Globe Valves | Kickoff

Globe Valves

7 Jun

A globe valve is a linear motion valve used to stop, start, and regulate fluid flow. A Z-body globe valve is illustrated in Figure 9. As shown in Figure 9, the globe valve disk can be totally removed from the flowpath or it can completely close the flowpath. The essential principle of globe valve operation is the perpendicular movement of the disk away from the seat. This causes the annular space between the disk and seat ring to gradually close as the valve is closed. This characteristic gives the globe valve good throttling ability, which permits its use in regulating flow. Therefore, the globe valve may be used for both stopping and starting fluid flow and for regulating flow. When compared to a gate valve, a globe valve generally yields much less seat leakage. This is because the disk-to-seat ring contact is more at right angles, which permits the force of closing to tightly seat the disk.

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 flow, the kinetic energy of the fluid impedes closing but aids opening of the valve.  When the disk closes in the same direction of flow, the kinetic energy of the fluid aids closing but impedes opening.  This characteristic is preferable to other designs when quick-acting stop valves are necessary.

Globe valves also have drawbacks.  The most evident shortcoming of the simple globe valve is the high head loss from two or more right angle turns of flowing fluid. Obstructions and discontinuities in the flowpath lead to head loss.  In a large high pressure line, the fluid dynamic effects from pulsations, impacts, and pressure drops can damage trim, stem packing, and actuators.  In addition, large valve sizes require considerable power to operate and are especially noisy in high pressure applications.

Other drawbacks of globe valves are the large openings necessary for disk assembly, heavier weight than other valves of the same flow rating, and the cantilevered mounting of the disk to the stem.


Globe Valve Body Designs

The three primary body designs for globe valves are Z-body, Y-body, and Angle.

Z-Body Design

The simplest design and most common for water applications is the Z-body.  The Z-body is illustrated in Figure 9.For this body design, the Z-shaped diaphragm or partition across the globular body contains the seat.  The horizontal setting of the seat allows the stem and disk to travel at right angles to the pipe axis.   The stem passes through the bonnet which is attached to a large opening at the top of the valve body.  This provides a symmetrical form that simplifies manufacture, installation, and repair.

Y-Body Design

Figure 10 illustrates a typical Y-body   globe  valve.This design is a remedy for the high pressure drop inherent in globe valves.  The seat and stem are angled  at  approximately  45 . The angle yields a straighter flowpath (at full opening) and provides the stem, bonnet, and packing a relatively pressure- resistant envelope.

Y-body globe valves are best suited for high pressure and other severe services. In small sizes for intermittent flows, the pressure loss may not be as important as the other considerations favoring the Y-body design. Hence, the flow passage of small Y-body globe valves is not as carefully streamlined as that for larger valves.

Angle Valve Design

The angle body globe valve design, illustrated in Figure 11, is a simple modification of the basic  globe  valve. Having  ends  at  right angles, the diaphragm can be a simple flat plate.  Fluid is able to flow through with only a  single  90  turn  and  discharge  downward more symmetrically than the discharge from an ordinary globe.  A particular advantage of the angle body design is that it can function as both a valve and a piping elbow.

For moderate conditions of pressure, temperature, and flow, the angle valve closely resembles the ordinary globe. The angle valve’s discharge conditions are favorable with respect to fluid dynamics and erosion.

Globe Valve Disks

Most globe valves use one of three basic disk designs: the ball disk, the composition disk, and the plug disk.

Ball Disk

The ball disk fits on a tapered, flat-surfaced seat.  The ball disk design is used primarily in relatively low pressure and low temperature systems.  It is capable of throttling flow, but is primarily used to stop and start flow.

Com position Disk

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.

Plug Disk

Because of its configuration, the plug disk provides better throttling than ball or composition designs.  Plug disks are available in a variety of specific configurations.  In general, they are all long and tapered.

Globe Valve Disk and Stem Connections

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

Globe Valve Seats

Globe valve seats are either integral with or screwed into the valve body.  Many globe valves have backseats.  A backseat is a seating arrangement that provides a seal between the stem and bonnet.  When the valve is fully open, the disk seats against the backseat.  The backseat design prevents system pressure from building against the valve packing.

Globe Valve Direction of Flow

For low temperature applications, globe and angle 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 steam service, globe valves are installed so that pressure is above the disk.  Otherwise, the stem will contract upon cooling and tend to lift the disk off the seat.

Ball Valves

A ball valve is a rotational motion valve that uses a ball-shaped disk to stop or start fluid flow. The ball, shown in Figure 12, performs the same function as the disk in the globe valve.  When the valve handle is turned to open the valve, the ball rotates to a point where the hole through the ball is in line with the valve body inlet and outlet.  When the valve is shut, the ball is rotated so that the hole is perpendicular to the flow openings of the valve body and the flow is stopped.

