Globe valves are linear motion closing-down valves in which the closure member is moved squarely on and off the seat. Usually the closure member is referred as a disc, irrespective of its shape. The seat opening varies in direct proportion to the travel of the disc. This proportional relationship between valve opening and disc travel is ideally suited for duties involving regulation of flow rate. Globe valves are most suitable for throttling and controlling fluid flow and are generally employed in small size piping.
Globe valve design necessitates two changes in the direction of flow and this causes resistance in liquid lines and objectionable pressure drop. The installation of globe valves is made so that the flow is up through the seat ring and against the bottom of the disc. This prevents accumulation of dirt and debris above the disc.
Globe valves may be used for most duties encountered in fluid handling process piping systems. Although the globe valve can be used as a block or isolation valve, it is primarily designed to regulate flow in the partially open position, while gate valves are designed for either the fully opened or fully closed position. When considering globe valves for on-off isolation service, design selection should receive careful consideration because maintaining a tight seal against the strong force pushing up on the disc is difficult. The globe valve, compared to the gate valve, has a short stem travel between the open and closed positions, has relatively little wear and is easier to repair.
A globe valve is primarily designed to stop, start and regulate flow. It is comprised of a movable disk and a stationary ring seat in a generally spherical body. The seat of a globe valve is in the middle of and parallel to the pipe, and the opening in the seat is closed off with the disk. When the handle is turned, manually or through an actuator, a disc is lowered or raised by means of the valve stem. When the disc is fully lowered, the fluid flow is shut off. When the disc is fully raised, the fluid flow is at its maximum rate. When the disc is in raised to less then maximum level, the fluid flow is regulated with proportion to the vertical travel of the disc.
There are three primary body patterns or designs for Globe valves, namely:
Standard pattern globe valve body design is the most common body type. The horizontal setting of the seat allows the stem and disk to travel perpendicular to the flow direction of the fluid. Due to its tortuous flow passage this design offers highest resistance to flow among all of the available patterns. This design has the lowest coefficient of flow and highest pressure drop. They are used in severe throttling services, such as in bypass lines around a control valve. Standard pattern globe valves may also be used in applications where pressure drop is not a major concern and only throttling is required.
Angle pattern globe valve body design is a modification of the basic standard pattern globe valve. The ends of this Globe valve are at an angle of 90 degrees, and fluid flow occurs with a single 90 degrees turn. They have a slightly lower coefficient of flow than oblique pattern globe valves. They are used in applications that have periods of pulsating flow because of their capability to handle the slugging effect of this type of flow.
If the globe valve is to be mounted near a pipe bend, the angle pattern valve body offers two advantages. First, the angle pattern design has a greatly reduced flow resistance compared to the standard pattern design. Second, the angle pattern design reduces the number of pipe joints and saves a pipe elbow.
Oblique Pattern globe valve body design is an alternative for the high pressure drop, inherent in globe valves. Oblique pattern design reduces the flow resistance of the globe valve to a minimum. Seat and stem are angled at approximately 45 degrees, what gives a straighter flow-path at full opening and offer the least resistance to flow. They can be cracked open for long periods without severe erosion. They are extensively used for throttling during seasonal or startup operations. They can be rod through to remove debris when used in drain lines that are normally closed.
Globe valves are made in three basic disc designs, namely :
The conventional type disc is the earliest type of disc and seat construction and is made of a ball shaped metal disc having a short taper which fits against a flat-surfaced seat in the body. This type of globe valve is fairly cheap and popular in low pressure service where severe throttling is not required. It is capable of throttling flow, but is primarily used to stop and start flow. Such a valve preferably should be used wide open or fully closed with little modulation of flow since the short tapered disc is subject to severe erosion and wire drawing. The seat and disc surfaces are easily reground if they are not too badly damaged.
The composition disk design uses a hard, nonmetallic insert ring on the disk. The insert ring creates a tighter closure. Composition disks are primarily used in steam and hot water applications. They resist erosion and are sufficiently resilient to close on solid particles without damaging the valve. Composition disks are replaceable.
The composition disc valve is an improvement over the conventional or ball type disc for many services, but still is not suitable for throttling purposes. Various types of composition discs are available making this type adaptable to many different services. This valve is easily and quickly repairable and requires less power to seat tightly. Small particles or foreign matter are not likely to cause any damage as they will likely imbed themselves in the relatively soft disc.
The plug type or disc globe valve is the best of the three types for throttling and hard service. The disc is a long tapered metal plug seating into a cone that produces a wide seating surface. This surface is not easily affected by foreign matter or by wire drawing and gives full flow when the valve is wide open. The construction of this valve permits easy and quick replacement of seat and disc if required.
The stem of a globe valve may be designed to rotate while raising or lowering the disc, or be prevented from rotating while carrying out this task. These modes of stem operation have a bearing on the design of the disc-to-stem connection. Also disc can be an integral part of stem causing the disc to rotate with the stem or disc can be designed to swivel freely on the stem. Accordingly we have following disc – stem configurations;
Most globe valves incorporate a rotating stem because of simplicity of design. The disc is an integral component of the stem in this case and the seating’s will mate while the disc rotates, possibly resulting in severe wear of the seating’s. Therefore, the main field of application of such valves is for regulating duty with infrequent shut-off duty.
