Globe Valve

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Globe Valve

What is a Globe Valve

Introduction of Globe Valves

As their name suggests, globe valves are linear motion valves with rounded globular shaped bodies. Since their shape is similar to other valve bodies, positive identification must be made based on internal piping. Recently globe valves have lost their traditional round body-shape. Globe valves have many advantages and disadvantages for users. They have excellent and precise throttling ability for high-pressure systems. The disadvantages include low-flow coefficients and a longer operating time because the operator must turn the handle and stem many times to ensure the valve is completely open or completely closed. Globe valves can be used in systems that require frequent stroking, vacuum, and systems that have a wide range of temperature extremes

A Globe valves is a linear motion valve and are primarily designed to stop, start and regulate flow. The disk of a Globe valve can be totally removed from the flowpath or it can completely close the flowpath.

Conventional Globe valves may be used for isolation and throttling services. Although these valves exhibit slightly higher pressure drops than straight=through valves (e.g., gate, plug, ball, etc.), they may be used where the pressure drop through the valve is not a controlling factor.

Because the entire system pressure exerted on the disc is transferred to the valve stem, the practical size limit for these valves is NPS 12 (DN 300). Globe valves larger than NPS 12 (DN 300) are an exception rather than the rule. Larger valves would require that enormous forces be exerted on the stem to open or close the valve under pressure. Globe valves in sizes up to NPS 48 (DN 1200) have been manufactured and used.

Globe valves are extensively employed to control flow. The range of flow control, pressure drop, and duty must be considered in the design of the valve to avert premature failure and to assure satisfactory service. Valves subjected to high-differential pressure-throttling service require specially designed valve trim.

Generally the maximum differential pressure across the valve disc should not exceed 20 percent of the maximum upstream pressure or 200 psi (1380 kPa), whichever is less. Valves with special trim may be designed for applications exceeding these differential pressure limits.

Types of Globe Valve

Body Designs of Globe Valves

There are three primary body designs for Globe valves, namely:

  • Tee Pattern or Z-body

  • Angle Pattern

  • Wye Pattern or Y-body body

Tee Pattern  or Z-shaped Globe Valve Design

Tee Pattern Globe valve design is the most common body type, with a Z-shaped diaphragm. The horizontal setting of the seat allows the stem and disk to travel perpendicular to the horizontal line. This design has the lowest coefficient of flow and higher pressure drop. They are used in severe throttling services, such as in bypass lines around a control valve. Tee-pattern Globe valves may also be used in applications where pressure drop is not a concern and throttling is required.

Angle Body Globe Valve

Angle Pattern Globe valves design is a modification of the basic Tee 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 wye-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.

Wye Pattern Globe valves

Wye Pattern Globe valves design, is an alternative for the high pressure drop, inherent in Globe valves. Seat and stem are angled at approximately 45 degrees, what gives a straighter flowpath 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.

 

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Typical Applications of Globe valves

The following are some of the typical applications of Globe valves:

  • Cooling water systems where flow needs to be regulated

  • Fuel oil system where flow is regulated and leaktightness is of importance

  • High-point vents and low-point drains when leaktightness and safety are major considerations

  • Feedwater, chemical feed, condenser air extraction, and extraction drain systems

  • Boiler vents and drains, main steam vents and drains, and heater drains

  • Turbine seals and drains

  • Turbine lube oil system and others

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A globe valve, different from ball valve, is a type of valve used for regulating flow in a pipeline, consisting of a movable disk-type element and a stationary ring seat in a generally spherical body.

Globe valves are named for their spherical body shape with the two halves of the body being separated by an internal baffle. This has an opening that forms a seat onto which a movable plug can be screwed in to close (or shut) the valve. The plug is also called a disc ordisk. In globe valves, the plug is connected to a stem which is operated by screw action using a handwheel in manual valves. Typically, automated globe valves use smooth stems rather than threaded and are opened and closed by an actuator assembly.

