Globe Valve Working Principle

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.

Standard Pattern or T – Pattern

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

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 or Y – Pattern

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.

Conventional Disc or Ball Disc

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.

Composition Disc

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.

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Plug Type 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.

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Inside and Outside Stem Screw

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.

Advantages

• Good shutoff capability.
• Moderate to good throttling capability.
• Shorter stem travel between the open and closed positions as compared to a gate valve.
• Available in many patterns, each offering unique capabilities.
• Easy maintenance, as the discs and seats are readily refurbished or replaced.

Disadvantages

• High  Pressure Drop or Head  Loss  from  two  or  more  right  angle  turns  of flowing  fluid.
• Large size glove valves require considerable power to operate and are especially noisy in high pressure applications.
• Large openings necessary  for  disk assembly.
• Heavier weight than other valves of the same flow rating.