is called "Cv". Cv will be very different for gate and globe valves of the same size because the gate valve has little resistance to flow when it is in the open position.
The shut-off mechanism is called the disc or plug and can be machined into many different shapes. By changing the shape of the disc, when the valve is opened, the flow rate through the valve can vary greatly depending on the number of turns of the stem. The more common or "traditional" curved disc design is used in most applications because it is better suited to the given movement (rotation) of the stem than other designs. The V-port disc is designed for fine throttling through various opening percentages and is suitable for all sizes of globe valves. The pin type is designed for absolute flow regulation, but is usually only suitable for smaller sizes. When absolute closure is required, a soft elastic insert can be installed in the disc or seat.
Stop valve internals
In a globe valve, the component that provides the actual component closure to the component is called the spool. The components that make up the globe valve internals include the seat, disc, stem, backseat, and sometimes the hardware that connects the stem to the disc. Correct valve internals design and material selection are critical to the proper operation and life of any valve, especially the globe valve due to its high fluid friction and spiral flow path. As the seat and disc move closer, their speed and turbulence increase. This increase in speed, combined with the corrosive nature of the fluid, can cause damage to the valve internals, resulting in severe leakage of the valve in the closed position. Sometimes, the defect is something that looks like a sheet on the seat or disc, which is called drawing. If not repaired in time, the initial small leak path can widen and become a serious leak.
On smaller bronze globe valves, the spool is usually made of the same material as the body, or in some cases a similar bronze alloy of higher strength. On cast iron globe valves, the most common spool material is bronze. The name of this iron valve internals is "IBBM", which stands for "iron body, bronze mounted". Steel valves have a variety of internal materials, usually one or more internal parts are 400 series martensitic stainless steel. Hard surfaces such as tungsten-chromium-cobalt are also used, as well as 300 series stainless steel and copper-nickel alloys such as Monel.
There are three basic modes of globe valves. The most common is the "T" type, where the valve stem is perpendicular to the pipe flow.
Angle valves are similar to T-valves, but the flow in the valve is rotated 90 degrees so that the Angle stop valve is used as both a flow control device and a 90 degree pipe bend. Angle shut-off valves are still commonly used on boiler tops and are also the type of final output regulator on oil and gas "trees"
The third design, the "Y" design, is designed to reduce turbulence occurring in the globe valve body and provide a more rigorous design for on/off applications. In this type of globe valve, the cover, stem, and disc are tilted at an Angle of 30-45 degrees, resulting in a straighter flow path and less fluid friction. This reduced friction also means less potentially invasive damage to valves and better overall flow characteristics of the piping system.
Valve cover design varies by application
Globe valves are available in a variety of cap designs, each with its own location and purpose. For small bronze valves, the internally threaded open rod design is the most popular. In this design, the stem threads are contained within the pressure/fluid housing of the valve cover. This design is easy to manufacture, but has a disadvantage - the threads are exposed to process fluids. This means that critical stem threads can be damaged if exposed to corrosive liquids or gases. Usually, this valve is used for water or low pressure steam, so this is not a problem.
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For valves larger than NPS 2, or when the valve is in a corrosive environment, a cap design with stem threads outside the pressure/fluid housing is preferred. The most common of these designs is called the external thread and yoke, also known as OS&Y. This is the main design of large industrial globe valves.
Body/cover connections on globe valves are available in a variety of configurations. Both threaded and loose joint caps are located on smaller bronze valves, while bolted caps are located on most steel and iron stop valves. Pressure seal valve covers for high pressure, high temperature globe valves, such as Y design.
Stop check valve
Shut-off valves have been installed on boilers for more than 150 years. The first valve on the boiler output line is usually the stop valve, also known as the stop valve or the boiler stop valve. The stop check valve is actually a two-in-one valve: a stop valve for regulating flow and a check valve for preventing backflow. To achieve the check function, the disc on the check valve is not connected to the stem, but is guided in the valve cover, allowing it to move freely up and down when the stem is raised. This regulates the flow rate, but when backflow occurs, the disconnected disc acts as a piston check valve and quickly closes to prevent counterflow into the boiler. If tighter closing is required, the check stem can be lowered by closing the stem, preventing the disc from moving to the fully open position. In addition to regulating boiler output, stop valves are also used in other applications, where the combination of check and stop valves makes sense for pipe designers.
Large diameter globe valve problem
Large globe valves may face challenges during the seat testing phase of hydrostatic testing due to some test requirements. When the cover of the globe valve is bent, problems occur because the hydrostatic test pressure is transmitted from the area under the disc to the stem, yoke, and then to the cover. Although globe valves are designed primarily to operate in a partially open position to regulate flow, in some test standards they are required to pass a strict hydrostatic seat test of 110% of their rated operating pressure.
The problem is the lack of stiffness in the caps of these low pressure large globe valves. In most cases, wall thickness requirements come from American National Standards Institute (ANSI) B16.34 "Valves - Flanged, Threaded, and Welded Ends," but this is usually not enough to prevent bonnet bending. Most basic globe valve designs and patterns were created when the American Petroleum Institute (API) 598 requirement for a globe valve "Valve Check and Test" seat test was only 90 psi air. Prior to the release of API 598, sixth Edition "Valve Testing and Inspection" in 1990, the problem did not exist when 90-psi air low pressure testing was a required test procedure for globe valve seat testing.
On large low-pressure valves, as the pressure increases, this can cause the disc to lift several thousandths of an inch from the seat, leading to leaks. The simple solution is to increase thrust (tighten) as the stem rises. This usually solves the problem, but the initial closing torque is usually exceeded during this operation. Due to the flexibility of the valve cover, large diameter manual stop valves may need to be retightened twice when used for switching service. Re-twisting may also be required during hydraulic tests.
To address this issue without increasing the thickness or ribbed of the valve cover, many manufacturers will offer hammer handwheels, manual helical gear operators, or electric actuators as standard with these large valves. The use of these methods usually works well, but do mask the actual force applied when closing the valve.
API Globe Valve Standard -API 623
API created a relatively new globe valve standard, API 623 "Steel Globe Valves - Flanged and butt-welded ends, bolted cap", requiring a wall thickness greater than ANSI B16.34, similar to the wall thickness in API 600. The additional thickness requirements are designed to address potential corrosion and erosion issues that often occur in petrochemical and refining applications, but this additional thickness also helps address bonnet flexibility issues that may arise in hydrostatic testing and high pressure closure applications.
Another point mentioned in the API623 document is that when the stem construction uses certain corrosion resistant alloys, a stronger stem is required to provide a tight seal. In highly corrosive fluid applications, many austenitic stainless steel materials, such as 316 stainless steel, are selected for the stem. However, these materials are sometimes much weaker than the 410 stainless steel stem usually specified as standard. This requirement results in the valve stem diameter in the file being significantly larger than the non-API 623 stop valve stem.
While reliable globe valves are still produced in large quantities around the world, newer quarter-turn designs such as butterfly valves and ball valves account for most of the previous market share. However, in many applications, shut-off valves are still top of the other designs, so the future is still bright for these long-favored flow control industries.
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