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Working of a Globe Valve

Globe valves usually have rising stems, and the larger sizes are of the outside screw-and-yoke construction. Components of the Globe valve are similar to those of the gate valve. This type of valve has seats in a plane parallel or inclined to the line of flow.

Maintenance of Globe valves is relatively easy, as the discs and seats are readily refurbished or replaced. This makes Globe valves particularly suitable for services which require frequent valve maintenance. Where valves are operated manually, the shorter disc travel offers advantages in saving operator time, especially if the valves are adjusted frequently.

The principal variation in Globe-valve design is in the types of discs employed. Plug-type discs have a long, tapered configuration with a wide bearing surface. This type of seat provides maximum resistance to the erosive action of the fluid stream. In the composition disc, the disc has a flat face that is pressed against the seat opening like a cap. This type of seat arrangement is not as suitable for high differential pressure throttling.

In cast-iron Globe valves, disc and seat rings are usually made of bronze. In steel-Globe valves for temperature up to 750°F (399°C), the trim is generally made of stainless steel and so provides resistance to seizing and galling. The mating faces are normally heat-treated to obtain differential hardness values. Other trim materials, including cobalt-based alloys, are also used.

The seating surface is ground to ensure full-bearing surface contact when the valve is closed. For lower pressure classes, alignment is maintained by a long disc locknut. For higher pressures, disc guides are cast into the valve body. The disc turns freely on the stem to prevent galling of the disc face and seat ring. The stem bears against a hardened thrust plate, eliminating galling of the stem and disc at the point of contact.

Flow direction of Globe valves

For applications with low temperature, Globe valves are normally installed so that the pressure is under the disc. This contributes an easy operation and helps protect the packing. For applications with high temperature steam service, Globe valves are installed so that the pressure is above the disk. Otherwise, the stem will contract upon cooling and tend to lift the disk off the seat.

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Globe Valve Parts

  • Body of Globe Valve

  • Bonnet of Globe Valve

  • Plug or Disk of Globe Valve

  • Cage of Globe Valve

  • Seat of Globe Valve

  • Stem of Globe Valve

Body

The body is the main pressure containing structure of the valve and the most easily identified as it forms the mass of the valve. It contains all of the valve's internal parts that will come in contact with the substance being controlled by the valve. The bonnet is connected to the body and provides the containment of the fluid, gas, or slurry that is being controlled.

Globe valves are typically two-port valves, although three port valves are also produced mostly in straight-flow configuration. Ports are openings in the body for fluid flowing in or out. The two ports may be oriented straight across from each other or anywhere on the body,or oriented at an angle (such as a 90°). Globe valves with ports at such an angle are called angle globe valves. Globe valves are mainly used for corrosive or high viscous fluids which solidify at room temperature. This is because straight valves are designed so that the outlet pipe is in line with the inlet pipe and the fluid has a good chance of staying there in the case of horizontal piping. In the case of angle valves, the outlet pipe is directed towards the bottom. This allows the fluid to drain off. In turn, this prevents clogging and/or corrosion of the valve components over a period of time. A globe valve can also have a body in the shape of a "Y". This will allow the construction of the valve to be straight at the bottom as opposed to the conventional pot type construction (to arrange bottom seat) in case of other valves. This will again allow the fluid to pass through without difficulty and minimizes fluid clogging/corrosion in the long term.

Bonnet

The bonnet provides a leakproof closure for the valve body. The threaded section of the stem goes through a hole with matching threads in the bonnet. Globe valves may have a screw-in, union, or bolted bonnet. Screw-in bonnet is the simplest bonnet, offering a durable, pressure-tight seal. Union bonnet is suitable for applications requiring frequent inspection or cleaning. It also gives the body added strength. A bonnet attached with bolts is used for larger or higher pressure applications. The bonnet also contains the packing, a wearable material that maintains the seal between the bonnet and the stem during valve cycling.