Most ball valve actuators are of the quick-acting type, which require a 90   turn of the valve handle to operate the valve.  Other ball valve actuators are planetary gear-operated.  This type of gearing allows the use of a relatively small handwheel and operating force to operate a fairly large valve.

Some ball valves have been developed with a spherical surface coated plug that is off to one side in the open position and rotates into the flow passage until it blocks the flowpath completely. Seating  is  accomplished  by  the  eccentric  movement  of  the  plug.The  valve  requires  no lubrication and can be used for throttling service.

Advantages

A ball valve is generally the least expensive of any valve configuration and has low maintenance costs.   In addition to quick, quarter turn on-off operation, ball valves are compact, require no lubrication, and give tight sealing with low torque.

Disadvantages

Conventional ball valves have relatively poor throttling characteristics.   In a throttling position, the partially exposed seat rapidly erodes because of the impingement of high velocity flow.

P ort P atterns

Ball valves are available in the venturi, reduced, and full port pattern.   The full port pattern has a ball with a bore equal to the inside diameter of the pipe.

Valve Materials

Balls are usually metallic in metallic bodies with trim (seats) produced from elastomeric

(elastic materials resembling rubber) materials.  Plastic construction is also available.

The resilient seats for ball valves are made from various elastomeric material.  The most common seat materials are teflon (TFE), filled TFE, Nylon, Buna-N, Neoprene, and combinations  of these materials.Because of the elastomeric materials, these valves cannot be used at elevated temperatures.  Care must be used in the selection of the seat material to ensure that it is compatible with the materials being handled by the valve.

Ball Valve Stem Design

The stem in a ball valve is not fastened to the ball.  It normally has a rectangular portion at the ball end which fits into a slot cut into the ball.  The enlargement permits rotation of the ball as the stem is turned.

Ball V alve Bonnet D esign

A bonnet cap fastens to the body, which holds the stem assembly and ball in place.  Adjustment of the bonnet cap permits compression of the packing, which supplies the stem seal.  Packing for ball valve stems is usually in the configuration of die-formed packing rings normally of TFE, TFE-filled, or TFE-impregnated material. Some ball valve stems are sealed by means of O-rings rather than packing.

Ball Valve Position

Some ball valves are equipped with stops that permit only 90   rotation. Others do not have stops and may be rotated 360 . With or without stops, a 90  rotation is all that is required for closing or opening a ball valve.

The handle indicates valve ball position.  When the handle lies along the axis of the valve, the valve is open.  When the handle lies 90  across the axis of the valve, the valve is closed.  Some ball valve stems have a groove cut in the top face of the stem that shows the flowpath through the ball.  Observation of the groove position indicates the position of the port through the ball. This feature is particularly advantageous on multiport ball valves.

P lug Valves

A plug valve is a rotational motion valve used to stop or start fluid flow.  The name is derived from the shape of the disk, which resembles a plug.  A plug valve is shown in Figure 13.  The simplest form of a plug valve is the petcock.  The body of a plug valve is machined to receive the tapered or cylindrical plug.  The disk is a solid plug with a bored passage at a right angle to the longitudinal axis of the plug.

In the open position, the passage in the plug lines up with the inlet and outlet ports of the valve body.  When the plug is turned 90  from the open position, the solid part of the plug blocks the ports and stops fluid flow.

Plug valves are available in either a lubricated or nonlubricated design and with a variety of styles of port openings through the plug as well as a number of plug designs.

Plug Ports

An important characteristic of the plug valve is its easy adaptation to multiport construction. Multiport valves are widely used.  Their installation simplifies piping, and they provide a more convenient operation than multiple gate valves.  They also eliminate pipe fittings.  The use of a multiport valve, depending upon the number of ports in the plug valve, eliminates the need of as many as four conventional shutoff valves.

Plug valves are normally used in non-throttling, on-off operations, particularly where frequent operation of the valve is necessary.  These valves are not normally recommended for throttling service because, like the gate valve, a high percentage of flow change occurs near shutoff at high velocity.  However, a diamond-shaped port has been developed for throttling service.

Multiport Plug Valves

Multiport valves are particularly advantageous on transfer lines and for diverting services.   A single multiport valve may be installed in lieu of three or four gate valves or other types of shutoff valve.   A disadvantage is that many multiport valve configurations do not completely shut off flow.

In most cases, one flowpath is always open.  These valves are intended to divert the flow of one line while shutting off flow from the other lines.  If complete shutoff of flow is a requirement, it is necessary that a style of multiport valve be used that permits this, or a secondary valve should be installed on the main line ahead of the multiport valve to permit complete shutoff of flow.

In some multiport configurations, simultaneous flow to more than one port is also possible. Great care should be taken in specifying the particular port arrangement required to guarantee that proper operation will be possible.

Plug Valve Disks

Plugs  are  either round or  cylindrical with a  taper.     They may have  various  types  of  port openings, each with a varying degree of area relative to the corresponding inside diameter of the pipe.