For all other duties involving rotating stems, the disc is designed to swivel freely on the stem. However, swivel discs should have minimum free axial play on the stem to prevent
the possibility of rapid axial movements of the disc on the stem in the near closed valve position. Also, if the disc is guided by the stem, there should be little lateral play between stem and disc to prevent the disc from landing on the seat in a cocked position.
In the case of non-rotating stems the disc may be either an integral part of the stem or a separate component from the stem. Non-rotating stems are required in valves with diaphragm or bellows valve stem seal. They are also used in high pressure valves to facilitate the incorporation of power operators.
The screw for raising or lowering the stem may be located inside the valve body or outside the valve body.
In Inside Stem Screw configuration, the threaded part of the stem is positioned inside the valve body, whereas the stem packing lays outside. With this design, the stem threads are in touch with the fluid flowing through the valve / piping. The inside screw permits an economical bonnet construction, but it has the disadvantage that it cannot be serviced from the outside. This construction is best suited for fluids that have good lubricity. For the majority of minor duties, however, the inside screw gives good service.
In Outside Stem Screw configuration, the external side of the stem is threaded while the part of the stem which is inside the valve is plain. The threads of the stem are isolated from the medium by the packing. This design keeps stem threads outside the body in order to avoid the damaging effects of high temperature, corrosives, and inline solids inside the valve. The outside screw can be serviced from the outside and is therefore preferred for severe duties. The only drawback is the packing, which is subject to wear because of the up and down movement of the stem, in addition to the turning motion.
Bonnets are other important components for the gate and globe type valves. By dismantling the bonnet, the maintenance staff has access to the internal mechanisms and can replace components like the seat, the stem, etc (trim). The Bonnet – Valve Body connection on globe valves comes in a variety of designs. Bonnets may be joined to the valve body by means of bolts, flanges, welding, or by means of a pressure-seal mechanism; or the bonnet may be an integral part of the valve body. Few of the bonnet – valve body connections are detailed out further.
The bolted (also referred as screwed-in) bonnet is one of the simplest and least expensive design. However, the bonnet gasket must accommodate itself to rotating faces, and frequent unscrewing of the bonnet may damage the joint faces. Also, the torque required to tighten the bonnet joint becomes very large for the higher size valves. For this reason, the use of bolted bonnets is normally restricted to valve sizes not greater than NPS 3.
If the bonnet is made of a weldable material, the bonnet connection may be made entirely by welding. Welded bonnets are not only economical but also most reliable irrespective of size, operating pressure and temperature. However a major drawback is that access to the valve internals can be gained only by removing the weld that may cause bonnet to distort. For this reason, welded bonnets are normally used only where the valve can be expected to remain maintenance-free for long periods, where the valve is a throw-away valve, or where the sealing reliability of the bonnet joint outweighs the difficulty of gaining access to the valve internals.
Flanged bonnet joints have the advantage over bolted joints in that the tightening effort can be spread over a number of bolts. Flanged joints may therefore be designed for any valve size and operating pressure. However, as the valve size and operating pressure increase, the flanged joint becomes increasingly heavy and bulky. Also, at temperatures above 350◦C (660◦F), creep relaxation can, in time, noticeably lower the bolt load. If the application is critical, the flanged joint may be seal welded.
The bonnet may also be held to the valve body by a separate screwed union ring. This construction has the advantage of preventing any motion between the joint faces as the joint is being tightened. Repeatedly unscrewing the bonnet, therefore, cannot readily harm the joint faces. As with the screwed-in bonnet, the use of bonnets with a screwed union ring is restricted to valve sizes normally not greater than DN 80 (NPS 3).
The pressure-seal bonnet overcomes weight disadvantage by letting the fluid pressure tighten the joint. The bonnet seal therefore becomes tighter as the fluid pressure increases. This construction principle is frequently preferred for large valves operating at high pressures and temperatures.
Globe valve have specific flow directions. Depending on the application, a globe valve will have fluid flow above or below the disc. Globe valves can be arranged so that the disk closes against or in the same direction of fluid flow. When the disk closes against the direction of fluid flow, the kinetic energy of the fluid impedes closing but aids opening of the valve. When the disk closes in the same direction of fluid flow, the kinetic energy of the fluid aids closing but impedes opening.
For low temperature and low pressure applications, globe valves are ordinarily installed so that pressure is under the disk. This promotes easy operation, helps protect the packing, and eliminates a certain amount of erosive action to the seat and disk faces.
For high temperature and high pressure applications, like steam, globe valves are installed so that pressure is above the disk. This helps to prevent the stem from contracting when it cools down and will keep the disc from lifting off the seat that may cause leakage. If the pressure on top of the disc is higher, a bypass valve may have to be provided that permits the downstream system to be pressurized before the globe valve is
opened.
Usually flow direction arrow is marked on the globe valve body for ease and simplification of installation purpose.
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