Disk

The most common disk designs for Globe valves are: ball disk, composition disk and the plug disk. Ball disk design is used primarily in low pressure and low temperature systems. It is capable of throttling flow, but in principle it is applied to stop and start flow.Composition disk design uses a hard, non-metallic insert ring on the disk, which ensures a tighter closure. Plug disk design provides better throttling than ball or composition designs. They are available in many different designs and they are all long and tapered.

The closure member of the valve, plugs are connected to the stem which is slid or screwed up or down to throttle the flow. Plugs are typically of the balance or unbalanced type. Unbalanced plugs are solid and are used with smaller valves or with low pressure drops across the valve. The advantages are simpler design, with one possible leak path at the seat and usually lower cost. The disadvantages are the limited size; with a large unbalanced plug the forces needed to seat and hold the flow often becomes impractical. Balanced plugs have holes through the plug. Advantages include easier shut off as the plug does not have to overcome static forces. However, a second leak path is created between the plug and the cage, and cost is generally higher.

Cage

The cage is a part of the valve that surrounds the plug and is located inside the body of the valve. Typically, the cage is one of the greatest determiners of flow within the valve. As the plug is moved more of the openings in the cage are exposed and flow is increased and vice versa. The design and layout of the openings can have a large effect on flow of material (the flow characteristics of different materials at temperatures, pressures that are in a range). Cages are also used to guide the plug to the seat of the valve for a good shutoff, substituting the guiding from the bonnet.

Seat

Globe valve seats are either integrated or screwed in to the valve body. Many Globe valves have backseats inside the Bonnet. Back seats provides a seal between the stem and Bonnet and prevents system pressure from building against the valve pakking, when the valve is fully open. Back seats are often applied in Globe valves.

The seat ring provides a stable, uniform and replaceable shut off surface. Seat rings are usually held in place by pressure from the fastening of the bonnet to the top of the body. This pushes the cage down on the lip of the seat ring and holds it firmly to the body of the valve. Seat rings may also be threaded and screwed into a thread cut in the same area of the body. However this method makes removal of the seat ring during maintenance difficult if not impossible. Seat rings are also typically beveled at the seating surface to allow for some guiding during the final stages of closing the valve.

Economical globe valves or stop valves with a similar mechanism used in plumbing often have a rubber washer at the bottom of the disc for the seating surface, so that rubber can be compressed against the seat to form a leak-tight seal when shut.

Stem

Globe valves uses two methods for connecting the disk and the stem: the T-slot and the disk nut construction. In the T-slot design, the disk slides over the stem, while in the disk nut design, the disk is screwed into the stem.

The stem serves as a connector from the actuator to the inside of the valve and transmits this actuation force. Stems are either smooth for actuator controlled valves or threaded for manual valves. The smooth stems are surrounded by packing material to prevent leaking material from the valve. This packing is a wearable material and will have to be replaced during maintenance. With a smooth stem the ends are threaded to allow connection to the plug and the actuator. The stem must not only withstand a large amount of compression force during valve closure, but also have high tensile strength during valve opening. In addition, the stem must be very straight, or have low run out, in order to ensure good valve closure. This minimum run out also minimizes wear of the packing contained in the bonnet, which provides the seal against leakage. The stem may be provided with a shroud over the packing nut to prevent foreign bodies entering the packing material, which would accelerate wear.v

Advantages and Disadvantages of Globe Valves

Advantages

  • Good shutoff capability

  • Moderate to good throttling capability

  • Shorter stroke (compared to a gate valve)

  • Available in tee, wye, and angle patterns, each offering unique capabilities

  • Easy to machine or resurface the seats

  • With disc not attached to the stem, valve can be used as a stop-check valve

Disadvantages

  • Higher pressure drop (compared to a gate valve)

  • Requires greater force or a larger actuator to seat the valve (with pressure under the seat)

  • Throttling flow under the seat and shutoff flow over the seat

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