Rectangular Port Plug

The most common port shape is the rectangular port.  The rectangular port represents at least 70% of the corresponding pipe’s cross-sectional area.

Round Port Plug

Round port plug is a term that describes a valve that has a round opening through the plug.  If the port is the same size or larger than the pipe’s inside diameter, it is referred to as a full port.   If the opening is smaller than the pipe’s inside diameter, the port is referred to as a standard round port.  Valves having standard round ports are used only where restriction of flow is unimportant.

Diamond Port Plug

A diamond port plug has a diamond-shaped port through the plug.   This design is for throttling service.  All diamond port valves are venturi restricted flow type.

Lubricated Plug Valve Design

Clearances and leakage prevention are the chief considerations in plug valves.  Many plug valves are of all metal construction.In these versions, the narrow gap around the plug can allow leakage.  If the gap is reduced by sinking the taper plug deeper into the body, actuation torque climbs rapidly and galling can occur.  To remedy this condition, a series of grooves around the body and plug  port openings  is  supplied  with grease prior  to actuation.          Applying  grease lubricates the plug motion and seals the gap between plug and body.   Grease injected into a fitting at the top of the stem travels down through a check valve in the passageway, past the plug top to the grooves on the plug, and down to a well below the plug.   The lubricant must be compatible with the temperature and nature of the fluid.  All manufacturers of lubricated plug valves have developed a series of lubricants that are compatible with a wide range of media. Their recommendation should be followed as to which lubricant is best suited for the service.

The most common fluids controlled by plug valves are gases and liquid hydrocarbons.   Some water lines have these valves, provided that lubricant contamination is not a serious danger. Lubricated plug valves may be as large as 24 inches and have pressure capabilities up to 6000 psig.  Steel or iron bodies are available.  The plug can be cylindrical or tapered.

Nonlubricated Plugs

There are two basic types of nonlubricated plug valves: lift-type and elastomer sleeve or plug coated.   Lift-type valves provide a means of mechanically lifting the tapered plug slightly to disengage it from the seating surface to permit easy rotation. The mechanical lifting can be accomplished with a cam or external lever.

In a common, nonlubricated, plug valve having an elastomer sleeve, a sleeve of TFE completely surrounds the plug.  It is retained and locked in place by a metal body.  This design results in a primary seal being maintained between the sleeve and the plug at all times regardless of position.  The TFE sleeve is durable and inert to all but a few rarely encountered chemicals.  It also has a low coefficient of friction and is, therefore, self-lubricating.

Manually Operated P lug Valve Installation

When installing plug valves, care should be taken to allow room for the operation of the handle, lever, or wrench. The manual operator is usually longer than the valve, and it rotates to a position parallel to the pipe from a position 90  to the pipe.

Plug Valve G lands

The gland of the plug valve is equivalent to the bonnet of a gate or globe valve.   The gland secures the stem assembly to the valve body.   There are three general types of glands: single gland, screwed gland, and bolted gland.

To ensure a tight valve, the plug must be seated at all times.  Gland adjustment should be kept tight enough to prevent the plug from becoming unseated and exposing the seating surfaces to the live fluid.   Care should be exercised to not overtighten the gland, which will result in a metal-to-metal contact between the body and the plug.  Such a metal-to-metal contact creates an additional force which will require extreme effort to operate the valve.

Diaphragm Valves

A diaphragm valve is a linear motion valve that is used to start, regulate, and stop fluid flow. The name is derived from its flexible disk, which mates with a seat located in the open area at the top of the valve body to form a seal.  A diaphragm valve is illustrated in Figure 14.

Diaphragm valves are, in effect, simple “pinch clamp” valves. A resilient, flexible diaphragm is connected to a compressor by a stud molded into the diaphragm. The compressor is moved up and down by the valve stem. Hence, the diaphragm lifts when the compressor is raised. As the compressor is lowered, the diaphragm is pressed against the contoured bottom in the straight through valve illustrated in Figure 14 or the body weir in the weir-type valve illustrated in Figure 15. Diaphragm valves can also be used for throttling service. The weir-type is the better throttling valve but has a limited range. Its throttling characteristics are essentially those of a quick- opening valve because of the large shutoff area along the seat. A weir-type diaphragm valve is available to control small flows. It uses a two-piece compressor component. Instead of the entire diaphragm lifting off the weir when the valve is opened, the first increments of stem travel raise an inner compressor component that causes only the central part of the diaphragm to lift. This creates a relatively small opening through the center of the valve. After the inner compressor is completely open, the outer compressor component is raised along with the inner compressor and the remainder of the throttling is similar to the throttling that takes place in a conventional valve. Diaphragm valves are particularly suited for the handling of corrosive fluids, fibrous slurries, radioactive fluids, or other fluids that must remain free from contamination.

Random Posts

Comments are closed.